Numerical Study of the Effect of Clay Structure on Piezocone Penetration Test Results

Studies of the effect of clay structure on piezocone penetration test (CPTU) results are beneficial for interpreting CPTU data in naturally or artificially cemented structured clays. The structured Cam clay (SCC) model was implemented into Abaqus, and the validity of the user-defined material (UMAT) subroutine of SCC was verified by predicting the experimental data for the laboratory oedometer test and drained and undrained triaxial shear tests of structured clays. A numerical model of piezocone penetration into structured clay was established, and numerical simulations showed that, compared with those for unstructured clay, the values of the mean effective stress near the cone tip were smaller, and the values of the excess pore water pressure (u) were larger for structured clay due to the additional destruction-associated plastic volumetric strain. The maximum deviator stress did not appear near the cone tip but appeared at a certain distance below and to the right of the cone tip, which manifested the strain softening characteristic of structured clays. The cone resistance (qc) values increased linearly with increasing structural strength (Sσ). The larger the value of Sσwas, the higher the value of u, but Sσdid not influence the patterns in dissipation curves of u. The value of the empirical cone factor (Nkt) decreased with increasing Sσ, since the clay structure caused a smaller increase in qcthan in the undrained shear strength, and a formula for estimating the Nktof structured clays was proposed. Compared with those for overconsolidated clay, the values of mean effective stress and deviator stress around the cone tip were smaller for the structured clay due to structural damage occurring at large strain, which resulted in a smaller value of qc, and the “nonstandard” dissipation phenomenon was less pronounced for the structured clay due to the shear-induced relatively large positive u.