NZS 3604 soil classes A–E

NZS 3604:2011 §5.3.3 sorts a site into one of five subsoil classes, from A (strongest) down to E (weakest). It matters because the class drives your bracing demand multipliers, your foundation design, and whether you can use NZS 3604 at all — so it’s worth pinning down before you quote or design.

The five subsoil classes

Each class describes the ground under the site and carries its own bracing multiplier. Higher-numbered classes mean weaker ground, more bracing demand, and more foundation work.

• A — Strong rock. Granite, basalt, hard sandstone; Vs > 1500 m/s. Common South Island bedrock. Most foundation options available. Bracing multiplier 0.8.
• B — Rock. Weaker or weathered hard sandstone; Vs 360–1500 m/s. Foundation cost similar to good ground. Bracing multiplier 0.9.
• C — Shallow soil site. The most common NZ residential ground — less than 3m of soil over rock, or firm dense soil more than 3m deep. Standard NZS 3604 path. Bracing multiplier 1.0.
• D — Deep / soft soil site. Soft sediment more than 3m deep, or a river plain. Bracing demand increases. Many Auckland flat sites sit here. Bracing multiplier 1.2–1.5 depending on EQ zone.
• E — Very soft soil site. Soft sediment more than 10m deep, or low-strength soils. Specific Engineering Design (SED) is likely needed — you cannot use the NZS 3604 prescriptive path.

Christchurch TC zones (post-quake)

In Canterbury, sites also carry a Technical Category from the post-earthquake mapping. Check the council GIS for the TC zone before you quote — foundation cost can vary 3× between TC1 and TC3.

• TC1 — Good ground. Standard NZS 3604 / NZS 4229 path is OK.
• TC2 — Moderate liquefaction damage potential. Engineered foundations typically — MBIE TC2 standard foundations.
• TC3 — High liquefaction damage. Always engineered foundations plus ground improvement — MBIE TC3 standard or an alternative geotech solution.

When you need a geotech report

Some sites push you past the prescriptive path and into a geotech engineer’s hands. Reach for a report when the site has any of these:

• Site Class D or E.
• TC2 or TC3 in Canterbury.
• Filled ground or recent re-contouring.
• Slope steeper than 1:5.
• Within 10m of a cliff edge.
• Known flood zone.
• Adjacent to a recent slip or landslide.
• A 2-storey build on a Class D site (often a “do you really need geotech” judgement call).

What a geotech report includes

A residential geotech report typically runs around $2,500–6,000 and covers:

1. Bore-hole logs at 2–4 points across the site.
2. Soil classification (NZS 3604 plus NZBC B1).
3. Bearing capacity (kPa).
4. Liquefaction potential.
5. Drainage assessment.
6. Foundation recommendation (slab, pile or raft).
7. PS1 (Design) for use with the consent application.