Hazard Zones and Cracking Addressed in Report        Waihi Subsidence Assessment

In 2002 GNS Science, responding to a Hauraki District Council brief, presented a report to the Hauraki District Council identifying hazard zones above portions of the historic Martha Mine workings. The investigation of the report followed the 1999 collapse and was being undertaken at the time of the 2002 Barry Road collapse.

In 2005 Council’s Pit Peer Reviewer recommended that “an update of the GNS Risk Assessment is necessary.” This recommendation was made following the 2005 Pit Peer Review on the basis that new information had become available from observations made during the excavation of the current mine pit for consideration since the 2002 report.

Acting on this advice the Hauraki District Council requested that GNS Science undertake the recommended review and reassess its earlier conclusions to see if they remained valid.

The report, presented to Council today, confirms that the identification of the hazard zones and the mechanisms resulting in the cave collapsing remain valid. The opportunity has been taken to extend the mapping of the hazard zones into the current pit. (This was not part of the brief for the 2002 report.) New hazard zones have also been identified east of the pit over the old Martha, Mary No 2 and North Branch lodes.

Council’s Chief Executive, Langley Cavers, stated that these new hazard zones did not involve any private properties and the hazard zones identified east of the pit were all on land owned by Newmont Waihi Gold.

The report identifies that appropriate land uses for the hazard zone areas include passive recreation that allows for walking tracks, roads and they have been confirmed as having an acceptable level of risk for these purposes.

Mr Cavers stated that this now allows the Council to recommence its work for the rezoning of the land for these purposes and also allowed for the proposed closure plans to be prepared with certainty and confidence. This work had been part of the District Plan Review and was put on hold pending the review of the 2002 Report.

Long term subsidence associated with the old, abandoned underground workings has been identified in the report as the most likely cause of the cracking and ground deformation that is visible in upper Seddon Street, Waihi. The report also notes the open pit cannot be ruled out as having a possible influence on the speed with which this creep is occurring.

GNS Science has advised that with complex geological and geotechnical processes such as those occurring in Waihi that it is not possible to say with certainty what the amount or extent of the tolerable ground subsidence may be and over what timeframe it may occur. Therefore they have recommended ongoing and long term monitoring be undertaken to ensure that there is early identification of any such movement. They have recommended the performance standards monitoring should meet as it needs to measure both horizontal and vertical movements.

The release of the report had been delayed as GNS Science had recommended that a new measurement method called DinSar (satellite-borne synthetic aperture radar (SAR) images) may be a valid method of measuring ground movement. This method was trialled but unfortunately the results were equivocal and the movement may be too small to be measured by this technique. This approach will be further investigated in the future.

The Mayor, Mr John Tregidga, stated that he was pleased that GNS Science has confirmed that there is no risk to public safety from the ground subsidence that is causing the cracking and he fully supports the introduction of long term monitoring to develop a better understanding of the subsidence mechanisms.

Council’s Chief Executive, Mr Langley Cavers, advised that the Council had agreed to institute the recommended monitoring regime as soon as possible and would be preparing a brief that met the performance standards recommended by GNS Science.

Mr Cavers further advised that the monitoring that was recommended was for the long term and the results of such monitoring may not be known for many years but that Council and the public would have the on-going results on a regular basis.

He noted that the underground mine working closed in the early 1950’s and that ground subsidence had been an ongoing issue since that time. The earliest visible sign of this movement was the formation of the well known “Martha Lake” over the milking cow subsidence area.
 

View the report - Waihi Subsidence Assessment

The report is also available from Council Offices and Libraries.

For further information contact Langley Cavers or Mark Buttimore on 07 862 8609