Undrained Solution for Spherical Cavity Expansion in Structured Clay and Its Application in CPT

Abstract

The cavity expansion theory is widely used in the analysis and prediction of cone resistance and lateral displacements in cone penetration test (CPT) and pile installation. Nowadays, the existing theoretical solutions for cavity expansion in structured clay cannot consider the influences of structure degradation on the mechanical behaviors of soil during the expanding process, which limits its applications in practical engineering to some extent. Therefore, taking the penetration of cone or pile tip as a spherical cavity expansion process in soil, based on the structured Cam-Clay (SCC) model and the large strain theory in plastic zone, the undrained spherical cavity expansion problem could be attributed to a boundary value problem of a system of ordinary differential equations about effective stress components. The equations then could be solved with stresses on the elastic-plastic boundary serving as boundary conditions. The results show that, with strengthening of soil structure, the plastic and critical state zones narrow, the internal cavity stresses increased, and the softening behavior and dilatancy of over-consolidated soil become more significant. For structured clay with the same initial stresses, the effective stress components adjacent to cavity wall overlap with that of the corresponding reconstituted soil, which indicates that the degradation of soil structure occurs during the soil-disruption accompanied expansion process. Finally, the internal cavity stresses derived from the proposed solution were used for theoretical calculation of cone resistance and pore pressure in CPT. A comparison of the existing solution with the results calculated indicates that the results obtained with consideration of soil structure are closer to the measured values.

Keywords

• structured clay
• spherical cavity expansion
• undrained
• elastic-plastic solution
• cone penetration test(cpt)