We review our endeavor to probe spatial variation in stress in seismogenic zones in deep hard-rock gold mines in South Africa. Highly stressed ground, especially in remnants or pillars, often increases the risks posed by normal-faulting earthquakes to mining operations. Some of these seismogenic zones were identified by AE monitoring, allowing us to explore them with drill holes shorter than several tens of meters in length. A M5.5 strike-slip earthquake, not a usual mining-induced earthquake, took place in 2014 on an unknown nearly-vertical geological structure, with the upper fringe of the aftershock zone several hundreds of meters below the mining horizons. The aftershock zone was elucidated by in-mine dense geophone network and surface strong motion network. This allowed us to drill two holes of 817 and 700 m length from 2.9 km depth. Integration of in-situ stress measurements by both conventional and new methods allows us to constrain the stress field accurately.

1 Introduction

Understanding and controlling the behavior of highly-stressed heterogeneous brittle material is a significant challenge. Complicated and difficult conditions are often encountered in deep mines, especially during excavations in highly-stressed rock masses penetrated by faults and dykes, or in the seismogenic zones of natural earthquakes (e.g. Ortlepp, 1997; Brown, 2012; Mori & Ellsworth, 2013). Tau Tona gold mine in South Africa, which Fairhurst (2017) describes as the world's deepest mine in the 20th century, was closed in 2017, and other deep South Africa gold mines will close sooner or later. With increases in depth or stress during mining, accurate quantitative stress information is critical. This keynote paper reviews our endeavor to obtain this information in deep gold mines in South Africa.

2 In-Situ Stress Measurements in South Africa

As reviewed by McGarr & Gay (1978), some important techniquesfor in-situ stress measurement have been developed in South Africa, such as the doorstopper or CSIR triaxial cell. Stacey & Wesseloo (1998), Wesseloo & Stacey (2006), and Handley (2013) reviewed the data base of hundreds of in situ stress measurements in South Africa. During 1998-2002, the DeepMine and FutureMine research programmes gained knowledge and developed technology to support ultra-deep gold mining in South Africa (e.g. Durrheim 2007). Their agenda included studies of the feasibility of in-situ stress measurement in rock masses prone to earthquakes and drilling-induced damage. The programmes measured stress at a depth of 3351.6m using 10 CSIR, door stopper and CSIRO cells, but only achieved one reliable result with vertical stress of 91±11 MPa. Difficulties were mainly caused by inflexibility in the overcoring procedure (especially drilling direction) in highly stressed ground, which is very prone to core discing or borehole breakout. A SATREPS project "Observational studies to mitigate seismic risks in mines (2010-2015)" downsized the Compact Conical-ended Borehole Overcoring (CCBO) technique (Sugawara & Obara, 1999) from N-size to B-size. It was under the auspices of Japan Science and Technology Agency and Japan International Cooperation Agency (JST and JICA, respectively). CCBO proved to be more suitable for adverse condition and difficult access from surface typical of deep level South African gold mines. Several sequential BX CCBO overcorings could be completed in the same time that it took the CSIRO HI technique to complete a single overcoring (Ogasawara et al. 2012, 2014a). Since then, the BX CCBO technique has been used at the deepest mining level in gold mines, in crush pillars in some platinum mines, and in a copper mine in South Africa. Ogasawara et al. (2014a,c) also discussed the effects of anisotropy and inelasticity on overcoring in highly stressed ground.

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