An 8-m-diameter storage tank is located on the ground surface of an area where a 4-m thickness of desiccated (firm) fine-grained soil is underlain by a very thick layer of similar fine-grained soil...

An 8-m-diameter storage tank is located on the ground surface of an area where a 4-m thickness of desiccated (firm) fine-grained soil is underlain by a very thick layer of similar fine-grained soil that retains a high water content and, as a result, is relatively weak and compressible. The modulus of elasticity for the firm upper zone is 10 times the modulus for the underlying layer. The tank bearing pressure is 125 kPa.

(a) Determine the increase in vertical stress beneath the center of the tank at a depth of 6 m.

(b) Compare the result from (a) to the value of stress increase if homogeneous conditions (Westergaard) are assumed.