Numerical study of the effects of strain rate on the behaviour of dynamically penetrating anchors in clay

The installation of torpedo anchors at high impact velocities imposes high strain rates in the surrounding soil. The high strain rates enhance the mobilised undrained shear strength compared to that measured statically by laboratory or in situ tests. To illustrate the implications of such high strain rates for the behaviour of dynamic anchors, large deformation Finite Element (FE) analyses were carried out. The numerical FE scheme is based on a dynamic coupled effective stress framework with the Modified Cam Clay constitutive model. The soil constitutive model is adapted to incorporate the dependence of clay behaviour on strain rata. In order to assess the validity of the numerical scheme, some laboratory tests on model free falling penetrometers have been simulated. The results indicate that overall the agreement between computations and measurements is good. It is seen that the generation of excess pore pressure around dynamically installed anchors and the frictional resistance at the soil-anchor interface are significantly affected by the strain rate. Moreover, increased strain rate dependency of the soil leads to a marked reduction in the embedment depth, reflecting a noticeable increase in the soil penetration resistance.