Subsidence fear over Three Kings quarry work

By JAMES GARDINER

Unexpected land subsidence and a bungled report by a mining company have alarmed residents in some of Auckland's most expensive suburbs.

Water has been pumped out of the ground at Three Kings Quarry for the past three years to allow Fletcher Building subsidiary Winstone Aggregates to continue quarrying scoria and basalt.

Residents environmental group Three Kings United (TKU) estimated up to 6000 homes in the suburbs of Balmoral, Epsom, Royal Oak, Mt Roskill and Hillsborough worth $1.8 billion are in the area affected.

Predictions of the quarrying's effect on groundwater levels in the nearby areas have proved wrong, raising concerns with residents and the Auckland Regional Council that the ground itself may sink across an area three times larger than originally expected.

Winstone said the dewatering would be confined to a 1km radius of the quarry but its own tests have indicated that the affected zone may include land up to 1.8km away.

The council has demanded tests on underground water levels be conducted further out from the quarry after rejecting Winstone's initial reports as "garbage".

Winstone has now brought in another company of consultants to assess the impact of its dewatering and is sinking more bores to assess water levels.

Water levels are an early warning indicator of likely land sinking. Both are being monitored.

So far land subsidence has been between 3mm and 17mm since 1999.

The council has given Winstone until November 30 to produce a satisfactory report on how much underground water levels are dropping, how much subsidence has occurred and over what area.

Winstone resource and environment manager Alan Happy said the houses were in no danger of sudden subsidence.

"There is variance from what was predicted and the council is concerned about that and this is why we're wanting to get the whole thing resolved before the drier season in the summer," Mr Happy said.

TKU chairwoman Corinne McLaren said her group had always opposed the dewatering and now believed the unexpected test results put "a huge cloud" over the neighbourhood. Most insurance companies would not cover damage caused by subsidence, she said.

Winstone has permission to mine a further 50m down to sea level. It is pumping 5 million litres of water each day out of the ground and into the city sewers.

Council water resource investigations team leader Alastair Smaill said one of the conditions Winstone agreed to when it was given consents to dewater was that it monitor the effects and report its findings.

Deficiencies were detected in the first two drafts of its first report, Mr Smaill said. "We've told them that their report's a load of garbage; they've gone away, got some further advice and are progressing quite satisfactorily, I think.

"The report did not accurately reflect conditions in the Waitemata sandstone; we didn't think they were accurately going to predict what was going to happen with the ground water levels in the future.

"During the original applications they made some predictions for the duration of the quarrying, how far the dewatering would go down, how fast it would go down, how far it would spread out and what rock formations it would occur in."

But the modelling assumed no dewatering would occur in the Waitemata sandstone (which lies below the volcanic rock) and proved wrong.

What the tests did show was that water levels were dropping over a much wider area of the city than originally expected and that land levels close to the quarry were not dropping as fast as expected, while further out they were dropping more than expected.

If the council was not happy with the report in November it would call a halt, limiting water extraction until levels stabilise.

"I would have preferred them to have done it quicker ... but we can't necessarily control it that closely."

james_gardiner@nzherald.co.nz