Question

A geotechnical engineer is evaluating the stability of a silty sand slope as shown (next page).

This evaluation is considering the potential for a shear failure at an infinitesimal point (soil
element) on the trial shear surface.

At this particular point, the soil is found to be subjected to a major principal stress of +450
kN/m2 and a minor principal stress of +50 kN/m2.

(A) Using the Pole Method, find the intensity of the normal and shear stresses acting on the shear plane making a 50 degree angle with the major principal plane of the infinitesimal element of soil. (Plot the Mohr circle to exact scale.)

(B) If the silty sand is found to have a cohesion of 240 kPa and a friction angle of 24 degrees, both assessed from Direct Shear (DS) testing, determine the factor of safety (FOS) against potential landsliding at this particular point on the trial shear surface.

Answer 1