Numerical simulation and field verification of inclined piezocone penetration test in cohesive soils
A large strain finite element analysis is performed to analyze the effect of soil anisotropy on the inclined piezocone penetration test in normally consolidated cohesive soils. The piezocone penetration is numerically simulated using the commercial finite element code ABAQUS. The saturated clay is modeled as a two-phase material and the effective stress principle is used to describe its behavior. A frictional contact interface utilizing Mohr-Coulomb's theory was chosen to represent interactions between the surface of the cone and the soil. The Anisotropic Modified Cam Clay Model (AMCCM) by Dafalias (1987) was chosen and implemented into ABAQUS through user subroutine UMAT. The piezocone penetration is numerically simulated by the three-dimensional finite element method using different inclination angles at different initial stress states. A field testing program of inclined cone penetration is also developed and performed in three different locations with varying soil characteristics in Louisiana, using the Continuous Intrusion Miniature Cone Penetration Test System (CIMCPT). The following conclusions are drawn from this study: 1) As compared to the previously conducted calibration chamber tests, the finite element analysis results based on Anisotropic Modified Cam Clay Model (AMCCM) are overall in good agreement with the actual measurements. This indicates that the soil anisotropy plays an important role during piezocone penetrations. 2) Initial stress state strongly affects the tip resistance, sleeve friction and generated excess pore pressures. Coefficient of lateral earth pressure K indicates the degree of initial stress anisotropy. If K=1, no difference is expected between inclined and vertical penetrations. However, for K‚1, the tip resistance, sleeve friction and generated excess pore pressures tend to increase (K<1) or decrease (K>1) when the orientation of penetration changes gradually from vertical to horizontal. Also, the soil classification derived from inclined penetration data may require special consideration. 3) The effect of anisotropic permeability on the tip resistance, sleeve friction and excess pore pressures during inclined penetrations is negligible for soils with very low hydraulic conductivities. However, it has significant effect on the dissipation of the excess pore pressure at the cone tip.
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