Finite element lower bound limit solutions for ultimate bearing capacity of subsoil under raft of composite piled foundations

When the composite piled foundation reaches the general ultimate bearing capacity status, an increment of the ultimate bearing capacity for natural foundation is induced by “barrier effect” of piles. The analysis of the problem can be simplified for both the sum of the vertical ultimate bearing capacity of foundation and the increment of the ultimate bearing capacity of foundation soil horizontally sliding around piles. Based on the finite element plastic lower bound limit method, the numerical solutions for the vertical ultimate bearing capacity of foundation and the horizontal resistance sliding around piles varying with soil depth are obtained, and the sliding resistance formula is also established. The sliding resistance coefficient ks reduces with the increase of pile spacing. The increment of the ultimate bearing capacity Δfu decreases with the increase of pile spacing. The results indicate that the resistance sliding around piles in the composite piled foundation is being and can not be ignored, and that the ultimate bearing capacity of the composite piled foundation has certain increment, which can be regarded as the security reserve of foundation.