Underground mining of gold-silver bearing quartz lodes was carried out in the great mines at Martha Hill, Waihi over 70 years from 1882 until 1952. Shafts and stopes reached a total depth of 600 m on 16 levels, with workings extending laterally some 1.6 km, and as new reefs were discovered the workings extended beneath parts of the township. Over their long life the mines produced 1217 tonnes of gold-silver bullion from processing 12.2 million tonnes of mined ore. In the early-1980s drilling proved large reserves of gold and silver ore remaining in stock-work zones between the previously mined lodes. This led to a new era of mining at Waihi with the opening of the Martha Hill open pit in 1988. A series of three sink holes have formed above unfilled stopes of the Royal lode. In 1999 a sudden collapse crater 40 m in diameter occurred near an occupied part of Waihi, placing in danger several houses and a busy street. This crater formed close to a similar crater that occurred in 1961. The 1999 collapse resulted in subsurface invesligalions and risk assessmenl sludies being commissioned by Ihe local Hauraki Dislricl Council. However, a further 50 m diameter collapse crater in December 2001 formed in a residential area, swallowing and wrecking an occupied house, fortunately withoul injuring its five occupanls. Two other houses were perched on the crater rim. The 2001 collapse led to an acceleralion of the work being carried out by the Inslilule of Geological and Nuclear Sciences Ltd. (GNS) to identify the extent of the old underground workings at Waihi. For the risk assessment detailed rock mechanics analyses were made to consider the stability of the underground mine slopes, in terms of the stope back, hangingwall, footwall and crown pillar failure mechanisms, as well as void migration. The collapse crater areas were analysed using using several methods, including analysis to simulate the probability of void migration from unfilled stopes to the surface using the @Risk program. The analyses show that in the cases of the actual collapse craters, the voids have a high probability of migrating to the surface, thus providing a method of lesling the void migration modelling. Modelling analysis applied lo the remaining slopes of the old underground mines show there are further areas where void migration from unfilled slopes has a high, medium and low probability of reaching the surface as a collapse crater. The locations of these probabilistic hazard zones were oullined using the @Risk modelling and delerministically by using the dimensions of the historical collapses applied along the lenglh of the slopes. Consideration of the 'incubation period' (the lime interval belween the cessation of mining and the occurrence of collapse) and comparisons with mine subsidence histories overseas, indicates that further collapses are likely lo occur over the next 50 years. Risk assessmenl estimates made using property values and occupancy levels wilhin the three hazard zones, were compared with Societal and Individual Risk criteria. Results indicate that safety within the high and medium zones may be unacceptable.
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