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Database shares soil’s secrets

EQC has renewed its funding for the New Zealand Geotechnical Database that lets engineers and planners “see” the soil under the ground they are working with.

The database shares the results of tens of thousands of soil tests that in the past, would have likely stayed in inaccessible project files, says Hugh Cowan, General Manager Resilience.

“The Canterbury Earthquake Recovery Authority (CERA) started the database back in 2012 (then called the Canterbury Geotechnical Database) to enable Canterbury recovery agencies to make the best use of the $30 million worth of soil testing EQC had funded and collected following the earthquakes.  CERA told engineers, planners and other interested parties they could download the information for free – meaning less testing expense for them – but in return, they needed to upload their own testing data as the quid pro quo.  This proved to be a very successful and internationally unique model and the data set quickly grew through the contribution of other public and private sector agencies to about $100 million worth of test data.  As a result, the innovative and internationally unique public and private data sharing model won the Association for Consulting and Engineering NZ (ACENZ) Innovate Gold award in 2015.

“With EQC’s support and co-funding, the Canterbury database was changed by MBIE to a national database, and what we have now is a fantastic resource of shared public and private sector information that lets EQC and other users see data, graphs and visuals of local soil conditions in many areas of the country before undertaking a project. We are now in a postion to get ahead of the game with regard to better understanding site conditions,” says Hugh.

Hugh says the information and data enables an improved understanding of the risk profile of a site and also optimises costs is doing so. “This leads to better decisions about things like where to place building foundations such as piles, how deep they need to go, and how to minimise a site’s natural hazards in general,” he says.

“For EQC, it also means we now have access to far more information than we could afford to get ourselves. It gave us an excellent practical base for our policy work in Canterbury.”

This month EQC and Tonkin & Taylor received the Gold Innovate Award from ACENZ for a world-leading approach to assessing the vulnerability of 140,000 Christchurch houses to increased liquefaction and flood risk. “This required analysis of a massive amount of soil data and would not have been possible without the information shared through the Geotechnical Database”, says Hugh.

“The vulnerability information also helps us understand what claims we could get in future, supports planning to reduce impacts, and speeds up post event recovery. It’s used by our reinsurers too, to help reduce uncertainty about their risk exposure in New Zealand.    

The New Zealand Geotechnical Database is managed by MBIE with a funding contribution from EQC of $150,000 over the coming year.

Seeing under the ground

The most common tests for understanding soil are drilling a borehole and the cone penetration test.

Borehole

Photo of a borehole drilling rig

A borehole is a hole drilled or dug into the ground to extract or investigate the soil or rock. The purpose is to understand the nature and strength of the ground and the location of groundwater. The depth of the hole depends on the nature of the project. Some boreholes are done by hand in soft ground, others need a machine-mounted drill rig. A borehole could be anywhere between 5 metres and 30 metres deep.  Soil is pulled to the surface for inspection/testing as the hole is dug. The data recorded from the borehole is then logged to give an accurate picture of the layers of the soil and rock and their likely strength.

Cone Penetration Testing

This diagram shows how information is reported back on the soil.

A small (around 35mm diameter) steel cone with a 150mm long “tail” mounted with sensors is pushed into the soil – usually by a specialised truck. It records data from the cone head on the force needed to penetrate the soil, and from the tail on the level of friction. This information is used to differentiate soil layers, their respective strengths and how vulnerable they are to hazards like earthquake-induced liquefaction.

Captions to photos:

Image 1: Shows information points where tests have been carried out in Shirley Christchurch

Image 2: Photo of a borehole drilling rig

Image 3: The diagram shows how information is reported back on the soil.

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