Viscoplastic discontinuum model of time-dependent fracture and seismicity effects in brittle rock

Abstract

A model is proposed for the direct mechanistic simulation of seismic activity and stress transfer effects in deep level mines. The model uses a discontinuum viscoplastic formulation to relate the rate of slip on a crack to the shear stress acting on the crack. A procedure is outlined for the solution of a collection of interacting cracks in a series of time steps and for the computation of energy changes in the crack assembly during each time step. Elastodynamic effects are not considered. In spite of the simplicity of the proposed slip law, it is shown that complex material behaviour can occur if the model is applied in a random assembly of cracks. A particular demonstration is given of the simulation of primary, secondary and tertiary creep phases in a uniaxially compressed sample containing an initial population of weak flaws. The model is next applied to the simulation of face advance steps in a deep level gold mine excavation and is shown to give favourable agreement between observed seismic activity and the length of fractures mobilized in a random mesh of cracks around the opening. The modelled closure between the excavation roof and floor, as a function of time, is also shown to be quantitatively similar to the observed field movements. A final example is given of the mining of a parallel-sided excavation at different rates which illustrates the tradeoff between high face advance and high seismic activity and low face advance but potentially greater damage in the rock near the stope.