ABSTRACT: In high stress environments, as the rock mass yields and fails, fracturing is initiated through intact rock and the extension and shear failure of pre-existing fractures (joints). When close to openings these fractures are expected to be tensile in nature, but further away with increased confinement, shearing is expected to become the predominant mode of failure. Associated with this fracturing in brittle rock masses, are microseismic and seismic events. This microseismic activity, although problematic to operators, can give an indication of the state of the rock mass, in relation to fracture initiation, accumulation of damage and ultimate failure. In this paper the authors examine the use of the spatial and temporal distribution of seismicity at two mine sites over a 6 year period of progressive failure into the post-peak region, relating the seismic event density, combined with the temporal examination of the principal components analysis (PCA), to characterize the extent and trend of the yield zone, and the significant stages of the failure process.
A great amount of research has focused on the properties and behaviour of laboratory samples of hard rock up to the peak strength and into the post-peak failure region. Similarly, much research has focused on the bulk properties of in situ rock masses up to the peak strength. This is largely due to the expense and technical difficulty in performing large controlled compressive tests at the rock mass scale and from a civil engineering standpoint failure is often considered just after the peak strength, when the material no longer fulfills its engineering function. In the hard rock mining industry, at high stresses the rock mass is often driven not just to the peak strength, but often well into the post-peak until ‘complete failure’ occurs.