Assessment of the Degree of Soil Stiffening From Stone Column Installation Using Direct Push Crosshole Testing

Abstract eng:
This paper outlines the results of a full-scale field testing program designed to assess the degree of soil stiffening resulting from the installation of shallow stone columns. Stone columns with similar area replacement ratios were installed in soils ranging from clean sands to sandy silts at a number of sites in Christchurch, New Zealand for liquefaction mitigation. Direct-push crosshole tests were performed before and after ground improvement using custom-built cone sensors to measure compression wave (V p ) and shear wave velocity (V s ) at 0.2 m depth intervals. Tests were performed across stone column to characterise the composite properties of the soil/column, and in-between stone columns to characterise the properties of the soil within the improved zone. Generally, the stone columns were effective at stiffening clean sand soil profiles, with the effectiveness of the technique shown to reduce in soils with higher fines content. Near the ground surface the stone columns were shown to be less effective at some locations, with the lack of confinement at these shallow depths preventing significant increases in stiffening, as indicated by V s . A significant lowering of V p was also identified at some sites after installation, indicating reductions to the degree of soil saturation. The change in the degree of saturation over time is a topic of interest for future study, as unsaturated soil conditions have the potential to increase liquefaction resistance.

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