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(b) A 30 m high plane strain slope inclined at 45° is shown in Figure Q4 (overleaf). The centre of the trial slip circle (O) is located 42 m above and 12.5 m to the right of the base of the slope. The cross-sectional area of the sliding mass is 929.4 m² and its centroid is located 16.7 m to the right of point O. Relevant soil properties are y = 18.5 kN/m² and c, = 120 kPa. (1) Calculate the factor of safety for this circle if there is no surcharge stress on the upper level, Le. q = 0. (Do not assume that the angle subtended at O is a right angle) [6] (1) Find the minimum value of the 20 m long surcharge stress q (kPa) that will cause failure of the slope along the same slip circle. [8] (ii) Assuming that there is a rigid stratum at a depth of 10m below the lower ground level in Figure Q4, re-solve part (1) using Taylor's stability chart (4) P.T.O. 3 12.5 m 20 m 42 m 30 m trial slip circle