Utilizing Direct-Push Crosshole Seismic Testing to Verify the Effectiveness of Shallow Ground Improvements: A Case Study Involving Low-Mobility Grout Columns in Christchurch, New Zealand

A full-scale field testing program was conducted in Christchurch, New Zealand, to assess the effectiveness of soil stiffening and/or densification caused by the installation of shallow, low-mobility grout (LMG) columns. The LMG columns were designed to be 4 m long and were installed at three different test areas for the purpose of liquefaction mitigation in loose silty sands. Direct-push crosshole (DPCH) seismic testing was performed on unimproved ground and on ground that was improved with the LMG columns. The purpose of the DPCH testing was to verify the integrity of the LMG columns and to identify any changes in soil stiffness created by their installation. The crosshole testing involved sampling the unimproved and improved ground conditions with horizontally propagating shear waves at depths ranging from 0.4 to 5.0 m in 0.2-m increments. The seismic verification method clearly delineated zones of change and LMG column variability. The verification results were confirmed by post-characterization trenching.