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8/13/2019 Instability of Sand under Plane-Strain Conditions
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ISSN 0219-0370 JANUARY 2004 NO.
17
8/13/2019 Instability of Sand under Plane-Strain Conditions
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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.
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