Scale effects of shallow foundation bearing capacity on granular material
This project investigated the scale effects of shallow foundation bearing capacity on granular materials to evaluate the observed decreasing trend of the bearing capacity factor, Nγ, with increasing footing width, B, for a given sand at a given density. Model and prototype scale square and circular footing tests ranging in size from 25.4 mm to 914.4 mm, were performed on two compacted sands at three relative densities. Results of model scale footing tests indicate that the bearing capacity factor, Nγ , is dependent on the absolute width of the footing for both square and circular footings.
Direct shear box tests were performed in three different size shear boxes. Results of the direct shear tests indicate that there is a substantial scale effect; i.e., as the box size increases the friction angle, &phis;, decreases. The results of the direct shear box tests also show that the Mohr-Coulomb strength envelope is non-linear. The curvature shows higher strengths at low normal stresses and lower strengths at high normal stresses. High strengths at low confining stresses found in the direct shear box can help to explain the scale effect seen in small footings.
Both the footing tests and direct shear tests were modeled using a lower and higher order Finite Element model. New Nγ values versus friction angle were generated using Drucker-Prager (1952) plane strain conditions. The values match the results of the footing tests from this study extremely well, while all other previously published solutions overpredict Nγ at high friction angles. The higher order model utilized the parameter, internal length, which has been shown to be a function of panicle shape, degree of interlocking and binder stiffness. The internal length parameter accurately predicted the direct shear, model and prototype footing test results.
Although the scale effect of shallow foundation bearing capacity on granular materials still cannot be quantified, it can be predicted with the addition of the internal length parameter and the understanding that particle interlocking at low stresses significantly influences the results of the strength of a small scale footing and of a shear test.