Investigation of Viscoplastic Regularization for Simulating Dynamic Impact Soil-Structure Interaction Problems

Lateral impact of a soil-embedded pile mobilizes pressure-dependent yielding, large deformation, and post-peak degradation of the surrounding granular soil. In a rate-independent softening formulation, the governing equations lose hyperbolicity once the tangent stiffness becomes negative, and deformation may concentrate in a mesh-controlled zone. This paper examines whether the Duvaut–Lions-type rate update of a pressure-dependent damage formulation changes this behavior in full-scale pile–soil impact. Three-dimensional explicit models were analyzed in pairs that differed only in the rate parameters: a rate-independent elastoplastic-damage variant (Model EP) and its rate-dependent extension (Model EVP). Two tests were analyzed: a lower-energy (51 kJ), rotation-dominated impact used to back-analyze the soil parameters, and a higher-energy (252 kJ) impact with combined pile rotation and structural bending used to assess parameter transfer. Model EP developed deformation trapping after the initial transient, producing a localized pile bend that was not observed in the lower-energy test and excessive sustained resistance in the higher-energy test. In the lower-energy case, this occurred even though the cumulative work approached the measured value. With the rate update active, cumulative force–displacement work at the last common displacement was within about 5% of the measurements in both cases, the rotation-dominated mechanism was reproduced, and the plastic-hinge location in the higher-energy case was within 5 mm of the measured position. Cumulative work alone did not identify the deformation mechanism; force history, integrated resistance, and permanent deformation must be assessed together. Within the investigated formulation, discretizations, and configurations, the improvement reflects the combined action of physical rate dependence and the numerical effect of the rate-dependent stress update.