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The caving process relies significantly on the natural fragmentation of the rock mass created by the network of fractures and faults. In order to help mitigate the risk associated with unfavourable cave propagation and fragmentation in stronger or less fractured rock masses, the use of preconditioning through hydraulic fracture (HF) generation has increasingly been used. In an attempt to provide a quantitative evaluation of the likely impact of various preconditioning strategies, a combination of discrete fracture network (DFN) and hybrid finite/discrete element method (FEM/DEM) numerical simulations have been undertaken. DFN simulations provide a means to develop an accurate description of the in situ fragmentation. The impact of preconditioning can then be included by adding to the model stress parallel HF of a certain design size and interval spacing. The resultant fragmentation resulting from both natural and induced fractures can then be determined. This can include both the full fragmentation curve as well as the proportion of the rock mass forming large residual blocks of poorly fragmented rock that represent an increased risk of oversize potential. Volumes of the rock mass prone to significant residual block formation can be identified from cave scale DFN modelling, allowing the mapping of these potentially problematic zones of the rock mass. The DFN models can be transferred to a stress analysis method to directly simulate, through synthetic rock mass testing, the impact of preconditioning on rock mass strength. By using a cave-induced stress-path history, synthetic testing of simulated rock samples with and without preconditioning provides a clear indication of the impact of preconditioning on rock mass strength and primary fragmentation. Large scale models are used for simulating cave initiation and growth using embedded DFN fracture traces, to investigate the effects of preconditioning upon caveability.
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