Failure loads on layered soils and mechanisms of collapse

Bearing capacity of layered soils remains an important problem in soil mechanics. A kinematic approach was successfully applied to address the problem of bearing capacity of a single weak layer over a rigid base, and the bearing capacity of a two-layer foundation soil system. Useful design charts were produced. The limit load of a weak layer under embankment determined using the slip-line method was found to provide a lower-bound to the true limit load. Comparison with the available upper-bound solution shows that the difference between the upper-bound and the lower-bound solutions is small. This indicates that both are acceptable in engineering designs, though the lower-bound solution obtained here is preferred by engineers because of its conservatism. Experimental measurements of sand deformation under footings were obtained to investigate the deformation process and failure mechanism of foundation soils with and without reinforcement. A new technique--image motion detection--was developed to find displacements of deformed foundation soils under loading. Some interesting features of sand deformation were obtained from preliminary analysis of laboratory tests. This new technique has significant potential applications in future research on patterns of flow of granular materials.