New Waihi sinkhole mystery

Drilling is restarting in Waihi East again today in a bid to discover what caused damaging land settlement at five properties in the area in November.

But property owners remained tight-lipped on the issue while they worked towards a solution with Hauraki District Council and Newmont Waihi Gold (NWG), which has an underground mine nearby and plans to mine the Correnso ore body beneath the residential area.

While there is no record of any underground mine workings under the affected area, the council drilled down to see if the cause of the event could have been a sinkhole into an old mine similar to a one that engulfed a residential section and its house in 2001.

They found that no voids existed below the area and that a sinkhole could "largely be discounted", but the true cause remained a mystery.

NWG sent a letter to residents on Christmas Eve to say a drilling rig would be reaming the same drill hole from today for up to three weeks to help the investigation.

The global mining giant has also offered to relocate all affected residents at no cost and to purchase the affected properties at the pre-event market value and to meet the costs of the sale.

At least one person has already sold.

But NWG had not admitted liability, Waihi East Ratepayers Group spokesman Terry Podmore said.

Podmore has seen the damage to two properties and says NWG's offer was to keep residents safe, "so there is a concern there".

He said the foundations of one property were cracked and the exterior weatherboards had buckled outwards as if the house was settling in the middle.

The ground had dropped in places along a strip of land that was easily visible, Podmore said.

He said there were three possible causes:

 - subdivision fill that was put in more than 30 years ago had settled but that was "unlikely".

- dewatering in NWG's existing underground mine had left a cavity underneath.

 - NWG's drilling in February.

"Contractors drilled a hole directly behind No 43 Gladstone Rd. And they drilled down to the top of the ore body. The suggestion is that as they were drilling down they went through what's called a void or cavity. What they've done is created a situation where the water around those houses is actually going down that drill hole into the void."

Podmore said NWG had done the right thing by putting those homeowners at ease now rather than waiting to prove who was at fault.

"I think it's safe to say the rest of the community around those houses are concerned about the potential for this sort of thing to happen in the future and they'll be waiting with interest to see what the outcome of the investigations are."

A working party consisting of representatives from the district council, the Waikato Regional Council and NWG will be formed to undertake further investigations into the probable causes of the settlement in the New Year.

Waikato Times