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The ability to understand and predict the effectiveness and evolving capacity of ground support in mine accesses is paramount to the success of an operation. This paper describes the results of numerical modelling studies of ground support performance in access drifts at Eleonore Mine. These drifts are located in competent rock with undulating sub-horizontal and vertical jointing present and are subjected to mining-induced stress. The studies employ Bonded Block Models (BBM) with explicit ground support represented by the newly developed hybrid bolts in 3DEC (Itasca Consulting Group, Inc. 2016a). These models are used to back-analyse and understand past falls of ground as well as to evaluate the adequacy of the current designs to handle future conditions as mining advances. The shallow dip and undulation of the sub-horizontal joints strongly impact the back behaviour in accesses, with joints susceptible to shearing and opening under high horizontal stresses. The results of the study elucidated that the falls of ground experienced at the mine are due to the bolts in the back (#6 rebar) being heavily strained locally through sub-horizontal joint shearing as nearby stoping took place, causing them to rupture. As foreseen by the mine in 2016, the model results confirm the need for bolts with higher shearing capacity to support the on-ore drifts. Both D-bolts and #7 rebar were analysed as alternatives with the model and shown to provide superior deformation compatibility, which agrees with experience on the mine. D-bolts offer a less stiff response (which increases survivability under highly localised deformations), while #7 rebar offers a higher yield strength. This paper describes how numerical models that allow for realistic fracturing and bulking of the rock mass along with explicit representation of pre-existing discontinuities can be used to more realistically model ground support performance in competent rock.

The design of effective ground support is critical to the success of the next generation of large cave mines. Typically, the behaviour of rock in this setting is controlled by shear and opening movements along fractures induced by the excavation and cycles of loading and unloading during mine development and operation. Support of highly stressed fractured rock shows two important aspects: (i) in blocky or fractured rock the majority of the support deformation is localised at fractures or joints and (ii) the resistance to fracture shear displacement offered by support is important. These observations have led to the use of the discrete element method based three-dimensional bonded block model (BBM) to represent the rock and the hybrid bolt model to represent the bolt support. The hybrid bolt model is an improvement on the classical cable bolt model, which features a more realistic resistance to fracture shear displacement and allows bolt installation in a fractured rock mass exhibiting open joints. This paper presents the application of the BBM and hybrid bolt numerical model to study the performance of tunnelling at depth under a caving-induced stress path. The effect of support pressure, bolts presence, and partial debonding of bolts, is explored. The influence of rock mass quality and support installation timing on the support efficiency is also investigated. It has been found that a small surface pressure is critical for tunnel performance. When a rock mass experiences large deformation and joint opening, local axial and shearing straining of bolts at joints intersection become important and can lead to rupture. Intense axial straining can be mitigated through debonding and intense shearing can be reduced through the use of bolts with higher shear resistance, such as rebars. Keywords: tunnel support bolt, numerical simulation, bulking, 3DEC