Objective Question & Answer

Sandy soil is essentially a cohesionless soil, so:  c=0

According to the Mohr-Coulomb failure criterion:

The angle of obliquity (q) of the resultant stress with the normal is given by:

At failure: q = f

Thus, failure occurs on the plane on which the angle of obliquity reaches its maximum limiting

value, i.e. the angle of internal friction f.