Rock Quality Designation (RQD) is a parameter used in rock mass classification systems defined as "the percentage of sound drill core with spacing of more than 10 cm". Due to its simplicity, RQD is collected by many mining operations, and correlations between large deformation ground behaviour and RQD (or bedding spacing) have been attempted. Even so, observations at CSA Mine indicate that RQD does not correlate well with squeezing ground behaviour in highly anisotropic siliceous siltstone rock masses. At CSA, case studies from QTS North and QTS Central indicate that large tunnel deformation occurs in both "poor" and "excellent" RQD materials. Large deformation was found to be driven by high stress states induced by the stope mining abutment, rather than low RQD. From the study, it was concluded that RQD alone insufficient to determine the likelihood of large deformations in high stressed areas.

1 Introduction

CSA Mine is an underground copper mine located near Cobar, NSW in Australia. The mine currently produces around 1.2 Mt/year at 4 % Cu. The CSA copper-lead-zinc orebodies consists of several steeply dipping parallel lenses that strike North-South. The orebodies occur within steeply dipping north-south trending shear zones which cut across the sedimentary rocks of the Upper Silurian-Lower Devonian Cobar Group (Tavakoli 1994).

The dominant historic and current mining method at CSA Mine is Sub-Level Open Stoping (SLOS) which is currently implemented down to 1700 metres below surface (mbs). The mining method relies on single lift stopes 25 m in height (sub-level interval was 30 m, transitioning to 40m below 8580 mRL (1620 mbs) mined under cemented paste and/or hydraulic fill. The maximum stope crown span and backfill exposures are generally around 25 m or less. The stope sequence is "centre in", "top down", with stopes on the hanging-wall leading to stress shadow stopes and access in the footwall.

The QTS Central is a newly found ore body which began access development mid-2018. The ore body lays between 8990 RL (1310 mbs) and 8515 RL (1685 mbs). The proposed mining method in the QTS Central is modified AVOCA with cemented rock fill (CRF). The proposed mining sequence is from both ends of the ore drive to the centre in 3 - 4 lifts. The ore body width is 6 m to 15 m and level spacing was designed at 30 m. Figure 1 below shows the layout of the QTS Central looking west and looking north.

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