PFEM modeling of CPTu tests in saturated structured soils

Abstract

The conventional interpretation of CPTu tests is typically based on empirical and semi-empirical correlations based on very crude descriptions of soil behavior, such as the total stress approach coupled with the Terzaghi-Rendulic pseudo-3d consolidation theory for modeling the time evolution of excess pore water pressure. The aim of this work is to show that a more rational interpretation of the coupled deformation and flow processes occurring in the soil during a CPTu test is possible by resorting to the numerical solution of the relevant governing equations, incorporating a realistic constitutive model for the soil. In order to deal with the large displacements and deformations induced by the cone penetration, the Particle Finite Element Method code G-PFEM, recently developed for geomechanical applications, has been used for this purpose. A key feature of the present work is the use of a finite deformation version of a non-associative isotropic hardening plasticity model for structured geoma terials -the FD_Milan model. The model is equipped with a structure-related internal variable which provide a macroscopic description of the effects of structure in natural, fine-grained soils. In order to deal with strain local ization, typically observed in structured geomaterials upon yielding, the model has been equipped with a non-local version of the hardening laws, which has demonstrated capable of regularizing the pathological mesh dependence of classical FE solutions in the post-localization regime. A number of PFEM simulations of CPTu tests on a soft structured natural clay has been performed in order to assess the effects of the initial bond strength and permeabil ity on the predicted results of the test, as well as on the spatial distributions of accumulated plastic strains, internal variables and excess pore water pressures. The results obtained represent a promising step towards a more rational interpretation of the CPTu tests in structured geomaterials and for their use in the calibration of advanced soil models.