Dependency of Nγ on footing diameter for circular footings

By incorporating a variation in soil internal friction angle ϕ with mean principal stress σm, the dependency of bearing capacity factor Nγ with footing diameter B has been evaluated for circular footings. The evaluation has been performed by a lower bound finite element limit analysis in combination with a linear optimization. Two ϕ–σm curves, available from literature, were used. The magnitude of Nγ has been computed for several footing diameters, covering almost the entire possible range of model and field footing sizes. Factor Nγ is seen to decrease significantly with an increase in B. For B approximately greater than (i) 0.7 m for smooth footings (δ=0) and (ii) 0.35 m for rough footings (δ=ϕ), Nγ varies almost linearly with B on a log–log scale. Once a relationship has been generated between Nγ and B, for a given design load, the required footing diameter can always be obtained.