TRANSPORT and ROAD RESEARCH LABORATORY

Department of the Environment Department of Transport

TRRL LABORATORY REPORT 901

THE STRENGTH OF CLAY SUBGRADES: ITS MEASUREMENT BY A PENETROMETER

by

W P M Black

Any views expressed in this Report are not necessarily those of the Department of the Environment or of the Department of Transport

Pavement Design Division Highways Department

Transport and Road Research Laboratory Crowthorne, Berkshire

1979 ISSN 0305-1293

Abstract

1.

2.

3.

4.

5.

.

7.

8.

C O N T E N T S

Introduction

The penetrometer

The relationship between CBR and undrained shear strength C u

The relationship between qc and CBR

Comparison of measured CBR values and values estimated by the penetrometer

5.1 Undisturbed overconsolidated soils

5.2 Remoulded soils

Conclusions

Acknowledgements

References

Page

1

1

2

3

4

5

5

6

6

7

7

© CROWN COPYRIGHT 1979 Extracts from the text may be reproduced, except for

commercial purposes, provMed the source is acknowledged

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on 1 st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

THE STRENGTH OF CLAY SUBGRADES: ITS MEASUREMENT BY A PENETROMETER

ABSTRACT

Improved methods of pavement design require that soil strength should be characterised in terms of fundamental parameters. Although the strength parameter CBR widely used in present methods of road design is empirical in origin, it is shown that it can be related to the undrained shear strength of cohesive soils by one of two linear relationships, one characterising soils in an undisturbed overconsolidated state and the other soils that are remoulded. After validating these relationships it is shown that a penetrometer with a small-diameter cone that is capable of measuring the shear strength of a soil can be used to estimate its CBR. The penetrometer can then be used to char- acterise soil strength in terms of either shear strength or CBR.

1. INTRODUCTION

The basic structural function of a road pavement is to reduce the stresses generated by traffic loading in the

soil over which the road passes to a level which the soil will accept without failure: at the same time, in

carrying out this function, the pavement itself should not be stressed sufficiently to bring about its failure.

A major input to all methods of design must therefore be a measure of the strength of thesoil, ie its ability

to resist the stresses imposed by traffic loading. The input is required for the design of new roads and for

the design of the total or partial reconstruction of damaged existing roads. Although not required in some

• methods of strengthening existing pavements by overlaying, knowledge of soil strength can often improve

the effectiveness of such methods.

Laboratory testing of remoulded soils under moisture conditions appropriate to those to be encountered

in service is normally adopted in designing new roads. This is dictated by the difficulty of carrying out in-situ

testing of the relevant soils under appropriate moisture conditions particularly in cut. On embankments yet

to be built, in-situ testing at the design stage is, in any case, an impossibility. When designing the reconstruction

or strengthening of existing roads it is obviously preferable to determine the strength of the subgrade, under

equilibrium with its long-term moisture regime, by in-situ testing.

The development of a structural approach to the design of new, strengthened and reconstructed pave-

ments, ie an approach based on the real stress-strain behaviour of pavements is required to provide the

flexibility necessary to accommodate changing technical and economic circumstances. The input characterising

soil strength of such a design method must therefore be in terms of fundamental parameters. A further

requirement of any design test is that it should be carried out with equipment that is either readily available

(or that is not unreasonably expensive to acquire) and that can be operated effectively by testing personnel

of normal quality. In order to minimise the considerable amount of testing inevitable on the varying soils

normally encountered on a road contract, the test should also have a good experimental reproducibility.

The strength of subgrades has been most commonly assessed by measuring the Californian Bearing

Ratio (CBR) of the soil. This arbitrarily conceived test has now been carried out both on remoulded soil

in the laboratory and in-situ, and used in many countries in correlative studies with pavement thickness.

In the present design recommendations for new roads 1 the design input is a test on a remoulded laboratory

sample and the great majority of experience is in terms of this type of test; in-situ values are however also

sometimes used, particularly in reconstruction problems. However the coefficient of variation of a large

number of CBR tests can be as much as 17 per cent and because of its indifferent reproducibility and the

size of sample required for a single laboratory test (5 kg), testing is time-consuming and the handling of

materials becomes a problem on a large job.

For these reasons it is not an ideal test for assessing the strength of soil under experimental pavements;

the removal of enough material to derive a good mean CBR value could be achieved only at the expense of a

great deal of excavation and damage to an existing road pavement. Equally the use of an in-situ CBR test

would require too many large holes to get enough useful information.

An alternative approach to direct CBR testing is to infer the CBR value from in-situ penetrometer

tests using a penetrometer of small diameter. The advantages of using such a penetrometer is that large

numbers of measurements can be made quickly. When testing an existing pavement a number of measure-

ments can be carried out through a conventional core hole without excessive damage to the pavement; •

also the strength variation with depth can be easily measured. The disadvantages at present are that the

commonly used penetrometer, described later, is empirically calibrated and has an undefined factor of

safety, ie the equipment reads strength values that are less than the real ones. Its calibration makes no

allowance for differences in the behaviour of remoulded and undisturbed soils.

This Report investigates the characteristics of a static penetrometer in use at the Laboratory; its

ability to measure undrained shear strength (the usual role of penetrometers in soil mechanics) is assessed

and related to the stress developed on the penetrometer cone, to the shear strength and to the CBR of the soil.

2. THE PENETROMETER

Historically the particular static penetrometer used at the Laboratory over a number of years was the Soil

Assessment Cone Penetrometer developed to measure the ability of soil to carry military vehicles. Itwas

comprised of a 30 °-included.angle cone 12.5 mm diameter at its base on the end of a 600 mm long, 9.5 mm

diameter steel shaft, and the CBR value was indicated directly by a pointer operated by a calibrated com- pression spring.

The range of the instrument was 0 - 1 5 per cent CBR, the rate of penetration being 25 mm/sec. The

force necessary to record 15 per cent CBR was 667 N for a spring compression of 25.4 mm. The instrument

has a non-linear CBR scale derived empirically with an unknown factor of safety. Figure 1 compares the

scale with the original linear scale used before production models were available.

From the spring calibration and the original linear CBR scale, the relation between CBR reading and the pressure on the penetrometer cone qc (in kPa), is

% C B R = _ _ . . . . . . . . . . . . . . . . . . . . . . . . . (1)

345

2

The manufacturers state that the instrument was for use in natural soils. Because the penetrometer

is normally used at or near ground level where the action of vegetation will always ensure that the soil is

overconsolidated to some extent, it can be assumed that the calibration provided applies to overconsolidated

soils.

At TRRL the penetrometer now used has been standardised to have the same cone and shaft dimensions

as the Soil Assessment Cone Penetrometer (SACP) so that Equation 1 still applies. It has some advantages

over the SACP. It is mounted on a weighted tripod so that considerably more dead load is available; it can

thus more easily displace small stones to allow its use in stonier soils. The rate of penetration can easily be

controlled when it is passing through soil layers of varying stiffness because the penetrometer is forced into

the soil via a rack-and-pinion drive and load measurement is by a stiff proving ring. The instrument was

designed by the T R R L Experimental Equipment Engineering Services. An outline drawing of the penetro-

meter is given in Figure 2.

Because most penetrometers relate the stress on the penetrometer qc to undrained shear strength Cu,

it is necessary to know the relationship between CBR of a soil and its undrained shear strength. These

relationships have been investigated both theoretically and experimentally as described below.

3. THE R E L A T I O N S H I P BETWEEN CBR A N D U N D R A I N E D S H E A R S T R E N G T H C u

The CBR of a soil is the ratio expressed as a percentage, at any value of penetration, of the force On the CBR

plunger to a standard force. The standard force 2 at a penetration of 2.5 mm is 13.24 kN, which on an area

of 1935 mm 2 produces a standard stress of 6842 kPa.

It has been shown 3 that in the CBR test carried out on remoulded soil the stress on the CBR plunger

at the standard penetration of 2.5 mm is approximately half o f the stress on the plunger at ultimate bearing

capacity (qu); hence

0.5q u CBR = _ _ x 100

6842 . . . . . . . . . . . . . . . . . . . . . . . (2)

Comparing this equation with the well known relationship between ultimate bearing capacity and undrained

shear strength

gives

qu = 6 C u

C u CBR = _ _

23

. . . . . . . . . . . . . . . . . . . . . . . (3)

. . . . . . . . . . . . . . . . . . . . . . . (4)

Many undisturbed overconsolidated soils have a much smaller strain to failure than remoulded soils,

the average strain to failure of undisturbed soils being only a quarter o f that o f remoulded soils according

to Skempton 4. The work of Skempton and an analysis of in-situ CBR tests carried out at the Laboratory

indicates that in undisturbed overconsolidated soils the stress beneath the CBR plunger at a standard

penetration of 2.5 mm is normally greater than 75 per cent o f the stress at ultimate bearing capacity and

that before a penetration of 5 mm is reached, ultimate bearing capacity has been attained. This being so

it will be assumed that the CBR test measures the stress at ultimate bearing capacity of undisturbed over-

3

consolidated soils, ie

C u CBR = _ _ . . . . . . . . . . . . . . . . . . . . . . . . .

11.5

Equations 4 and 5 can be compared with results from field measurements given in Table 1.

TABLE 1

The experimental relationship between CBR and C u

(5)

Source

Golder, Ref 5 Davis, Ref 6 TRRL unpublished TRRL unpublished

No. of tests

not known 8

15 15

State of soil

undisturbed undisturbed undisturbed undisturbed

C u

CBR

9.7 1 1 . 0

10.6 8.6

Scala, Ref 7 6 remoulded 27.6 Wiseman, Ref 8 150 remoulded 24.5

Considering all the variables associated with in-situ testing and the scatter of most of the measured

results, the derived correlation given in Equations 4 and 5 are considered to be validated by the results shown.

4. THE RELATIONSHIP BETWEEN qc AND CBR

Since the relationship between C u and CBR is established, the penetrometer relationship between CBR and

qc' as given in Equation 1, can be compared with other published data on penetrometers and with data derived

from tests and theoretical analysis on deep circular footings; these are given in Table 2. No differentiation

is made between a circular plain footing and a cone because the differences in behaviour are considered to be

small.

TABLE 2

The relationship between qc and C u derived from various sources

Source

Gibson, Ref 9

Meyerhof, Ref 10

Sanglerat, Ref 11

Sanglerat, Ref 11

Marsland, Ref 12

State of soil

effectively remoulded

remoulded

normally consolidated soil

overconsolidated

overconsolidated

Remarks

deep circular footings, theoretical analysis

deep circular footings, theoretical analysis, and experimental confirmation

penetrometers, analysis of world literature, experimental and theoretical

penetrometers, analysis of world literature, experimental

penetrometer, experimental study of London claY

%

C u

7.6-9.4

9.3

10

2 2 - 2 6

25 - 2 9

4

The behaviour of the top half-metre of soil immediately below formation is generally all-important

in determining road performance. The effect of overburden on the penetrometer values over this depth is

small and in Table 2 its effect has therefore been eliminated.

Because Equation 1 was derived for undisturbed overconsolidated soils, it is appropriate to combine

it with Equation 5 to give the relationship:

qc 345

Cu 11.5 - 3 0 . . . . . . . . . . . . . . . . . . . . . . . (6)

This is close to the upper bound of the relationship given by Marsland in Table 2 and very significantly

different from the relationship for remoulded soil. The results give reasonable credibility to the calibration

for the small penetrometer given in Equation 1.

Because the calibration for undisturbed overconsolidated soil of the TRRL penetrometer appears to

be confirmed by other work with penetrometers, it would be desirable to use published penetrometer data

to produce the likely calibration for the TRRL penetrometer for remoulded soils. The ratio of qc to C u

for overconsolidated soils was at the upper limit of published data (see Table 2), it therefore seems reasonable

to use the upper limit of the ratios quoted in Table 2 for remoulded soils; the relationship chosen was

%

C u = 1 0 . . . . . . . . . . . . . . . . . . . . . . . (7)

Introducing Equation 7 into Equation 4 gives the calibration of the penetrometer for remoulded soils, namely

qc C B R = _ _ . . . . . . . . . . . . . . . . . . . . . . . (8)

230

This calibration will be checked against experimental data.

5. COMPARISON OF MEASURED CBR VALUES AND VALUES ESTIMATED BY THE PENETROMETER

Two soil conditions are examined. If it is suspected that the particular soil under investigation does not fall

into either category, some calibration testing would be required.

5.1 Undisturbed overconsolidated soils

Although this section is headed 'undisturbed overconsolidated soils' the materials investigated were

soils that have a low strain to failure and these are most commonly the undisturbed overconsolidated soils.

However many compacted fills which wet up to a low equilibrium suction beneath roads, the usual British

condition, also come within this category; they do so because they take on aspects of overconsolidated

soils, one of which is that the act of shearing causes a negative change in pore water pressure; this results

in a small strain at failure.

Over the years parallel strength measurements have been made of in-situ CBR and penetrometer values

derived from the Soil Assessment Cone Penetrometer using the original linear scale. Some of the results have

been published 13; other unpublished work has been made on motorways and full-scale experiments; all the

results are given in Figure 4(a). The dotted line is the original penetrometer calibration given in Equation 1

and is not drawn as the best fit through the points, although it seems to fit well.

Most of the results in Figure 4(a) rely on single measurements of CBR; they are therefore associated

with a coefficient of variation of up to 17 per cent. It is likely that quite a lot of the scatter can be attributed

to this fact. An exception is the results from the M6 motorway, shown as filled triangles; these were based on

the mean of five CBR measurements for each plotted point; they all fall close to the calibration line. In

Key's work 13 the penetrometer readings were averaged over a depth of soil that was greater than that charact-

erised by the in-situ CBR test and he attributed some of the scatter in his results to the effect of changes of

soil strength with depth. Some areas tested had a dried crust and some had been locally wetted by rain. The

open circles in Figure 4(a) are those that Key considered to most likely be in error because of the effects of

strength gradients. The filled circles represent more reliable measurements. Below a CBR value of 10, the

measurements are closely grouped around the line of equality. Even better agreement is achieved if the

mean strength only of each section is plotted as shown by the + signs. This confirms that much of the scatter

in Figure 4 is the result of using too few measurements of CBR to produce reliable average values.

Allowing for the obvious causes of scatter Equation 1 provides a good calibration for the penetrometer

and the estimated values of CBR are of acceptable accuracy.

5.2 Remoulded soils

The open and filled circles on Figure 4(b) are unpublished results obtained by Sparks at TRRL on

remoulded Brickearth and London Clay statically compacted into a CBR mould. The data shown by triangles

were obtained by Gnanayutham 14 using a larger cone size to test London Clay; an experimental correlation

was used to adjust his measurements. The dotted line through the points in Figure 4(b) is the penetrometer

calibration given by Equation 8. It is a good fit up to a CBR of about 9.

Further confirmation that Equation 8 was appropriate for remoulded soils was obtained from Scala's

results 7. These measurements of CBR values and qc were made on compacted fill subgrades beneath roads

or in-situ soil subgrades that had been heavily rolled. An analysis of the 80 CBR test results showed that the

stress on the CBR plunger was on average 51 per cent of the stress on the plunger at a penetration of

12.5 mm; ie they behaved as remoulded soils (see Section 3 above). There was considerable scatter in Scala's

results but his mean correlation was qc ; this is very close to Equation 8. CBR =

221

6. CONCLUSIONS

1. On average, CBR values estimated from measurements of penetration resistance compare well with direct measurements of CBR.

. For pavement-design purposes, where high precision is not required in strength measurements, two

calibrations cover normal circumstances. For normal road subgrades of overconsolidated soils at

equilibrium with shallow water tables, the linear penetrometer calibration is based on

% CBR =

345

where qc is the ultimate bearing capacity of the soil measured by the penetrometer measured in kPa.

For remoulded soils the corresponding relation is

% CBR = _ _

230

3. The above relations are compatible with the assumption that the undrained shear strength of a soil

(C u) and its CBR are linearly related but the relation depends on the soil history. For undisturbed

overconsolidated soils the relation is

C u CBR = _ _ where C u is in kPa

and for remoulded soils

C u CBR =

J.3eSr

4. It is recommended that penetrometers should be used with linear calibrations of soil strength.

7. ACKNOWLEDGEMENTS

The work described in this report was carried out in the Pavement Design Division (Division Head:

Mr N W Lister) of the Highways Department of TRRL.

8. REFERENCES

1. DEPARTMENT OF THE ENVIRONMENT, ROAD RESEARCH LABORATORY. A guide to the

structural design of pavements for new roads. Road Note 29, Third Edition. London, 1970

(H M Stationery Office).

. BRITISH STANDARDS INSTITUTION. Britfsh Standard BS 1377: 1975. Methods of test for

soils for civil engineering purposes. London, 1975 (British Standards Institution).

. BLACK, W and N W LISTER. The strength of clay subgrades: its prediction in relation to road

performance. Conference on Clay Fills. London, 1978 (Institution of Civil Engineers).

. SKEMFrON, A W. The bearing capacity of clays. Building Research Congress 1951. (Building

Research Station). pp 180-190.

. GOLDER, H O. The relationship of runway thickness and undercarriage design to the properties of

the subsoil. Airport papers 4, 5. Institution of Civil Engineers, London, 1946.

7

.

.

.

.

DAVIES, E H. An investigation of defects in roads on the Isle of Wight using three methods of

flexible pavement design. Department o f Scientific and Industrial Research, Road Research Laboratory Note No. RN/1089/EHD. January, 1949 (unpublished).

SCALA, A J. Simple methods of flexible pavement design using cone penetrometer. Proc. 2nd

Australia-New Zealand Conf. Soil. Mech and Found. Eng. Canterbury University, 1954, pp 73-84.

WISEMAN, G and J G ZEITLEN. A comparison between CBR and shear strength methods in the design

of flexible pavements. Proc 5th Int. Conf. Soils and Found. Eng, Paris 1961. Vol 2, pp 359-363.

GIBSON, R E. Discussion on the bearing capacity of screwed piles and Screwcrete cylinders. Journal Inst. Ovil Eng. 34, 1950.

10. MEYERHOF, G G. The ultimate bearing capacity of foundations. Gebtechnique 2, 1951, pp 301-332.

11.

12.

13.

14.

15.

SANGLERAT, G. The penetrometer and soil exploration. Amsterdam, London and New York, 1972.

(Elsevier Publishing CO).

MARSLAND, A. A comparison of the results from static perietrometer tests and large in-situ plant

tests in London Clay. Department o f the Environment Building Research Establishment Current Paper CP 87/74. Garston, 1974 (Building Research Establishment).

KEY, J W. An investigation to compare in-situ CBR values measured on a heavy clay soil with a

prototype cone penetrometer against those obtained from a standard apparatus. Department o f

Scientific and Industrial Research, Road Research Laboratory Note LN/682/JWK, 1964 (unpublished).

GNANAYUTHAM, I. A laboratory investigation of the relation between CBR/dry density/suction/

shear strength/elastic modulus for London Clay. MSc Thesis, University of Surrey.

BLACK, W. A method of estimating the California Bearing Ratio of cohesive soils from plasticity data. Gebtechnique, December 1962, pp 271-282.

8

1 . 0 - -

2.0

3 . 0 -

4.0,==

Current product ion model

5 . 0 - -

6.0 ̧ - -

7 . 0 - -

8.0 u

C. B. R. 0

9.0 ==

10.0 -

1110 - -

12.0 " -

13.0 - -

=-1

- - ~ 2

- ' 3

- - 4

- - , 5 ¸

- - 6 Models used up to 1964

- 7

- 8

- 9

- - 1 0

- 1 1

- - 1 2

- 1 3

14.0 14

C.I . 0

- - 2 0

- - 4 0

- - ' 60

-18o

-- 1 0 0

- - 120 Present cone index scale

C.B.R. =.C. I , - - 140 20

-- 1 6 0

- - 1 8 0

- - 200

-- 220

" ;240

- - 260

280

Fig. 1 COMPARISON OF NON-L INEAR SCALE WITH THE O R I G I N A L L INEAR SCALE OF THE CONE PENETROMETER

25mm dia rack 1300mm long

Depth scale

Stop watch

Operating handle

E E

Q LO

Dial gauge (100 divisions -- CBR 10)

Proving ring (7kN capacity)

Tripod with folding legs

9.5 mm dia. penetrometer shaft

Platform for dead load/operators can stand on platform)

L i 30 ° cone 12.5mm dia

i~ 800ram = I I

Fig. 2 DIAGRAiVI OF T.R.R.L. PENETROMETER

0

.C~ d " "

C~ ~ 0

E ~

0

0~

CD C~ 0~

C ) ¢D 0~

0 OO v--

0 ~D

0

0 0~ v--

C ) O

0 O0

¢D r.O

0

0 0~

0

(}.ua:) Jad) "El "8 ":D

"I -

+=A

>

E

l -

z l.l.i

I - -

..l-

a z

z LI.I UJ

W ,.n

Z 0 I-.

.=.I I.I.I

n "

0

0 I.I.I

0 0 n.- a .

i.i=

E

t -

O . > . t~

r r nt~

15

10

5

/

/ CBR= qc ~ ~ t _d ~/~ O

345 O

o Qoo/S . o O

• O

Oxford clay, Alconbury By-pass O Cut (doubtful results) Single CBR measurements • Cut (good results) Single CBR measurements A Cut. Mean values per test section 8 CBR

measurements • Fill. Single CBR measurements

• Fill. Weald clay, TRRL compaction bay

• cut and fili. Keuper I~ar[, M-6 M0torway

[] Cut. Gault Clay, Maidstone By-pass

I I I 5 10 15

CBR

a) UNDISTURBED OVER CONSOLIDATED SOILS

20

E S ¢.-

O .

n -

~ 3

10

5

S 0 0

O

O O CBR = qc O O

230 ~ O Z~ A

A A

A A

O Brickearth • London Clay

A London clay (results corrected for cone size)

I I 10 15

CBR

b) REMOULDED SOl LS STATICALLY COMPACTED IN CBR MOULD

i 20

Fig. 4 RELAT ION BETWEEN MEASURED CBR VALUES AND CBR VALUES FROM THE PENETROMETER

(1533) Dd0536361 1,500 8/79 HPLtdSo ' ton G1915 PRINTED IN ENGLAND

ABSTRACT

The strength of clay subgrades: its measurement by a penetrometer: W P M BLACK: Department of the Environment Department of Transport, T R R L Labora tory Repor t 901: Crowthorne, 1979 (Transport and Road Research Laboratory) . Improved methods o f pave- ment design require that soil strength should be characterised in terms o f fundamental para- meters. Although the strength parameter CBR widely used in present methods o f road design is empirical in origin, it is shown that it can be related to the undrained shear strength o f cohesive soils by one of two linear relationships, one characterising soils in an undis turbed overconsolidated state and the other soils that are remoulded. After validating these relation- ships it is shown that a penetrometer with a small-diameter cone that is capable of measuring the shear strength of a soil can be used to estimate its CBR. The pene t rometer can then be used to characterise soil strength in terms of either shear strength or CBR.

ISSN 0 3 0 5 - 1 2 9 3

ABSTRACT

The strength of clay subgrades: its measurement by a penetrometer: W P M BLACK: Department of the Environment Department of Transport, T R R L Labora tory Repor t 901 : Crowthorne, 1979 (Transport and Road Research Laboratory'). In,proved methods of pave- ment design require that soil strength should be characterised in terms o f fundamental para- meters. Although the strength parameter CBR widely used in present methods of road design is empirical in origin, it is shown that it can be related to the undrained shear strength o f cohesive soils by one of two linear relationships, one characterising soils in an undisturbed overconsolidated state and the other soils that are remoulded. After validating these relation- ships it is shown that a penetrometer with a small-diameter cone that is capable of measuring the shear strength of a soil can be used to estimate its CBR. The penet rometer can then be used to characterise soil strength in terms of either shear strength or CBR.

ISSN 0 3 0 5 - 1 2 9 3