Notes on ground and consent.
Plain-English guides to the ground under a Canterbury build, and the terms on a consent, for the people who have to make the call.
The jargon, without the jargon.
A building consent throws a lot of acronyms at you, and most of them turn out to be simple once someone explains them straight. These are the ones we get asked about most, written for a client, not another engineer.
Which one you need.
Three guides. Which one is useful depends on where your project has got to.
If you are about to build
Start with the geotechnical report guide. It sets out what you will be handed at the end, section by section, so you can tell a thorough report from a thin one before you pay for either.
If your land has been classified
A technical category is not a verdict on your build, and it is not a number to accept on trust. The guide sets out what each category actually describes, and what still has to be confirmed on the site itself.
If you own an older building
The strength of an existing building comes back as a percentage, and that percentage is the whole conversation. The guide gives the thresholds, who does the assessing, and the point where a first look turns into a full one.
What a geotechnical report contains.
A geotechnical report is required for most building and resource consents. It is the evidence that the ground under a build has been investigated rather than assumed. Here is what is actually in one.
Section by section.
The ground profile
Every layer logged from the surface down, described and measured against the depth it was found at.
Site classification
The technical category (TC) and any liquefaction hazard, set from the testing.
Founding recommendations
What the ground will carry, and the foundation type the structural design can use.
Test data
Shallow testing to 3.0m and deep testing to 15m or more, recorded so it can be checked.
Bearing and settlement
The load the founding layer will take, and the movement expected under it. These are the two numbers the structural design starts from.
Where further work is called for
A recommendation for slope stability modelling or deeper investigation, where the ground says the shallow picture is not enough.
TC1 to TC3 explained.
After the Canterbury earthquakes, residential land was classified into technical categories so foundations could be designed to suit the ground. The category on your site sets how much investigation it needs and what the foundation has to do.
The three categories.
TC1
Ground where damage from settlement is unlikely. Standard foundations usually suit it.
TC2
Ground where minor settlement is possible in a large earthquake, designed for accordingly.
TC3
Ground where more significant settlement is possible. It needs the most detailed investigation and a specific foundation design.
Liquefaction
The underlying hazard the categories manage: sands that can behave like a liquid when the ground shakes.
What %NBS means for your building.
For an existing building, its seismic strength is rated as a percentage of the new building standard, or %NBS. It is the number that decides whether a building is legally earthquake prone, and how much strengthening it needs.
The number, and the thresholds.
Below 34%NBS
The building is an Earthquake Prone Building, and the risk has to be managed in a set timeframe.
34, 67 and 100%NBS
The tiers strengthening work is designed to reach, depending on the building and its use.
ISA and DSA
An Initial Seismic Assessment gives the first rating. A Detailed Seismic Assessment follows if it comes back below 34%NBS.
The standard behind it
Assessed to the MBIE Guidelines, against the life-safety requirements of Building Code B1-Structure.
Quick reference.
How the ground actually gets read, and the one threshold to remember.
- Shallow testing
- Hand auger, shear vane, Scala
- Deep testing
- Cone penetrometer, minimum 15m
- Where CPT cannot run
- Machine borehole tests
- Earthquake prone
- Below 34%NBS
Still not sure what your site needs?
Send the plans and the site address, and we will set out what your ground needs in the same plain English these guides use.