Coffey land reports
Do you need a geotechnical professional to guide you through the assessment and development of your property?
We have over 50 years of experience in geotechnical engineering and a local team in Christchurch who understands the Canterbury region.
We can take the mystery out of the ground conditions and help you rebuild safely.
For our specialised help, contact one of our team members and we will respond to your enquiry as soon as possible.
Frequently asked questions
What are the stages involved in getting my geotechnical report?
Before we see you - we’ll do a comprehensive desk study to better understand your property and surrounding area.
This involves using desk based resources to document background information on your property and the surrounding area.
On site - we’ll carry out a site walkover to better understand how your property performed during past earthquakes.
After the assessment – we’ll design a fieldwork program. This generally involves a shallow and/or deep soil investigation to address the repair or rebuild strategy determined by your insurer, structural engineer, builder or you.Back to top
What activities are involved with a fieldwork program?
Shallow investigation – we use hand augers and conduct Dynamic Cone Penetrometer (DCP) tests.
Hand augers are small drilling tools that we use to collect soil samples to a maximum depth of 4m and log them.
A DCP, or dynamic cone penetrometer test, involves dropping a weight onto a metal rod to measure the density of the ground.
Deep investigation – Cone Penetrometer Tests (CPTs) and machine boreholes
CPTs and machine boreholes are how we define the deep geotechnical properties of your land – the type we use will depend on the local ground conditions.Back to top
What is a geotechnical engineer or engineering geologist?
Almost all structures are built on or in the ground, so it’s important to understand how the earth will behave under certain conditions - such as an earthquake.
A geotechnical engineer understands the civil engineering component and an engineering geologist generally looks at the geology at your site.Back to top
What is liquefaction?
Liquefaction is when earthquake shaking causes some soils to behave more like a liquid than a soil. The shaking causes water pressure between soil particles to rise so that sand and silt is suspended in the ground water.
Sand and silt being thrown out of the ground is a good sign of liquefaction on your site – but absence doesn’t prove it hasn’t occurred below the ground.
This is when liquefaction isn’t contained to one area and spreads – usually because it’s near a ‘free-face’ such as a riverbank or open channel.
Future land performance
Even if your land and property wasn’t affected by liquefaction due to the latest earthquake events, this doesn’t mean that it won’t be in future.
We’ll assess what has happened, but will also provide you an indication of how your land might behave in future eventsBack to top
Why is Christchurch so prone to liquefaction?
The three basic ingredients that cause liquefaction are earthquakes, silty/sandy soils, and a high groundwater table - and Christchurch has them all!Back to top
What sort of foundation repair will I need?
This will be determined by a structural engineer with advice from us - this can vary from no action required to full foundation replacement.Back to top
What types of foundation repairs are there?
A foundation relevel can be undertaken in various ways and will depend on your foundation type.
This will generally involve either ‘packing’ foundation elements by inserting wedges above them to lift the floor to the required height; or by ‘uplifting’ from beneath the foundation using mechanical methods (hydraulic jacks or screw piles) or grout (resin or low mobility grout).
Full foundation replacement
For a full foundation replacement the house is entirely removed from the site, lifted above its current position, or propped in place, and the foundations are demolished.
New foundations will be constructed in accordance with current guidance and the house put back onto the replacement foundation.
Piles are used to transmit load from the building and its contents to the ground – shallow piles are usually used in ‘good ground’ (as defined in NZS 3604) and generally are founded at shallow depths (<1.2 m).
Used for rebuilding on TC2 damaged land – this concrete slab foundation system is designed to cope with predicted future land damage for TC2 sites.
Surface structure foundations
Surface structures can be used on most TC3 sites without ground improvement or deep foundation works. They adjust to varying levels of lateral spread and can be re-levelled in the event of future differential settlements caused by earthquakes.
These give you dependable vertical load capacity during predicted earthquakes – but aren’t suitable for all dwellings as they need to be founded in dense sand or gravel.
Many options exist – the choice depends on site specific criteria such as existing ground conditions, liquefaction susceptibility, size of the site, neighbouring properties, groundwater and cost.
Ground improvement allows you to build a shallow foundation, such as a concrete slab, without the concern of your land performing poorly during an earthquake.Back to top
Why should I build with lightweight materials?
Light construction materials for the roof, walls and floors significantly reduce the load on the ground – which means less chance of liquefaction-induced settlement damage.
For example, lightweight construction imposes as little as 30% of the imposed weight around the perimeter compared to a heavy roof, masonry cladding and concrete slab dwelling1.
1.Ministry of Business, Innovation and Employment (MBIE), December 2012: Repairing and rebuilding houses affected by the Canterbury earthquakes.Back to top
What about my new garage?
If your garage is attached to your home you’ll need to make sure your foundation is consistent across your house and garage so it doesn’t perform differently during an earthquake.
If detached, your foundation will depend on the technical category of your land. As these structures are considered uninhabited the foundations are designed slightly differentlyBack to top
What if my property is in the Port Hills?
The process is very much the same as with flat land, however liquefaction damage is a low risk.
It’s important you understand your property’s level of risk when it comes to slope stability and other potential geological hazard such as boulder roll, cliff collapse and mass land movement. Armed with the right information gained from a ‘site specific’ investigation, these challenges can be appropriately assessed and managed.
Read more about how to protect the value of your property if it’s zoned in Port Hills and the different options available to you.Back to top
This helps us to understand the past and future earthquake performance of your property.
We’ll also identify suitable site investigation locations.Back to top
We use a variety of equipment to investigate your land.
• hand augers for shallow soil sampling
• DCP tests to assess density
• CPTs to characterise deep site conditions
• sonic boreholes to drill through hard, dense materials
Once we have all the information we need - we'll write you a report.
This will be signed off by one of our Chartered Geotechnical Engineers.
This report helps your builder, project manager or structural engineer to design your repair, or rebuild for your home.Back to top
Construction monitoring & sign off
Construction monitoring & sign off
Geology is complex
As your construction progresses , your builder may find subsoil areas that are inconsistent with what we originally found with our initial investigation.
To manage this, we’ll work closely with your builder to regularly check soil conditions-keeping your construction on track and your development to code.Back to top