title of paper 2/issue 6/ijeit1412201212_47.pdf3 84.55 74.69 0.79 0.140 25.20 issn: 2277-3754 iso...

ISSN: 2277-3754 ISO 9001:2008 Certified

International Journal of Engineering and Innovative Technology (IJEIT)

Volume 2, Issue 6, December 2012

258

Study Of Instrumented Segmental / Meter Panels

For Basements: Comparison of Theoretical and

Actual Behavior D.K. Kanhere

Joint Executive Director, STUP Consultants Private Limited, Navi Mumbai, India

Dr R A Hegde,

Professor, Sardar Patel college of Engineering, Andheri-Mumbai,

Dr. V.T. Ganpule

V. T. Ganpule & Associates, Mumbai, India

Abstract: Growing need of basements for vehicles storage and

services in city of Mumbai is problematic. Proximity buildings,

services, narrow lanes, traffic and crowds preclude open excava-

tions or diaphragm walls. Building subways under crowded

roads with utilities and closure is not possible, led to using

segmental / meter panel construction. These are panels of small

width designed as cantilever sheet piles adequately anchored in

sub-stratum to develop necessary passive resistance and balanc-

ing couple. When constructed in succession, full length of the

wall can be constructed. This is a fast, flexible and cheap engi-

neering solution to the problems encountered in crowded locali-

ties. A number of such structures have been constructed. Some

panels were instrumented for verification of in-situ behavior.

Points of interest concerning stress patterns and distribution

were monitored. A comparative study of the observed readings

of the instrumented meter panels and the results of theoretical

analysis and design principles are presented.

I. METHODS OF ANALYSIS OF RETAINING

STRUCTURE/SYSTEMS

A. Analytical Methods

Usually, following methods for analysis of cantilever pile

walls, all of which involve simplifications of net pressure

distributions to allow calculations of critical retaining height

are in use:

a. Factored Moment Method (FMM)

b. King’s Method (1995)

c. Day’s Method (1999)

d. Water Lum’s Rules of Thumb (2000)

B. Empirical Design Methods

a. Strut Load: An important aspect in excavation sup-

port design is to estimate the strut load. Most of the me-

thods are empirical and based on limit equilibrium of wedge

(Terzaghi, 1941) and the apparent earth pressure diagrams

(Terzaghi. 1943; Peck. 1969)

b. Base Stability: Another important aspect is the ef-

fect of base heave. (Bjerrum and Eide 1956) proposed the

use of stability numbers against base heave.

C. Field Measurements:

Ground movement is a major concern in the design of

excavation support systems. In built-up areas where the

ground contains a soft layer, thorough investigation is re-

quired. The following observations are made for analysis:

a. Lateral wall deflections

b. Ground surface settlements

c. Stress state of soil near excavation

D. Finite Element Analysis

It is generally accepted that the Finite Element Analysis

(FEM) is the major technique used in the numerical analysis

of geo-technical problems. FEM in excavation related prob-

lems was first carried out between 1969 and 1971. Some

fundamental techniques in excavation process simulation

were developed in 1978.

E. Design Considerations

For general design purpose it is recommended to use the

trial and error method for analyzing the system and its sta-

bility. Meter panels like circular touching piles or diaph-

ragm walls are structural elements, which are constructed

underground. These are basically used as retention systems

and permanent foundation walls. These can be installed in

close proximity to existing structures, with minimal loss of

support to existing foundation. In addition construction de-

watering is not required. Structural meter panel can be em-

ployed for forming underground retaining or load bearing

walls for practically any purpose. Panels of small width are

designed as cantilever sheet piles adequately anchored in

sub-stratum to develop necessary passive resistance and ba-

lancing couple. When constructed in succession, full length

of the wall can be constructed. The stability of excava-

tion/meter panel, control of wall deflections, ground move-

ments/ ground water problems have been analyzed with in-

strumentation as well as with FEM models as a case study.

The most important consideration in proper design and in-

stallation is that the retained material is attempting to move

forward and down-slope due to gravity. This creates soil

pressure behind the wall. The same is to be balanced by

passive earth pressure on other side. Further couple formed

by active and passive earth pressure has to be balanced by

trial & error method. The section of meter panel happens to

be depth of cantilever and is function of excavation depth

and soil properties of retained earth. The enabling structure

derived its support from the passive reaction offered by




259

formation below excavation level and the resisting counter-

balancing couple developed on going further deep.

The forces on enabling wall are;

1. The active earth pressure from back of wall which

tries to push wall away or towards excavated side.

2. The passive pressure generated below dredge line

in direction opposite active pressure which resists

the movement of the wall

3. The passive pressure in the direction of active

pressure which will counterbalance the moment

formed by active pressure.

4. These passive forces are calculated by using equi-

librium conditions namely summation of moment

and summation of horizontal forces are zero at all

points.

5. The passive pressure can be idealized to rectangu-

lar block in case of stiff clay or rock and as first

trial considers suitable pile length below excava-

tion level depending upon the formation

6. Using equilibrium conditions the pressure intensi-

ties are calculated and they are not equal or

slightly less than permissible. The length of em-

bedment below excavation is changed. The pres-

sure is repeated till above condition is satisfied.

7. The maximum bending moment is calculated

where shear force becomes zero.

Fig 2. Step-1 – Active Earth Pressure - Soil

Fig 3. Step-2 – Equilibrium of Horizontal

Forces

Fig 4. Step-3 – Equilibrium of Horizontal Forces &

Moments – Soft Soils

II. PRESENTED STUDY

The past concerns about the stability of excavation have

now shifted emphasis on control of wall deflections and

ground movements under working conditions especially in

densely built-up areas. The numerical methods with appro-

priate consideration of relevant factors have the potential to

give relatively accurate solution for both stability and

ground movements for an excavation. However, a perfect

model to predict the collapse of support system and evalua-

tion of ground movements at some distance behind the re-

taining wall under working conditions is still not available.

III. INSTRUMENTATION SETUP

In order to physically verify the developed pressure both

active and reactive, worked out from theoretical considera-

tion, the retaining elements are instrumented as described

below; In arranging instrumentation of meter panel, several

aspects must be considered. The use of instrumentation is

complex process. Right from selection of appropriate tool

for gathering the data as an even a small mistake leads to

failure of investigation program In and around the city of

Mumbai, a number of basements/ subways have been and

are being instrumented and constructed. Normally, bending

L m

P3

P2

P1

Excavation level

0 m

Fig 1. Earth Pressure Diagram (Theoretical)




260

strains in wall are measured along its depth giving bending

moment response. The evaluated geo-technical properties of

soil and observed bending strains along with theoretical

bending moment, shear force, deflection and earth pressure

are presented and comparison made between theoretical and

experimental values. The measurements have been taken us-

ing electrical resistance strain gauge. These adhesive type

strain gauges were mounted on the panels, had the follow-

ing specifications:

Type: BE 120 – 6AA (11)

Gauge resistance: 119.9 0.1

Gauge factor: 2.08 1.0%

Make: Tokyo Sokki Kenkyujo Company Limited

A. Strain Gauge Arrangement:

Connections are made using red, black, yellow and green

wires as indicated in Fig.1.

B. Strain Measuring Instrument

A strain measuring device in the SYSCON product range

was used. It accepts input from 10 numbers of strain gauge

bridges simultaneously and signals are selected one by one

through an electronic channel selector to a signal condition-

er which consists of a high gain, stable, low noise, low drift,

instrumentation type amplifier, GF setting control and

bridge configuration select facility. The conditioned outputs

provide 5V DC signal for an input of 20,000 micro strains.

This output is connected to a micro-controller based 41/2

digit panel meter that can display strains up to 20,000 micro

strains with one micro strain resolution. This voltage output

is also made available to the user on the rear panel for pur-

pose of monitoring or recording. (Strain indicator SI – 38

MS)

IV. FIELD SETUP

As indicated in Fig.2, the meter panel is instrumented be-

fore concreting and all lead wires are taken out at the top of

the pile.

V. CASE PRESENTED

1. Soil profile at Andheri site (Fig.3)

2. Observed strains at each strain gauge station

i) Location of site: New Link Road, Andheri

ii) Size of Meter panel: 1000mm x 600mm

iii) Total depth of Meter panel: 9.00m from original

ground level

iv) Total depth of excavation: 6.00m

v) Total number of strain gauge installed: 5 on tension

side and 5 on compression side

Table No 1 Observed Strains

S t r a i n

g a u g e f r o m G L . ( m ) Tension side Compression side

Init

ial

Str

ain

Rea

din

g

Fin

al S

trai

n

read

ing

To

tal

trai

n

(m

/m)

Init

ial

stra

in

read

ing

Fin

al s

trai

n

read

ing

To

tal

trai

n

(m

/m)

0 0 0 0 0 0 0

1 36 43 7 38 54 16

3 -71 20 91 -126 -214 88

5 215 732 517 -394 11 405

6 254 1153 899 -1225 -240 985

8 345 70 275 -210 42 252

9 0 0 0 0 0 0

Calculated bending moment from observed strain: See table 2

Lo

cati

on

of

stra

in g

aug

e

fro

m G

L

Are

a o

f st

eel

(mm

2)

Lev

er a

rm

(mm

)

Str

ess

in

(N/m

m2)

Ben

din

g m

o-

men

t in

KN

-m

0 7238.23 0 0 0

1 7238.23 378.26 1.40 3.83

3 7238.23 409.48 25.20 74.69

5 7238.23 407.14 118.00 347.74

6 7238.23 408.51 179.80 531.64

8 7238.23 416.61 62.40 188.17

9 7238.23 0 0 0

Calculation of Deflection, shear force and earth pressure:

Deflection is second integral of bending moment; shear

force the first derivative of bending moment and earth pres-

sure the second derivative of bending moment. Therefore,

equation of observed bending moment was worked out by

using curve-fitting techniques. Origin Lab 7.0 Software was

used. Lorentz curve best fitted the observed bending mo-

ment. By solving the differential equations, observed def-

lections, shear force and earth pressure on meter panel were

calculated (See Fig.4).

VI. COMPARISON OF THEORETICAL (AS

MENTIONED IN 1.6 DESIGN CONSIDERATION)

AND OBSERVED BEHAVIOR

Tabulated results are given in Tables-3a & 3b, and for

graphical representation refer Fig.11, 12, 13 & 14. Table-3a

Lo

cati

on

of

stra

in

gau

ge

fro

m t

op

of

the

Pil

e in

m

Th

eore

tica

l E

arth

Pre

ssu

re k

N/m

Ob

serv

ed E

arth

Pre

ssu

re k

N/m

Th

eore

tica

l S

hea

r

forc

ekN

Ob

serv

ed S

hea

r

for

kN

0 6.7 4.85 0 10.5

1 13.9 9.25 10.70 17.273

3 64.0 45.24 87.09 61.199




261

5 33.85 63.7 186.21 251.06

6 -422.6 -532.74 227.38 -0.1598

8 442.1 105.64 -442.14 -133.33

9 0 2.3 0 -16.172

Table-3b

Lo

cati

on

of

stra

in

gau

ge

fro

m t

op

of

the

Pil

e in

m

Th

eore

tica

l b

end

ing

mo

men

t k

N-m

Ob

serv

ed b

end

ing

mo

men

t k

N-m

Th

eore

tica

l D

efle

ctio

n

mm

Ob

serv

ed D

efle

ctio

n

mm

Ten

sile

Str

ess

in S

teel

0 0 0 12.44 3.530 0

1 4.52 3.83 5.33 1.520 1.40

3 84.55 74.69 0.79 0.140 25.20

5 380.22 347.74 0.35 0.010 118.00

6 585.38 531.64 0 0 179.80

8 221.07 188.17 0 0 62.40

9 0 0 0 0 0

VII. COMPARISON OF THEORETICAL (FEM

MODEL) AND OBSERVED BEHAVIOR

Soil profile with properties is given below;

Table-4

Depth

(m.) C

EP

(kN/m2)

WP

(kN/m2)

Total

(kN/m2)

0 0 30 8 6.66667 0 6.66667

0.5 20 30 2 9.66667 0 9.66667

2 20 26 2 -24.3333 15 -9.3333

4 40 26 2 -12.2772 35 22.7228

6 100 30 4 -42.9061 55 12.0939

7.25 100 30 4 -162.5 55 -107.5

9 100 30 14 312.5 55 367.5

The water table is located at 2m below GL.

Results are graphical represented in Fig.15, 16, 17 & 18.

VIII. CONCLUSION

On the basis of field measurements, the observed flexural

behavior of cast-in-situ reinforced concrete cantilever seg-

mental/ meter panels leads to the following conclusions:

Theoretical and observed profiles of earth pressure, bending

moments, shear force and deflection in the structure due to

excavation are matching well. Nature of variation in theo-

retical and observed values of force functions along the

depth of the panel wall matches well with slight differences

in magnitudes. Differences in theoretical and observed val-

ues are due to variations in soil properties, inconsistent la-

boratory results of soil samples, changes in groundwater ta-

ble, etc. Magnitudes of observed values through

instrumentation are less than the theoretical values. The dif-

ference occurred because the theoretical magnitude of later-

al earth pressures and consequently theoretical bending

moment considered c value much less than actual. Earth

pressure diagram clearly shows that the actual passive earth

pressure developed is greater than the assumed theoretical

passive earth pressure in rock strata. This reflects a high

factor of safety taken for design of panels and an underesti-

mated c value of rock.

IX. SCOPE FOR FUTURE ENHANCEMENT

There is a need to extend the work further so as to collect

more information for inferring the design parameters from

the field and laboratory test results of cohesion and angle of

internal friction. The selection of design parameters from

soil properties at present is very much empirical and based

on subjective interpretation of the designer as per his expe-

rience. Some sort of relation needs to be established be-

tween the design parameters and properties of soil as re-

vealed by investigation to serve as a guide for designers.

The realistic evaluation of earth pressure, either active or

passive, is possible only by collecting the behavior data of

the instrumented piles / meter panels / diaphragm walls as

retaining elements, particularly since the laboratory results

conducted on soil samples is still a grey area. The authors

are making efforts in that direction.

Fig 5. Strain Readout Unit

Fig 6. Instrumented Plate




262

Fig 7. Strain Gauge Arrangement

Fig 8. Strain Gauge Locations

Fig 9. Soil Profile

Fig 10. Best Fitted Curve

Fig 11. BM Comparison




263

Fig 12. SF Comparison

.

Fig 13. Deflection Comparison

Fig 14. Observed Earth Pressure

`

Fig 15. SF Comparison (FEM with Instrumentation)

Fig 16. BM Comparison (FEM with Instrumentation)




264

Fig 17. Deflection Comparison (FEM with Instrumentation)

Fig 18. Theoretical (FEM) BM, SF and deflection.

REFERENCES [1] Charles Ng. W.W., Lings. H.L., Nash. F.T., back analyzing

the bending moment in a concrete-diaphragm wall, The

Structural Engineer/Volume 70/ No.23/24/8 December 1992.

[2] Yoo Chungsik, Behavior of braced and anchored walls in

soils overlying rock Journal of geotechnical and geoenviron-

mental engineering / March 2001.

[3] Charles. W.W., Douglas. B.R., Sean W.L., Lei.G.H. Field

studies of well instrumented barrette in Hong Kong, journal

of Geotechnical Engineering and Environmental Engineer-

ing, January 2000, 60-72.

[4] Symons I.F. and Carder D.R. Field measurements on embed-

ded retaining wall, geotechnique 42, No.1, 117-126.

[5] Kanhere D. K., Dr. Hegde R A, and Dr. Ganpule V T; Seg-

mental / Meter Piles for Basements and Subways; Proceed-

ings of Indian Geotechnical Conference December 13-15,

2012, Delhi.

[6] Kanhere D. K., and Dr. Ganpule V T; Behavior of ‘Segmen-

tal / Meter Panels’ for Basements and Subways; Published

and presented in FIB International Conference, Amsterdam,

2008.

title of paper 2/issue 6/ijeit1412201212_47.pdf3 84.55 74.69 0.79 0.140 25.20 issn: 2277-3754 iso...

Documents