A Model Study Of Dynamically Loaded Square Footings On Dry Sand

The investigation reported herein was undertaken to develop an approach to modeling displacements of surface and shallow-buried footings on dry sand subject to high-intensity, single-pulse loads. A hypothetical shallow-bured structure with an isolated footing loaded by airblast overpressure produced by detonation of a nuclear weapon was assumed for design of load pulses on nine model footings used: footing widths of 4.5, 6.0, and 7.5 in. and depth-of-burial to footing width ratios of 0, 0.5, and 1.0. The principles of similitude were used to scale length, force, and time in the models. The models were placed in mobile test bins of uniform, fine, dry sand (90 percent relative density) and subjected to dynamic and static loading. Nondimensional load-displacement relations dependent only on depth-of-burial ratio were developed relating maximum displacement to peak dynamic load, footing width, and soil shear strength gradient. When the dynamic response of the footings in the form of reaction-displacement curves was compared with static response, an increase in initial stiffness and ultimate strength was observed for dynamic loading. However, these dynamic increases were greatest when the static shear strength was lowest, i.e. footings on the surface, and were least for footings buried at a depth equal to its width where the static overburden produced a substantial increase in shear strength. (Author).