Vibrocompaction Design in Blenheim – Ground Densification for Loose Alluvial Soils

Clean to slightly silty gravels and sands with fines content below 12% compact very efficiently. The river terraces near Renwick and parts of the Fairhall area show excellent response. Silty layers with more than 15% fines often require supplementary techniques like stone columns, as vibratory energy alone cannot generate adequate pore pressure dissipation in those materials.

It can significantly reduce it when properly designed. By increasing relative density above 70% in the critical upper 12–15 metres, vibrocompaction moves the soil into a state where cyclic pore pressure buildup is minimal. We verify this through post-treatment CPT data and re-run the liquefaction triggering analysis using the improved soil parameters. In high-risk zones near the Awatere Fault, we sometimes combine it with stone columns for added drainage redundancy.

For a standard 800 m² lot in Blenheim, the compaction phase usually takes two to three days with a single rig, including set-up and demobilisation. The full timeline, including trial testing, design, production, and verification CPT soundings, typically spans three to four weeks. Larger industrial sites of course extend this proportionally.

We rely primarily on CPT soundings because they provide a continuous profile of tip resistance and friction ratio, letting us spot any thin uncompacted lenses. At least one CPT is performed within each compaction cell, and we often supplement with a limited number of SPT tests to corroborate the CPT interpretation. Cross-hole seismic testing is occasionally used on critical structures to confirm shear wave velocity improvement.