When the ground beneath a building turns to liquid during a major earthquake, no amount of structural engineering above grade can compensate for what happens below it. This lesson, learned at enormous cost during the 2010-2011 Canterbury earthquake sequence, has reshaped how New Zealand engineers approach problematic ground. Today, ground-improvement-techniques such as stone columns, deep soil mixing (DSM) and various densification methods are standard tools in the geotechnical toolkit, used from the liquefaction-prone silts of Christchurch to the hydraulic fill reclamations ringing Auckland’s waterfront.
This article examines three of the most widely applied ground improvement methods in the New Zealand context, explains when each is appropriate, and offers practical guidance for developers and asset owners navigating consenting and design under the NZ Building Code.
Why Ground Improvement Matters in New Zealand
New Zealand’s geology presents a particular challenge: young, loosely deposited alluvial and reclaimed soils sit directly beneath some of our most valuable urban land. Christchurch’s shallow groundwater and interbedded sands and silts proved highly susceptible to liquefaction, with MBIE’s post-earthquake mapping identifying tens of thousands of residential properties across Technical Category 3 (TC3) land requiring specific geotechnical assessment and, frequently, ground improvement before rebuilding could proceed.
Auckland presents a different but related problem. Large areas of the waterfront, including Wynyard Quarter, Bledisloe Wharf and parts of the Tank Farm, sit on decades-old hydraulic fill and reclaimed harbour sediments. These materials are typically loose, variably compacted, and prone to both static settlement under new loads and liquefaction-induced settlement under seismic loading, an issue explicitly addressed in Auckland Council’s regional geotechnical guidance and increasingly scrutinised through the resource consent process for waterfront redevelopment.
In both cities, the underlying design driver is the same: NZS 1170.5 seismic demand combined with MBIE’s Module 1-6 guidance documents on repairing and rebuilding foundations, plus the broader performance requirements of NZS 3604 and AS/NZS 1726 for site investigation. Ground improvement is rarely optional in these settings; it is the difference between a site being viable for development and being written off.
Stone Columns: Proven Performance for Liquefiable Soils
Stone columns, formed by vibro-replacement or vibro-displacement techniques, involve installing dense columns of compacted aggregate into weak or liquefiable ground at typical centre-to-centre spacings of 1.5 to 3.0 metres. The columns achieve improvement through three mechanisms: densification of the surrounding soil during installation, provision of a stiffer, more permeable load path that accelerates pore pressure dissipation, and reinforcement that shares vertical load away from the native soil.
In Christchurch, stone columns have been extensively used beneath residential rebuilds on TC3 land and beneath larger commercial and infrastructure projects where shallow liquefiable layers extend to depths of 6 to 10 metres. Typical outcomes reported in post-treatment CPT and SPT verification testing show normalised cone resistance (qc1N) increases sufficient to raise factors of safety against liquefaction triggering from below 1.0 to above 1.2 to 1.3 under design-level shaking, meeting the performance criteria set out in MBIE’s liquefaction assessment guidance.
Stone columns are typically the most cost-effective solution where soils are predominantly sandy and free-draining, and where settlement tolerances are moderate rather than very strict. They are less effective in soft, high-plasticity clays or peats, where the columns can “bulge” laterally without achieving adequate confinement, a limitation well documented in the geotechnical literature and reflected in NZGS practice notes on ground improvement selection.
Deep Soil Mixing: Precision in Variable and Contaminated Ground
Deep soil mixing uses rotating mixing tools to blend in-situ soil with cementitious binders, typically Portland cement or cement-fly ash blends, forming columns, panels or overlapping grids of significantly stiffened material. Unlike stone columns, DSM does not rely on the surrounding soil for confinement, making it far more versatile across soft clays, organic soils and variable reclaimed fills.
DSM has become a favoured technique on Auckland’s reclaimed waterfront sites, where mixed fill profiles, occasional contamination, and strict settlement criteria for adjacent heritage structures and underground services demand a more controllable outcome than vibro techniques allow. Wet mixing methods can achieve unconfined compressive strengths in treated soil ranging from 0.5 MPa to over 3 MPa depending on binder dosage (typically 150 to 300 kg/m³), soil type and curing period, verified through unconfined compressive strength testing on wet-grab and core samples at 7, 28 and 56 days.
A further advantage in the Auckland context is DSM’s compatibility with contaminated or heterogeneous fill, since it encapsulates rather than displaces existing material, an important consideration given the industrial legacy of many reclaimed sites and the requirements of regional contaminated land provisions under the Auckland Unitary Plan. The trade-off is cost: DSM generally carries a higher unit rate than stone columns or densification methods, and quality control demands rigorous field verification, including wet sampling, column continuity testing and, on critical projects, cross-hole sonic logging.
Densification: Vibro Compaction, Dynamic Compaction and RIC
Where soils are predominantly cohesionless and clean, densification techniques often provide the most economical solution by increasing relative density in place, without introducing foreign material. Vibro compaction uses a vibrating probe to rearrange loose sand grains into a denser configuration, typically achieving relative density increases from 30-40% to 70-80%, sufficient to substantially reduce liquefaction potential in medium-to-coarse sands.
Dynamic compaction, involving repeated dropping of heavy weights (typically 10 to 20 tonnes) from heights of 10 to 20 metres, has been applied on larger Auckland reclamation and industrial sites where deep, loose fill profiles extend beyond the practical reach of surface rolling. Rapid Impact Compaction (RIC), a lower-energy, more site-friendly variant, has gained popularity on constrained urban Christchurch sites and smaller commercial developments where noise, vibration and access restrictions rule out heavier dynamic compaction rigs.
Post-treatment verification for all densification methods typically relies on comparative CPT testing, with target qc1N values benchmarked against MBIE and NZGS liquefaction assessment procedures. As one Christchurch-based geotechnical engineer involved in the residential rebuild programme observed, “the single biggest driver of successful ground improvement outcomes wasn’t the technique chosen, it was the rigour of the verification testing that followed. You cannot certify what you haven’t measured.” This point is echoed in MBIE’s guidance, which places explicit emphasis on post-treatment confirmation rather than reliance on design assumptions alone.
Choosing the Right Technique: Key Takeaways
- Match the technique to soil type first. Stone columns excel in sandy, free-draining liquefiable soils (as widely applied across Christchurch TC3 rebuilds); DSM performs better in soft clays, organics and contaminated or variable reclaimed fill (as seen across Auckland’s waterfront precincts).
- Verification is not optional. Post-treatment CPT or SPT testing against MBIE and NZGS liquefaction triggering criteria should be built into project programmes and budgets from the outset, not treated as an afterthought.
- Cost and settlement tolerance drive selection. Densification methods generally offer the lowest unit cost for clean sands; DSM commands a premium but delivers superior control for sensitive adjacent structures and strict differential settlement limits.
- Consenting requirements differ by region. Auckland Council’s Unitary Plan provisions around contaminated land and coastal hazard overlays can materially influence method selection on reclaimed sites, while Christchurch City Council continues to reference MBIE’s TC-based guidance for residential and commercial rebuild consents.
- Early geotechnical investigation pays for itself. Comprehensive CPT and borehole coverage at feasibility stage typically reduces design contingency and treatment costs by allowing more targeted, cost-effective improvement layouts rather than blanket treatment.
Get the Ground Right Before You Build
Ground improvement is not a generic checkbox item, it is a technical discipline demanding careful site-specific assessment, appropriate technique selection, and rigorous verification against New Zealand’s seismic performance standards. Whether you are assessing a reclaimed waterfront parcel in Auckland or a residential rebuild on Christchurch’s eastern suburbs, the right ground improvement strategy, properly designed and verified, can be the difference between a resilient asset and a costly liability.
Chambers Consultants works with developers, asset owners and design teams across New Zealand to assess ground conditions, select and design appropriate ground improvement solutions, and manage verification testing through to consent sign-off. Contact our geotechnical team today to discuss how the right ground improvement strategy can de-risk your next development.