Ground response to dynamic compaction of dry sand

Abstract

The mechanics of dynamic compaction are studied in this paper using two-dimensional finite element analyses with a large-strain dynamic formulation and a cap model for soil behaviour. Comparison with centrifuge model results shows that stress wave attenuation and improvement effects are realistically predicted. The analyses show that, in the initial blows, stress wave propagation induces transient elasto-plastic K0 compression due to lateral inertia. This preserves the plane wavefront and reduces the attenuation rate of the dynamic stresses with depth. With multiple blows, the effect changes to one of triaxial compression: this sets a limit on the degree of improvement that can be achieved in the near field. Deeper down, the wavefront adopts a bullet shape, and the attenuation rate rises: this sets a limit on the depth of improvement. Both these phenomena are consistent with the existence of a ‘threshold’ state that has been noted in previous literature on dynamic compaction. The results also show that the depth of improvement is dependent upon the momentum per blow, as well as on the energy per blow.