Preconditioning by means of hydraulic fracturing is being used increasingly in deep block caves and has shown significant benefits. However, what does the hydraulic fracturing do to the rock mass? Does it change the stiffness and strength in a confined situation? How does it achieve the observed benefits, and how can the effects be modelled? Simple laboratory tests are described that demonstrate that, under confined conditions, fractures in the rock do not reduce the stiffness or strength of the rock. Mining involves unloading of the rock mass, and under these unloading conditions, associated with the induced and natural fractures, directionality of deformability and strength will result. Thus modelling of a hydraulically fractured rock mass is not trivial and the directionality, as well as alternative criteria for fracture and failure of the rock and rock mass, must be taken into account. Possible explanations of the benefits experienced after preconditioning are given.

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