Liquefaction and Your Section: What It Means for Your Foundations
What liquefaction actually does to your section
Liquefaction happens when strong earthquake shaking increases water pressure inside loose, saturated soil - usually sand or silt sitting above a shallow water table. For a few seconds to a few minutes, that soil temporarily loses its strength and behaves more like a liquid than solid ground. It's not about the soil "melting" everywhere; it's a combination of three things lining up: soil type and density, how shallow the groundwater is, and how strong and long the shaking is.
For a homeowner, the real-world effect isn't usually collapse - it's settlement. After the Canterbury earthquakes, many lightweight timber-framed homes survived the shaking itself but ended up with uneven or sunken floors that weren't cost-effective to repair, along with cracked paths, tilted fences, and damaged underground pipes from ground that spread or settled unevenly.
Why this changed how foundations are designed
Canterbury's experience led to a rule change: since 29 November 2021, Building Code clause B1/AS1 has required robust, engineered foundation design on liquefaction-prone sites nationwide, not just in Canterbury. Sites are now assessed against three vulnerability categories:
- TC1 (low vulnerability): standard foundation solutions are generally acceptable.
- TC2 (medium vulnerability): needs engineered foundation design, usually backed by shallow site investigation.
- TC3 (high vulnerability): needs a more detailed geotechnical investigation and a specifically engineered foundation, which can include deep piles or a stiffened raft.
We've covered these categories in more depth in a separate post - the short version here is that liquefaction vulnerability is one of the main things that pushes a section from TC1 into TC2 or TC3.
What a geotechnical assessment looks at
MBIE's guidance sets out a process that typically works through: understanding what you're building, checking the regional hazard maps your council holds, assessing how vulnerable the specific site is to liquefaction, carrying out site investigations, and then matching the foundation design to what's found. At minimum, that usually means establishing the near-surface ground conditions, the bearing capacity of the soil, and the depth to groundwater.
What this means for your build
- Ask your council for the liquefaction hazard information it holds for your area before you commit to a design.
- Don't assume a standard foundation will be accepted - on medium or high vulnerability land, a geotechnical engineer needs to be involved early, not after plans are drawn.
- Budget time and cost for site investigation (such as scala penetrometer testing, CPTs, or boreholes) - this isn't an optional extra on vulnerable land, it's what determines the foundation type.
- If you're buying an existing home on flat, low-lying, or reclaimed land near rivers, estuaries, or the coast, it's worth asking whether liquefaction vulnerability has ever been assessed for that site.
This is general information only, not site-specific engineering advice. Every section behaves differently, and only a proper geotechnical and structural assessment of your actual site can tell you what foundation your build needs.
Not sure what your section needs? Get in touch with Fil Engineers - we can help you work through the geotechnical and foundation requirements for your build.
