Mining-induced stress change and consequences of stress path on excavationstability - a case study

For stability assessments as well as for support design, it is important to understand the factors leading to detrimental stress changes. Stress chang es not only influence the demand on the rock support, they also change the support capacity of frictional support components such as plain cablebolts. Stress and stress changes are commonly predicted by numerical models but i t is rarely possible to verify these predictions. This study presents a pra ctical example illustrating the usefulness of stress change measurements in providing an accurate picture of the mining-induced stress changes and the ir Value for numerical model calibration. Stress changes, associated with m ining of the 565#6 stope at Winston Lake Mine, were measured by four CSIRO HI stress cells, two in the hangingwall and two in the back of a sill drift . In order to obtain the full stress history of the hangingwall, stresses a t the installation time were calculated by use of a three-dimensional bound ary element program - MAP3D. Two types of rockmass failure were observed at this mine: hangingwall delamination and collapse leading to ore dilution, and wedge-like failures in the backs of sill drifts. All stress cells were located such that they were affected by these failure mechanisms. Using mea sured and predicted stress paths, this study explains the nature of these t wo failure modes, as experienced at Winston Lake Mine. Excellent correspond ence between measured and predicted stresses is achieved for both the hangi ngwall and the back of the sill drifts. Modelled stress rotations in the ha ngingwall follow closely the stress rotations determined in the field, exce pt at the end of the monitoring campaign when the stress cells were affecte d by inelastic straining of the host rock. These measurements assist in und erstanding the failure processes and in defining the limitations of the ado pted numerical models. (C) 2001 Elsevier Science Ltd. All rights reserved.