pore pressures in clay embankments and cuttings by …

PORE PRESSURES IN CLAY

EMBANKMENTS AND CUTTINGS

by

HILARY JANE WALBANCKE

A Thesis submitted to the University of London for the degree of

Doctor of Philosophy in the Faculty of Engineering

Department of Civil Engineering Imperial College of Science and Technology

November 1975

SYNOPSIS

This thesis examines the pore pressure behaviour in cut slopes

in overconsolidated clays and fill slopes constructed using plastic

clays. In these cases long-term pore pressures are greater than those

at end of construction.

Pore pressure records since construction are presented for

Peterborough, Bough Beech and Grafham Water dams and records from

shallow piezometers installed during this project in Peterborough,

Grafham Water, Foxcote and Aldenham dams. From this data it has been

possible to show that equilibration rates in these clay fills are

comparable to those calculated from laboratory tests on large samples.

The Measurement of pore pressures in London Clay cuttings at

Edgwarebury and Potters Bar have made it possible to show that pore

pressure equilibration after construction is on the same time scale

as delayed failure and is probably a primary cause.

Equilibrium pore pressures are a function of the permeability

gradient within a clay layer and the boundary pressures. It has

only been possible to measure equilibrium pore pressures in very few

cases due to the long time scale therefore an indirect study of the

controlling factors has been used.

The effect of surface and base boundary pressures has been

considered and the seasonal variation of pore pressure beneath

grassed slopes with the effect of slope angle and roughness studied.

An assessment of the effect of surface drainage measures has been

made.

ii

Permeability gradients in in situ clays have been measured

previously. In situ permeability tests were carried out at Grafham

Water and Bough Beech as an attempt to measure permeability gradients

in clay fills. The pore pressures within clay slopes with permeability

gradients have been modelled on the electric analogue and an analytical

one dimensional solution proposed. The effect of internal drainage

within a fill slope has been considered.

Some recommendations have been made on the calculation of

equilibrium pore pressures for use in the design of cutting and

embankment slopes.

111

ACKNOWLEDGEMENTS

No research can be done in isolation and I am indebted to

many people without whom this thesis would not have been possible.

The project was suggested and supervised by Dr. P.R. Vaughan

whose continual interest and guidance I gratefully acknowledge.

He has always given generously of his time and ideas throughout

this research.

The work was carried out under the auspices of the Soil Mechanics

Section at Imperial College with the general guidance of Professor

A.W. Skempton and Professor A.W. Bishop.

Finance for the work on embankment dams and the development

of the equipment came from the Water Resources Board and for the

work on cuttings from the Science Research Council.

Permission for the work on various sites has been given by the

Central Electricity Generating Board at Peterborough, Bucks Water

Board at Foxcote, Great Ouse Water Division of the Anglian Water

Authority at Grafham Water, East Surrey Water Co. at Bough Beech,

Hertfordshire County Council at Aldenham, London Borough of Barnet at

Edgwarebury, and British Railways at Potters Bar and Oakleigh Park.

I am grateful to all those organisations for their permission to

work on these sites and to use their records.

The consulting engineers for these sites have generously allowed

access to their records. They are Binnie and Partners for Foxcote and

Grafham Water, Howard Humphreys and Sons for Edgwarebury, Rofe Kennard

and Lapworth for Peterborough, Bough Beech, Aldenham and Cow Green

and Harris and Sutherland for Canterbury.

iv

Ground Engineering Ltd. installed the piezometers at Potters

Bar, Aldenham and Oakleigh Park.

I would like to acknowledge the help and interest of many people

within these organisations, especially the following:

Mr. P.R. Walton the manager of the CEGB Land Reclamation Scheme

and his assistant, Mr. Thompson; Mr. K.J.H. Saxton, formerly manager

of the Great Ouse Water Division and his new works engineers

Messrs. W.O. Walsh and A.J. Seager-Smith; Mr. R. Pownall, manager

of Bucks Water Board and Mr. J. Mayer the pumping station superintendent

at Foxcote; Mr. J.S. Shinner, Chief Engineer, Mr. Williams and

Mr. K. Sharp of the East Surrey Water Co., Mr. W.J.G. Eveleigh, the

borough engineer of Barnet, and his assistants Mr. E.H. Start and

Mr. Jenkins; Mr. D. Ayres of British Railways soil mechanics section,

Mr. W.B. Emms of the Chief Civil Engineer's office, Eastern Region

and Mr. I. Warwick, the permanent way inspector at Potters Bar;

Mr. R.L. Brown of Binnie & Partners and Dr. H.T. Lovenbury of Rofe

Kennard and Lapworth. I am very grateful for the help with installation

given by Messrs. P. Bryan, F. Kindred and Dr. A.D. Burnett and help

with readings by Messrs. J.P. Apted, D.T. Evans and F.D. Evans.

My thanks go to Messrs. D.T. Evans, F.D. Evans and L.F. Spall

who willingly made and mended equipment, usually at short notice, and

Mrs. E.M. Gibbs who makes life in the Soil Mechanics Section better

for all of us.

V

I wish to acknowledge many helpful discussions with my friends

and colleagues especially Dr. R.J. Chandler, Dr. A.E. Skinner,

Dr. L.D. Wesley, Dr. M.L.G. Werneck, Messrs. M.W. Baldwin, W.M. Maguire,

R.S. Pugh, J.P. Apted, V.G. Sodha and E.N. Bromhead.

I would especially like to thank Dr. R.J. Chandler who generously

suggested that some of his unpublished data from Barnsdale could be

presented in this thesis.

My thanks go to Miss D.C. Lombard who typed the script with

speed and accuracy.

Finally I would like to thank my parents for their interest and

encouragement over the period of this research.

vi

PORE PRESSURES IN CLAY EMBANKMENTS AND CUTTINGS

CONTENTS

Page

SYNOPSIS

ACKNOWLEDGEMENTS iii

CONTENTS vi

SYMBOLS

CHAPTER 1 INTRODUCTION

1.1 Introduction 1

1.2 Development of research programme and questions studied 3

1.3 The design of clay slopes in practice 6

CHAPTER 2 SITES AND PROGRAMME

2.1. Boundary pore pressures in downstream slopes of plastic clay dams 9 2.1.1 Peterborough 9 2.1.2 Grafham Water 11

2.1.3 Foxcote 13

2.1.4 Installation of shallow piezometers 14

2.1.5 Direction of further research 16

2.2 In situ permeability in fills at equilibrium 16

2.2.1 Bough Beech 17

2.2.2 Permeability tests at Bough Beech 18

2.2.3 Permeability tests at Grafham Water 19

2.3 Equilibration of cut slopes 20

2.3.1 Edgwarebury 20

2.3.2 Potters Bar 22

2.4 The effect of vegetation 25

2.4.1 Oakleigh Park 25

vii

Page

2.5 Longterm pore pressures in clay dams 26

2.5.1 Aldenham 26

2.6 Other available data 28

CHAPTER 3 EQUIPMENT AND TECHNIQUES

3.1 Piezometers 30

3.1.1 Hydraulic piezometers 30

3.1.2 Electrical piezometers 34

3.1.3 Pneumatic piezometers 34

3.2 Piezometer installations in embankment dams 35

3.2.1 Foxcote 35

3.2.2 Peterborough 37

3.2.3 Grafham Water 40

3.2.4 Bough Beech 42

3.3 Installation of piezometers in boreholes 42

3.3.1 Casagrande standpipes 42

3.3.2 High air entry hydraulic piezometers 43

3.4 Measurement of pore pressure 45

3.5 Equalisation times 47

3.5.1 Piezometer response times 48

3.5.2 Equalisation after installation 51

3.5.3 Equalisation after de-airing 51

3.6 Falling head permeability tests 52

3.6.1 Theory 52

3.6.2 Equipment and test procedures 53

3.6.3 Calculation of permeability 55

3.7 Constant head permeability tests 57

3.7.1 Theory 57

3.7.2 Equipment and test procedures 59

3.7.3 Calculation of permeability 61

3.8 The electric analogue 62

\-e

viii

Page

-1

CHAPTER If

4.1

4.2

4.3

4.4

PRESENTATION OF DATA - EMBANKMENTS

Piezometric records from dams

4.1.1 Peterborough

4.1.2 Grafham Water

4.1.3 Bough Beech

Downstream boundary pore pressures

4.2.1 Slopes without drainage

4.2.2 Slopes with drainage

Permeability tests

4.3.1 Bough Beech

4.3.2 Grafham Water

Moisture contents

66

66

67

69

71

71

73

74

74

78

8o

CHAPTER 5 PRESENTATION OF DATA - LONDON CLAY CUTTINGS

5.1 Cutting slopes at equilibrium 82

5.1.1 Potters Bar - old side 82

5.2 Cutting slopes not at equilibrium 83

5.2.1 Edgwarebury 83

5.2.2 Potters Bar - new side 85

5.3 The effect of trees 86

5.3.1 Oakleigh Park 86

CHAPTER 6 DISCUSSION

6.1 End of construction pore pressures 87

6.1.1 Fill slopes 87

6.1.2 Cut slopes 89

6.2 Rate of equilibration 92


6.2.2 Cut slopes 100

6.3 Ultimate pore pressures 105

6.3.1 Cut and natural slopes 105


6.3.3 Stability of equilibrated slopes 108

ix

Page

6.4 Prediction of pore pressures in cut and fill slopes at equilibrium and during equilibration 110

6.5 Surface boundary pressures 112

6.5.1 Grassed slopes 113

6.5.2 Gravel layers 117

6.5.3 Effect of vegetation 117

6.5.4 Counterfort drains 119

6.5.5 Summary

121

6.6 The effect of permeability varying with depth 122

6.6.1 The effect on equilibrium pore pressures 122

6.6.2 The effect of a permeability gradient on equilibration rate 131

CHAPTER 7

CONCLUSIONS AND DESIGN RECOMMENDATIONS

7.1

End of construction • 133

7.2

Equilibration rates 134

7.3

Equilibrium pore pressures

135

REFERENCES

APPENDICES

Appendix A Peterborough drawdown records

Appendix B End of construction pore pressures in clay dams

140

148

149

B

SYMBOLS

A pore pressure coefficient

A E Vyw

a constant

a piezometer radius

a1 piezometer outside radius

a3 piezometer inside radius

constants a1,2,etc

B pore pressure coefficient

pore pressure coefficient — Au Aa

constants of intergration C1,2,etc

c coefficient of swelling or consolidation

cs coefficient of swelling

cv coefficient of consolidation

c' cohesion intercept in terms of effective stress

cf clay fraction

d diameter

E elevation

e voids ratio

e base of natural logarithms

F piezometer intake factor

f function

H thickness of clay layer

H height of embankment or depth of cutting

h hydraulic potential

I current

xi

i hydraulic gradient

K0 coefficient of earth pressure at rest

k coefficient of permeability

k0 permeability at depth z = 0

k1 soil permeability

k3 ceramic permeability

kc permeability measured in constant head test

kf permeability measured in falling head test

kH permeability at depth z = H

kH horizontal permeability

kv vertical permeability

L length

LL liquid limit

m coefficient of soil compressibility

mf coefficient of pore fluid compressibility

p pressure

PL plastic limit

Qt flow at time t

Q. steady state flow

R resistance

✓ radius

ru pore pressure ratio = yz

T time factor

t time

t90 time to 90% equilibration

u pore pressure

0 initial piezometer cavity pressure

'14

xii

ua pore air pressure

ug pore pressure in soil

ut cavity pressure at time t

uw pore water pressure

uz pore pressure at depth z

✓ voltage

v seepage velocity

w water content

z depth

a slope angle to horizontal

unit weight of soil (assumed = 2yw unless known)

Yw unit weight of water

A increment

equalisation ratio

n1,2 = 1/2{P±(112-401/2)

A = k1/k3 (a1/a3 - 1)

u stiffness of measuring system

a total stress

a1,3 major and minor principal stresses

a' effective stress

av total vertical stress

a' effective vertical stress

angle of shearing resistance in terms of effective stress

SZ ohms

1

Chapter I

INTRODUCTION

1.1 INTRODUCTION

The design of slopes in terms of effective stress requires a

knowledge of the pore pressures within them. Both shear strength

and strains are functions of effective stress a'

where a' = a — u 1.1

During construction, if no drainage occurs, the pore pressure

changes are a function of the total stress and they are a dependent

variable.

Au = f( Aa )

1.2

and the pore pressures are:

u = uo

+ f( Acr )

1.3

Total stress stability analysis can therefore be used.

Once drainage occurs, pore pressure is no longer a dependent

variable and total stress methods are no longer applicable. During

drainage:

Au = f (cv o; s, At,- boundary conditions) 1.4

and when the equilibration process to steady seepage conditions is

complete then

u = f(k, boundary conditions) 1.5

For clay slopes the construction process is nearly undrained

unless construction rates are very slow, the drainage paths are very

short or structures within the clay give it a high bulk permeability.

Bishop and Bjerrum (1960) describe the pore pressure changes

which occur due to an excavation in clay and the subsequent

equilibration (Fig.1:1). During excavation, the magnitude of the

pore pressure change is dependent on the pore pressure parameter A,

(Skempton 1954) for a saturated clay where B = 1 as shown in fig.1:1

where A & B are defined as:

= B { Aci3 + A(Acri - Aa3)} 1.6

In overconsolidated nays the end of construction pore pressures

will be lower than those at equilibrium and long-term design will be

critical for stability. In normally consolidated clays where A values

are larger (Skempton 1954) the difference between end of construction

and long-term pore pressures will be smaller.

In fill slopes the end of construction pore pressures depend

on B, where

Au = BAa1 1.7

which itself is dependent on plasticity and placement water content.

For all clay fills some suctions are set up at low stresses. At

higher stresses in the less plastic, sandy clays placed wet of

optimum, E is large and excess pore pressures are built up during

construction, for example Usk Dam (Sheppard and Aylen, 1957) and

Selset Dam (Bishop and Vaughan, 1962). The equilibration process in

3

these cases is mostly consolidation, with some swelling in low stress

zones, and the end of construction condition is critical for stability.

In plastic clays, unless placed very wet, the suctions extend

much deeper and quite high fills can have depressed pore pressures,

for example Bough Beech Dam (Hallas & Titford, 1971). The equilibration

process in these cases is predominantly swelling and stability

deteriorates from end of construction to the equilibrium condition.

This thesis concentrates on those slopes where the long-term

pore pressures are greater than those at end of construction and

long-term pore pressures are required for design. These are cut

slopes in overconsolidated clays and fill slopes constructed using

plastic clays.

1.2 DEVELOPMENT OF RESEARCH PROGRAMME AND QUESTIONS STUDIED

At the time this work started there was no established method of

predicting long-term pore pressures under these conditions except by

a conventional flow-net solution. By this date Sweeney (1970) had

shown that a flow-net solution was not valid for the pore pressures

in the Boulder Clay slope at Cow Green. Apart from this one case

there was no check on the validity of the methods in common use.

Therefore an attempt was made firstly to establish the actual mechanisms

of behaviour and secondly to establish some simple methods of prediction.

The initial work was an examination of pore pressures in

embankments of modern rolled clay construction, as long as possible

after construction, to obtain information on the extent to which the

steady state had been approached and information from which steady

4

state pore pressures could be predicted. This work on fills was part

of a programme supported by the Water Resources Board to examine end

of construction, steady seepage and rapid drawdown stability of

embankment dams which were constructed using plastic clays. Laboratory

testing of field compacted clays was carried out by Sodha (1974) as

part of the same programme. Laboratory testing was insufficient

because of problems of representative sampling, of modelling free

boundary conditions and of extrapolation from laboratory to field

scale. Therefore field measurements of pore pressure have been used

and initially the three dams, Peterborough, Grafham Water and Foxcote

were studied.

It became clear at an early stage that these dams were still a

long way from equilibrium, even Foxcote,then 15 years old, and it was

not going to be possible to measure equilibrium pore pressures directly

except where closely spaced drainage blankets had been used. Indirect

methods had therefore to be considered and in situ permeability tests

were carried out at Grafham Water and Bough Beech Dams.

This slow equilibration rate raised the question, did cuttings

in overconsolidated clays swell as slowly as the embankment dams and

was the equilibration rate controlling delayed failure? The work

was then extended to study cut slopes in London Clay, supported by

a grant from the Science Research Council.

The interdependent questions studied for both cut and fill slopes

were: What is (i) the rate of swelling, (ii) the magnitude of the

ultimate pore pressure and how can these two be predicted and controlled?.

1

5

In cutting slopes, where no observations are available, a simple

method of predicting end of construction pore pressures was required

before swelling rates could be ascertained.

The rate of swelling is of considerable interest and has received

little previous study. Swelling is accompanied by a reduction in

strength and failure may be delayed by the rate at which swelling

can occur. Hence the rate of pore pressure equilibration after

excavation of a slope is of major importance in the study of delayed

failure.

A number of mechanisms have been postulated to explain delayed

failures in overconsolidated fissured clays. For example, drained

strength may reduce with increased loading time (de Lory, 1957;

Henkel, 1957), or with the release of strain energy due to the reduction

of strength of interparticle bonds by weathering (Bjerrum, 1967).

These mechanisms only need be invoked if the delay cannot be fully

explained by pore pressure equilibration.

Comparisons between field rates of swelling and those predicted

--r

by laboratory tests can be made and from this comparison a method of

predicting field rates from the results of laboratory tests can then

be suggested.

Historically, ultimate pore pressures have generally been predicted

by the use of flow nets which assumes uniform permeability for the

clay layer and a phreatic surface which is a flow line. In clay slopes

this free surface concept is likely to be invalid as the clay can remain

saturated above the zero pressure line and pore water suctions exist.

6 -r

This zero pressure line, which has been equated with a phreatic surface,

is not a flow line and considerable flow from the slope surface occurs

across it. The flow across this boundary can be greater than the

flow through the body of the slope. Also the permeability is probably

not uniform due to both stress and weathering effects. The slope at

Cow Green (Sweeney, 1970) requires a permeability gradient of at least

5 to 1 to explain the perched water table and a flow-net will not

predict it. Thus, both the permeability gradient and the boundary

pore pressures are required to predict ultimate pore pressures.

The control of ultimate pore pressures by controlling the boundary

pressures either by surface drainage or vegetation and the control of

both equilibration rates and the ultimate pore pressures by internal

drainage have been considered.

1.3 THE DESIGN OF CLAY SLOPES IN PRACTICE

In Great Britain, the first major earthworks were those of the

Canal and then the Railway eras. Most railway cuttings were excavated

between 1850 and 1930 and James (1970), who made a study of cutting

failures, quotes a number of sections. The slopes to which the

cuttings were excavated are summarised in Table 1:1 for a variety of

overconsolidated clay formations. This table may give an emphasis

to the steep end of the range of slopes used as it is based on cuttings

which have failed. The steepest slopes used are of the order of 1 on 2

except in those clay formations which are somewhat cemented such as

the Lower Lias and Oxford Clays. The steepest slope quoted is 1 on 1.7

in the Lower Lias.

7

Most recent cuttings have been part of the motorway programme

where the more stringent geometrical requirements have resulted in

considerably more earthworks than had previously been used. Many

kilometres of motorway have been constructed using a standard slope

throughout for both cuttings and embankments. Some of these slopes

are quoted by Symons (1970) when discussing the cost of maintenance.

Many slopes on the M.1. motorway in the chalky Boulder Clay were

constructed at 1 on 12 or 1 on 2 in 1959. Symons (1970) quotes 97

cases of instability, see table 1:2. Steep slopes, 1 on 12, were

also used in Boulder Clay on the early part of the M6, built 1958,

where stability problems also occurred. Since the early 1960's clay

slopes have seldom exceeded 1 on 2 and there has been a tendency to

use flatter slopes. For example, the London Clay cuttings at

Edgwarebury where the slopes vary from 1 on 3 to 1 on 4 and the Lias

Clay cutting on the Grantham-By-Pass where 1 on 4i slopes were used.

Symons (1968) suggested guidelines for road cutting design using

residual strengths which, if followed, resulted in slopes of the

order of 1 on 4 or 1 on 5.

-r The slopes used for embankment dams are shown in table1:3 along with

drainage and slope protection measures. Where known, the soil

parameters are also quoted. These slopes are a better representation

of general design practice than those quoted in either table 1:1

or 1:2 as, except for Aldenham and Muirhead, these are at present

stable slopes. The majority of the dams are of Glacial Till of low

plasticity. In these cases the end of construction condition would

have been critical for stability and dams which are now stable should

remain stable; On the other hand, the dams constructed using clays

of high plasticity have not yet reached equilibrium except for

Aldenham, and problems of stability, if any, are still to come.

The slopes used at Aldenham are too steep and have given trouble

since construction. Attempts to maintain these slopes have met with

intermittent failures. The original cross section of Aldenham dam

is not known, therefore it has not been included in table 1:3.

Sections of the dam as it is in 1975 are shown in Figs. 2:13 and

2:14.

-r-

Table 1.1

Clay cutting slopes used by the railways

Geological Formation Slopes used

Chalky Boulder Clay 1 on 2 1 on 3.'

London Clay 1 on 2 -+ 1 on 3.5

Wealden Series 1 on 2 1 on 2.8

Kimmeridge Clay 1 on 3

Oxford Clay 1 on 1.8 1 on 2.5

Upper Lias Clay 1 on 2 1 on 2.5

Lower Lias 1 on 1.7 1 on 3

Note:-

Based on failed cutting slope data from James (1970).

-Table 1.2

Cutting and embankment slopes used on recent major roads

Road County Date Material Slopes used Remarks

M1 Herts to Northampton

1959 Boulder Clay 1 on 11/2 -+ 1 on 2 97 cases of instability quoted - 45 in cuttings & 62 in embankments, generally where H > 5m. 60% of slips occured in 1965-66.

M1 Leicester 1964-65 Boulder Clay 1 on 3 3 failures quoted, 2 in cuttings 1 in embankment.

M6 Stafford 1962-63 Soft clay over Coal Measures

1 on 2 3 cutting failures quoted.

M6 Cheshire 1963 Boulder Clay 1 on 2 or flatter Failure in 7 m deep cutting

M6 Lancashire 1958-63 Boulder Clay 1 on 11/2 in 1968 1 on 21/2 later

9 cutting and 6 embankment failures.

Al Huntingdon 1956 or later

Boulder Clay Lias Clay

1 on 2 1 on 3

Failures in 6 m deep cutting Failures in 10 m deep cutting

Al Lincoln 1956 or later

Clays, general Lias Clay

1 on 2 1 on 41/2

Probably both Lias & Boulder Clay. Grantham By-pass.

Based on Symons, 1970.

Table 1.3

Clay Embankment Dams in Britain

HURY: H = 30.5 m, completed 1894.

Construction material: Glacial Till, with puddle core

Slope protection: upstream, 0.5 m on upper third of slope.

downstream, none.

Unpublished.

2.5

BLACKTON: H = 24.4 m, completed 1896.

Construction material: Glacial Till, with puddle core.

Slope protection: upstream, 1 m.

downstream, none.

Unpublished.

Note (i) slopes shown as cotangent of angle to the horizontal.

(ii) parameters quoted are design values where known.

Sheet 2

Table 1.3

NUIRHEAD: H = 21m, completed 1943.


cf 18% c' LL

PL Cu 75 + 130 kN/m2

opt. w cv place. w - k -

Slope protection: upstream, 0.5m pitching over 0.15m

broken stone. downstream, none

Ref: Banks (1948).

cf 10%

LL 37%

PL 16%

opt. w 15%

place. w +5%

KNOCKENDON: H = 27.5m, completed 1946.


c'

c t Cu 36 4 62 kN/m

cv

Slope protection: upstream, pitching.

downstream, none.

2

)1.

Ref: Banks (1952).

v

FOXCOTE: H = 10m, completed 1956.

Construction material: Glacial Till.

Table 1.3 sheet 3

cf

LL 25% 41' 300

PL 17% cv 12.3 m2/yr

opt. w 10% k 1.7 x 10-9 m/sec

place. w 0 4- +2%

Slope protection: upstream, 0.1 to 0.15m concrete slabbing

on 0.1m gravel. downstream, none.

Ref: Sheppard & Aylen (1957).

cf - c'

LL 65% (1)' -

PL 29% cv -

opt. w 27% k -

place. w +1%

Slope protection: upstream, 0.15m concrete slabbing over

0.15m gravel. downstream, 0.3m gravel

under 0.5m topsoil.

Ref: Little (1958), Little & Vail (1960).

USK: H = 29.5m, completed 1954


c' 24 kN/m2

Sheet 4 Table 1.3

4

3.25

SELSET: H = 40m, completed 1959.


cf 18% c' 24 kN/m2

LL 30% (P I 24°

PL 16% cv 1.4 + 4.5 m2/yr

opt. w 11%

place. w -2 + +1%

Slope protection: upstream, stone pitching at top,beaching below.

downstream, none.

Ref: Bishop et al (1960), Bishop & Vaughan (1962),

Kennard & Kennard (1962).

V

SEAGAHAN: H = 27.5m, completed 1960.

Construction material: Glacial Till,

cf 8% c' 2.9 kN/m2

LL 38% 31.5°

PL 16% cv 1.0 + 1.5 m2/yr

opt. w 13% k 6 + 10 x 10-11 m/sec

place. w +3.5%

Slope protection: upstream, unknown.

downstream, rock toe extends up slope.

Ref: Lucks (1966).

3.5

4.5

cf

LL

PL

opt. w 20%

c' 0 (1) 1 27 0

cv 0.3 + 1.3 m2/yr

k -

Table 1.3 sheet 5

BLACK ESK: H = 19.5m, completed 1961.


cf 17% 0

LL 25% (1)' 28.5°

PL 14% cv 1.8 + 4.5 m2/yr

opt. w 12.6% k 1.3 x 10-10 m/sec

place. w +1%

Slope protection: upstream, 1.5 to 2.1m (material unknown).

downstream, 0.5m.

Ref: Lucks (1966).

PETERBOROUGH: H = 16m, completed 1963.

Construction material: Oxford and Kellaways Clays

place. .w 0 + +2% core, -6 + -2% shoulders.

Slope protection: upstream, 0.6m broken brick.

downstream, none.

Sheet 6 Table 1.3

GRAFHAM WATER: H = 25m, completed 1964.


cf c' 12 kN/m2

LL 58% 22.5°

PL 20% cv 0.3 m2/yr

opt. w 19% k 1 x 10-11 m/sec

place. w -1 + +2%

Slope protection: upstream, 0.15 to 0.2m concrete slabbing

over 0.15m gravel. downstream, 0.15m

gravel under 0.2m topsoil.

Ref: Hammond & Winder (1967).

WEST WATER: H = 30.5m, completed 1965.


cf 15% c' 0

LL 26% 33.5o

PL 14% cv 1.3 + 1.7 m2/yr

opt. w 12%

place. w -2.5 + +5%

Slope protection: upstream, 1.5 to 2.1m (material unknown).

downstream, 0.75m.

Ref: Lucks (1966).

Table 1.3 sheet 7

BACKWATER: H = 40.5m, completed 1968.


cf c' 9.5 kN/m2

LL 27% 32°

PL 17% cv 5.6 m2/yr

opt. w 11%

place. w +1.5 +2%

Slope protection: upstream, 0.75 to 1.1m, concrete blocks at

top, rip-rap below. downstream, 0.75m gravel.

Ref: Wilkinson et al (1970), Geddes et al (1972).

cf

LL 0' 19.5

PL cv 0.9 m2/yr

opt. w <10-10 m/sec

place. w -2 +2%

Slope protection: upstream, 0.85m gravel, concrete slabs at top.

downstream, 0.3m gravel under topsoil.

Ref: Hallas & Titford (1971).

BOUGH BEECH: H = 23m, completed 1968.

Construction material: Weald Clay

o' 17.7 kN/m2

Sheet 8

Table 1.3

ARLINGTON: H = 13m, commenced 1969.

Construction material: Weald Clay.

cf 46-, 77% c' 0 4.8kN/m2

LL 0' 20.5°

PL cv opt. w 22%

place. w opt.

Slope protection: upstream, 0.15m concrete slabbing on 0.2m

gravel. downstream, 0.15m sand under

0.5m topsoil.

Unpublished.

V

EMPINGHAM: H = 37m, completed 1975.

Construction material: Lias Clay.

cf -50% c' 0

LL 45 -• 63% 0' 23°

PL 20-* 27% cv 0.5 m2/yr

opt. w 21 -• 22% k -10-11 m/sec

place. w -1 -• +3%

Slope protection: upstream, 0.9 to 1.25m rip-rap. downstream, 0.15m gravel under 0.3m topsoil.

Unpublished.

on finalGWL

final PWP — end of construction PWP A =1

equipotential A =0

1-1- A=1

0 F c', 0' method )

0 time

final GWL

0 method applicable

A=0

Irapid )rpore pressure equilibrium excavation redistribution

initial PWP —

a)

a

The changes in pore pressure and factor of safety during and after the excavation of a cut in clay [Bishop & Bjerrum, 1960]

fig. 1.1

9

Chapter II

SITES AND PROGRAMME

2.1 BOUNDARY PORE PRESSURES IN DOWNSTREAM SLOPES OF PLASTIC CLAY DAMS

The measurement of boundary pore pressures required the choice

of several embankment slopes of modern construction and of known fill

type which might have approached equilibrium pore pressures. A

selection of different slopes, surface drainage and vegetation were

required, also the dams should be situated in areas with similar

rainfall and evaporation conditions.

Three dams were chosen: Peterborough, Grafham Water and Foxcote.

Their geographical location is shown in Fig.2:1.

2.1.1 Peterborough (TL 190950)

This embankment, a temporary structure, designed for a life of

about 30 yrs by Rofe, Kennard & Lapworth (Kennard 1967), was built in

a worked out brick pit for the Central Electricity Generating Board's

Land Reclamation Scheme. The old brick pits in the neighbourhood

are being reclaimed using P.F.A. from several East Midlands power

stations. The dry P.F.A. is brought to the site by rail. Reservoir

water is added before pumping the resulting slurry to its deposition

location. Excess water is removed and returned to storage. With

this recirculation, the reservoir water is now a weak acid.

Construction of the embankment took place between July and

October 1963. Impounding,commenced in November 1963, was completed

after the winter of 1964/65.

10

The embankment foundation is the remaining "i to 12 m of Oxford'

Clay left in the base of the pit underlaid by 4 m of Kellaway3Sand

then 2 m of Kellaway5Clay above the Cornbrash Limestone. The fill

material is Oxford Clay with some Kellaway3Sand and Clay intermixed.

The Kellaways Sand is mainly silt and clayey silt. The 'redeposited'

clay used for the core and downstream toe is 'callow' from the

brick pit. This 'callow' is the weathered overlying clays rejected

by the brick makers and dumped in the worked out pits. The natural

moisture content of this material is much higher than that of the

in situ Oxford Clay. A cross section through the embankment is shown

in Fig.2:2.

There are drains at 15 m intervals between the downstream shoulder

foundation and fill connected by a trench drain 15 m upstream of the

toe. Otherwise no filters or internal drainage is installed. The

'callow' core was used to guard against a possible sand layer through

the dam.

A single track railway, a road and several pipelines are carried

on the wide crest of the dam. Pitching on the upstream shoulder is

0.5 m of broken brick. The downstream shoulder is grassed but is

rough, unmown or grazed and rutted in places. The top 4-5 m, which

are above surrounding ground level and visible from the main London -

Edinburgh railway, have been planted with trees. Rabbits are also

active in this part of the bank.

During construction, thirty piezometers were installed in the

positions shown in Fig.2:2. They are all twin tube hydraulic piezometers,

with high air entry Imperial College type ceramic filters and polythene

coated nylon 11 leads. The readout is by mercury manometers. Details

of this installation are given in Vaughan (1965) and discussed further

in Chapter 3. Two additional Casagrande piezometers were installed

in the upstream shoulder during 1970 in the positions shown in

Fig.2:2.

2.1.2 Grafham Water (TL 171670)

Grafham Water (Diddington) Dam near St. Neots, Cambridgeshire, was

designed for the Great Ouse Water Authority (now part of the Anglian

Water Authority) by Binnie & Partners. The reservoir is part of a

pump storage water supply scheme, water being pumped from the River

Ouse and thence supplying parts of the East Midlands. The reservoir

is also put to considerable recreational use with sailing and trout

fishing. Details of the scheme are given in Hammond & Winder (1967).

The geology of the site is glacial till overlying Oxford Clay.

The till is a matrix dominated plastic clay derived almost completely

from the Oxford Clay in the area. The geotechnical properties of the

till differ very little from those of the remoulded parent material.

This till forms both foundation and fill for the embankment.

Construction of the road embankment commenced in the spring of

1963 and was completed that year. The construction of the main dam

commenced in the summer of 1963 and was completed by the end of 1964.

Impounding took about 15 months and was completed in spring 1966.

The dam shown in Fig.2:3, has a chimney drain downstream of the

core and a drainage mattress under the downstream shoulder. Both

shoulders have gravel drains at 1.5 m (5 ft) intervals. The 1 on 5

12

upstream slope has a 0.9 m gravel layer beneath 0.2 m concrete slabbing.

On the downstream shoulder, which is 1 on 4, the 0.15 m of gravel is

covered by 0.2 m of topsoil. The grassed slope is mown regularly.

The B661 road diversion is carried on an embankment 130 m

downstream of the centre line of the dam. This is of rolled clay

fill without any internal drainage and was used as a trial bank for

the main dam. The area between the two embankments has been filled

with-a rolled clay weight block.

Twenty piezometers in ten groups of two were installed in the road

embankment in the positions shown in Fig.2:4. One of each pair was

a hydraulic piezometer with a high air entry ceramic tip, the other

a Maihak vibrating wire electrical piezometer. During the embankment

trial the Maihak piezometers-were shown to be unsuitable for use in

partly saturated soil (Bishop et al, 1964) so none were used in the

main embankment fill.

In the main dam 46 piezometers were installed on two cross sections

(Fig.2:5 & 2:6). Those in the foundations are the Maihak electrical

type and the remainder are hydraulic. All the hydraulic piezometers

are of the Imperial College type and have high air entry filters and

Saran leads. In the gauge house the road embankment piezometers are

read by mercury manometer. The main dam hydraulic piezometers are

transferred at the gauge house to Maihak transducers and the signal

relayed, with those of the remainder of the Maihak piezometers, to

the pumping station. More detail is given in Chapter 3.

is

13

2.1.3 Foxcote (SP 714363)

Foxcote dam at Maids Morton near Buckingham was also built as a

pump storage reservoir drawing water from the River Ouse and it is

used to supplement the supply from the river during dry periods.

The scheme was designed by Binnie & Partners for the Bucks Water

Board (now part of the Anglian Water Authority). The quality of the

stored water is poor and the supply is seldom used.

The site is on glacial till overlying the Cornbrash. The till

is a matrix dominated plastic clay with the predominant constituents

said to be Gault Clay and some Forest Marble. This till forms both

the fill and the foundation of the embankment (Little 1958 and Little

& Vail 1960).

The dam is believed to be one of the first, homogeneous rolled

clay fill dams built in Britain (Civil Engineering & Public Works

Review, 1957), and was completed in 1956. Impounding was completed

by 1957/58.

Selection of fill materials was minimal although more stoney

clay was restricted to the downstream shoulder zone. No difference

was apparent when hand augering within the two zones. The 1 on

2.75 downstream shoulder slope (see Fig.2:7) has a 0.3 m gravel layer

below 0.5 m of top soil and the grassed surface is kept short by

mowing and grazing of sheep. A drainage blanket runs under the lower

three-quarters of the downstream shoulder.

Penman (1956) quotes Foxcote among the first British dams to have

the B.R.S. twin tube hydraulic piezometers installed. These have

low air entry ceramic tips, polythene leads and a Bourdon gauge pressure

14

measuring system. Twenty four of these piezometers were installed in

the fill, twelve in the positions shown in Fig.2:7 and a further

twelve in similar positions on a second section. Despite the

unsuitability of the piezometers for measuring negative pore pressures

(see Chapter 3) records of tips 2 & 19 show small negative pore pressures

in the fill at the end of construction. These negative pressures

apparently dissipated rapidly, a function of the piezometer rather than

the soil.

2.1:4 Installation of shallow piezometers

During June 1971 the manometer system for the main dam piezometers

at Peterborough was modified. The valves which had been installed at

the time of construction proved to be unsatisfactory and had to be

removed. This involved the complete removal of the de-airing manifold

and subsequently all de-airing has to be carried out with portable

equipment which is attached to individual piezometers. A complete

de-airing of the piezometers was carried out before readings were

recommenced. The equipment and techniques used are discussed in

Chapter 3. Piezometers 16 to 18, 21 to 23 and 27 had the input removed

from the mercury manometer and modified to be read with the portable

transducer equipment. These piezometers being considered the more

important for long-term monitoring. The modification ceased the

problem of de-airing and gave more frost protection.

At Peterborough in July 1971 six shallow twin tube high air

entry piezometers A to F, depths 0.9 to 2.5 m, were installed as

described in Chapter 3 in the positions shown in Fig.2:2. Two more

15

piezometers, G & H, were installed in a wide berm, with a slope of

1 on 20, also constructed of 'callow'. C & D are among trees at

the crest of the embankment. Readings on these piezometers and the

main installation have been taken from 1971 to 1974 except on D which

was lost in 1972 when it was incorporated in a rabbit warren.

On completion of the Peterborough installation nine shallow

piezometers of the same type as used at Peterborough were installed

at Grafham Water at depths between 0.9 and 2.2 m in the positions

shown in Fig.2:8. Three (P, Q & R) are in the 1 on 3 road embankment

slope and four,(U,V,W & X,)are in the 1 on 15 slope weight block where

there is no drainage. Two are in the 1 on 4 main dam, Y, midway

between two drainage layers and Z at the lower third point between the

next two drainage layers. Readings were taken on these piezometers

between 1971 and 1974 except at X which was damaged by a site vehicle

in 1973.

Six shallow piezometers 1to 6,of the same type as used at Peterborough,

were installed in Foxcote dam at depths between 1.2 & 4.2 m during

July 1971 in the positions shown in Fig.2:7. Readings were taken

on these piezometers between 1971 and 1974, all of them operating

throughout this period.

During August 1971, pump repairs at Peterborough required the

partial drawdown of the reservoir. With the aid of the CEGB

personnel it was possible to obtain daily, later weekly, readings

on the upstream piezometers to follow this drawdown and the subsequent

recovery. The drawdown results are presented in Appendix A.

16

2.1.5 Direction of further research

The preliminary results from this first part of the project

indicated two questions for further research. (a) Perched water

tables were being formed in the downstream slopes but it was still

uncertain whether they were a permanent feature of fully equilibrated

slopes. The Peterborough data indicated that they may well be. permanent.

If a permanent perched table is to exist, a permeability gradient

with k decreasing with increasing effective stress is required.

In a natural clay till slope at Cow Green, Sweeney,(1970) calculated

that a permeability variation of 5 to 1 was sufficient to form a

perched table. (b) The lack of equilibration in slopes without

closely spaced drainage blankets after as much as 15 yrs indicated

very low field values of cv & cs for the fill materials, of the same

order of magnitude as the laboratory values. The question then arose,

did this also occur in excavated slopes in in situ clay where,

according to Rowe (1972) the field permeability could be several

orders of magnitude greater than the laboratory values and the resulting

field swelling rates much more rapid.

2.2 IN SITU PERMEABILITY IN FILLS AT EQUILIBRIUM

Once it was discovered that equilibration was not complete in the

slopes under observation, in situ permeability tests were carried

out to examine the variation of permeability with effective stress

in order to obtain evidence for long term perched water table effects.

17

The measurement of in situ permeability in fills required a

suitable hydraulic piezometer installation which was still in operable

condition. This restricted the choice to dams and to those built

after 1959. Because of this short time scale only slopes with closely

spaced drains had any chance of being at equilibrium. The two dams

chosen for this part of the study were Bough Beech and Grafham Water.

The geographical location of these two 'dams is shown on Fig.2:1.

2.2.1 Bough Beech (TQ 492470)

Bough Beech dam, near Westerham, Kent, designed by Rofe Kennard

& Lapworth for the East Surrey Water Company (now part of the Thames

Water Authority) is a pump storage supply reservoir. The intake

is from the River Eden, a tributary of the Medway and the supply is

used to augment water taken direct from the Eden and from wells.

It is the largest pump storage scheme in South-East England (Hallas

& Titford, 1971). The reservoir is also used for sailing and trout

fishing.

The reservoir site lies entirely on Weald Clay, proved to a depth

of over 30 m across the site. Within the clay are a series of siltstone,

sandstone and limestone beds between 0.2 and m thick. A valley

bulge was exposed in the cut-off trench. Weald Clay from the reservoir

site is used for the embankment fill.

Construction of the embankment was commenced in 1967 and was

completed by the end of 1968. Impounding was started by a flood in

September 1968 but the reservoir was immediately drawn down again.

Impounding proper started during the winter of 1968-69 and was completed

by the end of the winter 1970-71.

18

The dam, which is curved in transverse section convex downstream,

is a maximum of 23 m high. The internal drainage consists of a

chimney drain downstream of the core, base drainage blankets of sand

and intermediate drainage blankets 0.3 m thick at 2.4 m centres both

upstream and downstream of the core as shown in Fig.2:9. The upstream

slope, (1 on 5) has a 0.8 m thick gravel layer while the downstream

slope (1 on 4 and 1 on 42) has 0.3 m of gravel covered by topsoil

and grassed. Sheep are grazed on the downstream shoulder, keeping

the grass cropped short.

Thirty-nine twin tube hydraulic piezometers with Imperial College

type high air entry tips and polythene coated nylon 11 leads were

installed during construction at the positions shown in Fig.2:9.

These include two clusters, tips 4 to 12 and 21 to 29, which are

placed in the lower drain, seven at 0.3 m intervals through the clay

fill and the ninth in the upper drain.

2.2.2 Permeability tests at Bough Beech

A complete set of readings was taken on the piezometers then 16 of

them, 5 in the downstream shoulder and 11 in the upstream shoulder,

were de-aired in June 1974. After allowing the pore pressures to readjust

after de-airing, the downstream shoulder still showed negative pore

pressures of up to 4.3 m of water. However, the upstream shoulder

showed equilibrium pore pressures within 0.3 m of reservoir level.

Therefore permeability testing was limited to the upstream shoulder

as permeability values at equilibrium were required.

19

Falling head permeability tests were carried out during July

1974 on the 11 upstream shoulder piezometers. Where possible constant

head tests were also carried out on the same piezometers as a check

on the consistency of the two methods. Details of the equipment,

testing techniques, and analysis methods used are given in Chapter 3.

The results are given in Chapter 4.

2.2.3 Permeability tests at Grafham Water

A complete de-airing of the hydraulic piezometers was carried

out at the beginning of September 1974. After allowing for re-adjustment

after de-airing the pore pressures in the downstream shoulder were all

slightly negative, only three tips showing more than 1 m negative.

The upstream shoulder, except in one case, all showed pore pressures

within 0.3 m of reservoir water level. Although the downstream

shoulder piezometers did not record equilibrium pore pressures they

were closer than those at Bough Beech and it was decided to carry out

permeability tests on both upstream and downstream piezometers.

It was decided after the generally good agreement obtained at

Bough Beech that only falling head permeability tests would be

carried out at Grafham Water. Details of the techniques are given

in Chapter 3and the results in Chapter 4.

20

2.3 EQUILIBRATION OF CUT SLOPES

In order to show clearly the lack of equilibration in a cutting

slope, the excavation needs to be of medium to large size so that

the reduction in pore pressure due to excavation is large. Previous

history of delayed failure indicates a time to equilibrium of approximately

60 yrs for small to medium size excavations in London Clay (de Lory,

1957; Henkel, 1957; Skempton, 1948, 1964, 1970 & James, 1970).

Therefore the cutting slope chosen was required to be in London Clay

for comparison with back analysis data and young enough for equilibration

to be incomplete. The choice of a site posed some problems as there

has been very few cuttings of any depth excavated in the London Clay

since the extension of the railway network in the 1930's (45 yrs old)

until the recent road programme. Edgwarebury, then 9 years old, was

chosen.

2.3.1 Edgwarebury (TQ 189946)

The cutting at Edgwarebury Lane (see Fig.2:1) on the Hendon Urban

Motorway length of the M.1, designed by Howard Humphrey and Sons, was

completed in 1964.

The site is in the upper part of the London Clay, just below the

base of the more silty Claygate Beds. Silty laminations were noted

in the boreholes in which the piezometers were installed. The junction

between the weathered brown and the unweathered blue London Clay,

encountered in five of the boreholes, is approximately 10 m below

original ground level. Skempton' et al.(1969) When examining the

fissuring of the clay a little further east along the same cutting

also found the junction at 10 m.

21

By interpolation from the Institute of Geological Sciences well

records and sections,it has been possible to estimate the sequence of

strata at the site.

Lower Eocene {London Clay Om to 58m

Woolwich & Reading Beds 58m to 64m

Thanet Sands 64m to 71m

Upper Cretaceous Upper Chalk

71m to 184m

penetrated at Aldenham House

At rest water levels in the wells, where available, suggest

that the ground water level is near the base of the Woolwich & Reading

Beds except in zones of considerable pumping. Thus the site is

completely under drained.*

Over most of the cutting length the side slope is 1 on 4 and

at the instrumented section the depth of excavation is 17 m as shown

in Fig.2:10. The surface of the cutting has been grassed and young

trees, mixed deciduous and coniferous, and gorse bushes have been

planted. There are no drainage measures in the slope.

Five twin tube piezometers with high air entry ceramic tips of

the same type as used in the dam slopes were installed in April 1972

in positions 1, 2, 3, 5 & 6 as shown in Fig.2:10. No.1 is close to

the blue-brown junction but otherwise all the piezometers were in the

blue clay. To remedy the lack of brown clay information two further

piezometers, 7 & 8 were installed in November 1974 in the brown clay.

Readings have been taken on piezometers 1 to 6 between 1972 & 1975

and on pi'Dzometers 7 & 8 during 1974-75.

*The proximity and level of the chalk outcrop is such that the site would be about half under drained before pumping.

22

Large negative pore pressures were observed in the cutting at

Edgwarebury which indicated that pore pressure equilibration is an

important factor in the delayed failure of London Clay cutting slopes.

However, the original Edgwarebury piezometers are all in the blue

clay whereas nearby all the case records of London Clay cutting failures

are of slips in the brown clay, often controlled by the blue-brown

boundary. Therefore to confirm that the brown clay behaved in a

similar manner to the blue it was necessary to look at other cuttings

in the brown clay. Also it was decided to look for both a rather

older site, say 20-25 yrs and a mature one, say 100 years old.

Potters Bar Railway cutting fulfilled all these conditions.

2.3.2 Potters Bar (TL 257004)

Potters Bar cutting is at the north portal of Potters Bar tunnel

on British Railways' east coast mainline out of Kings Cross. The

location of the site is shown in Fig.2:1.

The site is within the London Clay and the junction between

brown clay and the unweathered blue was encountered approximately

10.5 m below original ground level. The boreholes for the tunnel

duplication also show the junction at about 10 m. Claystones were

encountered in several boreholes. The sequence of strata at the site,

as interpolated from the Institute of Geological Sciences well records

is:

23

Lower Eocene

{OmLondon Clay to 49m

Woolwich & Reading Beds'


Upper Cretaceous Upper Chalk 60m to 190m

penetrated at Potters Bar Station

At rest water level in the well at Potters Bar Station was

recorded as 59 m 0.D. in 1946 and 64 m 0.D. in 1964 which is about

at the base of the London Clay. Therefore the site is now fully

under drained!

The line was first opened in 1850 and was two track throughout.

Between New Barnet and Potters Bar are three tunnels, a total length

of 1.67 km, built by Thomas Brassey. A rapid increase in traffic

soon after opening made duplication of the line a necessity. Most

of the widening work was carried out soon after 1882 but in view of

the great cost of duplicating the three tunnels the stretch between

Greenwood Box and Potters Bar Station remained two track until 1959

(Terris & Morgan, 1961).

Duplication work commenced on site in 1955 and it would be

reasonable to assume that widening the cutting at the portals was

carried out at an early stage of the contract. Therefore a date of

1956 has been taken for the widened (new) side of the cutting.

A section at 12 miles 3 chains is shown in Fig.2:11. The 1850

cutting was a maximum of 11 m deep with 1 on 3 side slopes. In 1956

the base width was increased by 16 m and the love' 6 m of the new

side cut at 1 on 4. The upper slope is 1 on 3. The old track has

*The site will always have been under drained to some extent due to the proximity and relative level of the chalk outcrop.

24

been lowered by about 1 m and a small retaining wall built at the toe

of the old slope. The maximum depth of the cutting is now 12 m.

The surface of the cutting is grassed with some areas of scrub

on the old side. The area instrumented is free from bushes. A few

shallow counterfort drains have been installed in the old side of

the cutting but none could be found in the vicinity of the instrumented

section. Counterfort drains in the new side vary between 2 and 3.5 m

in depth and are at 20 M centres. The instrumented section is midway

between two drains.

The new side of the cutting is stable at present except in a

rather steep area over the tunnel portal where several minor slips

have occurred, notably in February 1975. Some minor slips have

occurred in places along the old side but not at the instrumented

section. There has been no record of any major slips.

Thirteen piezometers were installed in the positions showh on

Fig.2:11 during October-November 1974. Four piezometers, 1, 2, 3& 5

in the brown clay on the new side, are twin tube piezometers with

high air entry ceramic tips. Piezometers 4.& 6, in the blue clay,

are of the same type while the deep piezometer 7, is a Casagrande

standpipe. On the old side the two shallow piezometers, 8 & 11, in

the brown clay, are twin tube. Piezometers 9, in the brown clay,

and 10, 12 & 13 in the blue clay are all standpipes. The piezometers

have been read from November 1974 to July 1975.

25

2.4 THE hkFECT OF VEGETATION

Piezometer C at Peterborough was among young trees and recorded

considerably lower summer pore pressures than E in an equivalent

position without trees. This result gave rise to the question whether

trees planted on a slope increase transpiration losses and therefore

reduce boundary pore pressures, thus reducing pore pressures within

the slope.

2.4.1 Oakleigh Park (TQ 272946)

The cutting at Oakleigh Park is south of Oakleigh Park Station

on the British Railways' east coast main line out of Kings Cross.

This part of the line is south of the Potters Bar to Greenwood Box

stretch which was widened in 1956 and would therefore have been

included in the 1882 Act widening programme. It is uncertain which

side of the cutting was widened and therefore a date of 1885 has been

assumed for both sides.

The site is within the London Clay and from a nearby well record

the blue-brown boundary is about 11 m below original ground level.

The well record also shows the following sequence of strata:

Lower Eocene

fOmLondon Clay to 37m

Woolwich & Reading Beds 37m to 49m


Upper Cretaceous Upper Chalk

34m to 137m

penetrated

26

At rest water level in the well is recorded at the base of the

Woolwich & Reading Beds in both 1935 and 1962.

The cutting section, a sketch of which is shown in Fig.2:12,

has a 1 on 3 slope overall on the western side which is tree covered.

The trees, which are mature, are mainly silver birch with some oak.

The eastern side has a 1 on 3 slope and is grass covered. The grass

is scythed occasionally. Burnt clay was found under the topsoil

in several of the boreholes.

Four Casagrande standpipe piezometers were installed in the positions

shown in Fig.2:12 in November 1974. Readings have been taken on them

from installation to July 1975. The two standpipes on the wooded side

are upslope and downslope of a large silver birch tree.

2.5 LONG TERM PORE PRESSURES IN CLAY DAMS

In January and February 1975 two drawdown failures occurred at

Aldenham dam, see Fig.2:1. The ensuing investigation produced the

opportunity to study the long-term pore pressures in an uncompacted

clay dam.

2.5.1 Aldenham Dam (TQ 169958)

Aldenham Dam, was built for the Grand Junction Canal Company to

supply compensation water to the mill owners on the River Colne.

More recently the reservoir has been used for water supply by the Colne

Valley Water Company. It has since been leased to the Hertfordshire

County Council and is used purely for recreation as part of the

Aldenham Country Park.

27

The geology of the site is 30 m of London Clay overlying the

Woolwich & Reading Beds (taken from the well record at Aldenham

House). The London Clay was used for the fill of the dam.

The dam in its original form was completed in 1795 and the

'headbank' was raised in 1802. It would seem that the dam gave a

fair amount of trouble as slips were continually occurring in the

'headbank' (Faulkner, 1972).

Jessop (1802),asked to report on the dam, says that there was

no danger as the slips were on the inside (upstream) and only occur

on drawdown. Therefore they can be repaired before refilling and

'repetition leads to greater safety'. He advocated that both faces

should be treated with a free draining layer of 8 to 10 inches of

sand and gravel to prevent cracking and that the downstream face should

in addition be covered with a thin layer of earth and sown with rye

grass.

The remedial measures were not satisfactory as it slipped again

in 1804. Proposals for further raising were abandoned and to combat

recurring troubles it seems that the water level was lowered not

long after. One other recent slip is known, that took place in

January 1959 in the upstream shoulder. Apart from the two upstream

slips which occurred in January and February 1975, a third slip is

appearing on the downstream side.

The dam which is a maximum of 7 m high is built of London Clay

which it is thought may have been compacted by horses, and has no

puddle clay core. Two sections, at peg 11 and peg 19, are shown in

28

Fig.2:13 and 2:4. These show the dam as it is in 1975. The wave

wall and concrete slabbing were added in 1933. Upstream of the

slabbing, the clay is protected by gravel and cobble beaching. The

downstream shoulder is grassed with tree roots. The trees were

felled in November 1972.

A drawoff pipe running in a brick lined culvert passes through

the dam close to section 19. The cinvert of the culvert is at about

original ground level at the portal, and this culvert is apparently

acting as a drain.

Seventeen piezometers were installed in the dam in February -

March 1975 and the positions of these are shown in Figs.2:13 and 2:14.

Piezometer 3, like 1 and 2, goes through the crest of the dam into

the in situ London Clay but is 106 m east of section 19 (section 11

is 75 m west of section 19). Piezometers 1, 2, 3, 6, 7, 8, 11, 15 &

17 are Casagrande standpipes while 9, 10, 12, 13, 14 & 16 are twin

tube hydraulic piezometers with high air ceramic tips. The remaining

two, 4 & 5, are also twin tube hydraulic piezometers but have low

air entry tips and were installed in sand pockets like Casagrande

piezometers.

2.6 OTHER AVAILABLE DATA

Pore pressure records have been taken on the main piezometer

installation at Peterborough during construction and until the end

of 1964. Further readings were taken during 1966-67 and again in

1971-72. Occasional readings have been taken at other times up to

1974.

29

At Grafham Water the piezometers have been read during construction

and at regular intervals until 1974. These records have been made

available by courtesy of Binnie & Partners.

The piezometers at Bough Beech were read regularly until 1971.

Readings have been irregular since. The readings until 1971 have

been made available by courtesy of the East Surrey Water Company.

Some field testing data is available for both Peterborough and

Grafham Water dams (Al-Dhahir, 1967; Bishop & Al-Dhahir, 1970) which

was carried out during or immediately after construction.

Other unpublished pore pressure data has been made available.

The piezometer records from Barnsdale by Dr. R.J. Chandler, from The

University of Kent by Harris & Sutherland and from Cow Green by Rofe

Kennard & Lapworth.

In the London Clay, de Lory (1957) and Skempton & Henkel (1960)

report pore pressures in cutting failures; the Road Research Laboratory

(Black et al., 1958) report surface measurements and Lewis (1972)

and Bromhead (1972) report reduced pore pressures in the cliffs at

Herne Bay.

In the Upper Lias Clay, Chandler (1974) reports pore pressures

in cuttings and Chandler et al. (1973) and Pachakis (1974) present

embankment data.

Records from other clays are presented by Muir Wood (1971) for

the Gault at Folkestone, by Lutton & Banks (1970) for the Panama Canal,

by Kwan (1971) for the Welland Cut and by Kankare (1969) for the

Kimola Canal.

A /

% ‘ ;

27.5; ; ,-- r' 22.5/

.„ ..._._, . ,

/ r/ ----__25 ____ _ _ 1

// 1

I / /

N N I

// % / 1

/ / N %

\IC) Peterborough I I

I 1 11 I f I I

% 1

\ )

\. , °Graf ham Water! , , 1

0 Foxcote

• \

Aldenhami /0 Potters Bar

Edgwarebur,y‘'' 00akleigh Park

275

. ._- - 30,-, -- _ ,\ - - , ..... - , - , - -- , .

32.5

- U7J1j1j1

22.5/

O'Bough Beech--

Contours of average annual rainfall - inches

Site Location Plan fig. 2.1

5 23• 24• 2 5• C1,

18 • 19• C2Y

12• 'Oxford' Clay 13•

22 •

11•

4 Ao 0B

17o `Redeposited' Clay

T.W.L. 3

1

29

-

Oxford Clay 10 50 Kellaways Sand

Kellaways Clay 2• Cornbrash Limestone e3

• 6 7 •4

8 'Redeposited Clay 9

1•

O 1971 piezometers • 1970 piezometers • 1963 piezometers

0 10 20 metres

30

PETERBOROUGH - Instrumented cross section

rolled clay fill - drainage mattress rolled clay core

T.W.L.

-original ground level — gravel drains -weight block

road embankment-

0 10 20 30 40 50 metres

GRAFHAM WATER Cross section at chainage 1707.5 N

••■•• ■Mi ■1011. 4■1

• R4 *R2 T R3 20 0 10

metres

OR1 Chainage 1707.5

Chainage 2000

Road Embankment Sections - showing piezometers

GRAFHAM WATER

I= • try , IMO MIN *re BIM ,;(11.:

1====a 0 10 20

metres

Hydraulic piezometers 0 Electrical piezometers 0

GRAFHAM WATER - Section I at ch. 2000 ft. showing piezometers

-t.

N 01

c=ernoimar 0 10 20

metres Hydraulic piezometers o high air entry 0 low air entry

Electrical piezometers o

GRAFHAM WATER - Section II at ch. 1707.5 ft. showing piezometers

T W L

\03 5 6%

o6 \ \120

7 0\

2.75 TJ

3.25

80 go 10° 110

rolled clay fill 20 3o 40 50 60

10

c=====-- 0 5 10 15 metres

piezometers o installed 1955, chainage 610E installed 1971, chainage 560E

FOXCOTE DAM Section showing piezometers

W & X

Tips U & V at 21 8 42m downstream of trapezoidal drain on weight block, 0.92m below ground level.

U &

Junction between downstream shoulder & weight block

15 1 D10

Chainage Tips ft. (approx.)

1707 D10, R6 1740 1760 X 1900 P.Q,R,U,V,Y &Z 2000 D4,R3,4,8,9&10

121===ant 0 1 2 3 4 5

metres Road embankment

R4 a

GRAFHAM WATER - Sections showing piezometer locations

N S

v Weald Clay fill

ti

Weald Clay

Detail of piezometer clusters 4-12 &

21 - 29 0 25

metres 50

Cross section at chainage 1820 - showing piezometers

BOUGH BEECH lD

=MOO MII=. ■■■• •••••••.

•••■■• AIWW1 ■• •MIlo Vma. Offiela •MINI ••■•■ 1•1■11. . ---

MO.

S N

\

Brown London Clay

Blue London Clay /A\

Original Ground Level

130

120 3 (1)

110

0 0

100

90 20 I IN !1.51 ■it,ah

0 10 metres

Twin tube hydraulic piezometer

Cutting Section at Chainage 64.50 - Showing Piezometers

EDGWAREBURY

Original Ground Level 110 3 (D

3 "` z,z1 1

U)

• - - • • - • - -

1956 Brown London Clay

4 100 1 ••••••..

•

W New Side Old Side

120

90 Blue London Clay

F •

0 10 20 metres

10 13

Twin tube hydraulic piezometer

Casagrande standpipe piezometer 80

Cutting Section at 12m. 3ch. - Showing Piezometers

4

POTTERS BAR

20

4

Casagrande standpipe piezometer

London Clay

t() As* MEI A-4 NM -"ft,.

0 10 metres

w E

•I■11 Original Ground Level

Cutting section showing piezometers

OAKLEIGH PARK

ALDENHAM - Section 11 showing piezorneters

S N

London Clay fill V

1 010

110

1 6

London Clay Twin tube hydraulic piezometer


Twin tube piezometer in sand pocket

===== 0 5 10

85 metres

90

105

100 3 CD

1 CD (/)

95

0

V

WIMP ,101.■ ■••■■ .•■■ OEM ■•■■• 11.■•

5

0 5 10 metres

16 15

••■•• •■•• =MD

London Clay f ill

013 14

■•■• OM ■■•■ ■mws ■■■•• 017 ■••■••■ =MM. ••■ .r■Ia ••••••• ■■•••■ ■••• ••■■■■ .•■• •■■ •■•■■• ••■•

MM. ,■■•

105

S N

100 3

(fl 95

0

85

90 London Clay 07 Twin tube hydraulic piezometer


Twin tube piezometer in sand 41=11••••

ALDENHAM - Section 19 showing piezometers

pocket

30

Chapter 3

EQUIPMENT AND TECHNIQUES

3.1 PIEZOMETERS

A piezometer works by measuring the fluid pressure inside a

cavity which is separated from the surrounding soil by a filter

element. This cavity fluid pressure is assumed to be equal to the

pore fluid pressure. In general the fluid used in the cavity is

water and in saturated soils will equalise to the pore water pressure.

In partly saturated soils it may equalise to either the gas or the

water pressure. Piezometers are limited in as much as they cannot

be used to measure pore water tensions (i.e. pore pressures below

absolute zero) as the free water in the cavity will not sustain

tension (Vaughan 1974). This section considers the various filters

and the methods of measuring cavity pressure.

3.1.1 Hydraulic Piezometers

The most basic hydraulic piezometer is the open borehole, sometimes

with a porous lining as at Hendon (Skempton & Henkel, 1960). These

have two major disadvantages, firstly that the response is extremely

slow and in a low permeability material,, can take years to reach

equilibrium because of the large volume of water required to raise the

water level. Secondly there is no way of ascertaining from what part

of the borehole flow is coming.

To overcome both these problems the Casagrande standpipe

piezometer came into use. This consists of a filter element made of

a low air entry value+ material which is installed in a sand pocket

Defined as the difference between the air pressure on one side of a saturated filter and water pressure on the other at which blow-through of air occurs.

31

of known dimensions and has a small diameter standpipe rising to

ground surface (see Fig.3:1). The borehole is backfilled with a

cement and bentonite grout seal. The standpipe diameter should be

as small as possible to reduce response time but should not be less

than 12 mm to ensure it is self de-airing. (Vaughan 1974). The

Casagrande piezometer is limited to measuring positive pore pressures

and generally requires access to the top for measuring purposes.

The development of twin-tube hydraulic piezometers in Britain

was started at B.R.S. in 1951 (Penman 1956) based on previous designs

by the U.S.B.R. (Armstrong, 1946) for Anderson Ranch Dam. The filter

elements in these piezometers are coarse pored ceramics having an

air entry value of less than 0.5 psi (see fig.3:1)• Where a

significant difference between pore air and pore water pressure

exists these instruments would be expected to measure pore air pressure

or behave erratically (Bishop et al.1964). Theoretically these

piezometers would not measure negative pore pressures in partly

saturated soils but significant negative pore pressures were in fact

recorded at Foxcote and Hanningfield (Little & Vail 1960). This was

probably because the constant de-airing of the system that had been

found necessary had created a wet zone around the piezometers free

from communicating air voids. A further complication was that the

polythene tubing generally used with this piezometer proved to be

permeable to air (Penman 1958).

32

In order to measure pore water pressures in partly saturated soils

a piezometer with a high air entry value filter was developed (Bishop

et a].1960). This piezometer has a porous ceramic filter which has

an air entry value of about 1 atmosphere and is tapered (see fig.3:1)

to improve the contact with the soil when it is installed in a shallow

hole made in fill with a shaped mandrel. A. more recent development

is the bull-nosed tip (fig.3:1) which is 'cleaner' inside to facilitate

de-airing and is less likely to leak. All seals must of course have

an air entry value as great or greater than that of the ceramic or

it will behave as a low air entry piezometer. The interpretation of

the results is more simple than with the low air entry type since the

pore water pressure is transmitted unless the value of ua - uw

exceeds the air entry value or unless the water pressure in the system

is low enough to cause cavitation. The need to de-air the system is

greatly reduced since the only way air can enter is by diffusion through

the water filling the pore space of the filter. This not only reduces

the maintenance but also minimises erratic readings (Bishop et al 1964).

The choice of suitable tubing for these twin tube piezometers has

proved a problem as polythene is permeable to air and nylon can

transmit water when subjected to differences in vapour pressure or

osmotic potential. Several types of nylon have been tried. Nylon 66

has a large water absorption and Nylon 6 has been found to be chemically

unstable. Nylon 11 has proved the most satisfactory and is now used

coated with polythene. Saran tubing, which is generally as good as

the polythene coated nylon 11, has the two disadvantages of greater

cost and brittleness when cold.

33

Measurement of pressure was originally by Bourdon Gauge but these

proved unreliable in a damp atmosphere due to electro-chemical

reactions between the different metals used. Also they were of

limited accuracy where small changes in large pressures were to be

measured and their calibrations were not stable under field conditions.

Mercury manometers were more reliable but had the disadvantage of

requiring a large gauge house, sometimes of considerable height for

large dams. The measuring systems used for the dams studied in this

project are discussed in Section 3.2. More recently, with the

increased reliability of electronics, pressure transducers have been

used very successfully. These only require one external pressure

source as a calibration check.

The TRRL used as a tensiometer, a form of single tube high air

entry piezometer (see fig.3:2), for measuring negative pore pressures

at shallow depths. The porous plate used was of sintered glass with a

pore size of 1 pm which had an air entry value of about 1 atmosphere.

Using plates of a smaller average pore size they were able to measure

negative pore pressures equivalent to 18 m of water. The tensiometer

had a standpipe and mercury manometer of glass capillary tubing. This

small diameter tubing meant that carefully de-aired water could withstand

some tension without cavitation. Once cavitation occurred or air

diffuses through the porous plate these instruments could not be

de-aired without removal and reinstatement (Black et al.1958).

34

3.1.2 Electrical Piezometers-

In electrical piezometers the cavity pressure is measured by

determining the deformation of a diaphragm between it and a second

chamber. This second chamber is usually sealed with a known pressure

in it or may be vented to atmosphere. As the movement of the

diaphragm is small the response time is fast. Vibrating wire strain

gauges have generally been used where the signal has.to be transmitted

long distances. Resistance strain gauges are now being used for short

term and short distance work.

The Maihak vibrating wire piezometer (fig.3:2) has a sintered

metal filter and is saturated with neatsfoot oil. The air entry value

is about 1.5 psi. Thus in partly saturated soils the Maihak instruments

behave in a similar manner to the low air entry value hydraulic tips

and record pore air pressure where this differs significantly from

the water pressure. An example of this from Walton dam is given in

Bishop et al (1964).

These piezometers have no facility for de-airing and in anything

except fully saturated soils, even if a high air entry ceramic filter

is used they can only measure pore water pressures for a short time.

Another disadvantage is that the piezometer requires a calibration

which cannot be checked after installation unless special provision

is made.

3.1.3 Pneumatic Piezometers

The pneumatic system involves two gas (usually nitrogen) or

hydraulic oil filled tubes which are separated by a valve behind a

35

flexible diaphragm which seals the piezometer cavity. The valve is

activated when the gas pressure applied is equal to the fluid pressure

on the other side of the diaphragm. In some systems the valve shuts

and in others opens (see fig.3:2). The operation of the valve

involves a small calibration factor and volume change. It has a

small response time but has the same disadvantage as the electrical

piezometers with de-airing. The inability to recalibrate is not so

critical and they are cheaper to install. They have an advantage

over hydraulic piezometers that the relative level of the tip and the

measuring system is not important.

A de-airable pneumatic piezometer has been designed (Marsland 1974).

This overcomes the de-airing problems but considerably complicates

the instrument. It requires four air lines, two to the transducer

and two to valves in the hydraulic system, and two hydraulic lines,

(see Fig.3:2). The relative levels of the tip and the measuring system

become important again for the successful operation of the hydraulic

parts of the piezometer.

3.2 PIEZOMETER INSTALLATIONS IN EMBANKMENT DAMS

The piezometer installations in the four embankment dams studied

in this project are described in this section. The piezometer tips

themselves were described in section 3.1.

3.2.1 Foxcote

The installation at Foxcote Dam was installed in 1955 and has

the original B.R.S. low air entry value piezometers (Penman 1956).

Twenty-four piezometers were installed in the dam, twelve in the

positions shown in fig.2:7 and a further twelve in similar positions

on a second cross section.

36

A temporary gauge house was set up at the downstream toe of the

dam and all leads from the piezometer tips to the gauge house are

5 mm O.D. polythene. Each tube led via a sleeve packed klinger valve

to a compound Bourdon Gauge with a pressure range of 30 in Hg suction -

20 psi pressure, each pair of tubes to one gauge. Via another valve

each tube led to the de-airing manifolds, one of each pair to the

input side, the other to the return (see fig.3:3). All pipework

was in 4 in bore copper tubing.

Measurement was made by opening the valve between the input limb

and the Bourdon Gauge and taking a reading. The value obtained was

the pressure in feet of water above atmospheric at the level of the

gauge. The pore pressure was this value less the height of the tip

above the return tank of the de-airing system which was used as datum.

As a check, after closing the valve on the input side, the return

was opened to the gauge and a second reading taken. If these readings

differed by more than 0.3 m of water it indicated that there was air

in the tubes and the pystem required flushing out.

The de-airing system originally installed at Foxcote was the

predecessor of that used on all the other dams and is shown in fig.3:3.

It consisted of two tanks, input and return, connected by a system of

valves to either de-airing manifold. De-aired water had to be made

separately from the apparatus and poured into the.pressure cylinder

via a filling funnel. This process gave the water every opportunity

of picking up air again. Pressure was supplied by a foot pump into

a bladder within the pressure cylinder and controlled on a small

Bourdon pressure gauge. On the return side suction was supplied by a

37

hand suction pump and monitored on a mercury manometer. There was

no direct facility for applying a pressure to the return side, the

only method was to allow the returning water to set up a pressure and

to control it by allowing the air to bleed via the valve on the suction

pump line.

It does not seem likely that de-airing pressures were carefully

chosen at first as long as there was a pressure gradient around the

system to allow flow. Little & Vail (1960) and Bishop et al.(1964)

point out the importance of keeping the pressure at the tip unchanged,

possibly allowing a very small excess pressure to create a small flow

through the filter element to flush it. Large pressure changes at the

tip during the de-airing can cause anomalous pore pressures for

sometime afterwards.

The gauge house and the de-airing system have since been removed

at Foxcote. Each piezometer has been separated from the manifold

above the valves and the two valves linked with a loop of copper tube.

The valves and Bourdon gauges are now suspended in a manhole and

require a complete refurbishing before any further readings could be

made on the main installation piezometers.

3.2.2 Peterborough

The piezometers at Peterborough were installed in 1963 and contain

the improvements made since the mid 1950's. All piezometers are of

the Bishop high air entry type described in section 3.1.1 and all

tubing leading from the piezometer tips to the gauge house is polythene

coated nylon 11,2.8 mm I.D. (3/16" 0.D.). The piezometer tubes each

38

went through a three way valve system to the measuring equipment

(see Fig.3:4). This was an improvement over the two valves used at

Foxcote as it allowed the piezometer to be isolated from the measuring

system. The valves used at Peterborough were lever operated Simplifix

cocks.

One limb from the three way valve leads to a mercury manometer

also in 3/16" O.D. nylon 11 tubing. A tube connects the atmospheric

limb of the manometer to a header tank of water which is filled to

a known level and used as the datum. The scales on the manometers

are calibrated to read directly in feet of water.

The second limb led to the de-airing manifold which is in 2" O.D.

nylon 11. The de-airing system, fig. 3:4, had some improvements over

that used at Foxcote. A steel air/water cylinder had been included

in the supply side so that water was pumped into the bladder instead

of air. Also a scale calibrated in volume was attached to both the

air-water cylinder and the return cylinder. The advantages were the

increased safety with the air pressure confined in a steel cylinder,

the large volume of air allows almost constant pressure to be maintained

with intermittent pumping and the volume of water supplied and returning

can be measured. A water trap was also included to prevent water

getting in to the suction pump. There was still no facility for

supplying a back pressure also an external de-aired water supply was

required. Details of the system at Peterborough are given in

Vaughan (1965).

39

The Simplifix lever operated cocks proved unsuccessful as they

became jammed and the levers broke. To continue taking readings on

the piezometers in 1971 the three valves were removed and the

piezometer leads connected directly to the manometer through a

tee-piece. A short length of tubing finished with a stop-end was

attached to the third limb of the tee, which was then used for

de-airing lines.

The de-airing pressure supply was a jerrycan of de-aired water

placed on the slope of the dam at an appropriate level to supply the

required pressure. O.D.3/8" polythene tube was used from the water

container to the gauge house. The back pressure on the return side

was supplied by another polythene tube with an outlet at the required

level. The return flow was collected in a calibrated bucket in

order to ensure a large enough volume of water had been circulated

(see sketch Fig.3:5).

De-aired water was made on site using the syphon method. A

sketch of the apparatus is shown in Fig.3:5. 5 gallon polythene

containers were used and the tubing was 3/8" O.D. polythene. If

the head of water, h, varies linearly along the tube, being 0 at the

lower container and H at the upper (see diagram fig.3:5) and if E

is the elevation of any point in the tube then the pressure in the tube

is

p = (h - E) yw 3.1

40

With the coil of tubing at the same level as the upper container,

E is always greater than h so p is always less than atmospheric. If

pmin , which occurs where the tube leaves the coil and drops down the

slope, is less than absolute zero the water cavitates and air comes

out of solution. It does not go back into solution but comes out as

air bubbles into the lower container. To obtain a vacuum the

relationship between the length of tubing in the coil and that up the

slope is important.

If the distance up the slope is H/sinf3 where p, is the

slope angle then the minimum value of L, length of coil in metres,

can be obtained from:

= 10.2 3.2 LtH/sins

In the case of Peterborough where 11, 15.8 m and 13 is 14.5°, L > 114 m.

A partial de-airing will occur with smaller values of L. To

improve the quality of the de-airing, the water can be run through

the system a second time. Containers of de-aired water should not

be moved unless they are completely full and sealedlor air becomes

re-dissolved.

3.2.3 Grafham Water

As outlined in Chapter 2, there are three different pore pressure

measuring systems in operation. The first type are the hydraulic

piezometers in the road embankments which have a mercury manometer

measuring system. These work in an identical fashion to those at

41

Peterborough. The only modification is the use of a three-way valve

block with key operated klinger sleeve packed valves which are

considerably more robust.

The hydraulic piezometers leads from the main dam come to the

section gauge chambers at the toe of the road embankment. Here the

pressure is transferred to a Maihak vibrating wire pressure transducer.

A sketch of the system is shown in Fig.3.6a. Using the three-way

valves the transducer can read the pressure on either side of the

piezometer. The transducer cable is taken to a slave unit in the

gauge chamber and from there the signal is transmitted through a multi-

core cable to the selector unit and receiver in the pumping station.

The transducers were calibrated before installation and it has been

possible to recalibrate some of them using an external pressure supply

applied through the de-airing system. There were small changes in

the calibration of most of these transducers tested but none great

enough to cause a significant difference in the measured pore pressure.

As this recalibration was eleven years after installation the transducers

can be, considered reasonably reliable.

The Maihak piezometers installed in the foundation have cables

taken directly to the slave units in each section gauge chamber and

from there the signals are transmitted as described above.

The de-airing system is portable as it needs to be used in two

gauge chambers but otherwise it has the same features as that used at

Peterborough. A sketch is shown in Fig.3:6b.

42

3.2.4 Bough Beech

The piezometer tips in Bough Beech Dam are all Bishop type high

air entry with O.D. polythene coated nylon 11 leads. The pressure

measuring. system is mercury manometers similar to those used

at Grafham Water, see fig.3:6a. The de-airing equipment is a wall

mounted version of that used at Grafham Water, see fig.3:6b, the

only modification being the use of electric pressure and vacuum

pumps. Having electricity in the gauge house also meant that

thermostatically controlled room heaters could be used to eliminate

any chance of freezing.

3.3 INSTALLATION OF PIEZOMETERS IN BOREHOLES

All 65 piezometers installed during this project, with the

exception of the seven deep piezometers at Aldenham Dam which were

installed using a shell and auger rig, are in hand augered boreholes

with a maximum diameter of 100 mm, generally 50 or 75 mm.

3.3.1 Casagrande Standpipes

Eighteen Casagrande standpipe piezometers were installed in

areas where positive pore pressures were expected and generally at

depths of 3 m or more where fluctuations were expected to be small.

The five deep standpipe piezometers at Aldenham were installed

in 150 mm diameter shell and auger boreholes. Asand pocket 0.7 m

long was used and the grout mix was 1:1:4, bentonite:cement:water.

This grout mix when used at Balderhead had a permeability of 5 x 101 m/sec

(Vaughan 1965). Vaughan (1969) calculated that the errors in pore

pressure due to grouts more permeable than the surrounding soil

are not significant for 1 or even 1z orders of magnitude difference.

4-3

The remaining thirteen Casagrande piezometers were installed

in 100 mm dia. hand augers boreholes with 0.5 m sand pockets. The

standpipe diameter used was I.D. for more rapid response. Otherwise

the installation was the same as described above.

Two twin tube piezometers with coarse low air entry filters were

installed at Aldenham (4 & 5). These were placed in sand pockets

0.7 m long. As the piezometers tips, being plastic, were very light

and the tubing flexible, it was necessary to weight the tips to stop

them floating out of the sand plug.

3.3.2 High air entry hydraulic piezometers

The top 0.3 m of borehole was augered 150 mm diameter then

reduced to 100, 75 or 52 mm diameter as shown in fig.3:7. The

hole was continued in this size to a minimum of 250 mm above the

required final depth. All but the last 150 mm were drilled in 52 mm

diameter and the remaining 150 mm in 38 mm diameter. The last 150 mm

where then tapered to fit a Bishop conical piezometer tip using a

special auger.

The piezometer tip had previously been prepared in the laboratory.

It had been de-aired and then pressure tested for any leaks using

compressed air. The required amount of polythene coated nylon 11

leads were attached and no volume change valves fitted to each lead.

The whole system was filled with de-aired water and the valves closed.

The tip was kept from loosing water by evaporation by wrapping it

in wet cloth then enclosing it in a polythene bag. In this way the

ready made up piezometers could be transported conveniently to site.

44

As soon as the borehole was ready the piezometer tip was lowered

down the hole on rods with a left-handed thread connector to the tip.

The tip was pushed firmly home into the pocket and the ground level

marked on the rods. The tip was then removed from the borehole. A

mix of plaster of Paris in about 0.5 1 of water was made to the

consistency of cream. This was then poured into the borehole to

partially fill the pocket. Immediately the plaster of Paris was in

place the piezometer tip was replaced in the hole and pushed back to

its previous position. This ensured the best possible constant between

the piezometer tip and the soil and any voids would be filled with

plaster of Paris.

Once the plaster of Paris had set the hole was backfilled with

grout, the same mix as described in section 3.3.1, to within 0.3 m

of ground level (see fig.3:7). A 0.3 m length of 150 mm diameter

plastic soil pipe was thsapushed into the top ofthe borehole until

it was about 50 mm below ground surface. A hole was then dug over

the borehole to take a 0.3 m square paving slab flush with the

ground surface. The ends of the leads, which were long enough to

extend 0.5 m above ground level were kept coiled inside the soil pipe.

The input lead is marked with tape to differentiate it from the

return lead. While the grout was setting, polythene bags were kept

over the valves to prevent them getting blocked inadvertently.

In some cases 'bull-nosed' piezometer tips were used instead of

the tapered Bishop tips. For these the final 250 mm of borehole

was drilled at 50 mm dia. without tapering. Otherwise the installation

was identical.

45

3.4 MEASUREMENT OF PORE PRESSURE

The measurement of water levels in Casagrande standpipes was by

electrical dip meter. This procedure was completely standard.

The twin tube hydraulic piezometers, without any gauge house

facilities required the design and manufacture of portable measuring

and de-airing equipment. A sketch of the equipment is shown in fig.3:8.

The pressure measuring unit is a pressure transducer mounted in a brass

transducer block. A battery operated Peekel strain meter was used to

energise and read the pressure transducer. The transducer is sensitive

to rapid changes in temperature and therefore it was necessary to

insulate it. The insulation had to have a polythene coating to

eliminate heat losses due to evaporation of rain.

Two no volume change valves are also mounted on the transducer

block. A length of small bore copper tubing, about 0.5 m long, was

connected to the first of these valves, the other end of the tube has

a male coupling with an '0' ring seal which would fit the female

coupling on the valves of the piezometer. Copper was used for this

connecting lead to keep the system as rigid as possible to reduce

the response time.

Attached to the second of the valves on the transducer block via

a peristatic pump was a polythene coated nylon collapsible bag filled

completely with de-aired water. A collapsible bag was used to keep

the de-aired water at atmospheric pressure and to allow flow of water

out of it without an inflow of air. A valve was fitted in the neck

46

of the bag. A by-pass line to the peristatic pump was also fitted.

The connecting tubing in this part of the equipment is 2.8 mm bore

polythene coated nylon 11. The equipment fits into a box which can

be carried and stood beside a borehole.

To take a pore pressure measurement the copper tube is attached

to the closed valve on the input side of the piezometer with de-aired

water being allowed to trickle through the system from the supply bag.

This stops any air being trapped between the transducer and the

piezometer. At this stage a reading is taken on the pressure transducer.

This is equivalent to the head of water in the de-aired water

container and this is used as the datum, the level of the water above

ground level at the piezometer being measured. Once the datum reading

has been taken the pump by-pass valve is closed and a suction applied

to the pressure transducer and copper tubing until the reading obtained

is approximately the same as the previous reading on the piezometer.

This reduces response time. The valve between the pump and the

transducer block is then closed and the strain meter watched for a

few moments to check for leaks. If the system is not losing suction

the valve to the piezometer is then opened. A reading is taken on

the strain meter as soon as it becomes steady, this seldom takes

more than 2-3 minutes. This strain meter reading taken from the

datum reading and multiplied by the transducer calibration gives

the pore pressure as a water level below the datum.

47

To de-air the piezometer first the valves between the water

container and the piezometer, and then the valve on the return line

are opened. The head difference between the de-aired water container

and the return valve causes a flow of de-aired water through the

piezometer. Occasionally when there was considerable air in the

piezometer the pump was required to start the circulation. De-airing

was performed after each reading. This regularity was very necessary

with those piezometers recording large negative pore pressures.

In the early stages of the project de-airing was carried out

1 to 2 hours before a reading. At Foxcote, the August 1972 reading

was carried out before de-airing and a significant reduction in the

measured pore pressures was observed, see Fig.4:35. This indicated

that equilibration after de-airing had not been complete on previous

occasions. After this the practice became to de-air piezometers on

completion of reading rather than before. This practice had the

disadvantage of there being some air in the piezometers but the

quantity was generally small enough to be insignificant. More detailed

discussion on the response of piezometers to de-airing is given in

section 3.5.

3.5 EQUALISATION TIMES

Hvorslev (1951) separates the response of a piezometer, into two

parts (i) hydrostatic time lag which is defined as 'the time required

for water to flow to or from the piezometer until a desired degree of

pressure equalisation is attained'. (ii) stress adjustment time lag

4or which is defined as the time fr-em the corresponding volume of water

to flow to or from the soil.

1+8

3.5.1 Piezometer response times

Hvorslev's hydrostatic response time is based on the simple

theory that the soil surrounding the piezometer is isotropic,

fully saturated, infinite in extent, incompressible and there are

no head losses in the system. With these assumptions the equalisation

ratio at time t is

Fkt u c g - ut = e Vyw ug - u

3 . 3

where is the equalisation ratio

is the pore pressure in the soil

ut

is the cavity pressure at time t

Uo is the initial cavity pressure

F is the intake factor of the piezometer

k is the coefficient of the permeability of the soil

V is the volume factor of the piezometer system defined

as the flow into the piezometer for unit pressure

change in the cavity

and yw

is the unit weight of water

Gibson (1963) considers a spherical porous element, radius a,

in a saturated homogeneous and isotropic material which is compressible

and obeys Terzaghi's consolidation theory. Gibson's equation for

the equalisation ratio, is

49

E — n1-n n1 1 { exp(n2T).erfc(n 1 T

2) — n2 2 exp(n2T).erfc(n 2T1/2)}

1 2

3 4

where

n1 = 1/2{1.1 ± (p2-40 1/2 } 3 . 5

2

p is the stiffness of the measuring system where

p = 47ra3myw/A 3.6

where rn is the coefficient of expansibility or compressibility

of the clay skeleton

and A E 17-1(1.4 3.7

and T = ct/a2 3.8

where c is the coefficient of consolidation or swelling of

the soil.

50

When the soil is incompressible (m = 0) the Gibson equation reduces

to:

6 = exp(-47rakt/A) 3.9

which is the Hvorslev (1951) equation. At high degrees of equalisation

the two theories give similar results.

Field piezometers are not spherical and to apply Gibson's

formula an approximation must be made. For a spherical piezometer

F = 47a

(Hvorslev, 1951) 3.10

therefore in eqn.3.9 a can be replaced by

a = F/471- 3.11

Hvorslev derived F for other geometries i.e. for a cylinder

1, F = 271

logA + (1 + (1c1)2)1/2 1

3.12

This is derived for steady state conditions but can be used as an

approximation in eqn.3.9.

Vaughan (1974) quotes typical response times :for piezometers,

table 3.1 and the influence of volume factor on response was

examined by, Penman (1960).

51

3.5.2 Equalisation after installation

Hvorslev's stress adjustment time lag is the equalisation of

stress changes in the soil brought about by the process of installation

of the piezometer, by the de-airing or by carrying out in situ

permeability tests. Soderberg (1962), in examining the behaviour

of piles, studied the equalisation of a zone of perturbed pore pressure

around an expanded impermeable cylinder. Depending on the amount of

plastic yield of the soil Soderberg found 80% & 95% equalisation at

the surface of the cylinder occurred at times of approximately 10 r2/c

and 30 r2/c respectively, where r is the radius of the, cylinder and

c the coefficient of consolidation. Except for standpipe piezometers

the equalisation of the perturbed pore pressure zone would therefore

be much slower than the piezometer.

At Potters Bar, the piezometers 6 m or more deep have taken

between 150 and 250 days to reach 90% equalisation. The piezometer

levels against log time are plotted in fig.3:9 for four of the

piezometers 6, 7, 9 and 12. Of these 6 is a twin tube hydraulic

piezometer, the other three are standpipes with much slower response

times. Thus the equalisation time is dominated by the reconsolidation

of the soil as suggested by Soderberg's work.

3.5.3 Equalisation after de-airing

The equalisation time after de-airing or in situ testing depends

on the period for which the excess or reduced pressure is applied

(Vaughan 1974).

52

In partly saturated soils, piezometers recording negative pore

pressures respond more slowly to de-airing than those measuring positive

pressures. At Bough Beech the downstream shoulder piezometers took

several days to equalise after de-airing for approximately 100 min.

Two examples, 3 and 10, are shown in Fig.3:10. These have tw

values of 4.3 and 5.2. days respectively. The response of the upstream

piezometers which recorded positive pore pressures and were closer to

saturation had equalisation times at least an order of magnitude

more rapid, e.g. for No.20 t100 < 5.5 hrs.

Equalisation tests on piezometer tip A at Peterborough are shown

in Fig.3:11. For both tests the de-airing time was 30 sec and in

the first test equalisation was from the de-airing pressure. In the

second test the cavity pressure was re-applied on completion of

de-airing. Immediately cavity pressure was recorded equalisation was

commenced. In this case equalisation was at least 1:'21- orders of

magnitude more rapid than in the first case. Thus resetting cavity

pressures can reduce response times considerably. Of interest is

the change of slope of the equalisation plot for the first test.

This occurs where the pore pressure changes from positive to negative.

3.6 FALLING HEAD PERMEABILITY TESTS

3.6.1 Theory

The Gibson (1963) equations for time lag can be used to evaluate

the coefficient of permeability of the soil around the tip. The

solutions of equations 3:4 are presented in graphical form of e against

pT or p 2 T for various values of p

The first family of

curves includes the limiting member p = 0 and the second includes

53

11 = m The first family, E against pT is plotted in fig.3:12.

It can be seen that for values of p ;<. 2 the curves coincide at

C = 0.1 (90% equalisation). Therefore at 90% equalisation the

Hvorslev formula (Eqn.3.3) can be used without any loss of accuracy

for p 2 . For p >2 it is necessary to use curve fitting to

evaluate p and use pT = 4Trakt/A 3.13

which was obtained by combining eqns.3.6 and 3.8

k c = --- my

and 3.14

3.6.2 Equipment and Test Procedures

Falling head permeability tests were carried out at Bough Beech

and Grafham Water Dams. At Bough Beech the piezometers all had mercury

manometers suitable for falling head tests and the de-airing system

could be used to apply the required pressures. These systems are

shown in Fig.3:6.

At Grafham Water the hydraulic piezometers in the shoulders which

were to be tested were those where the hydraulic pressure is transfered

to Maihak pressure transducers in the gauge chambers. It was therefore

necessary to design and construct a portable mercury manometer board

which could be connected to the three way valve of the piezometer in

place of the de-airing manifold. A sketch of the equipment is shown

in Fig.3:13. The board carried five manometers each with a two metre

scale. For transport the board consisted of two sections bolted

together and the lower half of the manometers could be fixed to the

board on site.

The atmospheric limb of each manometer was connected to a manifold

which led to a bucket which acted as a head tank. The other limb

was taken over the top of the board and down the back to a no volume

change valve. Beyond the valve there was a tee piece, one limb

leading to the three way valve block on the piezometer. The other

led to a second valve to which a wandering lead could be attached.

On this lead was a screw ram pressure pump protected by valves C & D

on either side. From the lower valve of the pump a lead went to a

container of de-aired water. All tubing, including the manometers

was 2.8 mm I.D. nylon 11.

For initial site assembly the two halves of the board were bolted

together and the lower half of the manometer pinned to the board with

cable clips. The manifold was then removed and all manometers filled

with de-aired water. Mercury was then added until each manometer limb

was filled to the 1 m mark, the water being displaced. The manometer

valves were then closed and the manifold replaced. All other de-airing

was done using the ram pump.

All piezometers had been recently de-aired and allowed to equalise

afterwards. To carry out a test on a piezometer all valves on the

three-way block were closed on the input side, the manifold lead then

being removed and replaced by the lead to a manometer. The wandering

lead was also connected to the same manometer and de-aired water from

the pump used to flush all leads. Valve A was then opened and with

valves B & C open the manometer was set to the measured pore pressure.

55

The next step was to open the two valves in the three way block

between the manometer and the piezometer and allow the manometer to

adjust to any mis-setting. When any necessary equalisation was

complete, the pump was used to apply the required excess pressure.

This pressure was generally 5 m of water at Grafham Water'and 6.1 m

at Bough Beech except on piezometers at low effective stress where

smaller values were used.

The pressure was held steady by means of the pump for 2 to 5 mins

then valves B and C were closed and timing commenced. As the test

proceeded the degree of equalisation was plotted against log time

until 90% equalisation was passed. Once the first 30 min of any test was

complete a further test could be commenced until all the manometers

were in use.

At Bough Beech it was possible to use the manometers already

installed in the gauge house and minor modifications were made to

the procedure used at Grafham Water. The valve on the return lead

of the piezometer was closed throughout the test and only the input

side used. The excess pressure was applied using the compressor and

control valve in the de-airing system through the de-airing manifold.

Once the manifold valve was closed readings were taken on the input

manometer and plotted against log time as before.

3.6.3 Calculation of Permeability

An overlay of the Gibson (1963) curves shown in Fig.3:12 was

made, plotted to the same scales as the test results. This was placed

over the test curve to obtain the best fit. Where p < 2 (which

occurred in all cases except one) the Hvorslev equation was used.

56

tk F = exp ( V --) Yw

for E = 0.1 (90% equalisation). The equalisation becomes

2.303 — Fkt Vyw

3.3

3.15

For a Bishop high air entry tip 100 mm x 45 mm dia. the shape

factor F = 0.402 m (see table 3:1) and \N for a 2.8 mm bore

mercury manometer is 2.44 x 10 7 m2.

Then

k = 2.303 x 2.44 x 10-7 m/sec 3.16 0.402 x 90

(sec)

k = 1.41 x 10-6

m/sec 3.17 t90 (sec)

For example

(i) Bough Beech Piezometer 25 (Fig.4:43)

p = 0.04 therefore Hvorslev eqn.(3.15) can be used

t90 = 537min then

k 1.41 x 10-6 m/sec

537 x 60

= 4.38 x 10-11 m/sec 3.18

57

(ii) Bough Beech Piezometer 27 (fig.4:44)

p = 4 by curve fitting and pT = 1.86 at e = 0.1

(90% equalisation)

— Fkt 3.19 Vyw

which is the same as eqn.3.13

t90 = 3.72 min and

k — 1.86 x 2.44 x 10-7

0.402 x 3.72 x 60

= 5.06 x 10-9 m/sec 3.20

3.7 CONSTANT HEAD PERMEABILITY TESTS

3.7.1. Theory

Gibson (1963) derives a solution for the case of a steady

pressure difference applied to a spherical piezometer:

Qt = 47a h f 1 + 1- ) Au Yw /TT 3.21

where Qt is the flow to or from the piezometer at time t.

Au is the pressure difference applied.

58

The other factors are as defined in section 3.5. As t tends to

infinity a steady state is set up and

4Tra Y — Au w 3.22

In this steady state the pore water pressure difference diminishes

according to the expression

u = uo + (—a ) Au 3.23

where r is the radial distance from the centre of the piezometer

tip. The radial and time variation of the pore water pressure in the

soil is shown graphically in Fig-3:14.

Eqn.3.22 was also derived by Hvorslev (1951) for an incompressible

soil. As with the response time theory an equivalent intake factor F

must be used for the non-spherical piezometer instead of 4Tra and

eqn.3:21 becomes

Fk Qt —yw Au { 1 + 2--- viTT,

3.24

If Qt is plotted against (t) 2 then according to the eqn.3:24

the relationship is linear with an intercept at t = t. of

Fk = Au w

3.25

59

The slope of the line, n, will be

Qc„,,F n =

3.26 4Tr"c2

Thus it should be possible to obtain both k and c from experimental

results plotted on this basis. But Gibson (1969) extends the theory

to show that c can only be obtained directly when the pore pressure

coefficient A = 1, / 3 3.7.2 Equipment and Test Procedure

Constant head permeability tests were carried out at Bough Beech

only. The equipment used is shown diagrammatically in Fig.3:15.

The constant head supply was obtained using a five gallon container

of de-aired water placed on the dam crest. A length of V O.D.

polythene tube ran from the water supply to the gauge house. Once

inside the gauge house the polythene tube was divided into two

branches, one of which was taken directly to the de-airing manifold

on the return side. The second branch was taken via a pair of double

burettes to the input de-airing manifold.

The burette system, the same as that used by Al-Dhahir (1967),

consisted of twin 100 ml burettes in outer cylinders which were

linked at the base. The two fluids used were paraffin (dyed red)

and water. Flow enters upwards through one burette, down through

its outer cylinder and into the outer cylinder of the second burette,

and finally downwards and out of the second burette. A four valve

cross-over is used to reverse the flow direction in the burettes. The

6o

use of two burettes allows the use of a downward moving interface

inside a burette for measurement at all times. These 100 ml burettes

were limited to measuring flows greater than 0.005 ml/min. Head

losses in the tubing became significant at more than 10 ml/min.

All piezometers to be tested had been de-aired and allowed to

equalise afterwards. Before the start of a test the valves connecting

the piezometer tubes to the manifold were closed and those to the

manometer opened. A reading was taken on the input manometer. To

commence the test the valve to the return manifold was opened and the

excess pressure applied. Readings were commenced after the first hour

of test. To take a reading the valve to the return manifold was closed

and that to the input manifold opened. After allowing five minutes

for any perturbations, due to switching the flow from one piezometer

lead to the other, to die away, a flow reading was taken on the burette.

By switching from one manifold to the other it was possible to run

several tests at the same time, a limit of 10 ml/min being kept on

total flow. Only one piezometer was allowed to flow through the

burette at any time. From time to time while the flow was being

measured in a piezometer its input manometer would be read and the

head difference applied calculated.

The rate of flow was calculated from the burette readings and

-1/2 plotted against t . Readings were continued for at least 5,

preferably 6 days, to t 2 (hrs) values of 0.09 or 0.08.

61

3.7.3 Calculation of Permeability

From the plot of Q against t 2 an extrapolation is made to

t = co (t 2 = 0). For most of the tests the plot showed not one

straight line as expected from the theory, but two with the change

of slope occurring about t 2 (hrs) = 0.2. It has been the second

part of the curve which is extrapolated to t = co. Possible reasons

for this double slope are discussed in Chapter 4.

The intercept on the Q axis at t = co is Q.3 .

All = Ah

3.27 Y

and 0 h is measured on the manometers during the course of the test.

F is the same as that used for the falling head tests for a Bishop

high air entry tip F = 0.402 m. Then from eqn.3.25

Q. k — 3.28

FAh

when Q is in ml/min and Ah is in m of water then

Q. x 10-6 k — m/sec

Ah x 0.402 x 60

Q. x 4.146 x 10-8 m/sec 3.29

Ah

62

For example

(i) Bough Beech piezometer 30 (see Fig.4:45) Q. = 0.060 mil/min

and Ah = 4.135 m.

k = 0.060 x 4.146 x 10-8 m/sec 4.135

= 6.02 x 10-10

m/sec 3.30

(ii) Bough Beech piezometer 35 (see Fig.4:49) Q. = 0.0058 ml/min

and Ah = 3.34 m.

k = 0.0058 x 4.146 x 10-8

m/sec 3.34

= 7.21 x 10-11

m/sec 3.31

The change of slope of the Q v t 2 plots made any calculation

of Cs extremely unreliable. Therefore these tests were not used

for evaluating cs

3.8 THE ELECTRIC ANALOGUE

The electric analogue is a versatile tool for determining ground

water response patterns. It relies on the flow relations described

by Darcy's law for flow in a saturated soil,

v = —ki 3.32

where v is the seepage velocity

k is the permeability

and i is the hydraulic gradient

63

and Ohm's Law for the flow of electric current in a conductor.

I = V/R 3.33

where I is the current

V is the voltage

and 1/R is the conductivity.

With geometric similarity between the model and prototype,

for representation of boundary conditions, voltages can be directly

scaled to total head and within the flow field, electrical conductivities

can be scaled to the corresponding soil permeabilities. The

resistance network analogue, the basic principles of which are

described by Herbert & Rushton (1966), sets up the flow field in a

finite difference form which involves a degree of approximation in

lumping the parameters so that seepage conditions will only be

defined at node points.

The electrical resistors are arranged in the form of a rectangular

grid. In order to represent two dimensional flow it is necessary to

lump the parameters for any point in both coordinate directions.

The vector flow areas associated with each resistor are indicated

in Fig.3:16.

Anisotropy can also be modelled. Using Ax and Ay as the

distance between nodes in the x and y directions as shown in fig.3:16

then the net flow through a unit of field is:

AV R = x and x

AV R - --Y

Ix 3.35

64

(Oh) x (Ah), kxAy and q = k Ax Ax Y Ay

3.34

and if

and AV « Ah and I « q then

C Ax Rx = and R = C AY— 3.36 kxAy k Ax

From eqn.3.36 the resistances can be calculated given the permeability

distribution, anisotropy, grid size and scale factor.

The scaled voltages are applied at the boundary nodes. For an

irregular boundary which passes between nodes it is necessary to

adjust the resistors on the perimeter of the network. Several

methods are available but Herbert and Rushton (1966) consider that

Redshaw's (1948) method is the most accurate. In this method the

resistance is scaled to that fraction of its length within the

boundary.

65

The resistance network used was that in the Rock Mechanics

Section at Imperial College, the technical details of which are

given by Sharp (1968).

The resistors form a rectangular grid of 16 rows by 22 columns

any part of which can be isolated by switches adjacent to the nodes.

The resistors are variable with a range over 10 turns from 100 Q

to 10kS2 which could be locked on intermediate values. The resistor

tolerance at 10M2 is 3% and the tolerance on intermediate linearity

is 0.25%.

70'potential sources are available to provide the boundary

conditions and the potential at each source can be varied between 0

and 5V to a resolution of ± 0.5 mV. Links to the grid nodes and

potential sources were terminated on a 'patch board' and connection

between a boundary node and a potential source could be made by placing

a length of wire between the two sockets.

Using a digital voltmeter the potential at any node could be

measured by plugging in to the appropriate socket on the patch board.

Table 3.1

Piezometer Response Times

Piezometer system V

cm5/gm

x10-4

F

cm

t

hrs

(6 = 0.05)

Casagrande piezometer in

sand pocket 0.6 x 0.15m

dia. 15mm dia. standpipe.

17,650 180 820

As above with 150m of 3mm

bore nylon tube & 3mm Hg

manometer

35 180 1.6

As above with electrical

transducer in the piezometer

cavity

0.05 180 0.002

As above with transducer

and with 100cm3 of air

in the cavity

400 180 19

Fill piezometer 100mm x

45mm dia. with 150m of

nylon tubing & 3mm bore

manometer

35 40 7.3

As above with transducer

in place of manometer

7 40 1.5

Coefficient of permeability of the soil, k = 10-10 m/sec

From Vaughan, 1974.

r

50mm

BRS 'disc'

A

3 3

38 mm

Bishop high air 'butt-nosed' tip entry tip high air entry

0 -3

3.

cD

0 3 0

CD 7

10 11

50mm

Casagrande standpipe

1

BR S 'pot' 50 mm

50mm >1 1 I

50 mm

•

38 mm

38 mm-el H

pneumatic pneumatic / hydraulic Marsland 1974 1

-,

0 C

(J)

-mm

N 0

CD

cD

(1)

Maihak vibrating wire electrical

f-12mm

tensiometer

-k

(Q

w w

0( I -+-1- return tank

suction pump ~

... I 1-1- filling funnel

.n ~ i- Hg manometer

compound Bourdon gauge I; >

~ 1-1-- pressure 6 pressure gauge./ cylinder

5mm 0 polythene tube

\~ " ~ ,'/

to piezometer tips

Piezometer installation

bladder

foot pump

FOXCOTE

-.,

to

W

"" ~

t:= -r--- ;:::=

( )

=®=! ( )

:

-~

~

(.1 to head tank

~ r-l-I-l-f-r-f-f-f-I-f-l- i--~-

I- ;:::

: ~

r---

'.

I

f- . --I-

-f--f- : - f- . ,'-

~

~

~ ( )

me ma

sec

~

de -airing manifolds

cury nometers

Ie

to piezometer tip

de-aired water

pressure gauge

pressure cylinder -

t to pressure pump

return cylinder

manometer

de-airing I~~ manifolds

~ to suction

\ pump -:-1'\

water trap inp'ut cylinder

PETERBOROUGH - Piezometer reading and de-airing equipment

.,.I Ia·

N

De-airing

Making de-aired water

PETERBOROUGH - Modified de-airing procedure

fig. 3.5

to head tank de-airing

manifold

oil water O

to piezometer tips

manometers

to slave unit INN

to pressure pump,,_ to 4- 0

vacuum pump

vacuum9 pressure

(P+ input

portable de-airing equipment

GRAFHAM WATER - Piezometer reading and de-airing equipment

Maihak pressure transducer

fig. 3.6

-- paving slab

\ \\\\\17\:, ,, \\.\\

-71\------150mm 0 plastic pipe

no volume change valves

2.8mm bore polythene coated nylon 11 tubing

52 - 100mm 0

cement / bentonite grout

plaster of Paris

52mm 0

tapered to 38mm 0

high air entry piezometer tip

Borehole piezometer with high air entry ceramic tip

fig. 3.7

no volume change valve A de-aired water in flexible polythene coated nylon container B 2.8mm bore polythene coated nylon 11 tubing C peristaltic pump D brass transducer block E pressure transducer F small bore copper tubing G connecter to valve on piezometer H battery operated strain meter

Portable pore pressure measuring apparatus

fig. 3.8

o_ a) -Q

a)

-o

3 0

0 a) -o

E

GL

4

5

6

2

3

7

8 10 50

days

1 •

e .

,.

. c"

'5.,,ZT

12 \t=

Standpipes Twin-tube hydraulic 6

7, 9 & 12 ,

100 500

Equalisation after installation

POTTERS BAR

fig. 3.9

10 t90

=5.2 days

3 tgo.4.3 days

49

48

47

46

C 0

45

X 44 a)

E 43

42

41 0.1

1

10

100 days

Equalisation after de-airing

BOUGH BEECH

fig. 3.10

0

20

40

0 _

.1 60

o-w

80

100

o n

-6— tx,... t ip

level

21---c---0-c)----

c,-- ....,.,,,a„ --a

0.1

1

10

100 minutes

1 Piezometer A, de-aired for 30 sec.

2 Piezometer A, de-aired for 30 sec. then original pore pressure replaced on

piezometer.

Equalisation after de-airing

PETERBOROUGH

fig. 3.11

r T

Variation of 6 with jjT

[Gibson 19631

to head tank A

ts,

V cp-

y to piezorneter

0 0 0

OD

Portable manometer board

GRAFHAM WATER fig. 3.13

u- uo au

fig .3.14

1·0

0·8

0·6

0·4

r/ a

6 8

Constant head" test - variation of pore pressure with time around an ideal spherical piezometer tip [Gibson J 1963]

parafin /water burettes

to head tank on dam crest

A

input manifold crossover valves 3-way

Valve 6--- return

to piezometer tip7.

Hg manometers

Constant head permeability equipment BOUGH BEECH

x, kx

-1

A region of the flow field showing vector areas of flow associated with typical resistors.

fig. 3.16

66

Chapter 4

PRESENTATION OF DATA - EMBANKMENTS

4.1 PIEWMEThR RECORDS ',ROM DAMS

4.1.1 Peterborough

The records for 29 piezometers installed in the dam and its

foundations are given in Figs./F:1 to 4:6. The height of fill on

the centre line of the dam and the reservoir. water level, when known,

are plotted with the piezometer records for comparison. Dates of

de-airing are also marked when known.

The foundation piezometers in the permeable Kellaways Sands and

the Cornbrash Limestone show a marked response to stress changes due

to both construction and reservoir level and rapid dissipation.

Dissipation of excess pore pressures in the clay is much slower. Tip

29 (fig.4:3) is beside a drain and has recorded atmospheric pressure

throughout, an indication that the drain is operating successfully.

At the end of construction all the piezometers in the shoulders,

including 13 & 25 which are in the upstream transition zone, recorded

negative pore pressure. Most of these piezometric records show

considerable fluctuation where water continuity begins to break down and

the piezometer reading tends towards pore air pressure. It is not

easy to recognise this break down unless de-airing is carried out to

restore continuity. A good example of this is piezometer 27 shown in

Fig.4:6. Thus records of negative pore pressure should be treated with

caution. These readings taken once equalisation after de-airing is

complete are the most accurate. By 1971 a zone of positive pressure had

67

built up on the upstream face and had progressed about 5 m. The

wet 'redeposited' clay in the downstream toe had also become a zone

of positive pressure and is the only part of the dam which can be

considered to be approaching equilibrium.

The core piezometers, shown in Fig.4:5,recorded positive pressures

_ at the end of construction which have dissipated and are at present

negative. The pressure dissipating sideways into the shoulders where

there are large negative pressures.

The pore pressures in the dam at end of construction and after

eight years are shown diagrammatically in Fig.4:6a.

4.1.2. Grafham Water

Piezometer readings from Grafham Water dam are plotted in Figs.4:7

to 4:16. The foundation piezometers, Figs.4:7 and 4:8, are all of the

Maihak vibrating wire type and only five of the original ten are still

in operation. Three were lost within the first two years. Readings

taken during the winter of 1972/73 are clearly in error. This was

due to the reading unit not being checked against the built in standard

and being considerably out of adjustment. The excess pore pressures

in the foundation are still dissipating.

At the end of construction the upstream shoulder piezometers

show a mixture of negative and positive pressures but they all respond

rapidly to the reservoir water pressure after impounding due to the

closely spaced drainage layers. By 1968 only piezometers U7, U8 & U11

were not recording reservoir level, these three being in zones where

the drain spacing was considerably greater than the standard 1.5 m. By

1972 the shoulder could be considered to be close to equilibrium.

68

In the downstream shoulder (see Figs.4:11 & 4:12) D8 and D1 had

positive pore pressures at the end of construction. These dissipated

rapidly and both tips were recording approximately tip level by

1965. The others, with negative end of construction pressures,

have dissipated more slowly. By 1967 they were close to tip level

but even in 1974 they are all recording negative pressures to some

extent, see table 4:3. This shoulder cannot yet be considered to

have reached equilibrium.

In the core (see fig.4:13 & 4:14) all piezometers except C2 &

C6 had positive pore pressures at the end of construction. Without

drains the equalisation process is slow and piezometers show equalisation'

ratios of between 40 and 60% assuming a linear head loss across the core.

Both Maihak and hydraulic piezometers were installed in pairs

in the road embankment. The Maihak piezometers, as discussed in

Chapter 3, were unsuitable for measuring negative pore pressures and

readings on them were soon abandoned. The records shown in Figs.4:15

& 4:16 are for the hydraulic piezometers only. As in the core,

pore pressure equalisation is slow without drainage and there is

evidence of negative pore pressures of the order of 5 m, e.g. R7

(Fig.4:16). These have not been sustained long after de-airing.

De-airing at 6 monthly intervals would be required for these piezometers

to have recorded correct pressures consistently. The de-airing which

was carried out in 1968 has not had any effect on the pore pressure

readings, very noticeably R6, R7 & R8, and it would appear that the

de-airing was not performed correctly.

69

4.1.3 Bough Beech

At Bough Beech records are available from 1968 to 1971 and a

further set of readings were taken in 1974. These are plotted in

Figs.4:17 to 4:25.

Piezometers 1 and 39 have been plotted as being located in the

foundation, as this is how they have been classified by the Resident

Engineer's staff. They are in fact in fill placed below original

ground level. This accounts for the swelling which No.39 is recording,

see Fig.4:17, and for its lack of response to construction. Those

on the upstream side, which are true foundation piezometers, show a

marked response to construction, No.31 having piezometric levels above

the fill level at that section. Some dissipation occurred before

impounding. Since then the pore pressures have been equalising to

reservoir water level, No.31 still showing pressures greater than

hydrostatic.

All the piezometers in the upstream shoulder now record reservoir

water level and the shoulder has reached equilibrium, see table 4:1.

Two piezometers, 21 and 29, in the upstream cluster are in the sand

drainage layers which are 2.4 m apart. The sand in these layers was

specially selected to be free draining and has a permeability of

3 x 10 5 m/sec (Hallas and Titford, 1971).

For the drains to be 100% efficient they required a permeability

a million times greater than that of the fill (Gibson and Shefford, 1968).

An acceptable efficiency would be obtained by a permeability ratio of

3 x 104. This should have occurred with the design permeabilities of

3 x 10-5 from the drains and < 10-10 m/sec from the clay which

gave a permeability ratio of 3 x 105 (Gibson, 1971).

70

However piezometers 35, 37 & 38 which are in the middle of clay

layers, at least 1.1 m from a drain have responded more rapidly than

Nos.21 & 29 in drains.

In 1970-71, in the cluster, 21 to 29 (Figs.4:20 & 4:21), the

lag of these piezometers in the middle of the clay layer (24 to 26)

behind the two piezometers in the drain, is only about 75% of the lag

of these drain piezometers behind reservoir water level. This indicates

a drain efficiency of about 25%. However, this drain design is based

on a drain's ability to remove water during consolidation rather than

to supply water during swelling. Therefore it is uncertain if the

inefficiency is due to design permeabilities not being obtained or to

the swelling process requiring a greater permeability difference.

The large negative pore pressures, as low as 7.5 m below tip level,

measured before impounding were not maintained well despite regular

de-airing so the pressures recorded before 1970 must be treated with

caution. Negative pressures were recorded in the drains.

Negative pore pressures were recorded by two of the core piezometers,

fig.4:22, at the end of construction, and by 1974 they show pore

pressures between 40 and 60% of the equilibrium values assuming horizontal

flow through the core.

In the downstream shoulder all piezometers have recorded negative

pore pressures, even 2, 4, 12 & 14 which are in drainage layers. All

pore pressures are tending towards tip level but the readings taken

after de-airing of tips 3, 6, 8, 10 & 13 in 1974 show this trend to

be partially the effect of water continuity breakdown in the measuring

system. Negative pore pressures of the order of 4 m of water still

exist, see Table 4:1 and figs.4:23 to 4:25, and the shoulder is far

from being at equilibrium.

71

4.2 DOWNSTREAM BOUNDARY PORE PRESSURES

4.2.1 Slopes without drainage

Three sites without drainage have been studied, Peterborough,

Grafham Water weight block and road embankment, and Aldenham section 11.

The records of the eight piezometers from Peterborough are plotted in

Figs.4:26 to 4:28. Readings were taken from July 1971 to March 1972

and from March 1973 to May 1974, thus obtaining two seasonal maxima

and minima.

The observations made using the shallow piezometers at Grafham

Water are plotted on Fig.4:29 to 4:31 for the same periods as those

from Peterborough. The road embankment piezometers,Q, shown in ag.429,

shows a pore pressure of -3 m of water. This was obtained consistently

until after October 1973. After that date the pore pressure reverted

to tip level, the volume of air obtained on de-airing in January 1974

was small,no greater than had been obtained on other occasions, thus

air in the system does not seem to be cause of the apparent breakdown

in measurement, which cannot easily be explained. However, considerable

surface cracking occurred during the very dry late summer of 1973.

This could have opened a fissure through the grout allowing water

access to the tip.

The four weight block piezometer records are shown in Figs.4:30

and 4:31. The pairW&X were installed in a wet patch (at Binnie &

Partners suggestion) to ascertain if there were high pore pressures

in that area. The results however are comparable with those from the

other pair U & V. V and X were installed using dry bentonite instead

72

of plaster of Paris. Except that X equalised more rapidly than W

after installation, the different method does not seem to have

effected the behaviour. Dry bentonite can therefore be considered

a suitable alternative to plaster of Paris.

The piezometric levels obtained on the deep piezometer„ in

section 11 at Aldenham are shown in Fig.4:32. Piezometer 3 which is

not on this section but has the same drainage conditions is also

shown. These piezometers are too deep to be much effected by seasonal

fluctuations and the pore pressures obtained can be taken as equilibrium. No.4

which is in the upstream foundation is effected by the reservoir

water level and no readings have been taken since the end of April

1975 because th U4 tube housing the piezometer leads is below reservoir

level and can no longer be found.

The shallow piezometer records for the same section are shown

in Fig.4:33. The readings during March, April and May 1975 are

winter values and after a very wet spring are maximum values. An

indication of the summer minimum values are given by the July 1975

readings. A complete annual cycle of readings are required to obtain

a mean value with accuracy approaching that obtained at Peterborough

or Grafham Water. The pore pressures in section 11 are shown diagram-

matically in Fig.4:34. Considerable surface cracking occurred during

the very dry June/July of 1975.

73

4.2.2 Slopes with drainage

Three sites were studied in this category: Foxcote, Grafham Water

main dam and Aldenham section 19, the drainage conditions differing

at each site. At Foxcote there is a 0.3 m gravel layer under the

topsoil and a base drainage blanket which extends beneath piezometers

1, 2 & 4. The pore pressures recorded on the six piezometers are

plotted on Fig.4:35.

The readings prior to August 1972 are suspect. This is because

they were in general taken approximately an hour after de-airing.

It was clear at an early stage that number 6 had not equalised after

an hour and this was usually given about four hours.

It was found on the July 1972 reading that despite four months

having elapsed since the previous reading, there was comparatively

little air in the tubes. Therefore, as a trial, the next readings

were taken before de-airing. The effect on piezometers 4 & 5 is

marked and it was this experience which led to the practice of allowing

at least a week between de-airing and reading as described in Chapter 3.

A section showing the post-July 1972 pore pressures in the downstream

shoulder is given: in Fig.4:36.

Piezometers Y and Z in the downstream shoulder of Grafham Water

Dam are in the middle third of the layer between 15 m spaced drainage

blankets and with a surface gravel layer beneath the topsoil. The

pore pressures recorded on these two piezometers in 1971/72 and 1973/74

are plotted on Fig.4:37.

71-1-

Section 19 at Aldenham is under drained by a brick lined culvert.

The deep piezometers, shown in Fig. 4:38, are recording equilibrium

pore pressures, however No. 7 is still responding to installation.

Piezometer 5 in the upstream shoulder is responding to impounding.

The shallow piezometers, shown in Fig. 4:39, have well defined

winter values but only one summer reading so mean values will not be

very accurate as discussed earlier. The pore pressLies in section 19

are shown diagrammatically in Fig. 4:40.

4.3 PERMEABILITY TESTS

Falling and constant head permeability tests were carried out

at Bough Beech and falling head tests at Grafham Water. The equipment,

techniques and methods of analysis used are discussed in Chapter 3.

4.3.1 Bough Beech

The lack of equilibration in the downstream shoulder, discussed in

section 4.1.3, restricted the permeability testing to the upstream

shoulder. Testing was restricted to those piezometers at or close to

equilibrium because (a) the degree of saturation in zones where

equilibration was still in progress would not be the same as in the

long term and (b) where pore pressures are negative the test is less

reliable and effective stresses are known with less accuracy. Of the

seven cluster piezometers installed in the fill, only three were chosen

for testing, otherwise all the upstream fill piezometers were tested.

The falling head tests were carried out first as no equalisation

time Was required on completion of a test. In practice a week was

allowed between two tests on any one piezometer. The test curves are

plotted in Figs 4:41 to 4:51, and the results summarised in table

4.2 and Fig. 4:52.

75

The Gibson (1963) curves of 6 against TIT are fitted over the-

falling head test carves plotted as equalisation against log time.

In some cases the Gibson curves are a good fit, for example No.25, 30

and 38 (Figs.4:43, 4:45 & 4:51). In other cases the fit is poor,

e.g. 23, 27 & 34 (Figs.4:42, 4:44 & 4:48). Where the fit is poor,

the best fit between 60 and 90% equalisation is used.

On 23 and 27 the lack of fit is on readings taken at half a

minute and less after commencement of the test. During this part

of the test when the manometer level is moving rapidly, small timing

errors are considerably magnified by the logarithmic scale. However

these errors would not account for the deviation from the Gibson

curve observed in 27. Incomplete saturation of the soil is likely

to cause some lack of fit as saturation will be increased as the

test proceeds and with it an increase of permeability to water.

The constant head test results can be divided into three groups.

The first group, and by far the largest, are 20,23,27,30,35 & 38.

These all have a Q v plot which consists of two straight

line portions, generally connected by a curve, the gradient increasing

as %p is approached. Theoretically the plot should be a straight

line of gradient n where

QF n - 4 . 1

47T 1.5 c 2

which is the same as eqn.3.26.

76

Gibson (1966) looks at the effect of a piezometer with a filter

permeability approaching that of the soil. His parameter A is

defined as

kl a = — ( —1 - 1 )

k3 a3 4.2

for a piezometer in direct contact with the soil

where k1 is the soil permeability

k3

is the ceramic permeability

al is the outside radius of a spherical piezometer

a3 is the inside radius of a spherical piezometer

-- The curvature of the Q v t 2 plot will increase with

increasing A , the gradient increasing towards the origin.

For a Bishop high air entry piezometer tip as used at Bough

Beech a1/a3 -1 = 0.5 and k3 10-8 m/sec.

The permeabilities calculated without correction for these

piezometers range from 6.40 x 10-9 m/sec to 7.21 x 10 11 m/sec

with corresponding A values of 0.32 and 0.0036.

For A < 0.05 (k < 10-9) the curvature should be very small

and the inaccuracy negligible. As may be expected 35, with k = 7.21 x 10 11

m/sec, is closest to a straight line in this group.

For A > 0.05 (k > 10 9) which is the rest of the group, the

curvature and inaccuracy increases with increasing A . At the

other end of the range where A = 0.3 the permeability calculated

from the Qv t 'curve may be an underestimate by as much as 20%.

77

The majority of the group may have an error of approximately 10%.

Within the scatter of results this has been considered negligible

and the permeability, calculated from the Gibson (1963) formula,

eqn.3.25, has been used without correction.

However, the curvature due to large values of x is not as

great as the curvature obtained in the test on piezometer 27, fig.4:44.

This piezometer also showed a bad fit on the Gibson curves for the

falling head test. This may be an effect of a low degree of saturation

and the dry lumpy structure of the compacted clay. This effect, which

is discussed further in section 4.2.3, may explain the reverse

curvature of the graph for piezometer 25, fig.4:43.

The third type of result is No.32 and 34 (Figs.4:46 & 4:48)

which have Q constant. This implies c = , an incompressible

soil. For 32 this can be explained by the impossible permeability

value of oN, 9 x 10 8 m/sec, which indicates either a leak, probably

in the piezometer tip itself as it is recording correct pore pressure,

or a fissure. The result of the test on 34 is inexplicable unless it

is also leaking, or there is a fissure. It is one of the piezometers

where the comparison between constant and falling head test results

is poor, but the permeability value is reasonable.

Due to an error during the field testing, the maximum pressure

applied to 5 of the piezometers during the falling head test was

greater than the overburden pressure. None of the test curves show any

78

indication of hydraulic fracture therefore the results are considered

acceptable. One of the constant head tests was also carried out

at a small negative effective stress without fracture occurring.

The constant head test on 37 (Fig.4:50) gave a scatter of

results through which no satisfactory curve could be drawn. This

was also run with a small negative effective stress and could have

been effected by hydraulic fracture. However the negative stress is

smaller than that in other tests where no effect can be seen.

The values of the coefficient of permeability, k, obtained by

the falling head method are plotted against those obtained by the

constant head method for the same piezometer. Eight results are

plotted in Fig.4:53. Five results lie very close to the kf

kc

line while the remaining three have kf

kc. Two of these three,

25 and 31, have kf/kc ?, 0.5 while for No.20 kf/kc = 0.18.

Except for No.20 the results of the falling head tests are

acceptably close to the more reliable constant head results. The

acceptability of the falling head test results meant it was possible

to carry out, falling head tests only at Grafham Water with some

confidence. The falling head tests being considerably more rapid

and simpler to perform.

4.3.2 Grafham Water

Eighteen falling head permeability tests were carried out at

Grafham Water, nine in the downstream shoulder and nine in the upstream

shoulder. At Grafham Water the downstream shoulder is closer to

79

equilibrium than at Bough Beech. The test curves are plotted in

Figs.4:54 to 4:62 and summarised in Table 4.3 and Figs.4:63 and

4:64.

Only D1 and D8, Figs.4:54 and 4:57, are a good fit on

the Gibson (1963) curves. A second group is a reasonable fit,

D5, D6, D7, U2, U3, U4, U7, U8 and U10 (Figs.4:56, 4:57, 4:58, 4:59,

4:61 & 4:62). Several of this group tend to drop across the Gibson

curves, p increasing, then rise again, p decreasing, to reach

quite low values of p by the end of the test. U10 (Fig.4:62)

is a good example of this. However, by 90% equalisation the fit is

satisfactory and the standard calculations can be used.

A third type show a typical equalisation plot but equalising to

pressures above the equilibrium pressure on the piezometer. This

group includes D2, D3, D4, U6 & U11 (Figs.4:54, 4:55, 4:60 & 4:62).

D4, the best example of this group, has t 90 = 7.5 mins, calculated

using the intercept of the two straight lines on the log plot as

100% equalisation, the test was allowed to carry on and after about

200 mins the equalisation began again. The curve crosses the 90%

equalisation line after 3070 minutes thus giving a second value of t9

The early part of the test was repeated on both D3 and D4 with

excellent repeatability. Dll and U5 do not fit into either group

but give reasonable values of permeability using the t90 obtained

in the Hvorslev equation.

8o

Plastic clays compacted at or dry of optimum have a lumpy

structure and a very variable degree of saturation. Initially this

may be very permeable to water locally, but as water is supplied

swelling occurs, closing voids. Also, if the degree of saturation is

low and air is continuous, the permeability of the soil to water may

be quite high. The permeability will reduce as saturation increases

to reach a minimum when the air becomes discontinuous then increasing

again towards saturation (Barden 1974). This may partly explain the

wide variety of permeability measured.

The supply of water to the downstream shoulder is much smaller

than the supply to the upstream one and the degree of saturation upstream

is probably greater and more evenly distributed. Saturation is a

slower process than pore pressure equilibrium and full saturation,

even with an abundant water supply, may never occur. This may explain

the comparatively consistent permeabilities in the upstream shoulder,

- generally between 10

10 and 10

-11 m/sec, see fig.4:63. The results

from the downstream shoulder give permeabilities either less than

- 10

-11 m/sec or greater than 8 x 10 10

m/sec, see fig.4:64.

Only the upstream shoulder shows any indication of a permeability/

stress relationship, see fig.4:63.

4.4 MOISTURE CONTENTS

Moisture content profiles were obtained for the shallow hand auger

holes in Peterborough, Grafham Water and Foxcote Dams at the time of

installation. Three additional holes at Peterborough were put down

during the winter to observe any seasonal changes. The results are

plotted in Figs.4:65 to 4:09.

81

At Peterborough there is considerable scatter of results which

is only to be expected with the variability of the fill material

used. The range of moisture contents lies between 20 and 35% with

some indication of an increase during the winter. Placement moisture

at A, B, G & H was between 20 and 22% and at C, D, E, & F between

14 and 18%.

The scatter.at Grafham Water is smaller, generally between 20

and 30% and the profiles themselves more consistent. The placement

moisture content was between 18.5 and 21.5%. The only deep hale,

Q, still shows these moisture contents.

Foxcote now shows moisture contents between 25 and 30% with

comparatively little scatter. This still compares well with the

placement moisture content of 28%.

Table 4.1

Bough Beech - Piezometer readings 17th July12A

Downstream shoulder

Piezometer

No.

Level

m 0.D.

Piezometric level

m 0.D.

Pore pressure

m of water

3 45.8 41.8 -4.0

6 49.6 47.3 -2.3

8 50.2 46.2 -4.0

10 50.8 46.5 -4.3

13 55.1 54.2 -0.9

Upstream shoulder

Piezometer

No

Level

m 0.D.

Piezometric level

m 0.D.

Pore pressure

m of water

20 45.8 62.2 16.5

23 50.1 62.1 12.0

25 50.7 62.3 11.6

27 51.3 62.2 10.9

30 55.7 62.2 6.5

32 47.6 62.0 14.5

33 50.0 62.0 12.0

34 52.5 G2.1 9.7

35 54.9 62.1 7.2

37 49.4 62.2 12.8

38 45.8 62.3 16.5

Piezometers de-aired between 24th June and 5th July

1974. Reservoir level 62.3 m 0.D.

Table 4.2

Bough Beech - Permeability test results

Piezometer

No

Effective stress kN/m2 Permeability m/sec

before test falling head

(min.)

constant head falling head constant head

20 23 25 27 30 32 33 34 35 37 38

152.8 110.0 103.7 100.7 55.0 95.7 71.7 47.6 23.0 32.3 38.9

93.0 49.9 43.9 41.0 -4.8 35.9 11.9

-12.2 -36.8 -27.5 -20.9

114.5 72.3 67.3 67.0 14.3 59.2 --

11.7 -9.9 -4.8 6.0

2.23x10 10

7.22x10 9

4.38x10 11

5.06x10 9

7.25x10 10 - 9.30x10 8*.

2.57x10-11 2.63x10-10

7.88x10-11

1.42x10 8

1.40x10-9

1.25x10 9

6.40x10 9

6.16x10-11

5.33x10 -9 6.06x10 10

8.90x10-8*

5.14x10-10

7.21x10-11

-- 1.58x10 9

* This is greater than the permeability of the ceramic of the piezometer tip. Therefore

there is probably a leak in the piezometer system.

Table 4.3

Grafham Water -Falling head permeability test results

Piezometer No

Level m O.D

Piezometric level m O.D.

Pore pressure m of water

Effective stress kN/m2 Permeability m/sec before test minimum used

U2 22.9 42.8 19.9 174.8 125.4 3.63x10-11

U3 22.9 42.5 19.6 237.2 200.1 2.64x10-11

U4 30.1 42.7 12.6 93.8 69.1 1.43x10-9

U5 30.3 42.7 12.4 144.9 95.5 1.68x10-11

U6 38.1 42.7 4.6 61.2 36.5 8.55x10-11

U7 25.9 42.6 16.7 142.5 93.1 2.79x10-11

U8 26.2 42.5 16.3 198.7 149.3 1.26x10-11

U10 34.9 42.6 7.7 100.1 50.7 1.77x10-11

Ull 41.1 41.9 0.8 54.4 17.3 1.62x10-10

D1 23.6 23.2' -0.4 404.0 354.5 3.74x10-12

D2 23.8 22.9 -0.9 228.3 178.9 8.74x10-10

D3 30.9 30.6 -0.3 249.8 200.4 4.85x10-9

D4 30.7 30.5 -0.2 74.4 49.7 3.40x10-9

D5 38.7 36.5 -2.2 93.8 69.1 -1 2 2.35x10

D6 26.8 26.7 -0.1 332.9 283.5 4.18x10-11

D7 26.7 25.3 -1.4 171.5 122.1 7.43x10-19 -

D8 35.6 35.2 -0.3 150.6 101.2 6.62x10-12

Dll 41.8 39.1 -2.7 45.5 20.7 2.75x10-12

Reservoir level 42.8 m O.D. Piezometers de-aired between 30th August and 6th September 1974

12 Fill on ~ v ,-r-- ,\ M

8 I : I V D I

/ I ~

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+' ~

..... cc l'

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12

8

r ~ 4 m

00

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v ~ r-- -"'\

I V*'---... I . ~ " .. 1 ,

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PETERBOROUGH - Core foundation, piezometric levels

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(Q

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ret)

< 4 et)

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Fill on ~

V r-' r--- "\ ...

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If~-;-~lp/~I I I I - ... --c-kL. I .<> I I~I . ---~ TIC---~. V I l~-

. C I r---.-;,-C-r-r '-.l ,

.~ . F" J-"'~

'lJ ro'

'N o

CD < CD

I I I

I Ii! I ,! -5·0 I I_I_~I IL_1_-'_-'

T I

I I 1 I I , I :

! -6·0 I I I I I I I I 1-W M I I! I I

-/·0

'\' !! '

~, ~{~J~(\~--_?! I 12 r IF! w" 1\\ i ~7r~d~-!f I

'" .

I \, !/ I , I !

1\./ ~

I I I I

I I

! I , I

I

I I

I

I 1 I

I IJ ,--__ L _________ ~_ I I I

"

PE-rER BOROUGl-l - Do\vnstream shoulder pie7 orneter.s

-t\

lO

.t'

~ (»

GL

G o -1·0

H -2·0 0

"'U -3·0 roe N 0

3 ro S- -4·0 ()

CD < ro -5·0

3 -6.0

..

ro .-+.., ro (Jl

-7·0

J A

~

r:t

~r

1971 1972 S 0 N 0 J F M M A M J J A

- .1"':l. I- ..

~ ~ -... ~ ... C)..

...........

~ ~" ~ ~ .-~\

n -"" ~

~ 1""""-"'"' ~

.~ H I~ -. M- _ ~ 1- ",[:.J ' ...

.",.~ 1-1

.,.---... ,.. ~

,

--I-.-~ -.~

PETERBOROUGH - Berm piezometers

1

1973 1974 S 0 N 0 J F M A

J. ~ ~ r---......

V ~.~

~ - V --ED -IV """t:I

,L I

~

to

+"

N LO

"I a LJ'i 1 ~ I I 1G7:) ~ Go._ 1973 ~19/4

I J IAlslo IN/oIJ '~~l IMIAIM J J lA's 10 'Nlol J I FJMIA Ge

l

'

l

I II I II,B_'i--i~:-i=o~-___ 1 I I I 1_ rJ--.1 I I I I I I ',; I I i I r'~ I i{~ I

R00P -1'0 ' I. I I II1I /i~~---r~ I. I 11 I I 14 ~'!~~ . ,~/ I I '-'\ i/\ , J,LLJ I i Ifl\~ I I j/ ..L:r Ii' r~~ I ~I ~ I I:· I I

tlq I \ _ I I I ~./I /1 1r~ j I I LJ ~:). I \l7 I I' I I' I, \ I ~~'cf I / 1/ I I I I I,

~. -3· 0 \1 I \I( I I ! ,I i I' I ! I II I \ I: 'I I /1 I II i i 3 d I I I \ Ii' ! I , i 1 ;?. il I I I! I I r-rr I I I i

:. -£.·0 I ¥ II ' .. ~!. H---Hi I! : II I l!t/H-1 II I I !. ~ ~1[-'4= I I I; I ~{)_' __ L I I I - ~ r0--! ·_--} .. -.2 I I I I I I I! I I I I I I 'I'

'" ,r-- I L..- -.....,.., , I L I .... , I I"

3 Q .-t-,

- f. J'/' I I t !---+-=!~.,.. __ I I . I • __ ~~""'''f=--==t....r::l I

-:;.01-1 J I I I I ,~--r~~'~~I! I ~I --+--+-+---~--7----i

~ -6·0 I I I I i I I I

-7.0

,.... 0 \ r H /\ P,,~ \ ~ I \ 1- r- r, n -.S .' J }.. I

LJ('\KC' /-\ivl fIJI-- t= K i-x.OGO e rnDQrl~rnerl( Die70meriers II

~

\0

:-...

w o

GL

u0v -1.0

-u -2.0 CD N o 3 CD

~ -3·0 n

CD < CD -4.0

3 -5·0 CD -+., CD (J)

r\

-6·0

-7·0

1971 1972 .. J A S 0 N 0 J F M

I Ii. .~ ~ I~ w:t.

~ --~ ~

~

1/ ,Vu

. !J .~ ~ III

~ 10 V 11 ~ i

M

~

" ~ ".:s T0-

A M

~

r---... r----.... r---..

J J A

~ ---~, + ro)

\ ,

1973 S 0 N

~ V ~

\ J

\ / '\ / ~ J~ V

. l!I

GRAFHAM WATER - Weig~t block piezometers

Aol }.

1974 0 J F M A

[1

r1rr-~ ----~~ ./

V V J

/ /

1

-

(

or-I

..... t (Y) I

I t.o L{) I

I

. level Piezornetrlc

(/) L. 0)

...:-.... QJ

E o 1'1 OJ

'0...

-d (1.)

~--d

.. 7-.-....

.1:: c.))

'(jj ~. >.

0:: UJ !--<l <.: --

fig.4.31

-E -

GL , V 6 .J:l fA-' ~- ~ -

~-L.:.J 1

2

3

Q. ,(3-. .. ----6). ------~-0 ~

3 ~ ...... -E) G 1 -

-Q)

> Q) roo

4 -0

6 I'"'

C ... :J l, 0 5 L-

en

3 6 0 -Q) ~

.0 1

..c. 7

...-c... Q)

0 8

n' II 113 .. u

9

10 M A M J J

1975

Piezometric Levels - Tips 1,3,4&6

ALD E.NHAM fig.4.32

-E

Q)

> OJ

1 ~-.. {----I

,~: N~

2 f_O,_8 ___ ~ ____ ~ ____ ~ ____ ~ __ ~_\ __ ~ o 11

3

~-----~----~----4-------·~----A M J J

u C ::J o t-tJ) G L

1975

o (lJ

..0

1

3

_.0-0-9 -10

4 ' __ .1--__ _

Pie zorn e t ric Level s - ef ip s 8 t 0 1 2

ALDENI-Il-\M

...... <.C

~

w ~

---- - - - - -I T - - - - - -. - - - - - - - _________ _

6

max 1T min """"I Piezometric level

r~ v::::tIiSP~,1 lj'tibE"r:iV'

o 5 metres

ALDENHAM - Seasonal pore pressure variations, section 11 downstream

t )- f- I

~

<.0

~

w U1

u CD N 0 3 ro -to-., -. ()

CD < CD

3 CD -to--, CD (j)

1971 1972 1973 I 1974

G L I JTAfSTolNloi J IF IMHMI J I J IAISlolNlol J IFIM[AjMI J I JIAISIOINlol J IFIMIAIMI J

-1·0 .h I ~~

10

2003 . -2·0

5°04

60

-3·0 I

-4·0

-5·0

~ L 1 ~ ~~ j ~pl I __ ~ I I I I I I ~':~T - I ---~ ~:L t~r I

r:;

i/~ !~ -II I ~18-g

\

~ - -- - ""' - - 1_" ~L-~ I I J ~ ~--~I 'F~r-

-7.0 iL· __ L-~ ___ ~~L--L--~~--~---~~---~~~~--~~--~--~--

FOXCOTE - Piezometers

T.W.L.

rotted clay fill

2.75 3

I-1 \ '---•4

=====t • piezometer

0 5 10 15 pore pressure (-ye) metres •---{ Oa (+ye)

FOXCOTE DAM - Section showing pore pressures

Opl

aWO

Zak

i

-3.0

A

...------!..- %1■50.......A.......;3*1..2

Z

-- L.-.

/

,

?

.-...-.‘..,

;

4

Li

M A M J J A S 0 N D J F 1 M A

.-c -- ----.....

, „..,....-.....,... r-P.------------ OM=

GL

-4.0

g -5.0

-5.0

-7,0

-1.0 Y Z 0

-2.0

1971 1972

1973 1974

GRA=HAM WATER - Downstream shoulder piezometers

M J J M A

7

2 5

7

2

GL

1

E

2

3 a)

4 -o

0 5 L._ cn

6 a)

_c

a) 0

9

10

8

1975

Piezometric Levels - Tips 2,5 & 7

ALDEN HAM fig. 4.38

-117

17

IV15

t.,_.:

..c

M A M J J 1975

G js , v7. 16 at---;-(------

1%-- 14

, _ =

,..0-0- 14 16

"---ES— 13

—0- 13

GL

1

E 2

■ •

> 3

4

0 G L

ta)

0 1 N n

3

Piezometric Levels - Tips 13 to 17

A LDENH AM

fig. 4.39

.•••••••• 0- 14

,•■ ■ 11

13

-0 13

16

•••••••••

=OM= Elmi■ ..■••

2 =woo II■me •■■••

OMmor. INI=M 11■111. MIMEO

IIMINNO *ammo

0 metres 5 max — min Piezometric level

-0 •=1•11=N, •••=f10 ■••••■•■

•••••••■ 111■0 ■■■■• al•■•■ ■••••■••• •••••• OEM.

ALDENHAM - Seasonal pore pressure variations, section 19 downstream

I-. '''' ■ .„"r•.N. 4, , ■ 1:-,—•‘.

0 0 k

.\

-1:_,„\s.

\ \.\ \

1/44\i„:\

1.14

2

\\ \,...,

te • Nk.:\i \\ , ,

-,,;:i.\ ..,,

20 u =1

k= 2.23x1010

t 90 = 0 = 1

m/sec

05 min

....

10

100

Time

( minutes)

— 20 O)

C 40 0

tn = 60 0 0-

80

100 0.1

0.2

E 0.1

0

0 LL

0

20 Q.cr-..

k-

0.117

1.25x10-9

mt /min

m/sec

0.2

0.4 0.6

0.8 1.0

1.2

1.4

/rf hrs

Permeability Tests - BOUGH BEECH

fig. 4.41

-. '•04 .

-. . .

\ 23 Jk

I.J .-- 0.2

= 7.22 x10

tgo = 3.35 -9 m /sec

min

\ ■ \ \

V 6 1 10 100

(minutes )

— 20 T3

c 40

1:5 tn

60 O

L1.1 80

100 0.1

Time 1.0

0.8

E 0.6

a 0.4

0

LT: 0.2

0

o

Q.= 0.59 23 k = 6.40

ml /min -s x10 m/sec

0 02 04 06 08 10 12

1.4 hrs


fig. 4.42

1.2 1.4 02 04 1

0 8 10 (hrs)

Clcc= 0.0055 25 k =6.16x10-11

ml / min

m /sec

,,,..

C)- --r- ....----a---''' -,-) „...------15

O

0

0.02

C

E

E

0.01 C

20

O

---- 40

._.--__. ..... -_,----_,- __ __ ,_. ...... k -I ..., .

- -.... "C \ '...•

''' ,, ''''‘:‘*••• 0

I.

.2 ON, \ tD\ \

) \ \

25 p = 0.04

k =4.38x

t90-= 537 min

0-11 m /sec

\ \ \ , .

10 100 1,000 Time

( minutes)


fig. 4.43

Time

(minutes)


0 02 04 06 08 10 1.2 1.4 1

(hrs)

0

— 20

40 0

N 60

Cr 80

100

... N

. ■

N

„ . .

. 2 \

N . '7 P '' 4 k=5 06X10_9

t 90=3•72

m /sec min

N \ 4 . 10

.

\ \ \ \

N N

N \ \

\ \

...., . ---. .....

0.1 1 10 100

2.0

C - E 1.6

E 1.2

.., t.

27 Q. = 0.44 k= 5.32X10

ml /min -9 m /sec

0.8 0

0.4

0

fig. 4.1.4

- -1 ---2. -. , _.

-------; ..

..-,-.., . ,.-) . 0 .-. •2 N

\ \

\ \--i) \ \

\

30 - u =0.02

k =7.25x10-1°

t 90= 32.4

m /sec

min \), \\N \\,‘

..., \-•(-D

20

---- 40

C 0 — 60

0 80 CT

LIJ

100

0.10 4.••■•••••

Eaos

I 0.06

o

30 Q. = 0.060

k = 6.0200

ml / 10 m /sec

min 0.04

0

0.02

0

0.1 1 10

100 Time ( minutes )

0

02

0.4 0.6

0.8 1.0

1.2

1.4 1/IF

( hrs)


fig. 4.45

t90 = 15.2 sec 3 2 k=9.3x10-8 m/see

0

20

40

C 0

60 0

72; 80 cr

1.1.1

100 0.1 1 10

100 Time

(minutes)

10 ••■•■...

E 8

6

0

4

0 LL

2

0,1'0 -0 -0 6 o 0 -0

3 2 Q, = 8.4

k = 8.90100-8

ml/min *Note:-

m /sec*

k

able leak tip =.1x10-8m/sec,

of piezometer

in system. prob-

02

04 06 0.8 1.0 1.2 1.4

1/11-t- (hrs)

fig. 4.46


20

40

C 0

-1-* GO 0 cr)

80 CT

100 1

--0-c-p- -

r-\\;„,-.,04 . \ .

\ • ' . \

■ .2-)

\\\ \

3 ,FI =

k=

•1

•57x10-11

t90=912

misec

min

10 100 1,000

Time

(minutes)

Note:- Constant head test equipment not suitable for measuring flows of less than 0.005 mi/min


fig. 4.47

0

20

40

0 6 0

tn

D 80 cr

100

.f.. 3

N. \

■ Ilk

\ 4\ N

\

4 \

\

\

\ c--

3 4 =0.1 t90 :: 89.5

k =2.63x10-1° m/sec

min \ \

y...

0.1

1

10

100 Time

(minutes)

3 4 Clec=0.047 ml/min

k = 5.14x10-10m/sec

0.10

E '0.08

0.06

0 0 0 0-- 0

0.04

0

0.02

0 0 0.2 0.4

1 0.6 0 8 1.0

( hrs ) 1.2 1.4.

fig. 4.48


fig. 4.49

0 02 1.2 1.4 8 1.0

(hrs) 60 4

14 0•G

20

40

C

60

80

Lil 100

:).„---:- -,..-',N, , .....-, N N

\ .)\ N \c,\\ .04\ -\\

\ \‘ \ • \

\ k

3 5 k =

ji.to.oi

7.88

t90= 2 98

x 10-11 m /se

min

\ \I

10 100 Time (minutes)

. 3 5 , . O., =0.0058 ml/min

k =7.21x10 -11 m/sec

-----5----

/co .,1)--- .

0.02

C

E

0.01 C

O

0


1.2

0

_ 0.8 0 LL

0.4

0

0 0

c

0

0 G 3 7 (ICC unobtainable

r

08 10

12

1.4 (hrs)

0

02

04 06

0

20

40

C 0 -.4.= 60 ci

6 80 0

100

. -2 .

\ .04 3 7 ji =0.2

k=1.42x10

t 90=1.66 -8 m /sec

min

. .7 .

\ \ \

\\ \\

,„,\ \ t

N 4 0.1

Time 1 10 100

(minutes)

fig. 4.50


20

0.5

C

C 0.4

E 0.3

C5

0.2

0

0.1

0.2

0.4 0.6

0.8 1.0

1.2

1.4

(hrs)

38 0...=0.128

k=1.58x10-9

ml/min

m/sec

(IL , ,-._...e..

0 0

0

- -.•-zi-----7--,_ -

--. L.

\ 1 ,,..,\

, 0 \ •2

\ j=0.04

k=1.40x10-9

t90 =16.6

m/sec

min

(

\ \

L \ \

\1/4

10 100 Time

(minutes)

0 :47 60 0

-.1E-5 80 CT

LIJ

10( 0.1


fig. 4.51

Permeability

(m/sec) le°

-20

60

0) cn. 80

■‘.0 (1)

100 a)

.&•0 120

CD

t4■

140

160

35

340 30

n `'

0 38

35

El 37

34 30W

38

33 CI

0 25

270

23 0

LI 2 5

020

27 ell

23

2013

-4010-11

1.

Minimum effective stress during fatting head test

Effective stress at piezometer tip

O Effective stress during constant head test

Vo.

Permeability Test Results - BOUGH BEECH fig. 4.52

10-8 /

/ /

/ /

/ /

/

/

38

23 cy d

27

/

\i-- / /

\t-G/ 0 N / /

/ 30

,, G 034 4-\

/

0

/

20 / /

/

/ / / /

- 35 .

N /

2"

//

/

U a) (f)

E

10-9

10-10 10-g

10-a

Constant head k c m /sec

Comparison of permeability tests

BOUGH BEECH

fig. 4.53

0

20

40

^ 60 • 0

80

ti 100

0

CT w

0 46

20

0) a) 0 40

60

80

L.e...0...

... ' ■ . i„.

p

„

Di =1 3 - k =47

t90 x10-12

= 6770 min

m/sec

. . . . . . ....

10

100 1,000

10,000

Time ( minutes)

1

10 100

1,000 1 I

t90 = 26.9 (1410) min

D2 k= 8-74 x10-1 (1.66x1011 )

-4.

46■.mimm■I.•■■•■ 100 — t9 0)

Falling Head Permeability Tests



GRAFHAM WATER

10

100

1,000 0 0

a) 20

D 40

GO

80

100

40

GO

80 0

0 100

w

D3 t 90=4.85

k =4-85 x

min

10-g m/sec

Run 1

2

o

4-

N. . t90 ' •-n-i71.41c I ..,9_+__ _......_+_...4

10

100

1,000

Time ( minutes

D4 t 90= 7.

k= 3.40 x10

5 (3070) 9

(7.65x10

min 12

) m/sec

Run 1 0

2 +

tor, -- \-- 0--<-4,_-::%7---(-r--c--------o---0,--..J...„,.. \ cb-

U90)

fig. 4.55

o

40

- 60 ~ 0 - I

o 5 }J =0·2 tgo= 10,000 min

k = 2 . 3 5 x 10-12 m I sec C 80 0

...-a

. (/) 100 -a 10 :J 0-

W 1

\of- 0 o ~ OJ OJ 20 "en ClJ o

40

60

80

100

---

06

100 1,000

Time ( minutes)

10 100 --- - ... .......

~ - - ~ - .... 0

~ r--.. "-- ........... ~"

.... ......

....... ..... " .. ~ ~

~ "-·7 " ' ", ··2 ~

\ , \

" ' .. ~ 1\

" ,

l

~ ~

~

~ , , ~ ,

\ \ , \ \

~

}J=0.04 tgo= 562 min '\~ ' \

~. ,

k= 4.18x10-11 m/sec " ,~ ., ~ "~

10,000

1,000

~ ~ ~


G R A F I~ AM W ATE R fig.4.56

100

---C-. 75- -,- - - ....

--- .. .

..--, ,, %., •2 \

7 \ 11")

\

2

min

"t-, ,... .

\

\ ,

\ \ -N, N

\ \. .,

, \,,,\.,

., D7 k

1)=-0.7

= 7•43

tin= 3160

x1012 m/sec

10 100 1,000 10,000

Time (minutes)

10 100

1,000

--o--;---i --- :*----

......____,- -. ..-

-..

.

-... --...

.2

N N

2

\, N.' \

N. \ -N,

D8 \

p=0.7 6=3550 min

l<=6 62 x 10-12 m/sec

20

40

60

80 C 0

C100

c5

CT w

0 0

a) 20

0)

A-

° 40

60

80



20

40

Dli t90= 8230 min

k = 2.75x10-12 m/sec

o 20 L._

W

40

60

80

100

10 100 1,000 10,000

1 10 100 1,000

s.. +.5...

s.

Al s.., -* \

.. \ \ 9:

%." '`. 's

5% ■ \

\

2 "\ . \ \

\ N \

\ \ t

U2 p=0.5

k =3.63 x10-11

tgo= 646

m/sec

min .\

\ \

\ \ .., N. \I\

\

_

"N \

N..

c80 0

(1) 100

0 0

1±1

96 0



••■■•■••• L-

80

-.-.

,, .....,„ .... ,s

'■

.. ....,•••„., .. .

1

1N

'`.

I II p = 0.2 t90 = 16.4 min U 4 k = 1.43 x 10-9 m /sec

"...Nt ,..4,

\

L\ 04

•7

0\ . \

' \

- }-ate-'

1

10

100

0

O

(1) C) ci)

20

0) 40

60

80

100

20

40

— 60

Equ

alisa

tion

100 1

'-'7,----:::•-7.--, - -- ---

- _21 ‘... -r....... .s" %.2....;-.

, --. ---

-‘,r. ---. ..,.. •...., _::_ -...

.04 - \ ' ,•2

--., .. .::--; .7

, „..

. ,..,,..

„

\

min /sec

\ \ k) \ %

\ Nt-\___\___

. \` \:.>

\ , U p =--

k = 0.2 2.64 x10-11

t90 = 891 m . .

. ,

...„ • . 10

100

10 00

Time (minutes)


GRAFHAM WATER fig 4.59

0 41.

20

40

'60 4 3

ai 20 0)

0

C 80 0

•-100 1

0

CT 11.1

U5 t90= 1400

k =1.68x1011 m/sec

min

,

. • t.

10

100

1,000

Time

(minutes) 10

100

1,000

t90

4-6 0

40

60 t90::: 275 min

U6 k= 8.55 x10-11 m/sec 80

100




GRAFHAN/1 WATER

C 80 0

(n 1 0 0

0

CT w

0

20

40

60

w.

1

Time 1 10

10 100

1,000

( minutes) 100 1,000

0 ...0 .".•

=a.

......

1 \...

.s.\\

1

\

2 ,--o.:. 4 "4.;

N -\.. N.

\

N \ N '

, N‘N

...... ,...........

U '7 J.J ..2 2

k= 2.79 x10-11

t90= 841

m

min

/sec

‘-o-o-J- ......

,....,,,G,t_ L

_, , ---- , \

., -:, \-- .. -, -,

\

-,, .2

\ \ \

\\\ ---\ \ \ \

\ \

\ \

■ \ .\ \

U8 L.1 = '

0 . 7

k=1.26x10-11

t90 :-- 860

m

min

/sec . ,

leak •

4. N,, ,,,,

N k:s .-..

0

a) a) 20

A

40

60

80

100

fig. 4.61

■Imemin.

t g

t90 =145 min U11 k=1. 62X10-10 m/sec

)72:■'...c—: -- -. ' -.. -- -- -..

■ . 4 \ \

\

\ \ \ \ \

\ \ s

\ \

\

\ N N

p = 2 U10 k =1.77

t90= 1330 min x10-11 m /sec

■ ..■ N

-.......- ss. -...

10 100 1,000

Time

( minutes) 10 100 1,000

0 0

0120 Cr) a)

40

60

80

100


0

20

40

60 o

C (80 0

c5 cn 100 • — 1 ci CT

1

-41


.•••••••••■

40

z

- 80

(n120

to

160 a)

1"...)200 a)

4▪ - 1-11

240

280 10-12

Li U7

U2

U6 Li w U11

UU4

aiU8

U3

10-1°

Permeability (m/sec)

rt

Minimum effective stress during test

t Effective stress at piezometer tip

Permeability Test Results - Upstream

Shoulder

GRAFHAM WATER

fig. 4.63

0

40 011

D4 (Ni

D5 E Z 120

D8 160

D2 240 0.)

280 4-

320

D3 0.3

D6

360

0 400-D

1

10-11 10-1°

Permeability ( m/sec)

Permeability Test Results - Downstream Shoulder


Moisture Content Prof ices

PETERBOROUGH fig. 4.65

0.4

0.8

1.2

E

1.6

2.0 -o

s B J my '71

' A/B Feb 72

A, July t

71—

,-,

4

0 10 20 30 0)

Moisture Content

0 -o

0.4 0

0.8

1.2

1.6

20

C/D, Feb '72 o . ..\.„ ,s.,

D, July'71-- C, July '71

r.

1), 1

40 50

0 10 20 30 Moisture Content

40 50 0 rn

PETERBOROUGH fig. 4.66

0.4

0.8

1.2 E

1.6

a) > a) 2.0

3 0 0 a)

— 0.4

0.8

1.2

1.6

2.0

Moisture Content Profiles

E, July '71

Feb '72

F July

1_

'71

H

Z

0.8

1.2

E

1.6

2.0

0 10 20 30

40

50 Moisture Content

V

0.8

0.8

1.2

1.6

0.4



belo

w g

roun

d

0.4

0

_c 0.8

a) 0

1.2

1.6

2.0

2.4

2.8

0.4

0.8

1.2 E

a)

a)

1.60 10 20 30

40

50

Moisture Content


fig. 4.68 FOXCOTE

6 c-5

•

0

0.4

0.6

1.2 E

1.6

a)

2.0

0

2.4 0 a)

_c 2.8

CD

3.2

3.6

4.0

3

4.40 10 20 30

40

50

Moisture Content


FOXCOTE fig. 4.69

82

Chapter 5

PRESENTATION OF DATA - LONDON CLAY CUTTINGS

5.1 CUTTING SLOPES AT EQUILIBRIUM

5.1.1 Potters Bar - Old Side

The six piezometers in the old side at Potters Bar have been

read since installation in October 1974 until July 1975. The

pore pressure readings are plotted in Figs.5:1 to 5:3.

The two shallow piezometers, 8 and 11, equalised after installation

but are fluctuating with seasonal pore pressure changes. 8 is

fluctuating over about 0.5 m but 11 is showing considerably greater

changes of about 1.0 m. A section showing the pore pressures in the

cutting slope is given in Fig.5:4. The pore pressures for 8 and 11

are shown with the fluctuation range.

The remaining four piezometers are Casagrande standpipes installed

in small sand pockets. Some recorded artesian pressures after

installation due to grout pressure. Therefore once the grout set the

equalisation process started with the standpipes full of water. The

equalisation process after installation has been slow, 9 and 12 for

example show equalisation times of between 200 and 250 days (Fig.3:9).

Neither of the other two have reached equilibrium as yet. However,

when pore pressure is plotted against log time, they have left the

straight line portion of the curve and it is therefore possible to

estimate the equilibrium pore pressure which will be achieved. Where

the last pore pressure reading, taken 10/7/75, differs from the

83

estimated equilibrium pressure, both are plotted on the section, Fig.5:4.

The maximum difference, as may be expected, is on the deep piezometer,

13.

Best fit pore pressure distributions have been sketched on Fig.5:4.

The depth of the zero pressure line has been estimated from Fig.6:24

which shows depth to zero pressure line against slope angle for

embankment, cuttings and natural slopes. For a one on three slope with

a rough grassed surface a value of 0.9 m has been taken. The same

distribution has been fitted to all three sections. Apart from 10,

the fit is good but the further movement on 10 to equilibrium may be

over estimated.

5.2 CUTTING SLOPES NOT AT EQUILIBRIUM

5.2.1 Edgwarebury

The readings taken on the five original piezometers between 1972

and 1975 are plotted in Figs.5:5 to 5:7. The records of the two

piezometers, 7 & 8, installed in 1974 are plotted in Fig.5:8.

The most recent set of readings on piezometer 1 (Fig.5:5) is

suspect as the inside of the casing has been used as a short tailed

voles' nest and the piezometer tubing has been eaten by them. It seems

probable that they have caused a slight leak as the piezometer has

been very'difficult to keep de-aired.

Piezometer 2 (Fig.5:5) shows pore pressures in 1973 higher than

those before or since, which is not reasonable. The period between

de-airing and reading for these three readings is about 2 months,

probably longer than the pei-iod for which this piezometer could sustain

84

its full suction. Therefore, for this piezometer, the 1972 values have

been used instead of 1973 on the pore pressure summary drawing (Fig.5:9)•

The other piezometers also show an inability to sustain suctions

over a long period, for example 3 and 6 which have on occasions reverted

to tip level.

A reading of 10.2 m of water below ground level is the limit

before tension occurs in the water at the transducer. This limits

the range of negative pressures measurable with the equipment used.

Piezometer 6 (fig.5:7) approaches this limit and on two occasions,

when the de-airing has been more successful than normal, has reached

or even passed it. Thus the water was carrying a small tension without

cavitating. From these readings it can be concluded that the pore

pressure at 6 is outside the range of the equipment and is more than

10.2 m below ground level and has not risen past this value during the

three years since installation.

Equalisation rates after installation cannot be calculated due to

the lack of readings in the second half of 1972 but 1 and 2 would

appear to have equalised in the first month while the others did not.

The two piezometers, 7 & 8, (Fig.5:8) have not completely equalised

in six months, but like those at Potters Bar it has been possible to

estimate the final pore pressures. The last reading and this estimate

are plotted on Fig.5:9 for these two piezometers.

For the other piezometers the 1973 and 1975 pore pressures, 9

and 11 years after construction, are plotted on Fig.5:9• Also plotted

is the equilibrium pore pressure distribution obtained from the old

side of Potters Bar cutting. The slope of 1 on 4 with a moderately

kept surface gives a mean value of 1.0 m (from Fig.6:24) below ground

level for the zero pressure line.

85

5.2.2 Potters Bar - New Side

The seven piezometers in the new side at Potters Bar have been

read since installation in October 1974 until July 1975. The pore

pressure readings are plotted in Figs.5:10 to 5:12 and summarised on

a section in Fig.5:13.

The three shallow piezometers 2, 3 & 5 reached equilibrium by

early 1975 and show a small fluctuation. The pore pressure range

is plotted in Fig.5:13.

The three deeper hydraulic piezometers, 2, 4 & 6 have been much

slower coming to equilibrium after installation. The plot of 2

(Fig.5:10) shows a rapid equalisation of the piezometer itself but

the soil response to boring and installation is very slow. The use

of water during boring may have increased the softening of the walls

of the borehole and in the very impermeable clay reconsolidation is

slow. Of this group, 4 is the only one to have reached equilibrium.

This more rapid response is probably due to the claystones which stopped

the borehole short at 6 m.

Piezometer 6 was very slow to show any response at all. During

transport to site the valves had been knocked open and a considerable

quantity of air had entered the piezometer. It was not flushed out

before installation and required three de-airings afterwards before a

reasonable pore pressure reading could be obtained. This excessive

de-airing at, perforce, a pressure well above the pore pressure, would

have forced water out of the tip increasing pore pressure. No.6 is

shown plotted against log time in Fig.3:9 where it can be seen to have

left the straight line portion of the log plot, thus it has been

possible to estimate the equilibrium readings.

86

The deep standpipe piezometer, 7, has also not yet reached

equilibrium but it too has left the straight line when plotted against

log time (Fig.3:9). On Fig.5:13 the equilibrium pore pressures on

those piezometers not yet at equalisation are estimated, the last

reading taken in July 1975 is also shown.

The fully equilibrated pore pressure distributions from the old

side at Potters Bar are also plotted on Fig.5:13, and the zero pore

pressure depth of 1.0 m taken from Fig.6:24.

5.3 THE EFFECT OF TREES

5.3.1 Oakleigh Park

The four comparatively shallow standpipe piezometers at Oakleigh

Park had equalised after installation by April 1975, see Fig.5:14.

Piezometer 1 was dry for the first two readings after installation.

The grass and thistles on the grassed side have grown to about

1 m in height but the seasonal fluctuation appears to be of the

order of 0.2 m at 3.5 m depth. The fluctuation on the tree covered

side is considerably greater than this, No.2 showing 1.2 m.

Equalisation was not complete at the end of the winter when

pore pressures are normally at their highest and only one summer

reading has been obtained therefore maximum and minimum pore pressures

have probably not been recorded.

fx

,$)

—0— ‹

—0- 9

S 0 N DJ F MA M J J A

GL

1

E 2

3 a)

a)

-0 4 C

0 0)

5

0 6

0

7 0

O

8

1974 11975

Piezometric Levels - Tips 88, 9

fig. 5.1

POTTERS BAR - Old Side

-E -

OJ >

GL

1

2

3

OJ 4 --0 C

~ 5 'rn

~ 6 o (l)

..c

..c::. 7 -+-c. OJ

o 8

9

1-0

-0-10

-0,-3

S I 0

,

'\ v

~ ..r::t

/~ ~ 10 ~

........

~ ~

N I 0 J I F MIA M I J J I A 1974 1975

Piezometric Levels - Tips 10 &13


fig.5.2

GL

1

E 2

3

a)

4 c

0 5

a) 6

8

t

11

1

1:). ........ ....,4 12

1 X11

x-,„ l ..x

-0- 12

.

SONDJIFMIAMJ J A 1974 1975

Piezometric Levels - Tips 11 & 12

fig. 5.3


EMEErn=EM 0 5

metres I I 1 1 111[1 0 2 1. 6 8

U m of water

pore pressure \.

x on 10.7.75

--I equalised

-H seasonal variation 12

110 •. 9

8

13* 0

POTTERS BAR Old Side - Section showing pore pressures

GL

1

2

E

3

4 a)

5

C

0 6

0)

0 7

-o

10

11

rcI\

1 r / /

-0— 1 2 x .

* _ -- --- —

r d

/ /

2 -....., ...__ ,

/

/ /

/ / ,

1972 19 73 19 74 19 75

Piezometric Levels - Tips 1& 2

fig. 5.5 EDGWARE BURY

GL

1

10

11

I' 03 / )3

,

0 /

/

,

\ . /

/ i

es....ee

. \ \

\ i / ve€03.6_

1972 1973 1974 1975

2

E 3

a) a)

c

0 s- 0)

4

5

6

. o 7 a) _a

8

0

9

fig. 5.6

EDGWAREBURY

Piezometric Level - Tip 3

Piezometric Levet - Tips 5 86

9 a) 0

05

...06.. lk

I

\ \ \ \

/

I i 1/ //

i

\ ‘ ■ \ X

cei

- -1-A9/

i/ I/ 41 c\,.., 2

..

‘ ,... _...c5

6 ‘-.-

V—'`-'„,....._:-

- - - -

--

1972 1973 1974 1975

GL

1

2

E

3

4 a)

5

0 6 0)

8

10

11

fig. 5.7

EDGWAREBURY

9

_c 8 Q.

0

GL

1

2

E 3

4 a)

N 5

C

0 6 01

0 7 a)

10

11 1974 1972 1975 1973

-0-

-0- 8

fig. 5.8


EDGWARE BURY

2

1 -8 -6 -4 -2 0 2 4

U m of water

Pore pressure

1973 (9yrs)

1— 1975 (11yrs)

x 23.5.75 where unequalised

--- equilibrium from Potters Bar

o5 \\

05

0 metres 5

EDGWAREBURY - Section showing 1973 & 1975 pore pressures

GL

1

E •••••••••

2

a) 3

0)

4

0 Co 5

O

6

-0-- 1

.. 1 - -

, 2

-0- 2

SONIDJIFMAMJ J A

fig. 5.10

1974 11975

Piezometric Levels - Tips 1&2

POTTERS BAR - New Side

3 , e

-0-3

.4.' L.11

a

O 4

ir.

7 -0- S 0 NID JIF M A M_J J A

GL

1

E 2

3

a)

a) 4

C

0 5 rn

0 6

cu

CL CD

CD 8

9

10

fig. 5.11

1974 1 1975

Piezometric Levels - Tips 3,4 &7


GL

1

E 2

a) 3 a)

r) 4 C

0

CD- 5

0 a) 6 .0

o_ 7

D

8

0 z

0

-0-5

x .0.6).....05

—6

S 0 NID JIF M A MiJ J A

fig. 5.12

1974 1 1975



0 5 metres

t 0 2 4 6 8

U m of water

Pore pressure

equalised 1975 (19yrs)

x 10.7.75 where unequatised

-1-1 seasonal variation --- equilibrium from Old Side

70 Ix

40-1

POTTERS BAR - New Side - Section showing pore pressures

2

1 "0=0-

SONDJFMAMJ J A 1974 1975

3 -0-

-0- 4

GL

1

E 2

a) > 3 a)

'a 4

0 G L

O a) 1

46_ 2 O a

3

4

Tips 1 & 2 in tree covered cutting slope

3& 4 in grass covered cutting slope

Piezometric Levels

OAKLEIGH PARK

fig. 5.14

87

Chapter 6

DISCUSSION

6.1 END OF CONSTRUCTION PORE PRESSURES

6.1.1 Fill Slopes

The magnitude of the end of construction pore pressures in fill

slopes is influenced by several variables, placement moisture content,

fill height, soil type, rate of construction and drainage conditions.

The effect of these variables is discussed by Sherman and Clough (1968).

They conclude, in agreement with Sherard et al (1963), that the major factor

is the placement moisture content. This influence is clearly shown in

Fig. 6.1a, which shows the end of construction ru

values plotted

against total stress for clays of low plasticity, optimum moisture

content <15%.The number beside each point is the placement moisture

content, %, relative to optimum. The result is similar to that shown

in Sherman and Clough (1968), Figure 9.

The end of construction pore pressure data used to plot Fig.6.1

is taken from the records of a world-wide selection of dams, either

from the literature or this thesis and presented in Appendix B. The

tropical soils data presented in the Appendix is not included on

Fig.6.1.

The influence of fill height can also be seen on Fig.6.1a. After

compaction ru will be negative unless the placement moisture content

is very wet of optimum (Lambe, 1961). With considerable scatter, which

is partially due to drainage, ru increases with increasing total stress.

This is better defined for the soils dry of optimum, again in good

agreement with Sherman and Clough (1968).

88

Sherman and Clough (1968) concluded that there was no correlation

between soil type and ru, and that other variables exerted an over-riding

influence. However, almost all the dams which they studied were

constructed using clays of low plasticity. Therefore they were unable

to observe the very different behaviour of clays of high plasticity.

At low stress levels negative ru

values are obtained, even for soils

placed as much as 3% wet of optimum, as shown in Fig.6.1b. Many of

the negative or zero values shown in Fig.6.1b may in reality be more

negative as they were measured using unsuitable low air entry value

piezometers. Positive pore pressures occur at higher stress levels,

as much as 600 kN/m2 for clays placed dry of optimum. Only those high

plasticity clays placed wet of optimum have, at moderate stress levels,

ru

values approaching those of the low plasticity.clays.

Where excess pore pressures are set up by compaction, the rate

of construction and drainage conditions are important. For same dams,

i.e. Usk (Sheppard & Aylen, 1957) and Selset (Bishop & Vaughan, 1962),

closely spaced drainage blankets, to allow considerable pore pressure

dissipation during the winter season, were used to obtain acceptable

end of construction pore pressures. For a plastic clay with construction

pore pressures below equilibrium, drains will tend to increase the

end of construction pore pressures, but not to a large extent unless

they can supply the water required for swelling:to occur.

To illustrate the very different behaviour of high and low plasticity

clays, Fig.6.2 shows theoretical end of construction pore pressures

that may be set up in a 30m high dam without drainage. The core being

placed 3% wet of optimum and the shoulders at optimum to optimum - 1%.

89

The total stress, pore pressure relationship being taken from Fig.6.1.

The plastic clay shows pore pressures ranging from -90 kN/m2 in the

centre of the shoulder to 350 kN/m2 at the base of the core. On the

other hand, the sandy clay shows a range of zero on the surface to

125 kN/m2 at the base of the shoulder and 450 kN/m2 at the base of

the core.

Thus a knowledge of the plasticity of the fill material, as well

as the placement moisture content relative to optimum, will indicate

the manner of pore pressure response to be expected. Laboratory

measurement of B and dissipation tests on field compacted fill, as

carried out by Sodha (1974), will increase the accuracy of the estimate

of end of construction pore pressure. However it is only important

in the sandy clays and very wet plastic clays where the end of construction

condition is less stable than long term. In the dry plastic clays

the long term design is the most important. The dams studied in this

thesis all come into the dry plastic clay group which swell rather

than consolidate after construction.

6.1.2 Cut Slopes

The calculation of end of construction pore pressures in a cut

slope required a knowledge of the pre-excavation stresses as well as

the response of the soil to unloading. Ko is required to obtain the

initial stress condition and the magnitude of the loads being removed.

The total stress changes due to excavation can be calculated

using finite element techniques, and this has been done by Duncan &

Dunlop (1969) and Eigenbrod (1972 & 1975).

90

From these stress changes, the change in pore pressure can be

calculated using:

Au = B 0a 3 + A ( Acri - Aa3)} 6.1

(Skempton 1954)

Most overconsolidated clays in Britain can be considered

saturated, therefore B can be taken as 1. A varies with the state

of stress, for a homogeneous, isotropic clay in the elastic range,

unloaded in plane strain A is 2 and for an overconsolidated clay at

failure, A may be as low as Thus A will vary with the magnitude

of the unloading, reducing as passive failure conditions are approached.

Using the stress changes obtained by Duncan & Dunlop (1969) &

Duncan (1970), end of construction pore pressures have been calculated

for their four cases, 3:1 and 12:1 slopes with Ko = 1.6 and 0.81.

B = 1 and A = 4 have been used and pre-construction pressures assumed

hydrostatic (ru = 0.5). The results are plotted in terms of

u ru yh ) in Figs.6:3 and 6:4.

Also plotted on the same figures are the ru values obtained using

the assumption that Au = yAz 6.2 Eigenbrod (1972) considered at a 100 ft (30.5 m) cutting is

overconsolidated clay with (i) Ko

= 1.0, A =1/3,(ii) Ko = 1.5, A =1/3,

and (iii) Ko = 1.5, A = 0 and preconstruction pore pressures which

were hydrostatic. His end of construction pore pressures are shown

in Fig.6:5, with again Au = yAz 6.2 as a comparison

91

The one dimensional solution, Au = yAz compares reasonably

with the two dimensional finite element solutions under the central

portion of the slope. However this is only a comparison between two

analytical solutions and must be compared with field measurements

of pore pressures. Case records in the literature are rare, but two

are available. Unluckily both are canal cuts in lightly overconsolidated

clays with comparatively steep slopes and are therefore not directly

comparable with the cuttings studied in this project which are in

heavily overconsolidated clay and slopes not steeper than 1 on 3.

The first is a 1 on 1 cut in the Welland Clay, near Welland )

Ontario and the results are reported in Kwan (1971). Amalgamating

the pore pressures from both sides of the cut, the pore pressures

under the slope are plotted in Fig.6:6 with those calculated from

Au = yAz for comparison. Even for this steep slope where the

influence of change of Lys much greater than for shallow slopes, the

comparison is reasonable under the central portion of the slope.

At the Kimola Canal in Finland (Kankare 1969) Au was measured

during the excavation of the lower half of the cut below a berm.

The side slope in this lower section is 1 on 2. The values of Au

obtained are plotted at the piezometer positions on Fig.6:7. Contours

of Au , assuming Au = yAz , are also plotted and once again

the comparison is reasonable under the central portion of the slope.

Eigenbrod (1972 & 1975) gets a reasonable fit throughout using his

finite element method with Ko

1.0 and A = 0.3.

92

At the base of the cutting Au = yAz will over estimate the

pore pressure changes, Kwan (1971) obtains Au = 0.75yAz under

the base of the Welland Cut. There are also pore pressure changes

in the crest of the cutting where yAz = 0 , due to horizontal

unloading. Here Au = yAz does not hold at all.

Thus under the central portion of the cutting slope only,

Au = yAz is a reasonable assumption.

6.2 RATE OF EQUILIBRATION

6.2.1 Fill Slopes

The rate at which a compacted clay structure swells or consolidates

from its end of construction to its long term pore pressure depends

on cs

or cv of the fill and the drainage.

Laboratory values of cv and c

s can be obtained from oedometer

and triaxial dissipation tests on both field and laboratory compacted

fill, preferably on large samples. In order to use laboratory values

of cv and cs

in design with any confidence it is necessary to compare

them with values obtained from pore pressure records of full scale

structures.

The calculation of field values of cv and c

s requires a measured

change of pore pressure over a known time period and the ultimate pore

pressure towards which it is moving. A knowledge of the boundary

conditions is required to calculate final pore pressures. Boundary

pore pressures are discussed in detail in section 6.5.

93

Where the pore pressures set up by the construction process are

positive and consolidation takes place, cv

can be evaluated reasonably

accurately. However negative pore pressures are known with less

certainty and breakdown of hydraulic continuity in the measuring

system can be confused with swelling (Walbancke 1974). De-airing

can also increase the amount of water in the soil around the tip

with a localised increase in the pore pressure.

Most field values of cv presented in the literature for earth

dams in Great Britain are for glacial tills of low plasticity, sandy

clays. The published laboratory and field values of cv in the fill

for six dams and a trial road embankment are summarised in table 6.1.

Comparing the mean values of cv,with two exceptions, the laboratory

values are between 70 and 100% of the field values. The first of

the exceptions is the main embankment fill at Derwent. It is uncertain

from Rowe(1970) whether the laboratory value comes from the core

material only or from a mixture of core and general fill. The general

fill material is more plastic than the core. The second exception is

Cow Green. Here the laboratory values overestimates the field results

by about 40%. Vaughan et al (1975) only_give a passing reference to

the core cv and have concentrated on the foundation behaviour. In

the foundation the laboratory values again overestimate cv. This

may be an effect of the considerable variability of the Cow Green

till, the range of values of cv obtained is much greater than for the

other dams.

94

A plastic clay,field compacted close to or dry of optimum has-

a very variable lumpy structure and in places the permeability can

be initially high. Al-Dhahir (1967) reported some very high values

for the fill at Grafham Water and Werneck (1974) found considerable

variability when carrying out hydraulic fracture tests at Empingham.

The tests reported in this thesis also show some high values in the

shoulders both at Grafham Water and Bough Beech despite pore pressures

tending to equilibrium values. The question arises of the effect of

this structure on the overall values of cs, as differential swelling

around the edges of voids should tend to close them as water becomes

available.

Some values of cv and c

s have been calculated from the records

presented in this thesis which are discussed below and summarised in

Table 6:2. Some laboratory test. data, available from the literature,

is also summarised in Table 6:2.

(i) Peterborough - core. The pore pressures set up in the wet

core have dissipated sideways to the dry fill. Horizontal flow

has been assumed. The width of the core has been taken as (a)

its true width 2B and (b) 3B to allow for the retarding influence

of the shoulders as there are no drains. The mean values of cv

obtained using the records of piezometers 14 & 20 were 0.8 m2/yr

in case (a) and 1.8 m2/yr in case (b).

(ii) Peterborough - upstream shoulder. The progress of the

wetting front from the reservoir into the upstream shoulder is

plotted on Fig.6:8. This shows end of construction, 1971 and

estimated long term pore pressure distribution perpendicular to

I

95

the dam slope. The initial values, which are negative in the

fill, are very scattered, thus only a rough estimate of cs

can be obtained. One dimensional swelling was assumed with a

two layer system. Values of cs

of 2.2 m2/yr in the positive

pressure zone and 0.7 m2/yr in the negative pressure zone were

obtained.

(iii) Peterborough - downstream toe. The downstream callow

toe zone was found to be close to equilibrium, this is discussed

in more detail in section 6.3.2. From the records, Fig.4.6,

piezometer 17 can be seen to be still swelling. An estimate

of final pore pressures has been made, see Fig.6:15, and from

this the degree of equalisation has been taken as 75%, which

gives a value of cs

of 2.4 m2/yr assuming vertical drainage.

(iv) Grafham Water - core. Assuming zero pore pressure in the

chimney drain downstream of the core, top water level in the

drains upstream, and horizontal flow, cv between 0.7 and 0.9

m2/yr was obtained. Some swelling has also occurred in the

core and cs

= 0.4 m2/yr was calculated.

(v) Grafham Water - road embankment. Using an average surface

boundary condition of zero pore pressure at 1 m below ground

level, approximate values of cv for the foundation of 1.2 m2/yr

and cs

for the fill of 0.3 m2/yr were obtained.

96

(vi) Bough Beech - core. Using the same equilibrium pore

pressure assumptions as Grafham Water core, piezometers 16 & 17

give cs

= 6.7 to 6.9 m2/yr. This may well not be a reliable

result, as piezometer 18, which should show swelling, is

showing consolidation, indicating considerable redistribution

of pore pressure within the core.

(vii) Bough Beech - upstream shoulder. Piezometers in the

drainage blankets show the response of the drains to impounding

to be rather slow and will effect the calculation of cs for the

fill. Piezometers 35 and 37 are close enough to the dam surface

for the retarding effects of the inefficient drains to be ignored.

These given cs

1.6 and 1.7 m2/yr respectively after completion

of impounding.

(viii) Bough Beech - foundation. Values of cv were obtained for

the upstream foundation piezometers from the end of construction

until impounding, ranging from 1.9 to 2.1 m2/yr.

Sodha (1974) quotes some laboratory cs

values for tests on field

compacted fill from Peterborough,otherwise all the laboratory values

are of cv. Only a design value has been obtainable for Bough Beech.

The field values, see table 6.2, are on average about twice those

from the laboratory. The values from Bough Beech core are suspect

as discussed earlier. At Peterborough, cv would appear to be smaller

than cs, while at Grafham Water this trend is reversed.

97

Thus the laboratory results in plastic clays are not quite so

good a guide to field performance as they are in the sandy clays

which may at least be partially explained by the following:

(a) There is less laboratory data available for the three dams.

(b) Where negative pore pressures occur, field values are harder to

obtain.

(c) The effects of low degrees of saturation. cs appears to be greater

in zones of positive pressure than in zones of negative pressure,

i.e. Peterborough upstream shoulder. Vaughan (1965) has suggested a

modification to the consolidation equation

k 222

6.3 yw (m + mf ) aye

where m is the coefficient of compressibility of the soil

mf ti It 11

II 11 pore fluid

mf will become zero as the soil is saturated and the equation will

revert to the standard form:

k @ 2 p = Dip ywm 9174

If cs is defined as

k cs — (m + mf )

6.4

6.5

it will increase as the degree of saturation increases.

98

However the results are close enough, in both sandy and plastic

clay fills, for laboratory values of cv and c

s to give a very good

indication of field behaviour. In the plastic clays the dry, lumpy

structure with localised high permeabilities does not appear to have

much influence on the bulk properties of the compacted clay, at least

if it is well compacted.

These values of cv and c obtained are small and without considerable

drainage the time to equilibrium conditions after impounding will be

long. Using the field values of cs

or cv

obtained, the time for 90%

equilibration (t90) has been calculated for Peterborough, Grafham Water

and Bough Beech. A reasonable value of cs

has been estimated for

Foxcote of 1.0 m2/yr. The times are tabulated in table 6:3.

Peterborough, without any internal drainage, will take some

160 yrs to reach E = 10% assuming vertical drainage only. Some

acceleration will occur due to the effects of two-dimensional drainage.

Two values are quoted for Bough Beech core, 60 yrs is considered

more, reliable as the high cs value of 6.8 m

2/yr is probably due to

pore pressure redistribution within the core as discussed earlier.

The values for the upstream shoulders of Bough Beech and Grafham

Water are for the zones with closely spaced drainage blankets and

are dependent on the behaviour of the drains. The piezometers in the

drains at Bough Beech, 21 and 29, show that the drains themselves take

about 3 years to respond fully to impounding. These drains were

designed according to Gibson & Shefford (1968) to be efficient in

consolidation. It would therefore seem that a drain requires a greater

permeability to supply all the water the clay requires to swell than

to remove water during consolidation.

99

The need for a drain to supply water, rather than remove it,

creates an interesting question of the efficiency of the drains in

the downstream shoulder. On both Grafham Water and Bough Beech there

is evidence that swelling of the shoulders has been retarded due to

shortage of water. If drains are used to speed the swelling of

plastic clay fills there could be some advantage in laying the drains

level or even reversing the fall. The gradient should however be

very small so that no large pressures are built up in the drains.

This could speed the ingress of water without any detrimental effects

on the equilibrium pore pressures.

The long time taken to reach equilibrium pore pressures has

considerable implications in plastic clays where swelling is occurring.

Over this period, which, for the small dams considered in this thesis,

may be more than 150 yrs and for large structures such as Empingham

several hundred years, the stability of the dam is deteriorating.

Thus the inspection of a dam under the Reservoir Safety Provisions

Act (1930) increases in importance as the dam grows older and monitoring

will be required over much greater periods than are at present envisaged.

A reliable working life of a buried instrument may need to be one

or two orders of magnitude better than they are today, i.e. 100 to

200 yrs instead of 1 to 10. Alternatively temporary instrumentation

installed at intervals will be required. In either case further

research and development of instruments is required.

100

6.2.2 Cut Slopes

For a cutting slope in the stiffer clays the equilibration

process is always swelling and the rate is dependent on the bulk cs

for the material in situ. In stiff fissured clays the behaviour

of discontinuities is critical. They will tend to open with the

stress relief of excavation which will greatly increase the permeability.

However entry of water will cause swelling of the walls of the

fissures and tend to close them again. If the clays are cemented,

as in the basement beds of the Upper Lias Clay, swelling may not

re-seal fissures, making the stress dependence of permeability very

marked.

The relationship between permeability and depth is shown very

clearly in Fig.6.9. In situ and laboratory measurements of k are

plotted against depth below ground level for the brecciated Upper

Lias and London Clays. The Lias data is from Chandler (1974) and the

London Clay data from Garga (1970). A straight line trend is shown

on the log permeability plot, the laboratory tests defining the lower

limits of permeability at any depth. k decreases by approximately

two orders of magnitude in 20 m. The variation is probably due to

both stress dependence and weathering, although in the case of the

London Clay the effect of weathering may be small. The data is from

the blue London Clay at Wraysbury where it is overlain by Thames

Gravels which have impeded the weathering process. As the effective

stress decreases during swelling the permeability will increase.

Some pore pressure records in the literature indicate very slow

equilibration rates after the excavation of cuttings or the degradation

101

of sea cliffs. Lutton & Banks (1970) quote some piezometric levels

below canal level in the Culebra and Cucaracha Shales at the Panama

Canal after about 60 yrs. Muir Wood (1971) and Hutchinson (1972) discuss

piezometric levels below sea level in the Gault Clay of Folkestone

Warren. Bromhead (1972) gives a good example of reduced pore pressures

in the London Clay in the cliffs at Herne Bay. Here, in the centre

of the 30 m section about 50% equilibration has occurred, no erosion

having taken place this century. Lewis (1972) also quotes piezometric

levels below mean sea level in the London Clay cliffs at Herne Bay.

Chandler (1974) and James (1970) both discuss an Upper Lias Clay

cutting at Wothorpe, near Stamford, where equilibration appears to be

almost complete after ten years. This indicates a bulk cs

considerably

greater than measured in the laboratory.

At the other limit Chinsman (1972) found that full equilibration

of pore pressures occurred in a experimental cut in the Gault Clay

in a few months. Rowe (1972) quotes the rapid consolidation of the

foundation of Ardleigh Dam on London Clay where consolidation was

complete 1 year after impounding. Very high values of cv were

measured in the 250 mm oedometer, however. He quotes many examples

of soils consolidating much more rapidly than would have been assumed

from laboratory values of cv.

This project has concentrated on the behaviour of London Clay

cuts as there is already a well documented history Of delayed failure

(Skempton 1948, 1964, 1970; de Lory 1957; Henkel 1957; & James 1970).

The early results from Edgwarebury, Figs.5:5 to 5:7, suggested that

equilibration rates could be very slow. With this information a

102

re-analysis was made of the data presented by James (1970) who analysed

the slips using the method of Morgenstern & Price (1967). The new

assumption made was that the strength parameters, in terms of effective

stress remained constant and were equal to c' = 0 and 0' = 20°.

With this assumption, the ru value required for failure could be

calculated. These values are plotted in Fig.6:10 against time to

failure for the cutting. and show an increase in deduced ru with

increasing time. The data from some sites where accurate measurements

of pore pressure were made are shown including the 9 yr values at

Edgwarebury. A distinction has been made between measurements in the

blue and brown London Clay. These measured values differ slightly

from the values assumed by James, who used the observations to justify

the assumption of steady seepage conditions, from which pore pressures

were calculated. These deduced and measured values are consistent

for the brown London Clay. This suggests that pore pressure equilibration

may be the dominant factor in controlling delayed failure. However,

it must be emphasised that the trend shown in Fig.6:10 does not

demonstrate a conclusive relationship between pore pressure equilibration

and'time to failure. For this a plot of degree of equilibration against

elapsed time would be necessary and there is insufficient data for

this to be prepared. For this reason failures of retaining walls

have been excluded as, in these cases, pore pressures after excavation

and at equilibrium are significantly different from those for slopes.

The values of ru

in Fig.6:10 tend to ru of about 0.3. There is

evidence that the ultimate mean ru is of this order for clay slopes

at average slip depths. This is discussed in more detail in sections

6.3 and 6.5.

103

The curve presented in Fig.6:10 is the same as previously

published (Vaughan & Walbancke 1973). The validity of the hypothesis

would be greatly improved if the measured ru value for a cut slope

in brown London Clay would fit on the curve. ru

values around

possible slip surfaces were claculated for both the New and Old sides

at Potters Bar. These slip surfaces were confined to the brown

clay as there is good evidence (i.e. Northolt, James 1970) that the

slips are restricted to the brown clay. The values obtained using

mean pore pressures were ru = 0.11 for the new side after 19 years and

ru

= 0.30 for the old side after 125 years. These are shown on

Fig.6:10 and are a reasonably good fit on the curve, being within the

scatter of the other data. The effect of the counterfort drains on

the new side has not been taken into account. From the analogue

model results (Fig.6:25) discussed in section 6.5.4., they would

probably reduce the ultimate ru

value midway between the drains by

about 0.03 and cannot account for the reduced pore pressures on the

new side. Thus it can be stated that the equilibration time scale

is the same as that for delayed failure leaving very little to be

explained by factors independent of change of pore pressure.

The long term mean pore pressures from Potters Bar, old side,

can be written as

uz = 0.82 (z - 1)yw 6 . 6

where u is the pore pressure at depth z . The pre-cutting pore

1o4

pressures were based on the same equation. End of construction pore

pressures were calculated using:

Au = yAz 6 . 2

Using a finite difference form of the Terzaghi consolidation equation

for a two layer material with cs

values of 0.85 m2/yr for the blue

and 2.55 m2/yr for the brown London Clay, pore pressure distributions

were calculated for each piezometer section at Edgwarebury and Potters

Bar, new side. The distributions calculated for elapsed times of 9

and 11 years are plotted for the Edgwarebury sections in Fig.6:11 and

those at 19 years for Potters Bar new side in Fig.6:12. The measured

values scatter either side of the distributions. At Edgwarebury the

calculated pore pressures are within + 15% of the measured values

with a standard deviation of 7%. Potters Bar is not quite as good

with a range of + 20% and a standard deviation of 11%. A value

of cs = 3.2 m

2/yr for the brown London Clay was given in Vaughan &

Walbancke (1973). With the increased information now available, it

has been possible to obtain a better estimate.

Laboratory values of cs for the London Clay are rare but for an

overconsolidated clay cv and cs are comparable. Apted (1976) quotes

cv and cs values from 76 mm oedometer tests which are of the same

order, see table 6:4. Larger size specimens give more realistic

values of cv (Rowe 1972). Table 6:4 gives some results from triaxial

dissipation tests on blue London Clay from Wraysbury (Garga 1970).

105

The 300 mm tests give results 1.4 times those from 100 mm samples.

Some brown clay tests on 100 mm samples (Skinner 1967) gave cv

values 1.8 times those from the blue with the same sample size.

The values of cs

for the blue London Clay obtained from field

data is comparable with cv measured on large samples in the laboratory.

The brown clay gives a field cs 2 to 3 times the laboratory cv value

but no large sample results are available. At the low stress levels

which will operate in the brown clay in the field the effect of

structure, and therefore of sample size, is liable to be more marked.

Thus, provided fissures remain closed or close up due to swelling,

which seems generally true for the London and brecciated Upper Lias

Clays, equilibration rates are slow and comparable with rates

calculated using laboratory values of cv or cs from larger samples.

These reduced pore pressures should not be relied upon for

stability of temporary excavations without detailed examination of

the sequence as described by Rowe (1972) because permeable layers

can cause rapid equilibration.

6.3 ULTIMATE PORE PRESSURES

6.3.1 Cut and Natural Slopes

The pore pressures in the old side of Potters Bar cutting, now 125

yrs old, appear to be close to equilibrium.* The distribution

of pore pressures on the three sections is shown in Fig.5:4. Within

the fluctuation zone the mean values are plotted which show ru

varying with depth and zero pressure on average 0.9 m below ground

level. At 6.5 m, where fluctuations have about died out, the ru

*With double drainage, one dimensional swelling, and c = 0.8 m /yr, full equilibrium will require something in excess of 500 yea

v rs. However,

two dimensional effects and open fissures at depth in cemented clay would decrease this time, and the effect of k decreasing with depth is to give pore pressure close to final equilibrium near the surface even when swelling at depth is incomplete (see Fig. 6:40). In these circumstances surface seasonal fluctuations may dominate the slight upward trend due to continuing swelling at depth.

106

obtained is 0.34. Around a typical slip surface, contained completely

within the brown London Clay, the average ru is 0.30. The winter

values of ru rise to about 0.45 in the top 2 to 3 m. The fluctuations

are discussed in section 6.5 on boundary conditions.

Chandler (1974) has deduced a winter maximum pore pressure

distribution based on piezometers in various Upper Lias Clay cutting

slopes, Fig.6:13. In the top 1 m, ru = 0.5 and reduces to approximately

0.3 at 4 m depth.

The London and Upper Lias Clay cuttings have similar long term

pore pressures in the top few metres which would indicate similar

boundary conditions. For the surface boundary this is probably true

as both areas have similar rainfall and evaporation characteristics.

Depth to the lower drainage boundary is 44 m at Potters Bar and

unknown for the Lias cases. All sites are underdrained to some extent

as the Marlstone Rock Bed of the Middle Lias is a major aquifer as

is the chalk under Potters Bar. Thus pressure at the lower boundary

and thickness of the clay layer could vary in each case.

Fig.6:14 shows the pore pressures in a Boulder Clay slope at

Cow Green infilling a buried channel. The slope is almost fully

under-drained, the piezometers in the limestone showing pore pressures

at,or just above,the level of the River Tees. The piezometers show

a perched water table in the Boulder Clay. Near the surface pore

pressures approach hydrostatic.

These cases all exhibit high pore pressures in clay layers at

equilibrium despite underdrainage,which is indicative of a permeability

decreasing with depth. The pore pressures near the surface, in the

range where most slips occur, are controlled almost entirely by the

107

surface boundary pressures and the permeability gradient, very

little by base boundary conditions.

6.3.2 Fill Slopes

As the equilibration rate in fill slopes is slow very few

modern embankments have reached equilibrium and less data has been

obtained from this project than had originally been anticipated.

One of the few cases available is Aldenham Dam, which was not

constructed by modern methods.

The equilibrium pore pressures in the section without drainage

are plotted on Fig.4:34. Here the dam section in the downstream

shoulder zone is very thin and is effected by seasonal changes over

its full depth. Here the perched water table is a transitory winter

feature which becomes permanent in the thicker central zone.

Where the embankment section is underdrained by the culvert,

Fig.4:40, the mean pore pressures near the surface of the embankment

are only slightly reduced and the fluctuations are of the same

magnitude as at Section 11. In these conditions the perched water

table is a permanent feature throughout the section.

The second case is the downstream toe section at Peterborough

which was constructed of the wet 'callow' with fairly high pore pressures.

An idealised section showing pore pressures is given in Fig.6:15.

The records of piezometer 10 plotted against log time show it to be

very close to equilibrium ( >95%). The other foundation tips have

also equalised. The records of piezometer 17 show swelling to be

still in progress after 11 years. Piezometers A, B and 22 show a

zone of fluctuation about 22 m deep at present. An estimate of

108

equilibrium pore pressure has been added to the figure. This indicates

that the perched water table formed in the upper part of the slope

will be a permanent feature of the equilibrated slope.

These two cases suggest the presence of a permeability gradient

in the clay fill which would account for the perched water tables.

The upstream shoulders of Grafham Water and Bough Beech dams

had both reached equilibrium and piezometers midway between drains

recorded top water level. The downstream shoulders, as discussed

in section 6.2.2, had not quite reached equilibrium but were tending

towards zero excess pore pressure between the drains. Any excess pore

pressures due to a permeability gradient would be very small because

of the closely spaced drainage blankets.

From the above cases it can be seen that clay slopes would

appear to have high equilibrium pore pressures independent of drainage

conditions on the lower boundary unless the clay layer is thin. The

magnitude of the pore pressure in the upper part of the clay layer is

dependent on (a) the surface boundary pore pressures and (b) the

permeability gradient. The effects of these two variables are discussed

in sections 6.5 and 6.6 respectively.

6.3.3 Stability of Equilibrated Slopes

Stability of the old side at Potters Bar and the downstream toe

at Peterborough have been considered. At Potters Bar, using both

mean and winter maximum pore pressures, the effective cohesion required

for stability has been calculated on slip circles of various depths

having the same entry and exit points on the slope, see Fig.6:16.

109

The effective angle of friction, 0', was assumed to be 20° throughout

and the Bishop (1955) routine method of slices was used. With these

conditions, the most critical slip depth was 3 m where, under winter

conditions, a c' of 6.5 kN/m2 was required for stability. Using an

infinite slope method (Skempton & de Lory 1967) the c'rel is 6.6

kN/m2 at 3 m. As this section has not yet failed, it must clearly

have a c' greater than 6.5 kN/m2. Other sections along the slope

have been subject to superficial slipping, therefore c' cannot be

much greater than 6.5 kN/m2. Chandler & Skempton (1974) show that

the design of London Clay cutting slopes on c' = 0, 0' = 20° is

a conservative assumption and suggest the use of c' = 1.5 kN/m2.

The stability of the downstream toe at Peterborough was

calculated using (a) Bishop and Morgenstern (1960) curves for a

circular slip surface and (b) infinite slope. Considering first the

top 2m, ru

is 0.25 and 0'req

is (a) 19° and (b) 18.4°. 0' from

laboratory tests was 27o (c' = 0) therefore the factor of safety is

1.47 and 1.53 respectively. For a deeper slip, involving the whole

toe, the ru

value reduces to 0.1 and, using Bishop & Morgenstern,

0'req

reduces to 16.5° (F = 1.72). Thus the factor of safety for

the toe is now close to the design value of 1.5. This may drop further

as equilibration is not yet complete. Superficial instability of

the toe may occur but the planned infilling of the brick pit with

P.F.A. in the near future is likely to forestall any possible trouble.

110

The rather steep 2.75:1 downstream slope at Foxcote is of interest.

No strength data is available but Vaughan (1975) has combined the data

for several compacted plastic clay fills (Kellaways & Oxford, Weald and

Upper Lias) obtaining the effective stress parameters c' = 10 kN/m2,

0' = 20°.

Using Bishop & Morgenstern (1960) curves, the ru values for F = 1

and F = 1.5 were calculated for various values of c', assuming 0' = 20°

c' ru

kN/m2 F . 1 F = 1.5

0 0.0 - 0.44

5 +0.36 - 0.04 10 +0.56 + 0.18

At equilibrium the pore pressures may be quite high in the upper part

of the slope and ru could reach as much as 0.25. If this occurs the

stability of the dam may cause concern as it will depend on some

c' operating. Therefore it is suggested that pore pressure monitoring

should form a regular part of the inspection of this dam and that

samples of the fill should be obtained for strength testing. With

the correct strength parameters it will be possible to tell if the

pore pressures reach values which may be critical.

6.4 PREDICTION OF PORE PRESSURES IN CUT AND FILL SLOPES AT

EQUILIBRIUM AND DURING EQUILIBRATION

The prediction of pore pressures during equilibration requires

first the prediction of equilibrium pore pressures but as Richards

and Chan (1969)_ point out, the ultimate flow pattern is not necessarily

that predicted by conventional flow nets. They state that this is

because the techniques of constructing flow nets for the ultimate

condition are based on assumptions which are correct only for coarse

111

grained materials in which capillary rise is insignificant. In clays

these assumptions are completely erroneous as (a) the flow parameters

are non-linear, resulting in the breakdown of the orthogonal Laplace

condition and (b) flow above the phreatic line in the partly saturated

regions is very significant. This latter condition means that the

phreatic line is no longer a flow line.

Richards et al.(1973), while examining the effect of tile land

drains, produced field evidence that equipotentials seldom intersect

the phreatic surface at right angles, showing that the phreatic surface

is not a flow line.

In clays which remain saturated, no true phreatic line exists

but there is a moving zero pressure line across which considerable

flow occurs. In the downstream shoulder of water retaining clay

embankments the flow across the free boundary can be considerably

more significant than the flow from the reservoir and can control

the pore pressures in that zone. Sweeney (1970) shows that a flow

net does not predict the measured pore pressures in the Boulder

Clay slopes at Cow Green.

Thus simple predictive methods are not valid and it is clear

from section 6.3 that the pore pressures are controlled by the

permeability gradient and the boundary pressures, especially the

pressures on the free surface boundary where the permeability is at

its highest. With knowledge of these controlling factors, ultimate

pore pressures can be calculated. In one dimension this can be done

by hand but in two dimensions an electric analogue or finite element

computer solution are required.

The equilibration rate is also controlled by these same factors

and intermediate pore pressures can be calculated using finite element

or finite difference solutions.

112

Thus, for prediction, a general study of the surface boundary

pressure and the variation of permeability is required. The control

of these variables can also be used to control pore pressures.

6.5 SURFACE BOUNDARY PRESSURES

The pore pressures at a free clay boundary are controlled by the

climatic conditions. In the area studied, South Eastern England and

the East Midlands, summer infiltration is generally less than

evaporation losses and boundary pore pressures drop. The reverse

occurs in the winter and pressures rise again. The pore pressure

at the surface reaches a maximum of zero unless ponding occurs and,

except in areas of very high rainfall and low evaporation, the mean

surface pressure is negative. In arid and semi-arid climates

vegetation can maintain a permanently desiccated slope and negative

pore pressures may be permanent (Blight, 1963).

The fluctuations which occur on the surface are transmitted into

the body of the clay. Both the magnitude of the fluctuation and the

depth of the zone in which fluctuation occurs are directly related

to Cr and cs of the clay.* Fig.6:17 shows the effect of a sinusoidal

pressure variation applied to the surface of a clay layer of constant

c (c = c ) for various values of c. This is based on Carslaw & v v s v

Jaeger (1959). Of interest is the time lag which occurs. This

indicates that pore pressures near the base of a fluctuation zone

should show maximum values in summer instead of winter. However,

piezometers with slow response times will also show a time lag.

*In this simple approach it is assumed that the clay remains substantially saturated and that the effect of shrinkage cracks can be ignored.

113

The existence of fluctuations does not indicate that equilibration

is complete but the magnitude of the fluctuation and the depth to which

it occurs will increase as swelling progresses as in these conditions

c is increasing. The magnitude of the fluctuations on the new and

old sides at Potters Bar show this behaviour (Figs.5:4 and 5:13).

The fluctuations can however mask the equilibration which is occurring.

This is demonstrated in Fig.6:18 which shows maximum and minimum

isochrones for a clay layer 20 m thick with single drainage,

cv cs 10 m2/yr, for t = 1, 2, 10 and 11 years. The figure shows

that, at later stages of swelling, equilibration rates would be

difficult to observe in the fluctuation zone. Thus piezometers

installed to measure equilibration rates should if possible be placed

below the fluctuation zone.

In practice the mean free boundary pressure in a clay slope and

its range depend on slope angle, vegetation and surface drainage as

well as climate.

6.5.1 Grassed Slopes

Many cutting and embankment slopes have no special treatment

but are just topsoiled and grassed. Pore pressures, fluctuating

on an annual cycle, have been recorded beneath such surfaces. The

shallow piezometers in Peterborough, Grafham Water weight block and

road embankment all show this fluctuation (Figs. 4:26 to 4:31).

Those at about 1 m depth are recording values close to equilibrium.

The deeper ones, about 2 m, despite incomplete equilibration show

small fluctuations and, in the cases of F and H at Peterborough, show

a time lag of several months.

Tensiometers were installed at 0.3 m intervals to a depth of

1.8 m in a shallow London Clay slope at Uxbridge by the Road Research

Laboratory (Black et al., 1958) and readings taken from 1954 to 1956.

The records of two of these tensiometers, at 0.9 and 1.8 m, are

plotted in Fig.6:19. Here negative pore pressures were measured by

the very shallow piezometers. On other sites where records of seasonal

fluctuations are available they are from rather deeper standpipe

piezometers with slower response times and unable to record negative

pressures. Fig.6:20 shows the records of two piezometers at 2.75 m

and 3.65 in the London Clay slope at Sudbury Hill ( Skempton & Henkel

1960). It is interesting to note that movement was noticed when the

pore pressures were at their maximum in February 1957.

Barnsdale and Gretton are two examples from the Upper Lias Clay.

The piezometers, P1 to P4, from Barnsdale (Chandler, unpublished)

are all at approximately 2.75 m depth and show similar fluctuation

patterns, Fig.6:21. The effects of an exceptionally wet July in

1973 shows clearly as it does at Peterborough and Grafham Water.

The installation of counterfort drains has effected P3 during 1974.

P4 does not fit quite the same pattern as the other three as it has

a lower mean level and smaller fluctuations. The 4.0 m deep piezometer,

C5B shows, as expected,a smaller fluctuation.

The three examples from Gretton (Pachakis 1974) all show the

theoretical pattern of the magnitude of the fluctuation decreasing

with depth (Fig.6:22). Piezometer 2, being a standpipe, dries out

in summer and its minimum can only be assumed with reference to piezometer 1.

A very clear time lag is shown by piezometer 5, but, being a standpipe,

poor response of the piezometer is probably contributory.

115

The maximum and minimum values from the piezometers from these

four sites as well as those from Peterborough, Grafham Water, Aldenham

and Potters Bar which can be considered at equilibrium are plotted on

Fig.6:23. They are all from grassed slopes without drainage measures.

Other data from the London and Upper Lias Clays are plotted and are

taken from Black et al.(1958), de Lory (1957) and Chandler (1974).

The results are fairly consistent despite various clays and slope

angles. In winter, with zero pressure at ground surface, ru approaches

0.5 and the fluctuation depth is some 6 m.* The mean zero pressure

is at 1 m below ground level and ru tends to 0.33 below about 4 m.

The minimum values are more variable and negative values are recorded

down to 2.2 m. Below the zone of fluctuation the pore pressures are

controlled by the mean boundary pressure.

If the depth of the fluctuation zone deduced from Fig.6:23 is

compared with the theoretical depth from Fig.6:17, cv/cs must be of

the order of 20 m2/yr for the top of the clay layer. If the

fluctuations are damped by poor piezometer response, this value

could be even greater. This must be compared with an overall value

for the brown London Clay back calculated from the Edgwarebury and

Potters Bar data of 2.55 m2/yr. Two reasons can be suggested for

this difference. (a) Equilibration is incomplete at Edgwarebury

and Potters Bar and, as stresses reduce, c v /cs is likely to increase.

The much smaller fluctuation at 2.5 m at Potters Bar new side compared

with those at the old side is good evidence for this.

*This depth of fluctuation appears to be valid for in situ brown London Clay, in situ Lias Clay and the uncompacted brown London Clay fill from Aldenham, suggesting that these materials have similar consolidation and swelling characteristics when fully equilibrated.

116

(b) The effect of cracking, which occurs in these clay slopes during

the summer, is to transfer the drainage boundary from the ground

surface into the body of the clay, thus increasing the depths over

which pore pressures are reduced. The apparent cv/cs obtained,

assuming the drainage boundary at the surface, will be higher than

the true value. Cracks of 0.7 m were recorded during piezometer

installation at Peterborough in a zone of incomplete equilibration.

Deeper cracking is not unlikely. With both these effects, a realistic

value of c v /cs in the top few metres would be 5 to 10 m

2/yr.

For slope stability the maximum pore pressure values are critical

and most slips occur during the winter. James (1970) showed 80%

of his quoted slips occurred in the six months October to March and

are often triggered by wet periods raising ru to exceptional values.

Apart from the retaining wall failures, none of the London Clay slips

quoted by James are more than 8 m deep thus most of the slip surface

will be effected by seasonal fluctuations. Chandler (1974) also shows

fluctuating pore pressures at slip depths.

On grassed slopes, both slope angle and surface roughness will

effect the relationship between rainfall and run-off. Mean piezometric

level and surface conditions for piezometers from the dam sites are given

in table 6:5. All piezometers are shallow, between 0.9 and 1.7 m

below ground level. The mean piezometric levels from these and other sites are

plotted against the slope, cot 0) in Fig.6:24. The depth of the mean

piezometric level below ground level decreases (a) as the slope flattens

and (b) as the slope becomes rougher. At a slope of 1 on 3 the mean

piezometric level is 1.0 + 0.2 m and at 1 on 20 rises to 0.6 + 0.1 m.

117

6.5.2 Gravel Layers

A layer of gravel beneath the topsoil as at Foxcote reduces the

magnitude of the fluctuations, piezometer 3 (Fig. 4:35) at 1.7 m

depth shows a variation of 0.3 m compared with 1.8 m under grass

(from Fig.6:23). The two piezometers Y and Z at Grafham Water

(Fig.4:37) with a drain beneath them as well show about 0.2 m fluctuation.

The effect of the gravel which has low transmissability at

operating degrees of saturation, is to stop capillary rise and hence

most of the transpiration and evaporation losses and can also retain

some free water increasing minimum pore pressures. Maximum values are

also reduced if the gravel can drain freely, maximum piezometric levels

being at, or just above the base of the gravel. The mean value for

number 3 at Foxcote is consistent with its slope and well kept grass

cover.

The increased minimum values reduce the likelihood of cracking,

also the gravel itself, being non-cohesive, cannot sustain tension

and is in the most susceptible zone. The use of gravel layers to stop

cracking has been known for a long time. Jessop (1802) suggested it

as a remedy for the bad cracking of Aldenham Dam.

6.5.3 Effect of Vegetation

An excellent demonstration of the effect of grass cover is

reported by Black et al (1958). Tensiometers were installed in two

areas in a 1.8 m layer of brickearth at Harmondsworth, one with bare

soil and one with grass cover. During the winter period the maximum

values are comparable but in summer pore pressures of -9 m of water

were recorded below the grass compared with -2 m below the bare soil

118

at depths of less than 1 m. This increased water loss due to

transpiration reduces the mean boundary pressure and thus the pore -

pressures below the zone of fluctuation. Good control of the grass

by mowing or grazing will improve run-off therefore decreasing the

infiltration.

The obvious reduction of mean pore pressures by grass leads to

the question, how much more effective are trees? The data presented

in this thesis for deciduous trees at Peterborough (Fig.4:27) and

Oakleigh Park (Fig.5:14) is not at all conclusive but tentative

conclusions can be drawn. In winter the pore pressures would appear

to be similar to those under grass but with lower summer values due

to increased transpiration. The root penetration will increase the

depth over which pressures are lowered. The mean boundary pressure

is again reduced with a resulting reduction of pore pressure at depth.

A further advantage of trees is the effect of roots on shear

strength. In situ shear tests on soil pedestals containing plant

roots carried out by Endo and Tsuruta (1969) and laboratory tests

carried out by Manbeian (1973) and Kassif and Kopelovitz (1968) are

reported by Gray (1974). Their tests show root fibres increase the

apparent cohesion of the soil. This increased cohesion will be in

the critical top few metres where winter pore pressures are still

high. Field proof of the advantages of trees is presented by Bishop

and Stevens (1964) on the increased frequency and magnitude of slides

in a glacial till slope in Alaska after clear-cut logging.

119

Trees can be used to their best advantage on cutting or embankment

slopes where equilibration rates are slow. Young trees planted at

the end of construction will mature as the slope equilibrates and

reduce the equilibrium pore pressures. The annual water loss by

transpiration must be maintained if it is to be relied upon and

replanting organised to replace losses.

According to Twort et al (1974), on embankment dams with internal

drainage measures, trees can be a disadvantage as the roots can

interfere with drains and filter layers. Therefore they are best

considered for homogeneous clay dams without internal drainage such

as Peterborough. Removal of mature trees from older structures is

likely to be harmful as pore pressures will increase.

6.5.4 Counterfort Drains

Counterfort drains are used to reduce the surface boundary pressures.

The effect of those in the new side slope at Potters Bar was studied

using the electric analogue. The clay layer at the site was taken as

44 m thick and fully underdrained with a 10:1 permeability ratio over

the top-.12 m. With these assumptions, a one dimensional solution

gave a reasonable fit with the old side pore pressures, Fig.6:25.

A two dimensional section between drains was then considered with

drains 3 m or 5 m deep and 20 m apart. Fig.6:25 shows the pore pressure

distribution midway between drains for drains 3 m deep as at Potters

Bar. Ratios of horizontal permeability to vertical permeability of

1:1, 2:1, 5:1 and 10:1 were modelled. At base of drain level, the

* maximum pore pressure reduction on this section is 0.7 m. However,

*This could be as much as 1.0 m. The convergence of the solution shown in Fig. 6:25 could be an effect of using the analogue model at the limit of its range in the top few metres of the clay layer. The solution converges again at depth as the base boundary pressure remains constant.

120

the permeability ratio is more probably 2 to 4 then the pore pressure

reduction would be of the order of 0.3 to 0.4 m of water. The pore

pressures recorded on the new side are also plotted, showing that the

present pore pressures are well below those which can be accounted for

by the drains alone.

At Barnsdale 3.5 m drains at 12 m centres were installed between

20th September and 8th October 1973 and pore pressures have been

recorded for 1973 and 1974. Fig.6:26 shows the records from six

piezometers at 3.5 m depth. The magnitude of the fluctuations is not

much changed by the drainage but both maximum and minimum values have

dropped. Combining the results of all six piezometers, an average

reduction of 0.5 m has been achieved on the maximum values. The

reduction of minimum values is 0.9 m overall but these values are not

so well defined as the installation of drainage coincided with the

rather late pore pressure fall after the wet July/August of 1973,

see Fig.6:26. The reduction of the peak values is probably a

better guide to the performance of the drains. This would be

equivalent to a value of about 0.3 m for the shallower, more widely

spaced drains at Potters Bar.

The pore pressure distribution between drains at base of drain

level has been plotted for both 3 m and 5 m drains in Fig.6:27. These

show the influence of the drain becoming more marked as the permeability

ratio increases. A field example, from solifluction lobe D at

Sevenoaks (Weeks 1969) with 5 m drains at 20 m spacing is also shown

on Fig.6:27. This site has a permeability discontinuity between the

solifluction layer and the underlying Weald Clay and is therefore not

121

directly comparable but shows the same type of distribution.

6.5.5 Summary

The boundary pore pressures in grassed clay slopes in South

East England and the East Midlands can be summarised as follows:

(i) The mean zero pressure level is approximately 1 m below ground

level giving a negative surface pressure and is effected by slope

angle and surface roughness. The depth increases as the slope

angle increases and decreases . as roughness increases. Pressures in

the body of the clay layer are controlled by the mean surface pressure.

(ii) Seasonal fluctuations occur. The zero pressure line rises to

ground level in winter and can drop to 2-3 m below ground level in summer,

the boundary pressure varying from zero to a large negative value.

The effect of these boundary pressure changes is transmitted to the

body of the clay and fluctuations can occur to approximately 6m. Both

the depth and magnitude of the fluctuations is probably increased by

cracking.

(iii) Boundary pore pressures can be controlled by:

(a) Gravel layers under the topsoil and grass. These reduce

the magnitude of the fluctuations and mean values are likely to

be close to the base of the gravel layer. Thus they improve

superficial stability by reducing winter pore pressures within

the fluctuation zone but may increase mean pressures depending

on the depth of gravel. The gravel does not transmit capillary

pressures and can hold some free water thus reducing cracking.

122

(b) Trees. Deciduous trees reduce the summer pore pressures

but do not appear to have much effect in winter. Thus the mean

value, and therefore pressures at depth will be reduced. In

the fluctuation zone, where high winter pore pressures still

occur, some additional apparent cohesion is achieved by root

reinforcement. They are a good solution on cutting slopes but

should be used with a little caution on dams as they may damage

drains and filters.

(c) Counterfort drains, unless they are deep and closely spaced

have far less effect on pore pressures than the seasonal pressures

on the surface. Their most effective use is in cases of slopes

close to instability where they can by a small decrease of pore

pressure increase a critical factor of safety. They have a

disadvantage in cutting slopes of taking free water into the soil

and increasing the equilibration rates.

6.6 THE EFFECT OF PERMEABILITY VARYING WITH DEPTH

6.6.1 The effect on equilibrium pore pressures

The profiles of in situ permeability variations with depth for 1.

the Upper Lias Clay and the London Clay (Fig.6:9) suggest a relationship

of the form:

k z = e-az 6 . 7 0

A

123

Since effective stress varies with depth in these profiles this

might also be written

k z e -a1 Acc v ko 6.8

Alternatively a relationship may be constructed as follows:

Typically (Lambe & Whitman, 1967, p.286)

log k cc e 6.9

and

log cs;.7 hence

k = C1 (a' ) -n

e 1 cc 6.10

6.11

Eqn.6.8 implies a straight line on a log k/av plot and eqn. 6.11

implies a straight line on a log k/log aily plot. Both relationships

are compatible with laboratory results presented by Bishop & Al-Dhahir

(1970). Thus we have three equations 6.7 , 6.8 and 6.11 which

may be used to define a permeability-depth relationship in a soil.

Equation .6.7 implies a permanent structure varying with depth, such

as might be caused by weathering. Equations 6.8' and f'6.11 imply

that permeability is a function of effective stress.

These equations may be used to predict changes in pressure with

depth in one dimensional flow. Consider the situation shown on Fig.6:28.

Flow is between two horizontal drainage layers, H apart. The

pressure in the upper drain u = 0, the potential h = H and the

permeability at the upper drain is ko. The pressure in the lower drain

u = uH, potential h = hH and the permeability at the lower drain is kH.

124

For vertical flow, by Darcy's Law

= C2 6.12

Using eqn.6.7 and substituting in6 .12

an k . e-az = C2 az • 0

and from 6.7

kH -aH 177 = e 0

and k 1 a = In ( -o ) H

Integrating 6.13

C2 1 h = . eaz + C3 ko a

C2 and C3 are given by the boundary conditions

z = 0, h = H

z =H, h = hi/

The pore pressure u at depth z is given by

6.13

6.14

6.15

6.16

6.17

6.18

u = { h - (H - z) } yw 6.19

Using eqn.6.8 and substituting in 6.12

az . ko . = C4 6.20

4

125

Supposing the effective stress at the upper boundary to be (0-1:7) 0

then at depth z

(a') z = (a')0 + Hyw + z(y - yw

) - hyw v v 6.21

or

Aa' = a + bz - ch v 6.22

where

a = Fh(14

b = y - yw

c = yw

Substituting 6.2 in 6.20

6.23a

6.23b

6.23c

Dh k . e(-a1a - a1bz + a1ch) = C4 6.24 3z ' 0

Integrating

C4 a a a1 c.ea1ch k = --.e 1 .a1.b.ea1bz + C5 0

a, may be evaluated from 6.8 for given values of

k0 and kH at the two boundaries.

C4 and C5 may be evaluated from the boundary conditions

6.17 and 6.18.

6.25

z = 0, h = H 6.17

z = H, h = hH 6.18

126

Using eqn.6.11 and substituting in 6.12

6.26

Substituting 6.21 and 6.23 in 6.26 with a2 = a + (0-1"7 ) 0

Dh -5.7.C1 (a2 + bz - ch) n = C6

This equation can only be integrated by series and will not be

6.27

proceeded with. However, for horizontal flow, with a constant,

and

a' = a3 - ch

Dh = 6.az (a3 - ch)n C1

6.28

6.29

Equation 6.29 may be integrated by substitution.

Fig.6:29 shows values of pressure calculated according to

eqns.6:16 and 6:25 for a permeability decreasing from 10 to 1 and

for complete underdrainage ( uH = 0 ). It can be seen that there

is little difference between the pore pressures predicted by the two

solutions or by the assumption of a permeability decreasing linearly

with depth.

The effect of the magnitude of the permeability ratio on ru is

shown in Fig.6:30. With u = 0 at the surface of a fully underdrained

clay layer a permeability ratio of 8 or more will give an ru

value

of 0.3 or greater in the upper 20% of the layer. For ru?„. 0.3 in the

upper half of the layer a k ratio of the order of 20 is required.

127

Curves for half underdrainage are given, showing that, at high

permeability ratios, the upper layers are insensitive.to underdrainage.

These theoretical solutions are one-dimensional and it is of

interest to compare them with two dimensional analogue solutions.

Fig.6:31 shows the analogue solutions for a 1 on 4 slope, 12 m deep,

cut in a 48 m thick clay layer. A logarithmic permeability gradient

of 10 to 1 was modelled for the top 12 m and constant below that

depth. A horizontal to vertical permeability ratio of two was used.

The boundary conditions were zero pore pressure at 1 m below ground

level and no flow on base in case A or half hydrostatic pore pressures

on base in case B. The pore pressure distributions at three sections

in case B are plotted in Fig.6:32. One dimensional solutions using

eqn.6:7 are plotted for comparison. Good agreement is obtained.

A more severe test is the embankment dam downstream shoulder

section modelled, Fig.6:33, which has a vertical chimney drain and a

base drainage blanket. A logarithmic vertical permeability variation

of 10 to 1 over 21 m and a horizontal to vertical ratio of 2:1, 4:1

and 8:1 were used.

The ru

values obtained for these three conditions are plotted

in fig.6:33, which shows the effect of anisotropy is small. The

maximum ru ranges from 0.23 in the 2:1 case to 0.20 in the 8:1 case.

However, without the vertical permeability gradient, the pore pressures

would be zero throughout.

One dimensional solutions using eqn.6:7 are plotted for four

sections through the slope in fig.6:34 and compared with two two-

128

dimensional solutions. Close to the vertical chimney drain the

one dimensional solution gives higher pore pressures as may be

expected, the influence of horizontal drainage not having been

considered. Out of the zone of influence of the vertical drain the

one and two dimensional solutions give comparable answers. The two

dimensional analysis carried out by Sweeney (1970) on an infinite

slope with a radial variation of permeability gave pore pressures

close to his one dimensional solution. Therefore for most slopes,

except the steepest and most anisotropic, a one dimensional solution

gives a good approximation of the pore pressures.

There could be some argument that the high pore pressures obtained

in the old side slope at Potters Bar are residual pore pressures due

to the pre-pumping high pore pressures in the Chalk. In this context

it is illuminating to consider the pore pressures in the London Clay

at the University of Kent, Canterbury. Here, close to the escarpment,

the 24 m London Clay layer fully underdrained by the .Oldhaven Beds

which outcrop within 800 m of the site. The measured pore pressure

in the Oldhaven Beds is 1.2 m of water at the base of the clay. The

water table in the overlying gravels is now 1 m above the top of the

London Clay, but was 2 m at the time of the original site investigation.

The section is shown in Fig.6:35. A pore pressure of 7.9 m of water

was recorded at 10.5 m below ground level (7.5 m into the London Clay).

ko To obtain this a permeability gradient of= 100 is required

ko using eqn.6:7 and 200 using eqn.6:8 with 1m of water in gravels.

kH

This reduces to 30 & 50 respectively with 2 m of water in the gravel.

129

The pore pressure in the London Clay is not likely to have

responded to the recent change in water level in the overlying gravels.

Therefore the values based on 2 m of water are the more realistic.

Thus permeability gradients do exist in the London Clay and appear

to be comparable, if slightly smaller than that measured by in situ

testing in the brecciated Upper Lias Clay (Fig.6:9).

The Potters Bar section is now fully under-drained. To obtain

the measured pore pressures, using eqn.6:7, ko/kH = 30 is required

through the 44 m clay layer, see fig.6:36. The permeability gradient

is the same as that at Canterbury thus no residual pore pressures are

required to explain those measured.

The pressures in the underlying Chalk were probably never more

than half hydrostatic before pumping due to the proximity and level

of its outcrop. The pore pressure distribution with ko/ku= 30

using eqn.6:7 and half hydrostatic pressures on the base is also

plotted on fig.6:36 showing that in the top 10 m the influence of

base pressure is very small.

The Potters Bar data indicates the same permeability ratio as

at Canterbury. This may be fortuitous, since in the upper part of

of the section the effect of a variation in the permeability ratio is

small once the ratio is greater than 10:1.

The attempts that were made to measure a permeability gradient

in the embankment fills at Grafham Water and Bough Beech were

inconclusive. The results obtained are plotted in figs.4:53, 4:63

and 4:64. The results are shown to scatter over three orders of

magnitude with only the upstream shoulder at Grafham Water showing

any indication of a relationship between permeability and effective stress.

130

Both these embankments were constructed with moisture contents

at or below optimum and the resulting dry lumpy structure can give

large local variations in permeability. Also the layered construction

can cause some degree of anisotropy.

However, the pore pressures in the downstream toe at Peterborough

Fig.6:15, and in the downstream shoulder at Aldenham, Figs.4:34 & 4:40,

show that perched water tables can develop in fills which indicates

that a permeability gradient must exist. To obtain the recorded pore

pressures at Peterborough, ako/kH ratio of at least 10 is required.

The results from an analogue model of a fully under drained embankment

shoulder with k0 /kH

= 10 have been discussed earlier and are shown in

fig.6:33. They show the permeability gradient increases ru from zero

to as much as 0.23 in a 21 m slope.

These excess pore pressures can be controlled by the use of internal

drainage layers in anembankment. The effect of drainage has been

examined for a one dimensional case using eqn.6:7. A clay layer 30 m

thick with koitkil 30, fully under drained and with zero pore pressure

at the surface was considered.

Without internal drainage, ru rises to about 0.45 at the surface

and is > 0.40 for the upper 8 m, see fig.6:37. A single drain placed

at 8 m depth gives a maximum ru in both layers of 0.17. The optimum

placing of two drains is at 5 m and 15 m with a maximum ru in all

layers of 0.075.

The effect of drainage layers at 3 m intervals is shown on fig.6:38-.,

The top drain is the most efficient in reducing ru values with the

second having nearly as marked an effect. The drains become less and

less effective as each one is added.

131

Probably the most cost effective drainage system for controlling

long term pore pressures in downstream slopes of clay embankments is

a series of stub drains as have been used in the upper part of the

shoulders at Grafham Water. They should extend far enough into the

body of the dam so that all vertical sections pass through at least

one drain, see fig.6:39.

Drainage near the base of a clay layer is inefficient and may

have negligible effect near the top of the layer. In deep cuttings

the installation of drains in horizontal drifts at the base of the

slope are of comparatively little value. They would probably be more

efficient and cheaper if placed further up the slope or if used on a

stub drain system similar to that suggested for an embankment (Fig.6:39)•

6.6.2 The effect of a permeability gradient on equilibration rate

Schiffman & Gibson (1964) consider the consolidation of non-

homogeneous clay layers. They plot the excess pore pressure isochrone

for 50% consolidation in the case of a polynomial decrease of permeability

with depth, constant my. The distribution is considerably skewed from

the conventional theory and can deviate from it by as much as 30%. The

skewness depends primarily on the magnitude of the permeability

change throughout the layer.

The diagram in Fig.6:40 shows the upper 10 m of a 20 m clay layer

swelling after excavation. Both the ultimate pore pressures and the

50% isochrones, based on Schiffman& Gibson (1964), are shown for the

permeability distribution in the inset sketch. The increased equilibration

rates in the upper half of the clay layer are shown. Swelling is

retarded in the lower half of the layer.

132

In practice the effect of a permeability gradient in a cut slope

is to increase the rate of swelling in the zones where the pore

pressures are critical for stability thus reducing time to failure

from that calculated by conventional methods assuming constant k,

and hence cs. The two layer system used for Potters Bar and

Edgwarebury goes some way towards calculating this effect but will

not model it completely. Therefore times to substantial equilibrium

near the surface, calculated using the two layer, system are still

overestimated.

In fills the gradient will have the same effect of reducing

equilibration time scales in the critical top few metres where most

failures occur.

Table 6.1

Sandy clay embankments

Site Laboratory cv m2/yr

Field cv m2/yr

Remarks

max min mean max min mean

Usk 8.9 12.3 Skempton (1957)

Selset 3.1 0.9 2.0 3.6 1.4 2.7 Bishop & Vaughan

(1962)

Derwent 1.3 1.7

3.7

1.0

1.6

1.4

2.3

core Rowe

fill (1970)

Balderhead 15.8 2.3 9.0 11.9 9.3 10.6 Bishop & Al-Dhahir

(1970)

M6 Kendal 3.8 1.4 2.7 5.8 1.9 3.8 as above

Backwater 13.0 1.5 6.9 13.9 3.6 7.2 Wilkinson etal

(1970)

Cow Green 2.9

7.2

0.2

0.1

1.4

1.7 3.3 0.4

=1.0

1.1

Core

foundation

Vaughan et al

(1975)

All values for fill unless noted

Table 6.2

Plastic clay embankments

Site Laboratory cv m2/yr

Field cv m2/yr

Remarks

max min mean max min mean

Peter- 0.6 0.3 0.5 Sodha (1974)

borough 1.3 0.4 0.9 cs 11 to

0.6 0.5 0.2 Al-Dhahir (1967)

1.8 0.8 1.3 core

2.4 0.7 1.8 cs upstream shoulder

& downstream toe

Grafham. 1.5 0.4 0.8 1.3 0.8 1.1 Al-Dhahir (1967)

Water 0.5 cs, Bishop &

Al-Dhahir (1970) 0.9 0.7 0.8 core

0.4 0.3 0.4 cs, core & road

1.2 foundation 1.4 1.0 1.2 " Bishop &

Al-Dhahir

Bough 0.9 design cv, Hallas

Beech & Titford (1971)

6.9 6.7 6.8 cs, core 1.7 1.6 1.6 cs, shoulder

2.1 1.9 2.0 foundation

All values- for fill unless noted.

Table 6.3

Embankment dams - Time to equilibrium

Site Part of structure cv or cs m2/yr

Drainage direction Drainage path

length m

Time to t90

years

Peterborough upstream shoulder 1.8 vertical 17 =160

Foxcote central section 1.0 vertical 10 =100

estimate

Grafham road embankment '0.4 vertical 7 =120t

Water core 0.4 horizontal 2H = 11 =80

upstream shoulder 0.4 vertical 2H = 1.5 =1.5*

Bough core 6..8 horizontal 2H = 20 =15 •

Beech core 1.6 horizontal 2H = 20 =60

upstream shoulder 1.6 vertical 2H = 2.2 =1*

* These values depend on fully efficient drains and should be taken as times after pressure

in drains reaches top water level.

Assuming zero pore pressure line to be lm below ground level (mean).

Table 6.4

London Clay - cv and cs from laboratory tests

Test Size mm cv m2/yr cs m

2/yr an kN/m2 Reference

Blue London Clay, Triaxial dissipation. 300 dia. 0.97 138 Garga, 1970

Wraysbury vert. drainage 0.51 372

100 dia. 0.79 185

0.26 508

Brown London Clay Triaxial consolidation 100 dia. 0.99 190 Skinner, 1967

vert. drainage 0.92 550

Brown London Clay Oedometer 76 dia. 1.07 160 Apted, 1976

Ockendon vert. drainage 0.29 2,600

0.10 100

0.94 2,600

Table 6.5

Boundary pore pressures in fill slopes

Site Piezometer Piezometric level, m below G.L. Slope Surface

max min mean

Peterborough A 0.2 1.3 0.8 1 on 4 Rough, uncut grass

C 0.6 4.0 1.6 1 on 3 Rough, young trees

E 0.7 1.4 1.05 1 on 3 Rough, uncut grass

G 0.2 1.8 0.55. 1 on 20 Rough, uncut grass

Grafham U/V 0.2 5.0 0.8 1 on 15 Well kept grass

Water W/X 0.15 1.8 0.8 1 on 15 Well kept grass

P/R 0.2 4.5 1.2 1 on 3 Well kept grass

Foxcote 3 1.1 1.4 1.2 1 on 2.75 Well kept grass over

gravel layer

Aldenham 9/10 0.3 1.8 0.8 1 on 4.5

approx

Very rough, uncut grass

and tree roots

Based on piezometers 0.9 to 1.7 m below ground level.

0+3

• r3 ;;.• 04-3

0+3 434.3 64.3 6.3 o+3

• e#4. 6,3

0.4

00

044.

9+3

Sandy Clay Fills „4.1.k ,0

Opt. m/c

f .0,1 • • lb+,

• •12.-i 0

• alj.

• • 43

eS 411

oil

6_1

(a)

•

I (b) 'ot3 or3 (0.3

42/ 04

42

042

*0

0-4 0-1 /

1'1 41 4 :#3 43 0+3 40•3„. DO

+3 °V O-4 3 10-1 .3

/14

/

0+1 ; • -1

•-;

Plastic Clay Fills

Opt. m/c >15%

.-4 1

0.8

0.6

0.4

0 47-0 0.2 cr a)

cr) 0 (no)

0

ci

0.6

0

0

0.4 C 0

°'2

C

0

-0.2

-0.4

-0.6

-0.8

0 200 400

Total

600

Stress

800

kN /m2

1,000 1,200

0 200 400 600 800 1,000 1,200

fig. 6.1

Plastic Clay Fill Shoulder:- opt. -1 Core:- opt.+3

30m

0 50 100 150 200

- 300

Sandy Clay Fill Shoulder:- opt. Core:- opt.+3

0 - -r-

- 1"-- 100

----200 30m 1-:300

-400

Theoretical End of Construction Pore Pressures in Clay Fills

-1.0

-0.5

0

0.3 ru = 0.4 Ko= 0.81

Based on stresses from Duncan & Dunlop (1969)

0.2

-1.0

-0.5

0

0.1 0.2 0.3 ru = 0.4

Ko = 1.6

_4.

-1.0 -0.5

01 0.2 0.3 0.4

05 = ru

Based on Au = X Az

End of construction pore pressure in a 1 on 1.5 cutting slope

fig. 6.3

-1.0 -0.5

0

-1.0 -0.5

0.1 0.2 0.3 0.4 ru = 0

-

5

-1.0 -0.5

0 0.1

0.2 0.3 ru -- 0.4 Ko = 1.6

0.1 0.2 0.3 ru -- 0.4 Ko=0.81

Based on stresses from Duncan & Dunlop (1969

Based on Au =)rAz

End of construction pore pressure in a 1 on 3 cutting slope

fig. 6.4

End of Construction Pore Pressures [Eigenbrod, 19721

1 2 3 1. 5

Au = XAz assumption fig. 6.5

O.G.L. O.W.L.

3 , ■ \ ■ \ 4 -- 3 1.2

0 0 2.4

■ \ \

5.5 6.1 4.6 5.8 \ \ 0.8 I oe 6 • • •\ dry0 \ \ edry \ 0 ■

• measured based on measured

values ---using Au -7.)5Az

Pore pressure, m of water 7.3 ■

10.9 11.8 • •

7.3 e 0 \ ‘

0 metres 5

16.10 • 15.8 • 14.3

\` 3.6 ■ 04.1 1.8•

5-2_.... \ \ 3 X5.5 3.05

u = 3 m of water r== • 12.2 10.7•

1

WELLAND CANAL CUT - End of construction pore pressures [after Kwan, 1971]

2 7//

1 Q1

- 0% - 0e.7 I -1.51 •

0 5 oI 0

• metres -0.9 1-2- 5 -4.1 -08 •

-1.1 I 1 -2% 0 -2 - -6

Pore pressure change - metres of water o measured

----Au r-nz

-8 -10

KIMOLA CANAL - Pore pressure change due to excavation [after Kankare, 1969]

14 —'OG L

16

18

2 foun

datio

n

2

4

6

8

u m of water 4 8 12

16

23 -0-

; /

.

0 \ \

24 -0-

4 • possible

t =0 \possible

\

t =co

-0- 19 -0-

Cl

I

x \ \

12 -0-

(ii) t =9 yr \ ‘

C2 -0-

f d rct. x 1

I ti

\ . I I

\

\

I /

/ \

-0- 8 \

\

N.

\

\ -°- 4-5 0 End

x After 9 years

of construction \x

■N,

GL

10

m

12

8 -4

2 -0- (1) 1-2 _J

PETERBOROUGH - Pore pressures in

fig. 6.8 upstream shoulder section

0 x 0.•1,11 1PS;q:, 111 0

0 16-

X

0 0--0 18

1 I IFS'S, s lost I- ;III

2 x +

cm* o 0_4. 0

0

4. c 0

0 4

6 0 0 0 0 00

8

o % co 10

E 12

0

Upper Lias Clay Empinghom: Rising head tests

Constant head tests • London Clay Laboratory tests X

Wraysbury : Constant head tests 0 Wothorpe Rising head tests 0 Laboratory tests es Constant head tests 0

Laboratory tests *sits 11.11,

CO

22

24

0

o •

A

Coefficient of Permeability, k. m/sec.

10-11 10-10

Permeability test data , Upper Lias and London Clay

from Chandler, 1974 & Garga 1970 1

fig. 6.9

co

n 0

-0.4

- 0.8

Potters Bar Sudbury Hill

Potters Barb

Northolt Wembley Hill

dry standpipe piezometers

Edgwarebury

London Clay Cuttings - Variation in pore

pressure ratio, ru ,

(Edgwarebury ____ci I I I I

0 20 40 60

Time to failure years

with time

bulk strength c'= 0

Brown London Clay

Blue London Clay

• cb'= 20°

ru • calculated from slip

O measured ru o measured

80 100 120

Clay

.82 1

9.75m

3 •\

blue London

-14 -12 -10 -8 -6 -4 -2 0 2 4 OEL. —

\t=co

t =0

8 -10 -8 -6 -4 -2 0 2 4 6 8 pore pressure m of water

GL

OGL

o piezometer measured pore pressure + 9 years • 11 years

A

OGL

T 5 5m

GL GL

\ \ \

s -1\ \, '

- 6 \ \

9 yrs \ . -8 "--11 yrs \ %:-11 yrs \

E -10 \t co - o -12 \

‘ \

\ t ...a\s. -14 --E

. \ c) \

-16" t= 0 \

-18

-20

12m

1

I 0 I 0

EDGWAREBURY Pore pressure isochrones at piezometer locations - calculated using c5 = 0.85 m2/yr in the blue and cs= 2.55 m2/yr in the brown London Clay

fig. 6.11

OGL

4.2 m GL -0

-2 brown

-6

-8 10 blue

London 12 Clay

-14

-16

18

\ \

t =0 19 yrs' .co

6.4m \(-6 x measured pore

pressure, 19 years

o piezometer

10 - - - -2 0

E

-o -2

-4

-6

-8

GL 'o

N

19 yrs -10 -c

-12 .8 t=co•

-14

-16

A

I I -8 -6 -2 0 2 4 6 8 10 -8 -6 -4 -2 0

pore pressure m of water

OGL

2 4 6 8

POTTERS BAR - New Side Pore pressure isochrones at piezometer locations - calculated using c5= 0.85 m2/yr in the blue and cs= 2.55 m2/yr in the brown London Clay

fig. 6.12

0

1

E

2

5

A

Pore Pressure m of water 0 1 2

3 4

5

1\\\ t\\\ I \ \ \\

\ \ ‘ -1._

\ \ c \ ,. \ %.

1 \ ,‘

\\ • \

\ \ '‘,. \

\ \ 1 \

k

\'\ • • ' \'. \ \ " \ \\ \

1 ‘• • 4 ‘ \

it k

\

1

\77-.\ 7\

\ • \ \

\ \

\ \

\

‘

0.2

\ \\ ‘

\ • .\

\* \

\ \ \

\

\ 0.3

\

\

\

\ 0.4

\

\ \ ♦.

0.5

V---

1

• 1 ru=0.1

Range of maximum pore pressures, Upper Lias Clay cuttings. [ after

Chandler, 19741

fig. 6.13

COW GREEN - Pore Pressures in a Boulder Clay Slope

°-... N, -‘, •-■ \ \

x \-\\ ,c-

.t\I

x

: 6.

, %_. \

Probable equilibrium pore pressure distribution

After 11 years *-.

--p:.. '..

1 1 i

*.- I

.

0 . i i':..1\

N.

o. N•

x

\ \• \

. \ \

x

\\

Nu •

GL +2

OD

2

4 SGL

6 cu

-8

-10

12

-14

a

Foun

dation

A0 220

B0

170

10

60

30

1‹.-- Annual fluctuation

-4 -2 0 +2 +4 +6 +8 +10 Pore pressures metres of water

0 Piezometer level x Pore pressure - end of construction o — minimum recorded

- after 11 years

PETERBOROUGH - Pore pressures in downstream toe & foundation

fig. 6.15

0 2 4 6 depth of slip surface m . .. / ... . .. /

..- 2 ....- .....- / /

....- ...-- .....- - , ..-- / / ..-- ...- .-- ..-- ....- , , ..--• .. / , ....- , .... , - 4 -- , , ,

....- - - ...- . / _ ---__ --- - ..-- - ----- _ , - 5 -- . / _ .._.. _...- ..- „ .. .. _ ...- - - ___ _- , .. ,.,-,__-___

- -- „ 6 _., --- , , ..., - _______ - ___ - ....-... ---- ..__ _. ___ _ _ ...... . 7- -' .... - -... .-- edil -. __ ...... - ...... ...... - ..M.Mb ,... ... ■•■••

8

kN/m2 2

4

0

6

c' required 20°

pore pressure kN/m2

0 40 80 120

6- m

8 -

10-

winter

I

I

/ 7

/**•-•• / /

2 winter

meaii\ 4-

POTTERS BAR - Old Side - Stability analysis using Bishop (1955)

routine with mean and winter pore pressures

CV =CS = 1 2 /y r 5 m2/ yr • 10m2, yr 50 m2/yr

1

2

3

4

5

6

7

8

9

10

1" E

1

2

3

4

5

6

7

8

9

10

• Pore pressure fluctuations in a , semi-infinite clay layer due to an annual surface -ressure variation of ±1m.

seasonal variation at surface

6

8 tr)

10 E

12

14

16

18

20t

Relationship between rate of equilibration of pore pressure and seasonal variations

fig. 6.18

22 a) .c)

E 3

713

24

0

(C)) 5 N a) 0

1954 1955

1955 1956 M J J A S O N D J FMAMJ J AS 0 N D J FM A M

... N

-0-3

"....% %.

'N •• or .•■••■ " .....1

....p, l......' 1 N. %..... •• ...... —.. --. .........0".". ...... ........ .• . . \

l'‘. 1

,..._/

... -- it I

NJ - ...N

■ ,

-0-6 .

S. ...... \ 6

, 0.--. v -. .. \ ii01 ..0

1 ii

I i .

3

6

cn UXBRIDGE - Tensiometer Readings after Black, Croney & Jacobs, 1958]

1956 1957

GL

1 Depth m

2

3

4

5

JFMAMJ J ASONDJ FMAMJ J A

, N.... --, ,---- ......... ., ..-- ... , . _.. ...--

. . --- .

.--... --. 9

12%- e 1

9 i -0- i l

... ....' 1r

—0 -2 •

SUDBURY HILL - Annual pore pressure fluctuations [ from Skempton & Henkel, 1960 ]

C5B

c 5B

GL

1

2

3

.

A -a ,

/.$ 1 rI $ %, , %

N

% , ‘ '\

\ \ i\ --V---\

% \

_ i

A

I' % v.% s. s S ■....

P2 s%. ... s

PP'12

\ \I

19 72 197 3 1974

GL

E

1

s 2

a)

3 GL

1

2

3

4

MAMJJASONDJFMAMJJASONDJFMAMJJA H -.)

P3 nstalled installed

. ■

.1. .• %,......* w•

•

.

I

P3 Cr'

v

3 . 4 •-------

I , V Ii `, P4 r". I t , ii

i % . v . I . ‘ \ ,.\ j •••■•• ...

I

BARNSDALE - Annual Pore Pressure

Fluctuations I data from Chandler, unpub.]

fig. 6.21

1971 1972

GL

1 Depth

m 2

NDJFMAMIJJASONDJFMAMJ

- •••••..., ... .',... i/

...- ...•..

•

-----------• 5 ..... -.'" ...., ........... • ..... --_ .

1.-- 2 dry 'I

GRETTON - Annual pore pressure fluctuations [ after Pachakis, 19741

0 (D

"II

3

0

0

2

3

1

pore pressure 07 -6 -5 -3 -2

4

5

6

II■11 .■■■

0

,I. ....

..IMIO

❑ ... , .... 0

...-- ~ -.0 El CO CI CCI 1=1 r: o -.. ... -..

CI.4,, 0 0 ■

0 •

F GC L g■P .

- .

in

Natural Natural Cutting

Annual

Embankment

clay slopes

slope slope

,

pore

with

1

pressure under

head

1

grass

min. o + A o

, 1

fluctuation

max. 0 x

a

1

cover

N

Q IP

0O 0 \A

y \

' )( ‘

A \ l'- A AA 0 .

\ 0 ' N

N 0 \ A

A `\ 0 \

\ A \ x o

‘° \ A A A a &

\ E3

AoA \ \ \

0.5

A \ ' ‘ 0.4

‘

ru =0.1 ' 0.2 0.3 \P‘ A• • \

...t

m of water 0 1 3 5

4 8

12 16

20

24

GL

0.4

E 0.8

1.6

• ._.._ , ..._. ..••••••.' - .....- ....-. ....-

....... _.-- .......

a © V „ --

.- ....- ..-

increasing

....- rough ess

...-- ....4-34

2.0 0

Slope Cot

Natural Slope A Gretton [Pachakis, 1974] a Uxbridge [ Black et al. 1958]

Cutting Barrowden [ Chandler, 1974]

Embankment V Seaton 0 Aldenham

[ Chandler, 1974]

+ Foxcote x Grafham Water 0 Peterborough

All piezometers between 0.9 and 1.7 m deep

Depth of Mean Zero Pore Pressure in Grassed Clay Slopes

fig. 6.24

pore pressure m of water

00 1 2 3 4 5 6 7

1

o E1

20 m ,I "

~ ..

Pore pressures

Potters Bar - old side

Potters Bar - new side

11~----~----~----~----~----~----~--~

P'OTTERS BAR - Effect of counterfort

drains" analogue solution

fig. 6.25

E

.c. o+-J

0-w

0

fig. 6.26

GL~----~----~----~------~----~----~ I I

1

2

3 03 &

04

I4-le--1 drains installed

I I

1973 1974

GL~~~-+~~~~,~,~~~~~~~~~~~~~

1·

2

3

GL

1

2

3

05 &

06

08 &

010

I I I I 1 I

I '\, .... J 1

I I , I

, 1

BARNSDALE - Effect of counterfort drains [ChandlerJ unpub.l

A

k • k Fr V

• • • • " :.• / •

1:1

•••., `•••

2 :1 •••••.

••••- ••••,.

T

•••

•••••• ••••••• •.•••• 5:1

10 :1

0

Pore pressure distribution at base of drain level - 3 & 5m deep drains at 20m centres ----- with drains without drains

Analogue solution •

' : • : • . , • • : • : : •••••. : - . . • . ' t ' •.•••• " • • • • --....;-1.......:`

• : . : . : . i • :::: .... . : . -- ••••••. ---

- - - --- ....... -- - - -

0 -

- _ ... ...... - . -- -- .... -... -•••••••

- • - -

SEVENOAKS Lobe D [ from Weeks, 19691

Effect of counterfort drains

—.••••• ••••••••

fig. 6.27

0

90°0 ° C;cf.D°. 4.o° 0

e DG, 04e0 c.C70 0 =0, - k=k0,

- -r

Clay layer

Cz 0

0 0 0 0 0 0` ;'

cz, ecs 00o et,e• 0 0 c.c. cz) /5

u = uH, k = kH, hw= H

fig. 6.28

• -

k z ko k z ko

k z = k o - a3 z

10

5

kz kH

= e-az

0.1 "' 0.2

0.3

T"

0.2

0.4 z H

0.6

0.8

1.0

1 I I I J 0 100 cr, (H=i0m) 200

v

The effect of various permeability distributions on pore pressure

f ig. 6.29

2 5 10 20 50 100 200 500 ko / kH

The effect of permeability ratio on pore pressures in fully and half underdrained clay layers

fig. 6.30

0.35- --4

0.4 or.

L

— 0.3 ,

—J

•••

••

H 12m

1-0 flow

Case A

Case B Boundary conditions

H 212m

••• ••• ••• ..... ■■•

•M/ ■••• .........• ..

..... -------0.4---------- •• ••• •••

co•

1

•••■■• ■■■■ ■■••■• •■■■• •■■ ■■■••

Case A :- average ru on slip surface = 0.29

Case B average ru on slip surface = 0.28

Equilibrium pore pressures in a 1 on 4 slope with a permeability gradient of 10: 1

fig. 6.31

0 4 I 8 I 12 u m of water

Case B (fig. 6.31) comparison of pore pressures from one and two dimensional solutions

......... ............•=

........ .............

.....

...• ......,... .....

..........................,........./ .., , .... •••• )

/ I I ..." ........ .■... i ....T.... I .... / /

..% .... •••••• ■•••;..... .... ....

.......... I' ...••• I

—■•• ..... .... ••••• 0 .2 ..-, / / .... -- — --- -- -- --

, --- „ - 0.15 ' _-- '' /

--- —' l — — — _— /

-- -- ■ —____ ______

— — — — — 0 .1 ' ..- '

kH = 2kV

••■• ■•• ■■•■ ■■••■ •■■ ■■• .1■■ •■• ■•• ■■■ •■■■ ■■• ■•• ■■• .■ 0 . 0 5_

••■• •••••••

k0

=0

A

M.O. 0....

..:1 ' . 0.... 1... ..... ..... ..... •■ •••-• ..... ■.... ■ ,..

........... .... —0.05 ,... ...

c_— _ _ _ _—_— _ _.7_ _ ----- — — — — — — — — — — — —

— .... ....

kH = 4kV

0....... a.... 0,...,0. .../O

...... ...,.. ........... 1■0 ...... st \

....... ....

..... ........ ,.. .A.. , ........... W..

....00 •■• / j 1

.10...../. 00.

_... ..... ./... .... ,,,B. .......... ••■•• ...... .......... ...... ,

...- ..... ...... ...o. 0..

........... ..... • I . . . . . . .... .... 0.. ..... 0 . 2 ---

.-- / / __ .... ....

,............. — _ ..-

..... .—_. _ ,......

C. _ ..... ....- ..— 0 .1 5

...-- /

..--

.......... — / _— ......__ .... —

..— _ — — .... ••••, ••••••

— — — pore pressure ratio, ru

The effect of anisotropy on a fully drained clay slope with a permeability gradient of 10 to 1

fig. 6.33

0 4 u, m of water

Pore pressures in a fully drained clay slope - comparison of one and two dimensional solutions

m of water 4 8 12

pore pressure 8 12

_120e o 0 -=-• 00 a0 0 0

C Z fill ."' gravel Ill

U) —I -< 0 --1,

M London Z ---I Cloy

A21nEld3INV3

(xi

pore pressure m of water 0 4 8 . 12 16 20

24

4

8

12

16

20

E

24 -c

as -0 28

32

36

40

\

_

pore

underdrainage

underdrainage

measured

full

half

pressure

kH \ \ 20

30 \\ 50

100

3 0

0

1

i

I I /

I /

/

POTTERS BAR - Calculated pore pressure distributions using:-

fig. 6.36 kz /ko = e-az

24

8

u m of water 0 4 8 0.1 0.2 0.3 0.4

28 ILII

The effect of 1 and 2 drainage layers on the pore pressure in a 30m clay layer with ko /kH = 30

fig. 6.37

u m of water ru 8

0.1 0.2

0.3

0.4

The effect of drainage layers at 3m

intervals on the pore pressure in a

30m clay layer with ko/kH = 30

fig. 6.38

L

Embankment shoulder showing stub drains

T - co t

H2

H = 20 m my const.

pore pressure metres of water -4 -3 -2 -1 0 1 2 3 4 5 6 7

The effect of a polynomial variation of permeablity with depth after Schiffman & Gibson, 19641

-r

fig. 6.40

133

Chapter 7

CONCLUSIONS AND DESIGN RECOMMENDATIONS

7.1 END OF CONSTRUCTION

The end of construction pore pressures in clay fills depend on

the plasticity of the clay as well as placement moisture content

relative to optimum. Excess pore pressures are set up in the sandy

clays and very wet plastic clays. In these cases the end of construction

pore pressures are required for design. These are best obtained by

laboratory measurement of B and pore pressure dissipation tests on

field compacted fill.

In plastic clays, unless very wet, at low stresses negative pore

pressures are set up and even at high stress the pore pressures may be

less than the equilibrium values. Therefore for design purposes the

long term pore pressures are critical.

The pore pressures in excavated slopes in overconsolidated clays

are always less than the long term values and are therefore not

critical for design. However, in the central portion of the slope,

a first order estimate can be made using

Au = yAz 6.2

This eqn. does not hold for very steep cuts and will overestimate

the pore pressure change under the base of the excavation. At the

crest of a cutting reduction in pore pressure occur due to horizontal

unloading which is not modelled at all by an eqn. based only on

vertical unloading.

131+

7.2 EQUILIBRATION RATES

In compacted clay fills the values of cv or cs obtained from field

performance data are comparable with the values obtained from laboratory

tests. In plastic clays the dry lumpy structure, which produces large

local variability of permeability, does not appear to influence the

bulk properties of the fill.

Without considerable drainage measures, equilibration times will

be long, for large structures several hundred years. For the smaller

dams considered in this thesis the time scale is of the order of 100

years.

In dry plastic clay fills, the pore pressures are increasing

during this period and the stability decreasing. This highlights the

importance of inspection under the Reservoir Safety Provisions Act

(1930). Provision must be made for monitoring a dam for at least

100 years. Permanent installations would require considerable

improvement or occasional temporary installations could be used to

overcome this problem.

Swelling is dependent upon an adequate water supply and upstream

slopes of dams with internal drainage swell more rapidly than downstream.

The equilibration rate of excavated slopes in clays where swelling

will close discontinuities is, at least in the early stages, comparable

with the rates calculated from laboratory values of oir or cs measured

on large samples. In the London Clay the equilibration rate is

comparable to the delayed failure time scale. Therefore it is unnecessary

to presume a significant drop in drained strength with time to

explain the failure. The presence of a permeability gradient can

increase the rate of swelling in the upper part of a clay layer.

135

7.3 EQUILIBRIUM PORE PRESSURES

For excavated slopes in overconsolidated clays and dry plastic

clay fills the equilibrium pore pressures are those required for

design. From the results presented in this thesis some recommendations

can be made:

A one dimensional treatment, using vertical sections will give

acceptable results except in zones where horizontal drainage

predominates, i.e. close to vertical drains and the intersection

of the slope and horizontal drains.

To calculate pore pressures, four variables are required for

a clay layer of thickness H:

(1) Average surface boundary pressure ho

(2) Magnitude and depth of seasonal fluctuations

• (3) Base boundary pressure hH k

(4) Permeability variation kH

The majority of excavated slopes, road embankments and some

embankment dams have no surface drainage measures and are just top

soiled and grassed. In this case some firm recommendations can be

made. The mean surface boundary pressure, ho, in metres of water

(taken as minus the depth to the mean zero pressure line) can be

obtained from fig.6:24, for the appropriate slope. The magnitude

of the seasonal fluctuation and the depth to which it extends can

be taken direct from fig.6:23. This is shown diagrammatically in

Fig.7:1 for a base pressure of hydrostatic relative to the mean

surface boundary pressure. For design purposes only the maximum

values are required.

136

These values are based on data from areas where the rainfall is

500 to 800 mm/yr. In areas of higher rainfall ho will increase with

a resulting decrease in the fluctuation magnitude. In the limit 110

will tend to zero and there will be a negligible zone of fluctuation.

For other surface drainage conditions the recommendations can

only be tentative.

(a) Counterfort drains increase the depth to mean zero pressure

line by between 0.3 and 0.7 m for drains 3 to 5 m deep at 20 m centres.

Decreasing the spacing will increase effectiveness, 3.5 m drains at

12 m spacing increased the mean depth from 0.3 m to 0.5 m. On the

surface the drains will not effect the boundary pressures which will

remain the same as for the grassed slope. A curvature of both the

mean and maximum pore pressure distribution will therefore occur.

The magnitude of the fluctuation will remain the same as under a

grassed slope. See Fig.7:1.

(b) Trees The data on deciduous trees indicates winter maximum

pore pressures equivalent to grassed slopes. Mean values can be

reduced by 0.5 m, based on young trees, perhaps more for mature trees.

Fluctuations are therefore larger than for grass and would appear,

from Oakleigh Park, to go deeper,possibly an additional metre, see

Fig.7:1.

Trees would appear to give an equivalent reduction of pore

pressure at depth as do considerably more expensive couterfort drains.

Within the fluctuation zone, deciduous trees are not as effective

in winter as the drains but reinforcement by tree roots will increase

c' to compensate. A further advantage of trees on London Clay cuttings,

or on fill slopes of plastic clay, is that swelling rates are comparable

137

with growth and young trees planted at the end of construction will

be approaching maturity as equilibrium pore pressures are reached.

(c) Gravel. Free draining gravel layers on the surface (i to 4 m

under topsoil) appear to maintain zero pore pressure close to the

base of the layer and, from the Grafham Water data, almost damp out

the fluctuation. At Foxcote a mean pore pressure below the base of

the gravel is recorded with small fluctuations rising t the- base

of the gravel layer. This is probably caused by the gravel not being

as free draining at Foxcote as at Grafham Water. As the fluctuations

are damped, pore pressures within the top few metres are improved.

At depth, pore pressures may actually be increased. See fig.7:1.

3) The base boundary head, hH, is not required with any great

accuracy and is best obtained by measurement. If the boundary is

an aquifer it may be possible to obtain an accurate enough pressure

from local well records or from the outcrop level.

If the base pressure is hydrostatic, with respect to the mean

pressure line, then the pressures throughout the layer are hydrostatic

and there is no problem obtaining suitable design pore pressures.

If the base is underdrained, the pore pressures in the upper

half of a thick clay layer (20 m or more thick) are not effected

greatly by the base pressure. For example at Potters Bar the difference

between full and half underdrainage was a maximum of 0.5 m in the top

10 m of clay.

4) Permeability variation. For a London Clay layer 20 m or more thick k

a — value of 30 fits the limited data available. This is based on kH the Canterbur y and Potters Bar data.

138

The in situ brecciated Upper Lias Clay permeability data k

(Fig.6:9) gives -2 of 100 over 20 m. In this material the use kH ko of a higher ratio would be reasonable. However, for values of --

kH of 20 or more the effect of increasing the ratio is small.

No evidence is available on the behaviour of thinner clay

layers but it seems reasonable that for H < 20 m.

k0 loge (1--) = — 20 loge30 7 . 1

ko hH and --- can then be used with either eqn.6:16 or 6:27 to obtain kH h and therefore the pore pressure at any point in the clay layer.

The results obtained with effective stress eqn.6:27, are very similar

to those obtained using the more simple eqn.6:16. This will give

the mean pore pressure distribution to which the maximum values in

the fluctuation zone can be added as shown in fig.7:1.

The pore pressure in free draining layers within the clay

formation which outcrop within the excavation will be equivalent

to outcrop level or base of layer level whichever is the higher.

These should then be used as additional boundary conditions in

eqn.6:16 or 6:27.

The recommendations presented above are for excavated slopes.

Although there is less substansive information, they can probably be

extended to embankment slopes. The Aldenham and Peterborough results

show permeability gradients do exist. In water retaining structures

only the downstream slope is being considered here. For embankments

the following should also be considered:

The values of ho, corrected as required for surface drainage,

139

Where there is a free draining blanket the embankment can be

treated on its own. In other cases the fill and foundation must be

considered together. If the permeability of the foundation is an

order of magnitude or more less than the fill then it can be

considered as a no-flow boundary. On the other hand, if the permeability

of the foundation is several orders of magntiude greater than that of

the fill then it may act as a complete drain but requires checking

for the particular geometry of any case.

The effective stress relationship, eqn.6:27 can be used for

both fill and foundation if thepermeabilities are similar.

Free draining intermediate drainage layers should be treated

as additional zero excess pore pressure boundaries. Drains are most

efficient in reducing equilibrium pore pressures in the upper, more

permeable, part of the fill.

An interesting extension of this work would be to calculate

the slopes required for stability using pore pressures calculated

from these design suggestions and making a comparison with the slopes

at presented used in practice. The slopes presented in section 1.3

are a representative selection of embankment slopes but those from cuttings

are somewhat biased as they are generally based on failed slopes.

m of water pore pressure

Free-draining Gravel

cohesion increased by roots

°r°cz (750? . .Z9ca.,2_ce=2_02.

Trees

Counterfort Drains

The effect of surface treatment on boundary pore pressures

fig. 7.1

1 140

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11+5

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r^-

147

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1

148

APPENDIX A

Peterborough drawdown records

The effect of drawdown was monitored on ten upstream fill and

foundation piezometers at Peterborough between July and November 1971.

The recorded pore pressures are plott'ed with the reservoir level in

Fig.A:1.

The drawdown was commenced a week after completion of de-airing

and before equalised readings could be taken so no pre-drawdown

pore pressures are available. The response of 18 after de-airing

is exceptionally slow.

Piezometer 23 started to leak at the beginning of August and

was unsuccessfully repaired. After a second repair in early

September it returned to recording sensible pore pressures.

~

to

» ~

Q) > ~

L. (l)

a; E o N .~ a..

19 0

0 8

1 0(-11·6)

+12

I ,

r- \IReservoir level

.10 t 1 111-~ .. 11 J J J ~ L td::t4~f~t:;L'~;,:·~~,~L.J~1 ~ 1 1 11 +8 1~2:b-b-bJf:4=t-:.:i=+l:r:~.tt-~~~+---t-+-~~-,+-t~-~~

t-k I I I I I: ..... ~~ ..

........... ~.....-/"~ I J 6 --, IV<:,.::.F':::..·" -=. - ... ~ ... - ... " I I 1 + ~ ~I --. -";"-.. ~--- -_ .... --- -"',.--- .. ~ -~ - - 1--..... ~ ..... 1----r.18

I ~I • "V ~ 1/1 ~ ~ ! -'''. ......... 1 (l) -"""::- I--.!= I ..

(l) " ~: J E +4 ~

Q)

> (l)

+2 1.._.:

~

.. ·· .. ···,··· ...... , .. · .. ····1·· .. ·· .. ·, ......... , .. . ~ ""'" t·- """- ................. , ... --.. -............... " .. _ .. , ....... -.. , E /' .}--- _ .- - .. - .-- ....... -~--". --- -='~""'f----to-

-1- f.- .... "~....... ± I --- ...,f 1"-=-1' I =F . ........... '+ ~ ~-, 19

~ 0.0. ,/ , (l) 19', I I l I I I I ._' I 1 I 1 1 1 I.-J.....J... I ' I a.. , __ ~ L- ,I I -, ·r- I I 1 I ITT 71 I 1

7

-21 " , I I ~ I I , 18, "

I 1 112

"1 ~ i ~ i i t ~ ~ b ! f ~ - 4 . . 8-.,,:"<;<~~~~~~·:~ :~=:~~.~ .. :~:~~ :=:=~=<=~·:-~=·:===:~><;:::~·~=·~~:· .. ··t~::~~::~,,··· .... I·········I·· .. ···"~,, .... ·l .. ·· .. ·I···~:+~~:·t::4 1 ~ I I I

-61 JUly 'I Aulgus t I~ ~ _ Se~tern~e~ _~ I ___ O~t«:)b~~_ No'vemher

PETERBOROUGH Response of up-stream piezQmeters to reservoir draw-down 1971

Appendix B Sheet B.1

End of construction pore pressures in earth dams

Dam Fill Material LL PL Opt. w Place. w No. an

kN/m2 ru Remarks

USK Glacial Till 25 17 9.8 12.2 A 217 0.27 Dissipation during Wales B 517 0.39 construction. (Little &

C 283 0.41 Vail, 1960)

HANNINGFIELD London Clay 68 19 25 28 7 65 -0.05 BRS low air entry tips No 2 12 86 -0.16 (Little, 1958, Little England 5 144 0.12 & Vail, 1960)

4 131 -0.07 6 69 -0.09 15 143 -0.08

FOXCOTE Glacial Till 65 19 27 28 1 176 0.25 BRS low air entry tips. England 2 62 -0.06 (Little, 1968, Little

6 144 0.06 & Vail, 1960) 12 121 0.09 19 81 -0.15

VALAJASKOSKI Moraine clay - - 8 6.5 7 343 0.52 Dissipation during Finland to 8 343 0.33 construction.

10.1 9 520 0.44 (Arhippainen, 1964) 10 520 0.42

SEITENOIKEA Moraine clay - - 8.8 10.2 4 275 0.37 Dissipation during Finland 5 471 0.20 construction.

6 471 0.29 (Arhippainen, 1964)

Dam Fill Material LL PL

Appendix B

Opt. w Place. w No. an

kN/m2 ru

Sheet B.2 ■11 0

Remarks

ROSSHAUPTEN Glacial Till 7 A 461 0.36 Dissipation during Germany B 461 0.44 construction. Clay fraction

C 461 0.38 12%. (Treiber, 1968) 392 0.51 589 0.65

TOOMA Residual soil 30.5 3 18 17 P3 344 0.19 Dissipation during Australia (Weathered to P5 388 0.21 construction.

Biotite Granite) 19 P8 275 0.19 (Pinkerton & McConnell, P10 275 0.21 1964,)

SEYMOUR FALLS Canada

Clayey silt 31 22 23 to

275 0.61 Dissipation during construction. (Ripley

28 & Campbell, 1964)

BRIDLE DRIFT Weathered 39 16 16 opt. 1 520 0.19 (Blight, 1970) South Africa shale to to 2 460 0.27

20 -1 3 540 0.17 4 530 0.16 8 370 0.07 10 370 0.07 11 165 0.15 18 245 -0.14 19 200 -0.24 20 265 -0.09

MANJIRENJI Sandy clay 52 34 15.8 • 14.9. 2 335 0.47 Soil C, clay fraction South Africa (Weathered to to 3 385 0.39 48%. (Blight, 1970)

Gneiss) 19.8 18.9 4 385 0.48 5 460 0.39

r4.

Dam Fill Material LL PL

Appendix B

Opt. w Place. w No. a n kN/m2

r u

Sheet B.3

Remarks

31 20 9 7 10 300 0.16 Soil X, clay fraction to to 11 335 0.12 220. 13.2 11.2 12 380 0.10

14 to 370 0.08 17 18 215 0.50 20 200 0.51

LESAPI Residual soil PI=19 17.5 13 2 750 0.30 Variable material, Rhodesia Granite/Dolerite 17 18.9 5 750 0.30 upper part of core less

- - 7 600 0.22 plastic than lower part. - - 8 600 0.17 (Mackellar et al, 1974)

15.5 15.7 12 450 0.13 15.8 13 13 450 0.13 15.6 14 16 830 0.15

IDAS VALLEY Decomposed shale 35 24 16 -0.5 7 270 -0.08 (Mackellar et al, 1974) South Africa to to to to 8 310 -0.16

45 27 20 -1.0 10 280 0.08 opt. 11 260 0.08

RAUBENHEIMER Residual clayey 20 14 11 0.5 3 550 -0.11 (Mackellar et al, 1974) South Africa shale to to to to 6 370 0.11

39 24 18 -2.5 7 380 -0.16 opt. 8 230 0.26

9 230 0.02 10 390 -0.03

Dam Fill Material LL PL %

Appendix B

Opt. w Place. w • % • %

No. an. kN/m2

ru

Sheet B.4 vi

Remarks

XONXA - 35 20 16 13.5 2 480 -0.02 (Mackellar et al, 1974) South Africa to 4 360 -0.06

15 5 380 -0.05

LLYN CELYN Wales

Glacial Till 10.8 11.2 28 750 0.55 Dissipation during construction. (Crann,1968)

SASUMA Residual soil 87 54 50y opt. f 300 0.04 Dissipation during Kenya (Halloysite) mean approx o 180 0.04 construction. (Dixon

c 330 0.05 et al, 1958) g 290 0.07

COW GREEN Glacial Till 30 13 10.5 +2 430 0.65 End of 1st season values. England to to to to 310 0.55 (Vaughan et al, 1975)

50 18 13.5 +4 240 0.42 opt. 90 0.64

500 0.74 450 0.67 340 0.60 220 0.59 120 0.66

CASTILETTO Glacial Till - - 7.5 7 390 0.45 Clay fraction, 8% Switzerland to to 690 0.27 (Schiltknecht & Bickel,

8.5 8 980 0.40 1957) 1270 0.50

SELSET Glacial Till 32 18 9 +1 47' 110* 0.45 Dissipation during England to to to to 515 0.20 construction. *End of

34 20 11 +5 54 70* 0.38 1st. season. (Bishop opt. 450 0.10 et al, 1960)

Dam Fill Material LL PL 0

Appendix B'

opt. w Place. w No. an

kN/m2 ru Remarks

Sheet B.5

71 250 0.55 P2 285 0.35

OTTER BROOK Sandy clay 28 17 11.3 14.3 3A 620 0.52 (Sherman & Clough, U.S.A. lA 655 0.32 1968)

MAD RIVER Sandy clay U.S.A.

20 14 10 11 2 705 0.26 (Sherman 1968)

& Clough,

NORTH HARTLAND Low plasticity U.S.A. clay

17 13 9.5 9.6 9 670 0.15 (Sherman 1968)

& Clough,

HILLS CREEK 53 22 14 15.6 1 1725 0.46 (Sherman & Clough, U.S.A. 3 965 0.46 1968)

BOUGH BEECH Weald Clay -1 15 530 0 England to 17 255 -0.35 core

+2 18 145 0.10

-2 20 315 0 to 30 120 -0.42 shoulder +2 8 245 -0.28 opt. 13 100 -0.10

PETERBOROUGH Oxford & 20 20 14 255 0.40 England Kellaways Clays to 20 175 0.34 core

22 26 95 0

14 15 370 -0.20 to 16 155 -0.34 shoulder 18 22 40 -0.75

Appendix B Sheet B.6 cy;

Dam Fill Material LL PL PL Opt. w Place. w ' %

No. an kN/m2

r Remarks

GRAFHAM Glacial Till 58 20 19.5 19.5 C2 450 -0.10 WATER to C4 360 0.42 core

England 21.5 C6 150 -0.32 C7 45 0

18.5 U2 310 -0.07 to U4 165 -0.09 shoulder 21.5 U6 100 0

U8 340 0.19

EMPINGHAM Upper Lias 62 26 22 25 C6B 440 0.47 England Clay C7B 355 0.47

C8B 295 0.45 C9B 235 0.06 B fill 51B 355 0.44 52B 225 -0.06 53B 195 -0.12

21 C14B 705 0.01 C15B 675 0.11 C16B 640 0.07 C fill C17B 590 0.05 C19B 490 -0.02 C20B 390 -0.12

DERWENT Glacial Till 35 16 11.9. 13.7 E21 475 0.42 Core and blanket England E23 590 0.51

42 20 12.7 14 E8 335 0.24 General fill E10 395 0.24 (Buchanan, 1970)

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