Stone Columns Ground Improvement - A Trial in Silty Sands-Paper

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The study evaluated stone-column ground improvement for liquefaction mitigation in Christchurch silty sands, using vibratory and non-vibratory installation methods. The site had a shallow water table and soft clayey and organic soils over loose silty sand and sand. Columns were installed to 4 m depth with different layouts and area replacement ratios (ARRs), then assessed using CPTs, seismic dilatometer tests, cross-hole shear-wave measurements, and plate load tests.<br /><br />Both methods produced some improvement in the granular layers, but changes in CPT resistance between columns were generally modest. The non-vibratory method showed clearer improvement in fine-grained soils as ARR increased, while increasing ARR had little additional effect in the sandy layers. CPTs driven through columns reached only 20 MPa, suggesting the columns were less dense than expected. Plate-load stiffnesses were also low for both methods, possibly because weak near-surface soils provided insufficient lateral confinement. Cross-hole tests found higher shear-wave velocity across columns than between them, but velocities remained below the 200 m/s threshold used in the study to indicate low liquefaction potential.<br /><br />Overall, neither installation method showed strong evidence of suppressing liquefaction. The authors suggest that the soft clayey surface layer may impede pore-pressure dissipation and that soil fines and limited confinement may reduce column effectiveness. They recommend further trials and broader assessment methods, including geophysical surveys across improved areas and plate-load tests on groups of columns rather than individual columns.

Keywords

stone-column ground improvement

liquefaction mitigation

Christchurch silty sands

vibratory installation

non-vibratory installation

area replacement ratio

cone penetration tests

shear-wave velocity

plate load tests

pore-pressure dissipation