A 30-ft-diameter storage tank is supported on the ground surface at a site where a 15-ft-thick layer of dense sand overlies a very thick clay layer. The modulus of elasticity for the upper layer is...

A 30-ft-diameter storage tank is supported on the ground surface at a site where a 15-ft-thick layer of dense sand overlies a very thick clay layer. The modulus of elasticity for the upper layer is ten times the modulus for the lower layer. The tank imposes a pressure of 3 ksf.

(a) Determine the increase in vertical stress below the center of the tank and at a depth of 30 ft.

(b) Determine the stress increase at the 30-ft depth if subsurface conditions were homogeneous (assume Boussinesq), and compare the results with (a).