Abstract

It is of interest to numerically simulate hydraulic fracturing in block caving as an aid to underground mining. UDECTM software was used to simulate the behavior of a two-dimensional jointed rock mass with a defined undercut under biaxial in-situ stresses. The effect of stress ratio on flow into the joints has been studied as a function of injection distance from the top of the undercut. Given the difference in situ stresses, joints reopen under the induced pressures dominantly parallel with the maximum principal stress, a consequence of work minimization. In addition, the effect of injection distance on the stimulated area has been studied. It seems that there is an effective injection distance beyond which rock mass response remains approximately constant. Applying hydraulic fracturing further from the effective distance would precondition the hard rock but it would take longer injection times, rates or pressures to reach the caved block.

1 Introduction

Hydraulic fracturing as a treatment method to aid mining has been suggested for several decades in the coal and metal mining industry. Different applications have been suggested as being potential contributions to safety and economic benefits. Recently, hydraulic fracturing has been applied to precondition rock or to induce caving in hard rock around mines. Fractures during mining are conventionally induced by blasting to help the broken rock mass flow from conditioned stopes under gravity forces. More and more, massive caving processes are used to extract entire ore bodies, involving large-scale rock mass fragmentation through massive stress redistribution, and leading to induced caving and flow for the entire orebody, while trying to reduce the amount of blasting needed and also reduce risks of uncontrolled bursts and inadequate fragmentation. Preconditioning of the rock mass to aid this process via hydraulic fracturing involves pre-weakening of orebodies with low friability that are not caved easily (Vyazmensky et al. 2010 and Rahman et al. 2002). Hydraulic fracturing also provides the possibility of stress relief and redistribution, reduction in the stiffness of rock masses, and other associated effects that can increase rock mass caveability to allow it to cave continuously in a controlled stable manner (Jeffrey et al. 2009).

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