Contractors have been excavating a 2 m deep trench along the toe of an existing 5 m cutting. The cutting is sloping at 25° from horizontal and was constructed in a heavily over consolidated clay soil over 100 years ago. The soil deposit was formed millions of years ago. Having excavated the trench, the soil from the adjacent slope has started to move into it. It appears to be moving along a planar failure surface (between points A and B on Figure Q2), which is located 1 m below the ground surface in the trench
(point A) and acts at 12 ° from horizontal. At the crest of the slope a vertical failure plane has formed (between points B and C in Figure Q2) and the soil within the moving mass is no longer in contact with the soil on the right hand side of this vertical failure plane (hence you do not have to consider the forces acting on this vertical plane)

The apparent effective cohesion (c’) of the soil along the failure plane is estimated to be 3 kPa, and the unit weight of the soil is 18 kN/m3,

Calculate the angle of friction of the soil acting on the failure plane. Assume there is no water table near the failure plane.

+ 1 2 3 m B 25 A 12