Proposed Modification to CPT-Based Liquefaction Method for Post-Vibratory Ground Improvement

Abstract

The NCEER 1997 CPT-based liquefaction analysis uses the CPT Soil Behavior Type Index, Ic, to infer grain characteristics, such as fines content and plasticity of fines. However, after vibratory ground improvement, the in-situ horizontal effective stresses are typically increased (i.e. higher Ko) and are no longer linked to vertical effective stress in the same manner as the case histories. This change in Ko has an influence on the CPT results and can result in a reduction of the measured Ic value, and a corresponding decrease of apparent fines content. However, it is impossible for the vibratory compaction process to produce a decrease in fines content The authors have performed extensive CPT, SPT, and soil sampling during recent vibroreplacement (stone column) projects in southern California. The Ic values and fine contents of the soil were compared before and after ground improvement. The authors propose a correction method in order to compensate for the shift in Ic and to maintain the same fines content in the pre- and the post-treatment CPT based liquefaction analyses.