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CivilEngineeringResearchJan

uary2004

Instability of Sand under

Plane-Strain ConditionsJ. Chu (cjchu@ntu.edu.sg)

D. Wanatowski(darek@pmail.ntu.edu.sg)

Introduction

Singapore is a country with very limited land resources.

Offshore land reclamation has been carried out using mainly

hydraulically deposited granular till in the past to cater for

further economic expansion. A study of the engineering

properties of the granular fill , in particular its instabilitybehaviour, has been conducted.

Experimental studies on sand are usually carried out under

axisymmetric stress conditions using a triaxial cell. However,for most practical problems, soil is subjected to a plane-strain

or three-dimensional stress condition. Therefore, studies onbehaviour of sand should be conducted under plane-strain or

other general stress conditions. Furthermore, the effect of shear

band formation on the behaviour of sand can only be studied

under plane-strain or three-dimensional conditions.

Plane-strain apparatus

A plane-strain apparatus has been developed at NTU for

conducting more advanced soil testing. The design of the plane-

strain cell and the testing arrangement are shown in Figure 1.

In this apparatus, a 120 mm in height and 60 x 60 mm in cross

section prismatic specimen is tested. Both the vertical andhorizontal loading platens are enlarged and lubricated to reduce

the boundary constraints and to delay the occurrence of non-

homogeneous deformations. Local stress and strain

measurements are made. A reconstituted sand specimen isshown in Figure 2.

ResultsSome typical testing results obtained from plane-strain tests

on sand are shown in Figure 3. In all the tests, the specimens

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Figure 3. Typical plane-strain testing results:

(a) effective stress paths; (b) stress-strain curves

(b)

Figure 2. A specimen used in the plane-strain apparatus

were first K0consolidated to the same initial stress conditions

and then sheared along different stress or strain paths.

The results of a drained, undrained, and a strain path test

under dv/d

1= 0.5 are presented in Figures 3(a) and 3(b).

The influence of stress or strain paths on the stress-strain

behaviour of sand is clearly shown in Figure 3(b).

Shear bands were observed during plane-strain tests. The

typical mode of the shear bands observed at the end of a

plane-strain test is shown in Figure 4.

Another plane-strain test showing the instability behaviour of

sand is presented in Figure 5. In Figure 5b, the axial strain

shoots up at point A indicating that the specimen had become

unstable at point A. It can be seen from Figure 5a that the

stress state at point A is below the failure line. Therefore, it

is a type of prefailure instability.

Conclusions

A plane-strain apparatus has been developed at NTU to enablethe stress-strain behaviour of soil to be studied under more

(a)

Figure 1. The arrangement for plane-strain test

Figure 4. Typical mode of shear bands observed

in plane-strain tests

(a)

(b)

Figure 5. Instability behaviour observed under a plane-strain

condition: (a) effective stress path followed;

(b) development of axial strain with time.

generalised stress conditions. Some typical results obtainedfrom tests on a granular fill are presented. The results showthat stress or strain paths affect the stress-strain behaviour of

sand considerably. Instability in the form of a sudden increase

in strain rates can occur at a stress state below failure.

GEOTECHNICS