Behavior of Ring Footing on Two-Layered Soil due to Various Loading Positions

The primary focus of the present article is to evaluate the bearing capacity of ring footing placed over two-layered soils and subjected to vertical loadings applied over certain portions of the footing, namely, inner half, middle half, and outer half. The layered soils are chosen as (i) sand over sand and (ii) undrained clays overlying on another layer of undrained clays. The combined effect of layer strength, ring geometry, top layer thickness, soil-footing roughness, surcharge loading, and loading positions are extensively verified by using axisymmetric lower-bound theorem of limit analysis in conjunction with finite elements and nonlinear optimization. An interior-point method based on logarithmic barrier function is employed to carry out the nonlinear optimization. The sand and the clay layers are modeled with the smoothened Mohr-Coulomb and Tresca yield surfaces, respectively. The results are reported in terms of normalized bearing capacity and efficiency factor. The magnitude of normalized bearing capacity appears to be highly influenced by the position of applied loading, whereas the efficiency factor and the optimum upper layer thickness are found to be unaffected by the zone of loading positions. Failure patterns are drawn for several cases.