Bearing capacity of ring footings placed on dense sand underlain by a loose sand layer

Abstract

The ultimate bearing capacity of a ring footing on loose sand deposit, with an inclusion of a dense sand layer on its top, has been determined numerically in a bound form by following the finite element limit analysis (FELA). The friction angles ϕ1 and ϕ2 were varied from 40° to 46° and 30° to 36° for the top dense sand and the bottom loose sand respectively. The Mohr-Coulomb’s yield criterion has been assumed to be applicable. The bearing capacity has been found to increase quite extensively with an increase in the thickness of the dense sand layer thickness up to a certain optimal thickness before attaining a constant value. The magnitude of has been found to lie generally between 1.5 and 4.7, and the corresponding maximum value of the bearing capacity with an inclusion of the dense sand layer varies in a wide range of 2.32–82.45 times the corresponding value for an unimproved loose sand deposit; the parameters and refer to the inner and outer radii of the ring, respectively. Similar to a ring footing on homogeneous sand deposit, the bearing capacity for different chosen combinations of internal friction angles of the two sand layers, tends to become maximum generally for a value of closer to 0.25. For the validation, the results for a ring footing on two-layered sand deposit were also determined by using the displacements based elastoplastic finite element (EP-FE) method. The results from the FELA were found to compare well with (i) the obtained solutions based on the EP-FE method, and (ii) the published results by using experiments and the method of stress characteristics.