New Zealand Ground Conditions guide: Do I need a geotechnical engineer?

Ground conditions throughout New Zealand vary greatly. Geologically, we live where tectonic plates merge, so the land we live on was once the ocean floor. So, we have good, hard, soft, liquefiable, expansive, and sloped ground. You may read, hear, or see even more ground conditions depending on where you are in the country. But what exactly does it all mean? And how does it impact my foundation? These are the questions I’ll answer in this article. 

Ground Conditions Quick guide

  • How to get the best result from a Geotech Engineer for your slab?
    • Engage slab contractor early
    • Ensure your Geotech and structural engineer work together.
    • Verify the ground conditions in your area yourself using the GNSS maps.
    • What to do when?
      • TC2 / Soft Ground = Design an Xpod Slab - $185-$200 per m2
      • TC1 / Good Ground = Design a Ribraft - $155 per m2
      • TC1 / Soft Ground = Design a Ribraft on timber piles or an Xpod - $185-$225 per m2

Good Ground Explained

What is good ground, and why is it essential to your foundation?

“Good Ground”, as defined by the NZS3604 Building code, is “any soil or rock capable of permanently withstanding an ultimate bearing capacity of 300kPa” (kPa is a unit of pressure). Although not an absolute deal breaker, you want this ‘good ground’ on your site to ensure the most cost-effective build.

What is the best ground to build a structure on?

The best ground to build your structure on requires the following four conditions:

  1. “Good Ground” 
  2. Non-expansive soil
  3. Technical Category 1
  4. Flat site

Why is correctly assessing your ground conditions so important? 

The ground is the foundation of your foundation. Although it will not always start as good ground, if your ground is assessed properly, you’ll find out what improvements need to be made to ensure you can safely build your foundation.  

Having said that, if you incorrectly assess your ground conditions, you could under or over-engineer your slab. Both situations could cost you a lot of money and put the home at risk. 

So it pays to get good advice and take the time to assess your ground correctly.

What is a good kPa for soil?

“Good Ground”, as per NZS3604, is when the ultimate bearing capacity is 300kpa or greater as verified by a geotechnical engineer. This is the amount of potential pressure the site can withstand. 

A 300kpa ground bearing will allow a standard 3604 foundation or a Codemarked pod floor (Such as Ribraft or Allied Superslab) to be specified without the need for a Structural Engineer (provided there is no liquefaction risk) 

This is the cheapest ground for building. If your site requires a specific Engineered Design, it will not only cost you for that design, but it will also be a more expensive slab to build.

Getting a building consent

You need to confirm the ground conditions on your site before the council can issue a building consent. 

In the past; a scala penetrometer test confirming the bearing capacity of the site was all the council would need. This was cheap and easy.

You either proved that the ground exceeded 300kpa and a structural engineer was not required, or if you were less than 300kpa, the corresponding results instructed the engineer how deep the piles needed to go.

However, after the 2011 Christchurch earthquakes and the years following, MBIE created guidelines for each BCA (Building Consent Authorities, aka your local councils) on how to interpret ground conditions better. They say who can do the ground testing and how the results need to be reported. They also created a framework for interpreting liquefaction risk. Although it is a good framework,  the inconsistency of interpretation across the BCAs has caused some chaos. 

We have 66 BCA’s NZ-wide and they have all taken their time to implement the guidance from MBIE. This means in some regions you need to provide more than just your standard scala Penetrometer to confirm your ground’s bearing capacity. You’ll likely need to engage a Geotechnical engineer to conduct a site-specific investigation which could include CPT testing and test pits and cost up to $15k

This cost is a burden for some would-be home builders. The worst part is that consistency is still yet to be found across all BCA’s on how the tests are completed, and how the results are interpreted. Meaning in some areas people are paying $15k, in the region next door they are doing the most basic test and only paying $2k.

It pays to get good advice here and align yourself with a knowledgeable slab contractor and their team of Geotech and structural engineers.

Liquefaction Explained

Liquefaction explained (image from Christchurch City Council)
Liquefaction damage during Christchurch Earthquakes

In 2010 a series of earthquakes struck Christchurch. Unfortunately, many homes suffered liquefaction. This was a shock to the building industry. As a result, changes were made to ensure our foundations are better equipped to handle the results of seismic activity. The technical categories were during the recovery and rebuilding stage to minimise the possibility of future damage to Christchurch (and now New Zealand). The technical classifications TC1, TC2, and TC3, are in the ‘green zone’ which means human occupation is possible on this land.

TC1, TC2, TC3: Technical classifications explained

What does TC1 mean?

TC1 means that future damage to the land in question is unlikely. This means you can use standard foundations. 

What does TC2 mean?

TC2 means that earthquake damage and liquefaction from minor to moderate is possible. You can use standard timber-piled foundations for houses with lightweight cladding and roofing. You can use suspended timber floors or enhanced concrete foundations.

What does TC3 mean?

TC3 means that in large earthquakes moderate to significant damage is possible. This means the site needs to be examined by geotechnical experts. There is no one-size-fits-all solution for TC3 site classification, meaning the solution will be site-specific. 

Expansive Soil Explained

Expansive soil means the ground you are building on is likely to expand and/or contract with the changes of wet and dry periods. It only impacts a specific type of soil. In New Zealand, this is mainly found in Auckland. When the soil dries out, it will contract and heave. When it gets moist, it will swell.

This movement in the soil will cause damage to your foundations and ultimately, your house, so it requires a Specific Engineered Design. The insurance claims in the USA for damage to housing caused by expansive soil in any one year is greater than all natural disasters combined - it is a big deal.

Your geotechnical engineer will ensure that this risk is correctly understood and provide guidance to your structural engineer on how it might be combatted. One of the best solutions for expansive soil in New Zealand is the Xpod waffle slab system.

How do I know what type of ground I have myself?

Every Regional Council has handled this differently. There is not one data set holding all of the information for all of New Zealand. Search your specific area and ask your council what records they have available. As an example - The Greater Wellington Regional Council has invested in collecting all of the historical data into one Arc GIS map as per the link below.



Hawkes Bay



You can search your address and find out if it is a low, moderate, or high liquefaction risk. This is a great start for getting the big picture but is ultimately very conservative. You will still need a Geotechnical Engineer to interpret the ground conditions of your specific site and give their conclusion.

In many situations, you will find that the risk is lower than what is stated on this map when an experienced Geotech conducts their testing. Again, you can use resources like Arc GIS maps to do a high-level self-assessment during due diligence on a land purchase.

However, nothing can replace good advice from an expert.

When do I need a geotechnical engineer?

In almost all cases, you will need to engage a geotechnical engineer for the Building Consent Authority to be sure that the ground conditions have been adequately considered when designing your foundation.

The only exception to engaging a geotech yourself is when you are building in an established subdivision.  In this case, there is likely going to be a Geotechnical Engineer involved in the design and construction of the entire subdivision.  They may be able to provide you with a comprehensive geotechnical report for the subdivision, and this may go as far as to provide site-specific certification of each building platform (if you are lucky). 

If you don’t have the site-specific platform certification provided to you, but you do have the Subdivision Geotechnical report. You can get away with a simple ground-bearing test on your site, but it still needs a geotechnical engineer to oversee this.

So if you are buying a section in a new subdivision, it will pay to get your hands on the Geotechnical Report commissioned by the developer.

What is involved in testing the ground conditions?

A geotechnical engineer can assess the ground conditions in several different ways and provide you with their analysis and foundation recommendations.

They can use a Scala penetrometer, hand augurs, shear vane, test pits, or a full CPT. There are a lot of jargon-sounding words here,  but essentially it is your engineer's job to choose the right tool for the ground conditions and then provide an expert analysis and recommendation. 

There is not a one-size-fits-all approach: in some ground conditions a scala penetrometer just will not work and a sheer vane will be used. Sometimes you can rely on historical CPT in the areas,  in other situations, you will need to get one done yourself. 

It pays to have an expert here to guide you. 

NB:  A scala spectrometer test is when you hit a steel rod into the ground and measure the blows it takes to move a set measurement into the ground. This tells you how hard the ground is in your location.

How much does a geotech report cost in NZ?

Ground test costs can vary significantly depending on where you are in the country, the condition of the ground, and the type of building you are building.

Gone are the days when you could provide the council with the Scala penetrometer results and pay the structural engineer $500 for the trouble of completing them. We now need some more significant analysis completed by a Geotechnical Engineer, taking into account the greater liquefaction risk—and this will cost a bit more than $500.

For a Ground Test accompanied by a Geotechnical memorandum, you can expect to pay anywhere from $2,500 - $7,000, depending on the engineer and where in the country you are.

If you require a CPT (Cone Penetration test), the on-site test and the accompanying Geotechnical analysis will cost $5k—$15k.

If you are on the side of a hill then a slope analysis is also required, and this could add up to $2,000 to the bill.

Long story short - a geotechnical report will cost money. But remember that ground is the foundation of the foundation. This is one step you will need to get right, and it ultimately pays to get good advice about how you are going to get your build-out of the ground - so don’t cheap out here. 

Having said that - the most expensive geotechnical engineer will not always be the best for your job. Be sure to work with a geotechnical engineer that both your structural engineer and foundation contractor have collaborated with in the past. This relationship is key. 

What can go wrong? 

  • You assume 3604 Good Ground without testing it and submit it to the council for building consent. The council then requests proper testing and your project is delayed as a result. 
  • Your geotechnical engineer uses the liquefaction risk maps provided by the council without assessing the site-specific risks and bases their recommendation on a more conservative approach due to limited information. This may save cost at the ground test stage but cost more in the foundation construction as a result. 
  • Your geotechnical engineer uses the incorrect test protocol for the ground conditions on site and provides a foundation recommendation that is not fit for purpose.  You may have some serious trouble if your foundation fails. 
  • You choose a geotechnical engineer who does not agree with the approach of the structural engineer and you get a very conservative recommendation. This will cost more to build the foundation than otherwise may be possible. 
  • You pay a significant amount of money,  wait a significant amount of time to get your report and the foundation recommendation ends up being the most expensive build possible. 

It pays to get a team together that you know will collaborate.  Your geotech engineer,  your structural engineer, and the contractor who will build the slab. If they are all on the same page from the get-go,  the result is a fit-for-purpose cost cost-effective result. 

Foundation Options as a result of the Ground conditions

  • Standard Dig out - If liquefaction risk is low and 300 kPa good ground is found below the topsoil layer, all that is required is for the topsoil to be removed (likely 150mm layer) and replaced with a compacted base course. Now, you can build a “Good Ground” Codemarked Ribraft (or any 3604-approved foundation). This is your cheapest option for both the earthworks and the slab, and no engineer is required.
  • Piles - If the Good Ground depth is greater than 500mm below the ground, you will likely choose to go for Concrete augured piles or timber-driven piles that embed into the Good Ground and then put a SED Ribraft slab on top. This is essentially a suspended slab now and will require some enhanced steel detailing in the slab compared to the standard rib raft. Bear in mind, that you always need to remove the topsoil as you cannot build a slab on top of organic material.
  • Gravel Raft - If Good Ground is at a depth of 500mm or greater, you are in TC2 ground conditions (or your topsoil layer is greater than 150mm). You may choose to have your Geotechnical engineer design a Gravel Raft instead of piles. This is where you excavate under the foundation between 500mm - 1200mm deep. Under the supervision of a geotechnical engineer, you then lay Bidim cloth or geogrid and backfill with approved aggregate in layers of approximately 200mm. You’ll need compaction testing at each layer. This essentially creates an engineered ‘raft’ for the foundation to sit on in a sea of bad ground conditions. 
  • Ribraft Slab - Ribraft slabs can be built on as low as 200 kPa ground without requiring any more enhancement. However, if the ground is TC2 or TC3, the slab will need to be enhanced so it can sustain a couple of metres of the ground liquifying under the slab. Enhancement may mean additional steel enforcing, fibre-reinforced concrete, or a deeper slab. 
  • Xpod slabs - These are plastic pod slabs with ribs at closer centres to Ribraft. This has created a stiffer slab. In many cases, Xpod is your best option for a TC2 ground as it may not require a Gravel Raft to be built under it. Also, it can be designed for ground as low as 100 kPa ground. If the ground is soft or it is TC2, go with Xpod. For all other situations, Ribraft is likely best.
  • TC3 slabs - if you are building your house in an area with a high risk of liquefaction then a TC3-compliant foundation will be needed. Simply,  it is highly likely that in an earthquake the soil under your slab will liquefy and your house will need to be releveled. So, your foundation will need to incorporate jacking screws within the slab to give you that option.  Both Ribraft and Xpod have a standard detail for this type of foundation. 

What should I do?

  • There are so many different types of ground conditions and variables to contend with so don’t try to be the expert here. Find an expert to help you…
  • Contact an experienced slab contractor before conducting any Geotechnical site investigations. Ensure that they are a reputable business that offers Design and Build packages. 
  • Request a fee proposal to conduct the ground test and reporting to meet the local council's requirements and the likely design philosophy of the structural engineer.
  • Do not engage your structural engineer until your Slab contractor has interpreted the results and has provided a likely slab design and an estimated cost to build the slab.
  • In most cases, your slab contractor will do this interpretation step for free and with no obligation for you to work with them beyond obtaining the reporting. They are incentivised to make sure this foundation type is as cost-effective for you as possible so that you will then choose to engage them.

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