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Joining forces for a close look at South Dunedin ground

Dunedin (Tuesday, 22 January 2019) – The ground in South Dunedin will come under close scientific scrutiny in a new research project on soil conditions and below-surface water levels.

EQC, GNS Science, QuakeCORE, the Dunedin City Council, the University of Otago and the Otago Regional Council have joined forces on a project to test the soil and install instruments that can take water level measurements every 10 minutes.

EQC Senior Advisor John Scott says results from the study will help understand the effects of rainfall and tidal movement on the water level under the ground, and how the soil may behave in an earthquake.

“With solid scientific data from this project, the community can make better decisions on how to reduce risk,” he says. “For example, we will have a much better idea of how wide and deep building foundations should be to handle conditions in an earthquake.”

ORC Engineering, Hazards, and Science Director Dr Gavin Palmer says the soil will be tested by cones pushed about 15 metres deep into the soil at about 20 different locations, mostly on road reserve, and water level monitors will be installed in tubes, called piezometers, at some of these locations.

“As well as giving us a better understanding of what’s going on now, the testing will help develop better computer models of the impacts of future storms and sea level rise,” Dr Palmer says. “This in turn lets us better understand options to mitigate against and adapt to these effects in future.”

DCC Group Manager 3 Waters Tom Dyer says the DCC has budgeted $35 million on flood alleviation in South Dunedin over the next 10 years.

“The piezometer data will help inform options for exactly what that money should be spent on.

“The information will also be useful in preparing for an imminent flood, as response agencies will have a better idea of how much capacity the ground has to hold rainwater at any given time,” Mr Dyer says.

Installation is expected to start in the week of 28 January and data gathered from this project will be available on request from the Otago Regional Council.

• South Dunedin soil is made up of a mixture of sand and silt that absorbs water from rainfall. The underground water has been measured as usually sitting about half a metre below the ground surface.

• The groundwater table rises and falls with tides, rainfall and runoff.

• Piezometers are tubes that record the water level under the ground. Some of the piezometers will have automated sensors that will record the water level every 10 minutes allowing precise tracking of water movement under the ground that can then be matched much more accurately against rainfall and tide timings.

• The piezometers are around 50mm in diameter and will be set into the ground to allow ongoing monitoring. They are sealed with a lockable cap on the surface, and look similar to a water toby cap.

• The soil testing will be done via “cone penetration testing”. This involves a small (around 35mm diameter) steel cone with a 150mm long “tail” mounted with sensors being 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. The cone is then removed.

• Most of the soil testing and piezometer locations are on road reserve.

• The piezometers will be located across the South Dunedin area to help understand the influence of different processes at different locations. Both sea-level (tide) and hillslope run-off are expected to influence groundwater levels, so this work will help to better understand such linkages.

• The piezometers will supplement the four current Otago Regional Council groundwater monitoring sites in South Dunedin (Bathgate Park, Culling Park, Kennedy Street and Tonga Park).

• Results from the testing will be included in the New Zealand Geotechnical Database and available on request from the Otago Regional Council.

ends

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