Model tests of the influence of ground water level on dynamic compaction

The effect of groundwater level on dynamic compaction is significant but remains poorly understood. Model tests of dynamic compaction on sand with different groundwater levels were conducted to investigate the effect of groundwater depth in dynamic compaction. The crater depth, the dynamic stresses, and the pore water pressures induced by dynamic compaction were recorded and analyzed. The soil movements caused by the impacts were monitored using a high-speed camera, and the corresponding shear strain fields were generated employing the digital photography technology. For cases where the groundwater level is below the surface, the crater depth, the dynamic stresses, the normalized peak porewater pressures, and the shear strain response all increase as the water level becomes lower. Dewatering, as expected, is beneficial for ground improvement. Besides, the shear strain distributions indicate that the unsaturated sand above the water table experiences compression-induced compaction, previously observed in dynamic compaction on dry sand. Whether a compaction zone and the associated shear bands can be formed or not depends on the thickness of unsaturated soils above groundwater. By comparison, the saturated sand immersed in water displays the liquefaction-induced compaction. The densification of saturated sand in dynamic compaction is less efficient than that in dry or unsaturated sand in terms of energy utilization.