This paper describes a field and numerical investigation on the response of overburden strata to underground longwall mining, with a focus on overburden strata movement and stress concentration in shallow strata. Subsidence related high stress concentration is believed to have caused damages to river beds in the Illawarra region, Australia. In the field study, extensometers, stressmeters and piezometers were installed in the overburden strata of a longwall panel at West Cliff Colliery. During longwall mining, a total of 1000mm tensile deformation was recorded in the overburden strata and as a result horizontal bed separations and gaps were formed. Bed separations were observed to start in the roof of the mining seam and gradually propagate toward the surface as the longwall face advanced. Substantial increase in the near-surface horizontal stresses was recorded before the longwall face reached the monitored locations. The stresses continued to increase as mining advanced and they reached a peak at about 200m behind the longwall face. A numerical modelling study identified that the angle of breakage (i.e. the angle of the boundary of caved zone) behind the longwall face and over the goaf is 22° to 25° to vertical. This is consistent with the high gradient of stresses and strains on the surface 150m to 320m behind the mining face in the monitoring results.


Mining induced ground damage can significantly increase mining costs where major surface structures, facilities and natural environments need to be protected from ground movements. Longwall mining under river systems, gorges, cliffs, power lines, pipelines, communication cables, major roads, railways, bridges, and other significant surface facilities has occurred at a number of underground mines in Australia. Increasingly, mine subsidence and ground damage are becoming a major issue of community concern, particularly in New South Wales. To date, subsidence under sensitive surface features is often controlled by leaving large blocks of unmined coal behind. This method not only sterilizes the coal resource, but also increases mining costs as a result of production loss and longwall relocation. Remedial and mitigation measures to manage damage caused by subsidence can be very costly. West Cliff Colliery is mining coal at a depth of 500- 600m using the longwall method. A regional river, Georges Rive, traverses much of the mine lease, and is sensitive to damages from mining activities. Mine induced subsidence and stress concentration are believed to be the key factors in the observed river bed fracturing and valley closures that have occurred in the past. Overburden grouting injection [1] is considered as a potential technology to minimize the impact of underground mining on the river system. However, successful application of this technology relies on a good understanding of the overburden strata movement and the near-surface stress change during mining. A research project sponsored by Australian Coal Association Research Program (ACARP) and BHP Billiton Illawarra Coal has been carried out with the specific focus to understand: • strata movement and bed separation mechanism occurring in the overburden strata above the coal seam during longwall retreat.

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