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 $(H)$ of the dense sand layer thickness up to a certain optimal thickness ${(H}_{\mathrm{opt}})$ before attaining a constant value. The magnitude of $H_{\mathrm{opt}}/(r_{\mathrm{o}}-r_{\mathrm{i}})$ 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 $r_{\mathrm{i}}$ and $r_{\mathrm{o}}$ 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 ${r_{\mathrm{i}}/r}_{\mathrm{o}}$ 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.

Keywords:

• Bearing capacity
• finite elements
• failure
• limit analysis
• ring footings
• layered sand

Disclosure statement

No potential conflict of interest was reported by the author(s).