Question

QUESTION 5
A consolidated drained triaxial test was conducted on a normally consolidated clay.
Following are the results of the test:
• Cell or Chamber pressure = 130 kN/m2
• Deviator stress at failure = 230 kN/m2
a) Determine the consolidated drained friction angle
b) What is the pore water pressure developed in the clay specimen at failure?
c) What is the angle θ that the failure plane makes with minor principal stress?
d) Determine the normal stress and shear stress on the failure plane

Answer 1

We know that, when a drained triaxial test (C-D) is conducted on a normally consolidated clay, it behave like sand, with no cohesion. Hence c = 0.

Given, cell pressure,   = 130 kN/m2

and, deviator stress, = 230 kN/ m2

a) We shall find the consolidated drained friction angle as follows:

Therefore, the consolidated drained friction angle is 27.99 degree.

b ). Since consolidated drained test is conducted here, the valves will be open in both confining and shear stages of triaxial test. Thus, the pore water pressure that get developed will dissipate out. Hence, the pore water pressure at the time of failure will be zero.

We can mathematically also show that the pore water pressure will be zero, as follows:

c ). The angle made by failure plane with the minor principal axis can be determined as follows:

Therefore, angle between failure plane and minor principal axis is 31.005 degree.

d ). Normal stress and shear stress on failure plane is determined as follows:

Thus, the normal stress and shear stress on failure plane are 191.046 kN/m2 and 101.56 kN/m2 respectively.