Pre- and post-failure ground behaviour: case studies of surface crown pillar collapse

Five case studies of collapse of surface crown pillars from Western Australia and the Northern Territory were analysed. It was found that there was a pattern in the geotechnical behaviour of the rock mass prior and after the collapse. The pattern indicated that the hangingwall was deteriorating years or months prior to collapse. Weeks before, level pillars were collapsing and broken rock rilled down the stopes. Within days or even hours before the collapse, rock noises and rock falls were reported in various parts of the mines. To monitor different behaviour of the rock mass it is recommended that appropriate geotechnical instrumentation and monitoring be used. Deterioration in ground conditions can be triggered by external sources like mass blasting or a seismic event. After the collapse, the postfailure behaviour in a from of further ground collapse and gradual movement continued for a number of years.