Effect of footing roughness on bearing capacity factor Nγ

The effect of interface friction angle (delta) between the footing and underlying soil mass on the bearing capacity factor N gamma, was examined by using the upper bound limit analysis, finite elements, and linear programming. The analysis was carried out by employing velocity discontinuities along all the interfaces of the chosen triangular elements. The development of the plastic strains within elements was incorporated by using an associated flow rule. It was clearly noted that an increase in delta leads to a continuous increase in N gamma. With delta=Phi, the magnitude of N gamma becomes almost the same as that for a perfectly rough foundation, that is, when no slippage takes place between the footing and underlying soil mass. The size of the plastic zones increases with increase in delta and Phi. The obtained values of N gamma, for perfectly smooth and perfectly rough footings, compare quite favorably with those reported in literature. The study demonstrates that in the case when delta is smaller than Phi, the assumption of a perfectly rough footing will lead to an unsafe prediction of the ultimate bearing capacity.