A tank of diameter 5 m is to be located on a deep deposit of lightly over consolidated homogeneous clay, 25 m thick. The vertical stress imposed by the tank at the surface is 60 kPa. The soil parameters are λ = 0.26, k = 0.06, OCR = 1.8, and ∅'cs = 24°. The average water content is 42% and groundwater level is at 1 m below the ground surface. Assume the soil above the groundwater is saturated.
(a) Calculate the excess pore water pressure at depths of 2, 5, 10, and 20 m if the vertical stress were to be applied instantaneously.
(b) Estimate the undrained shear strength at a depth of 2.5 m.
(c) Estimate the surface stress to cause the soil at a depth of 2.5 m to yield.