CPT Readings, VisCPT Observations, and Advanced FEM modeling of Penetration through Soil Interfaces

By virtue of its ability to take nearly continuous readings with depth, the CPT presumably provides exceptional stratigraphic delineation, and specifically, precise detection of interfaces. However, the tip and sleeve elements respond to resistance of soils from different elevations, and not just because the sleeve is physically located above the tip at a central distance of 10 cm. The tip resistance is influenced by soils ahead of the advancing probe while the sleeve responds to the soil immediately surrounding it. As such, there is a disjoint between the two readings often resulting in a pronounced friction ratio spike near the interface. The present paper confirms the presence of such spikes by: 1) Evaluating the theoretical tip and sleeve resistances as the cone approaches and passes an interface; 2) Performing numerical simulations of the problem using an advanced autoadaptive remeshing FEM program; 3) Comparing to actual field data, including piezocone readings; and 4) Verifying the interface locations by optical observation using the vision cone penetrometer, VisCPT.