A Rock Engineering System Based Abandoned Mine Instability Assessment Index with Case Studies for Waihi Gold Mine

Mining as one of the fundamental elements of the economy has left tens of thousands of mines abandoned. With increased urbanisation and population growth, industrial or residential sites become prone to post-mining geotechnical hazards such as subsidence from abandoned mining sites. Instabilities of abandoned workings may, also, interfere with active mining, imposing risks and considerable delays in the operations. Proper knowledge of these geotechnical hazards and the mechanisms of instabilities are critical for the classification and prioritisation of the sites for rehabilitation or repurposing of abandoned mining sites.The long-term stability analysis of abandoned mine workings is a complex task for engineering geologists and rock engineers. Numerous factors have been identified here as affecting the stability of underground mining excavations including depth, age, shape, and geometry of the mining excavations, underground water, in-situ stress state in the area, geotechnical characteristics, discontinuities, seismic loading and most importantly weathering and deterioration of the rock masses. To provide an efficient approach for assessing the complex phenomena of geotechnical hazards associated with shallow abandoned underground stopes in hard rock environments, with interconnected parameters, this study proposes a systematic approach by adopting and modifying the Rock Engineering System, RES, (Hudson 1992b). An Interaction Matrix is developed, the critical factors and dominant modes of instabilities are identified, and the hazard potentials are determined. An empirical index known as the Abandoned Mine Instability Index, AMII, is proposed to enable a quick and preliminary assessment of geotechnical instability and subsidence hazards in post-mining areas. This systematic approach was tested against one case history involving multiple crown pillar collapses and sinkhole subsidence incidents related to historic underground stope mining followed by a modern adjacent open pit at the Waihi mine in New Zealand. Information from relevant literature was used to assess the post-mining geotechnical hazards. The outcomes of the study indicate that the methodology can be successfully applied to differentiate between stable and unstable areas at the same mine. Future comparisons of mines in different regions are required to fully confirm the calibration of the AMII.