Question

Explain, with reasons, whether you can use the following tests to simulate the behavior of sand in liquefaction

i) Unconsolidated Undrained Triaxial Test

ii) Direct Shear Test

iii) What is the most suitable type of triaxial test that should be used? (Does not have to be (i) or (ii))

Answer 1

i) To Simulate the behaviour of Sand in Liquefaction , Unconsolidated Undrained Test can be performed as the test is suitable for Soils of Low permeability/or when loading is very fast.

1. Drainage is not permitted during 1st and 2nd stage ( Sample is initially saturated )

2. when specimen is placed under triaxial cell , the initial pore water pressure is negative due to capillary tension . Total Stress being zero and effective stress positive. ( +)

3. After application of allround pressure,the effective stress in the specimen remains unchanged becaue under undrained condition,increase in allround pressure results in equal increase in pore water pressure.

4. In deviator stress also, effective stress doesnot change because again undrained conditions prevail.Due to Undrained Conditions,  

Total Stress analysis is performed as it is undrained Test.

Shear Strength =

ii) To Simulate the behaviour of Sand in Liquefaction , Direct Shear Test can be performed as the test is suitable for sands and Gravels where drainage conditions prevail.

Vertical Load is applied through Loading Plate.

Soil is sheared at constant rate of strain by applying horizontal force gradually.

This test does not allow the control and measurement of pore pressure and confining stresses that a triaxial test allows.

The load applied is an actual shear due to the offset positions of the upper and lower plates as opposed to axial loading of a sample in which shear stress result through coordinate transfer so the failure plane can occur at any position along the length of the axial sample and the angle of the failure plane will be oriented according to the combination of friction coefficient and cohesion.