A common slip we notice in Nelson is when contractors rush straight to mobilising a vibroflot without first pinning down a design calibrated to the local geology. The Moutere Gravels and the alluvial sands of the Waimea Plains behave differently, and a generic specification can leave you with patchy compaction and settlement later on. We have seen projects on the Port Hills where an inadequate design led to rework costing far more than a proper investigation upfront. Getting the vibrocompaction design right means understanding the grain-size distribution, the depth to water table, and the seismic demands of a region that sits within a high-hazard zone—Nelson experienced a damaging M6.5 earthquake in 1968 and ground conditions haven’t got any easier since. Before finalising a specification, we often pair the design with a CPT test to map the loose lenses continuously, which tells us exactly where the vibroflot needs to work hardest.
A vibrocompaction design built on regional grain-size data and seismic site class targets prevents the all-too-common problem of deep densification missing a loose intermediate lens.
Service characteristics in Nelson
Critical ground factors in Nelson
Under NZS 3404 and the NZGS module on ground improvement, vibrocompaction design must explicitly account for the risk of differential settlement if pockets of fine-rich material are missed. Nelson’s coastal terraces are notorious for this—a metre of clean sand can abruptly transition into silty sand that does not respond to vibration alone. Without a design that identifies these transitions through a dense pattern of investigation points, you can end up with a treated zone that passes a single CPT check but still settles under load because a bypassed layer consolidates slowly. The 1968 Inangahua earthquake, which caused significant structural damage as far as Nelson, is a stark reminder that loose sands in this region are susceptible to cyclic mobility. A solid design therefore specifies not just the compaction parameters but also the post-treatment verification criteria tied to a minimum factor of safety against liquefaction, typically 1.2 to 1.3 for commercial structures.
Our services
Our vibrocompaction design work in Nelson covers the full sequence from initial feasibility to post-treatment sign-off, always anchored in the specific subsoil conditions of the Tasman region.
Feasibility assessment and design basis
We review existing geotechnical data, run sieve analyses on samples from the target zone, and determine whether vibrocompaction is the right tool—or if stone columns or rigid inclusions would suit a silty profile better.
Detailed specification and grid optimisation
Using CPT and MASW baselines, we produce a full design package with grid geometry, vibrator selection, backfill gradation, lift heights, and hold criteria tailored to the Nelson ground model.
Post-treatment verification and compliance
We design and supervise the verification programme—CPT, MASW, or pressuremeter testing—comparing pre- and post-treatment data to confirm that the specified relative density and seismic performance targets have been met across the site.
Frequently asked questions
What does vibrocompaction design typically cost for a site in Nelson?
Design packages in Nelson generally range from NZ$2,320 to NZ$9,140 depending on site size, depth of treatment, and the volume of existing geotechnical data available. A greenfield site on the Waimea Plains requiring new CPT soundings and a MASW survey before grid design will sit at the upper end of that range, while a small infill lot with good existing records comes closer to the lower end.
How do you decide if vibrocompaction will work on a Nelson site?
The key is the fines content. We look for less than 12 to 15 percent passing the 75-micron sieve, a coefficient of uniformity below 3, and a water table deep enough to allow drainage during vibration. The Moutere Gravels often meet these criteria, but the silty lenses near the coast require careful screening with CPT and a few check boreholes.
What verification tests are required after vibrocompaction in Nelson?
We typically specify a combination of CPT soundings at the centroid of the compaction grid and a before-and-after MASW survey to confirm the increase in shear wave velocity. Where the design is driven by liquefaction resistance, the NZGS guidelines ask for a minimum of one CPT per 200 m² of treated area to check that the target relative density has been achieved consistently.
What is the typical design life for a vibrocompacted foundation in this region?
When designed to achieve a relative density above 70 percent and verified with post-treatment CPT, a vibrocompacted granular foundation in Nelson is expected to perform for the full 50-year design life of the structure without significant settlement, provided the groundwater regime does not change drastically—something we assess during the initial site investigation.