ABSTRACT:
Overburden grout injection is a cost-effective technology used to control coal mine subsidence by injecting fine waste material (e.g. flyash) into the overburden during longwall mining. The injected material fills the bed separation gaps formed above the mining horizon and reduces the propagation of the overburden movement to the surface. The success of this technology relies on a good understanding of the bed separation process in the overburden strata during longwall mining. A research project has been recently completed by CSIRO in which the feasibility of applying the overburden grout injection technology to Australian underground mining conditions was investigated. Within the study, the overburden strata of two operating underground coal mines were monitored using deep-hole surface extensometers. The location, magnitude and timing of bed separation development during mining were identified. A 3D numerical code developed by CSIRO for modelling stratified rock mass was used to predict the overburden movement at several mines, and the numerical results compared favourably with the surface subsidence measurements. The code was improved to simulate the effect of grout injection on the overall overburden movement and the final surface subsidence. The study also included a field trial in which flyash grout was injected into shallow ground above a mined panel. The injection process was monitored using pressure sensors and flowmeters. The flow of grout in the mining induced fractures was observed using a downhole camera and Ground Penetration Radar (GPR).
1 INTRODUCTION
Mining induced subsidence can significantly affect mining costs where major surface facilities and natural environments need to be protected. Longwall mining under river systems, gorges, cliffs, power lines, pipelines, communication cables, major roads and bridges, and other significant surface facilities has occurred at a number of underground mines in Australia. Increasingly, mine subsidence is becoming a major issue of community concern.
