Dugald River trial stoping, overall hanging wall behaviour

Authors: Hassell, R; de Vries, R; Player, J; Rajapakse, A Download Paper

DOI https://doi.org/10.36487/ACG_rep/1511_08_Hassell

Cite As:
Hassell, R, de Vries, R, Player, J & Rajapakse, A 2015, 'Dugald River trial stoping, overall hanging wall behaviour', in Y Potvin (ed.), Design Methods 2015: Proceedings of the International Seminar on Design Methods in Underground Mining, Australian Centre for Geomechanics, Perth, pp. 185-198, https://doi.org/10.36487/ACG_rep/1511_08_Hassell



Abstract:
Concerns regarding the uncertainty of stope dilution at MMG Limited’s Dugald River Underground Mine led to a trial stoping program to acquire full-scale comprehensive geotechnical information and to test the validity of the proposed geotechnical and mining parameters. As part of the trial, a geotechnical instrumentation program was designed to improve the understanding of the rock mass response to the mining of the trial stopes. This included an array of instruments installed from dedicated hanging wall drives, hanging walls that included 66 multipoint borehole extensometers (MPBX), 23 time domain reflectometers, 24 geophones, six accelerometers and 15 observation holes. Trial mining consisted of 19 sublevel open stopes with cemented rockfill (CRF) that extracted 335 kt of ore, with a panel height of 130 m and strike of 100 m. All stopes were monitored with a minimum of two MPBXs at the open span with a MPBX average density in the panel hanging wall of 180 m2. The MPBXs were 19 m in length and consisted of six nodes. Instrument displacement information was recorded every 30 minutes by dataloggers to a resolution of 0.12 mm. This provided a high quality, detailed and extensive rock mass response to stope extraction. The displacements were interpreted as hanging wall relaxation and shear on structure from specific stope firing and expansion of the mining front. Rock mass creep was also recorded in the hanging wall. The mechanism of displacement consisted mainly of strike-slip movement on high angle bedding and faults that are orientated sub-parallel to the dip of the orebody. Overall hanging wall behaviour was assessed by investigating the near (stope hanging wall boundary to 6 m) and far (>6 m from stope boundary) displacements as the sequence progressed. The primary rock mass response stage was the extraction of the crown pillar. The secondary responses were associated with the cable bolt arrays mitigating some of the deformation of the hanging walls with more movement mid-span compared to the cable bolt horizon. Finally, the displacement results post-filling showed the backfill mass was being slightly compacted by ground movement.

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