Auckland's growth over the past three decades has pushed development onto the Waitematā Harbour's reclaimed edges and the low-lying alluvial flats of the isthmus. Much of this ground consists of loose sands, silty fills, and soft marine clays that settle unevenly under load. Before the first tower went up on the Viaduct Harbour, geotechnical teams had to figure out how to densify metres of hydraulically placed sand. Vibrocompaction design became the go‑to solution for those sites, and it remains essential today for projects on similar ground across the city. Combining deep vibration with water or air jetting, the method rearranges loose granular soils into a denser state, cutting void ratio and boosting bearing capacity. In areas like the old railway yards near Britomart, we have relied on this technique to bring reclaimed ground up to specification without excavating and replacing the fill.
Proper vibrocompaction design can cut post‑construction settlement by more than half on loose granular fills typical of Auckland's reclaimed land.
Methodology and scope
A typical Auckland project we worked on involved a medium‑rise apartment block on reclaimed land along the North Shore. The geotechnical investigation had shown a 6 m layer of loose silty sand over Waitematā sandstone. Standard shallow foundations would have settled 40 mm or more under design loads, so we opted for vibrocompaction design to densify the sand in situ. A depth vibrator was lowered into a pre‑drilled hole, then withdrawn in lifts while the surrounding soil collapsed and compacted. The process increased relative density from around 40 % to over 75 % within the treatment zone. We often pair this work with a densidad-cono-arena field test to verify compaction uniformity and with corte-directo lab tests when the soil contains fines that affect shear strength. The whole sequence was completed in under two weeks for that site, and the building's raft foundation now sits on ground that meets the NZS 4203 seismic performance criteria.
Technical reference image — Auckland
Local considerations
The most common mistake we see on Auckland sites is assuming vibrocompaction can densify any sandy soil regardless of fines content. When the passing #200 sieve exceeds about 15 %, the silt particles clog the pore spaces and the vibrator loses its ability to rearrange the grain skeleton. We have visited several projects where the contractor ordered vibrocompaction without a proper grain‑size analysis, and the result was barely a 5 % density gain. That wasted time and budget, and the foundation still needed piles. A thorough pre‑treatment investigation, including sieve analysis and natural moisture content checks, prevents that misstep. In our experience, a quick field check with a densidad-cono-arena after the first few compaction points can catch problems early and avoid a full‑scale rework.
Standard vibrocompaction design for granular fills
This service covers the full design workflow for clean sands and gravels with less than 10 % fines. We calculate the required compaction grid, vibrator type, and energy input using site‑specific CPT and SPT profiles. Our deliverables include a compaction plan, post‑treatment testing protocol, and settlement estimates. Typical applications include storage yards, residential subdivisions, and low‑rise commercial buildings on reclaimed or alluvial ground in Auckland.
02
Vibrocompaction with vertical drains for silty sands
When fines content sits between 10 % and 15 %, the vibration alone may not dissipate pore pressure fast enough. We integrate prefabricated vertical drains into the design to accelerate drainage and improve densification. The method has been used successfully on several Auckland sites near the Manukau Harbour where the natural soils contain a higher proportion of silty sand. We provide detailed spacing calculations and installation sequencing to keep the project on schedule.
Applicable standards
NZS 4203:1992 (general structural design actions, including seismic), NZGS Guideline for the Development of Liquefaction Hazard Maps (2016), NZS 4402 (deep soil compaction using vibratory probes)
Frequently asked questions
What is the difference between vibrocompaction and dynamic compaction?
Vibrocompaction uses a deep vibrator that penetrates the soil and vibrates at a controlled frequency, typically 30 Hz, to rearrange grains without impact. Dynamic compaction relies on a heavy weight dropped from a crane to generate surface impacts. Vibrocompaction works better for deeper treatment (up to 15 m) and for soils with a consistent water table, which is common in Auckland's reclaimed land. Dynamic compaction can be cheaper on large open sites but often leaves loose zones at depth that require additional testing.
How much does vibrocompaction design typically cost for an Auckland project?
For a typical medium‑rise site in Auckland, the design and supervision scope ranges between NZ$2.480 and NZ$9.300, depending on treatment volume, required testing, and site access. This covers the compaction plan, field verification with density tests, and a final settlement report. Larger subdivisions or projects with higher fines content may push toward the upper end of that range due to additional drainage design work.
Does vibrocompaction eliminate liquefaction risk on Auckland's reclaimed land?
It significantly reduces the risk but does not guarantee zero liquefaction under a major earthquake. By increasing relative density above 75 %, the soil's resistance to cyclic loading improves markedly. However, if the site contains thin silt lenses or buried organic layers, those zones may remain loose. We always recommend post‑treatment CPT or SPT soundings to confirm the achieved density profile and to identify any untreated pockets. For critical structures, a site‑specific seismic hazard assessment is still advisable.
Location and service area
We serve projects across Auckland and its metropolitan area.
