New Zealand Ground Conditions guide: Do I need a geotechnical engineer?
Ground conditions. Expansive Soil, Liquefiable ground conditions, TC1, TC2, & TC3 explained.
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.
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.
The best ground to build your structure on requires the following four conditions:
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.
“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.
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.
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 means that future damage to the land in question is unlikely. This means you can use standard foundations.
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.
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 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.
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.
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.
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.
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.
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.
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.