Question

An unconfined compression test was performed on a soil sample and the axial compressive stress at failure was 42 psi. a) What is the undrained shear strength of the soil; b) State if this test overestimates or underestimates the field shear strength of the soil. Why? c) Which type of soil is most suitable for this type of test.

Answer 1

In an Unconfined Compression test, Minor principal stress is zero. The mohr circle passes through origin and failure envelope is horizontal. The cohesion intercept is equal to the radius of the circle.

a)Therefore undrained shear strength of soil = s = (Axial stress at failure/2) = 42/2 = 21 psi

b) Merits of the test:

1. The test is convinient, simple and quick.

2. It is ideally suited for measuring the unconsolidated undrained shear strength of intact saturated clay. (Answer to question c)

3. The sensitivity of the soil may be easily determined by conducting the test on undisturbed sample and then on the remoulded sample

Demerits of the test:

1. The test can not be conducted on fissured clays

2. The test may be misleading for soils which the angle of shearing resistance is not zero. For such soils, the shear strength is not equal to half the compressive strength.(Answer to question b)

Thanks