Soil & ground conditions: is it good ground?

NZS 3604’s whole simple-foundations approach rests on one assumption: that you’re building on “good ground”. If the ground doesn’t meet the definition, you fall out of NZS 3604 for the foundations and need a geotechnical report plus a PS1 and Specific Engineering Design (SED). Confirming this before you quote a slab is one of the highest-leverage checks on a job.

What counts as “good ground”

Under NZS 3604, ground is only “good ground” if it ticks all of these:

• Allowable bearing capacity of at least 300 kPa.
• No liquefaction potential — that’s saturated loose sands or silts that turn fluid when shaken.
• No lateral-spread risk — ground sliding toward a free face, like a river bank.
• A stable slope (broadly 1:5 or flatter) that isn’t creeping or slipping.
• Not uncontrolled fill, not expansive or reactive clay beyond limits, and not flood- or inundation-prone.

If any one of these fails, the ground isn’t “good ground” and the simple-foundation rules no longer apply. See NZS 3604 Soil Classes A–E for the bearing-class detail.

Canterbury TC zones (post-quake)

In Canterbury, sites are mapped into Technical Category (TC) zones that reflect liquefaction risk and drive the foundation type:

• TC1 — low liquefaction risk, effectively like good ground; standard NZS 3604 is usually OK.
• TC2 — moderate, with minor-to-moderate settlement possible; often an enhanced or engineered slab (for example a TC2 raft).
• TC3 — high liquefaction risk; site-specific geotech and an engineered foundation, often with ground improvement.

Check before you quote

Foundation cost can vary several-fold between TC1 and TC3, so pin the ground down early:

1. Check the council GIS or geotech maps for the TC zone (Christchurch) or any hazard overlay before quoting.
2. If the site fails the good-ground test, plan for a geotechnical report, PS1 and Specific Engineering Design (SED).

Reference: MBIE — Using NZS 3604 (good ground).