The TC framework, quickly.
Christchurch's Technical Categories were introduced after 2011 to triage residential foundations on liquefaction-prone land. The three categories cover an increasing scale of risk:
- TC1: ground performance acceptable; standard NZS 3604 foundations are usually fine.
- TC2: minor-to-moderate liquefaction risk; standard foundation enhancements (specifically the MBIE TC2 details) apply.
- TC3: significant liquefaction risk; specific engineering design required for every building.
Outside Christchurch, the TC framework isn't legally binding — but the underlying ground assessment principle is widely accepted by councils in Wellington, Hawke's Bay, Tasman and parts of the North Island east coast.
What to specify by ground type.
TC1-equivalent ground
For sites assessed as low liquefaction risk on engineered fill, gravel or competent soil:
- Standard waffle pod slab to MBIE detail.
- Or strip footings to NZS 3604 for stiff clays.
- Geotech report still required by most councils, but the recommendations are usually thin.
TC2-equivalent ground
For sites with moderate liquefaction risk — saturated silty sands, soft alluvium, near rivers and estuaries:
- Enhanced waffle slab with deeper edge beams (375 mm typical), thicker raft, and steel mesh both top and bottom. The MBIE TC2 detail is your reference.
- Robust connections between slab and superstructure — high-tensile holding-down bolts, full perimeter ring beam.
- Expect $15,000–$30,000 added cost over a TC1 slab for a typical 200 m² single-storey home.
TC3-equivalent ground
For sites with significant liquefaction risk — deep loose sands, peat layers, high water table:
- Specific engineering design. No off-the-shelf detail applies.
- Screw piles are the most common solution for residential — typically 6–12m deep depending on competent bearing layer, with a structural floor spanning between pile caps.
- Stone columns or rammed aggregate piers as ground improvement, with a conventional raft above — competitive for larger footprints.
- Concrete piles to bedrock or competent gravel — most expensive, sometimes the only viable option in deep peat.
- Expect $40,000–$120,000 cost over a TC1 baseline depending on depth and footprint.
"Outside Christchurch, there's no TC stamp on your title. The geotech report is doing all the work — make sure it answers the right questions."
How to brief the geotech.
The most common foundation cost surprise we see is a geotech report that doesn't address liquefaction directly because the brief didn't ask. If you're building in a known liquefaction-prone area outside Christchurch, give the geotechnical engineer three specific instructions:
- Liquefaction susceptibility assessment using the NZGS guidance — SPT, CPT or DMT data, not just borehole logs.
- Settlement estimate at design-event ground motion. This is the number that drives foundation choice.
- Foundation type recommendation with cost-band comparison. Most geotechs will do this if asked but won't volunteer it.
The insurance question.
EQC and private insurers price liquefaction risk into premiums and excesses post-Canterbury. If you're building on borderline TC2/TC3 ground, the difference between a screw-piled and conventional foundation can be a 20–40% reduction in your annual premium over the life of the home. We've seen owners pay back the piling cost in 8–12 years on insurance savings alone for higher-risk sites.
What we typically recommend
For coastal Hawke's Bay, eastern Bay of Plenty, parts of Wellington's harbour edge, and most of the Canterbury Plains east of SH1 — we default to a TC2-equivalent enhanced waffle slab unless the geotech says otherwise. For known peat areas (south Auckland, Waikato river margins, parts of Westland), we go straight to screw piles or specific engineering and don't bother costing a raft.