If you own property in Christchurch you’ll have seen TC1, TC2 or TC3 on your LIM or insurance papers. Here’s what they mean for your build – in plain English.
What is liquefaction?
Liquefaction happens when loose, water-saturated sandy soils lose their strength during strong earthquake shaking. At the surface it shows up as sand boils, settlement, tilting and cracking.
Why Christchurch is different
The 2010–2011 Canterbury earthquake sequence triggered widespread liquefaction, with very uneven damage. In response, the Government produced the Technical Category (TC) zoning system that classifies land by its risk of liquefaction-related damage.
TC1, TC2 and TC3 explained
| Zone | Liquefaction risk | Investigation | Typical foundation |
|---|---|---|---|
| TC1 | Low | Often none site-specific | Standard NZS 3604 solutions |
| TC2 | Moderate (most common) | Site-specific geotechnical report | Thickened-edge or waffle slab |
| TC3 | High | Detailed investigation (CPT, liquefaction modelling) | Deep piles or ground improvement |
How your TC zone affects your project
- Whether you need a geotechnical investigation
- What foundation type is required
- The cost and complexity of the engineering
- Your consent process and timeline
Can liquefaction risk be managed?
Yes. Depending on the site, options include ground improvement, deep piles founded below the liquefiable layers, raft or stiffened slabs, and drainage. The right solution comes from the site data and is designed to the New Zealand Building Code.