A three-storey commercial build on Eliot Street hit refusal at 12 metres. Beneath the fill, a layer of saturated volcanic ash was deforming under load. The structural engineer needed a solution that kept the slab on grade without piling. We proposed a grid of stone columns, designed after running a CPT test to map the soft layer thickness and a series of triaxial tests on the ash to calibrate the friction angle. The Ashburton-derived gravel we used compacted to a stiff column, transferring load to the Egmont-sourced lahar deposits below. In New Plymouth, where the topography shifts from coastal terrace to volcanic apron, stone columns bridge the gap between poor near-surface soils and competent deeper strata.
Stone columns in New Plymouth's volcanic ash achieve a replacement ratio of 15-20%, cutting settlement by half and providing a drainage path that shortens consolidation time by months.
Methodology and scope
Local considerations
NZS 3404 and the NZGS ground improvement guidelines frame our design checks. In New Plymouth, the biggest risk is differential settlement where stone columns terminate on a sloping lahar interface. The volcanic stratigraphy dips seaward, and if columns bear on a thin ash layer above a hard lahar, shear can concentrate at the interface. We run CPT soundings at every column location on critical projects. Another risk is fines migration into the column. The local ash has up to 20% silt content, so we specify a filter fabric wrap when fines exceed 12%. Without it, the column clogs, drainage drops, and pore pressures build under seismic load. Liquefaction assessment under the NCEER method feeds directly into column spacing, with a factor of safety of 1.2 as our minimum.
Applicable standards
NZS 3404: Steel Structures (ground improvement references), NZGS Ground Improvement Guidelines, NCEER Liquefaction Assessment (Youd-Idriss), ASTM D5778 (CPT), ASTM D2850 (Triaxial)
Associated technical services
Stone Column Design Package
Full geotechnical analysis including CPT-based column layout, settlement modelling, liquefaction assessment, and bearing capacity verification. We deliver signed PS1 documentation and construction monitoring specifications.
Post-Installation Verification
Load testing on selected columns, CPT cross-checks between columns, and permeability testing to confirm drainage performance. We provide an as-built report with ammeter logs for council sign-off.
Typical parameters
Frequently asked questions
What does stone column design cost for a standard New Plymouth commercial lot?
For a typical 800-1200 m² commercial footprint, the design package including CPT investigation, lab testing, settlement analysis, and PS1 documentation runs between NZ$2,520 and NZ$8,370. Projects with high liquefaction risk or variable lahar depths fall at the upper end due to additional investigation and modelling.
How deep do stone columns need to go in New Plymouth's volcanic soils?
Most columns we design in the city penetrate 8-14 m, passing through the soft ash layer until they seat on competent lahar or alluvial gravels. We confirm the bearing strata with CPT refusal data before finalising column depth.
Can stone columns replace piling for a multi-storey building?
For buildings up to about 4-5 storeys on the ash soils, stone columns often avoid piling by improving the bulk ground stiffness. Taller structures or those with high column loads may need a piled solution, which we assess during the investigation phase.
Do stone columns prevent liquefaction in New Plymouth?
They reduce liquefaction susceptibility by densifying the soil and providing drainage to dissipate pore pressure during shaking. We design the grid spacing using CPT-based liquefaction analysis following NCEER methodology, targeting a factor of safety of 1.2 or better.
How long does settlement take after stone column installation?
The columns act as vertical drains, so primary settlement under load typically completes within 2-4 months in New Plymouth's ash soils. Without columns, consolidation could extend past 12 months due to the low permeability of the silt-rich ash.