Roof Support in Gateroads in Multiple Seam Longwalls - Lessons From Ultraclosse Mining at Gibson's Colliery

Gibsons Colliery is mining the Balgownie Seam at less than 10m below the extracted Bulli Seam. The mine enters the Illawarra Escarpment and the depth of cover increases rapidly over the first 500m until it plateaus at approximately 220m. A number of roof falls have developed in intersections – the roadways have mostly been free of falls unless aligned parallel and coincident with roof joints. Underground observations are that the falls are associated with a roof that has no horizontal confining stress – the falls are either joint bounded blocks or failed cantilevers. A roof support strategy based on forming thick cantilevers and if necessary standing support has been developed. A model for how similar stress regimes could develop in multiple seam longwalls has been developed.