Cone penetration in a thin medium dense sand layer sandwiched by different clay layers – LDFE analysis

The cone penetration test (CPT) has been the most common site investigation practice for decades. This paper presents the results of large deformation finite element (LDFE) analysis that modeled a CPT into soil comprising a thin medium dense sand layer sandwiched between two clay layers of different undrained shear strengths. An extended critical state Mohr-Coulomb model is utilized to capture the stress-dependent drained behavior of sand, while Tresca failure criterion is adopted for the undrained behavior of clay. Because the sand layer and the surrounding clay layers work as a system that defines the mobilized soil stiffness and soil interface deformation, two soil layer profiles are analyzed to study cone penetration resistance responses, including soft clay on top and stiff clay at bottom, and vice versa. It is found that a peak resistance is registered in the thin sand layer, through the top and bottom clay layers show impact to the peak as well. The impact on the peak resistance from the clay layer ahead (i.e. bottom layer) weights more than the clay layer behind (i.e. top layer). The evolving soil flow mechanisms are revealed to explain the findings.