Undrained Limit Analyses for Combined Loading of Strip Footings on Clay

This paper applies numerical limit analyses to evaluate the undrained stability of surface footings on nonhomogeneous and layered clay deposits under the combined effects of vertical, horizontal, and moment (V, H, M) loading. The analyses are able to resolve the true collapse loads to within ±5% by computing rigorous upper and lower bound solutions for these plane strain problems using linear programming methods and finite element discretization. The results focus on the geometry of the three-dimensional failure envelope and illustrate the effects of underbase suction and non-homogeneous undrained strength profiles. The existing empirical bearing capacity factors for inclined, eccentric loading are conservative, often underestimating the exact collapse solutions for footings on homogeneous clay by more than 25%. However, the same correction factors can become completely unreliable when there is a significant undrained strength gradient. The numerically-derived failure envelope for footings on homogeneous clay is described approximately by curve fitting techniques using relatively simple functions that can be used to update the existing bearing capacity factors.