A five-storey apartment block near the Te Aro basin. The borelog showed 9 metres of compressible silts and soft clays before hitting weathered greywacke. Standard footings would have settled unevenly. We designed a grid of stone columns to transfer the load past the weak layer. The NZS 3404 framework guided the steel requirements, but the ground improvement logic came straight from the NZGS module on vibro-replacement. In Wellington's harbour-fringe geology, this scenario repeats itself on almost every reclamation site. Our lab runs the triaxial and consolidation tests on the native soil before any column spacing is decided. Data first. Design second. That sequence avoids over-design and keeps the contractor's stone volume within budget.
A stone column is only as good as the soil it replaces. We test both before designing the grid.
Common questions
What depth of soft soil can stone columns treat in Wellington?
We design stone columns for depths between 4 and 18 metres. Beyond 18 metres, the installation cost increases sharply and we evaluate alternatives like driven piles. The weathered greywacke bedrock in Wellington often limits the treatable depth naturally.
How much does stone column design cost for a residential lot?
For a typical residential block in Wellington, the design and laboratory testing package ranges from NZ$2,260 to NZ$8,800. The final figure depends on the number of boreholes, the lab tests required, and the complexity of the settlement analysis.
Can stone columns prevent liquefaction in a Wellington earthquake?
Yes, when designed correctly. Stone columns densify the surrounding soil and provide drainage paths that reduce pore pressure build-up during shaking. We run liquefaction triggering analyses using NZGS guidelines and the Wellington seismic hazard spectrum to verify the design.
What QA/QC tests do you specify after stone column installation?
We specify plate load tests on single columns and groups, SPT or CPT verification between columns, and post-treatment grain-size checks on the installed gravel. Every test is referenced to the lab-characterised baseline properties established during the design phase.
