Borehole source of Waihi property damage

A geo-tech drill rig at the site in Walmsley Rd, Waihi

Waikato Regional Council and Hauraki District Council have received the final report on the damage to five houses in Gladstone Rd, Waihi.

In November 2012, HDC and Newmont Waihi Gold were told of land settlement and damage to houses at 47, 49, 51 and 53 Gladstone Rd and later damage was noted at 48 Gladstone Rd.

The properties were all located close to where an exploration borehole known as CGD008 was located in Walmsley Rd, at the back of these residential properties.

Hauraki District Council, Waikato Regionald Council, and Newmont Waihi Gold worked together to investigate the settlement and damage.

An investigation report was prepared by Dr Trevor Matuschka, Engineering Geology Ltd as to the cause of the settlement and recommendations to prevent such future events.

Tonkin and Taylor did a Subsistence Geotechnical Assessment of the areas in Gladstone Rd and reports were also prepared on groundwater levels in the wider area.

At the same time, as these reports and investigation were being carried out, Newmont Waihi Gold was applying for its resource consent for its proposed Correnso Mine which lay within the area of concern.

There were daily inspections of the properties affected to monitor the cracking and differential settlement of the houses and settlement and cracking of the ground.

Engineering Geology Ltd (EGL) investigated the likely cause of damage and provide advice with re-grouting of drillhole CGD008.

The work also included work to determine subsoil and groundwater conditions, drilling of hand-auger boreholes and installation of standpipe piezometers, laboratory testing on selected samples from the boreholes and drillholes along with hydraulic conductivity in drillhole CGD008, review of settlement and groundwater monitoring records plus the assessment of the likely cause of damage to the houses.

It was found that the cause of the settlement was the borehole had not been adequately grouted which allowed the groundwater from the upper level system to drain into the relatively permeable lower quartz andesite material and ultimately into the Martha Pit dewatering facility.

This was not noticed until the damage to property and buildings had occurred.
The borehole was constructed without resource consent, but rather in reliance upon permitted activity rule 3.8.4.6 of the Waikato Regional Plan.

The Working Group found that if the conditions attached to this rule had been met, this incident would not have occurred.

Waikato Regional Council and Hauraki District Council have accepted the Conditions of Consent for the Correnso Mine will deal with any future risks related to differential settlement caused by dewatering arising from the Correnso Consent.