Authors: Duran, A

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Duran, A 2021, 'Horizontal drain holes: a case study', in PM Dight (ed.), SSIM 2021: Second International Slope Stability in Mining, Australian Centre for Geomechanics, Perth, pp. 415-428,

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Horizontal depressurisation holes (drain holes) form one of many options that are available for the depressurisation of mine pit slopes to improve stability. Whilst horizontal drain holes have been employed for over 80 years (Royster 1980), the literature commonly indicates their benefit to slope stability through the causal relationship of slope movements decreasing after drain holes have been installed (Seegmiller 1979; Rahardjo et al. 2003; Tsao et al. 2005; Beale et al. 2013). Clermont Coal mine is located in an atypical coal mine setting where a major thrust fault in the west wall results in Cambrian aged foliated metamorphics (basement) being thrust over Permian aged coal measures. Geotechnical studies have identified an area of the west wall where a significant exposure of foliated metamorphics will be exposed over most of a 200 m high slope with foliation dipping towards the east. Stability analyses have inferred the potential for unfavourable stability at the overall pit wall scale. Owing to existing infrastructure at the crest, a cutback or unloading were not considered as viable design alternatives whilst leaving a buttress of coal at the toe was considered economically less desirable. As such, a depressurisation program was considered, with horizontal drain holes put forward to depressurise the slope and improve stability. This paper presents an overview of the results of the depressurisation program and with particular emphasis on the interpretation of pore pressures within the slope based on the combination of both vertical and horizontal multi-nested vibrating wire piezometers.

Keywords: depressurisation, drain holes, horizontal piezometers, slope stability, open pit, coal mining

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