A 30-m-long closed-end steel pipe pile group is driven into layered

undrained clay. The pile cap is square, and the nine piles are evenly

spaced. The layout of the pile group is shown in Figure 4.25. The topsoil

layer has a unit weight of 18 kN/m³, undrained cohesion of 100 kN∕m²,

and a thickness of 10 m. The second layer has unit weight of 19 kN∕m³,

undrained cohesion of 150 kN∕m², and it extends to great depth. The

groundwater table is at the ground surface. Determine the ultimate

bearing capacity and pile group efficiency of the pile group.