Geology and Geotechnical Superintendent, Anglo American – Grosvenor Mine
Trainee ERZ Controller, Anglo American – Grosvenor Mine
A series of floor heave and gas inrush events have occurred during the development mining process in MG103 and MG104 at Grosvenor Underground Coal Mine. These events have exposed coal mine workers to elevated levels of methane preventing safe mining operations.
The presence of an undrained source of gas in the immediate floor, geotechnical floor characteristics, loading environment and various other factors have contributed to the dynamic floor failure. Methane released during these events are originating from the underlaying thin Goonyella Middle Lower (GML) seam which is a thin carbonaceous layer with high ash content. The 1m – 5m interburden thickness between the GM seam and the GML has an increased likelihood of the floor gas release events.
Based on the analysis of these gas events, creating a conduit in the interburden between the GM seam and GML will allow the gas to freely release to the development roadway during development drivage. This will prevent the build up of gas within the interburden creating a floor gas release event.
The proactive interburden fracturing was initiated using water pressure generated from a longwall salvage pump. The current UIS drilling equipment was retrofitted with a series of subs, packers and a fracturing tool to initiate a hydro fracture within the drilled UIS borehole. Once the packers are fully inflated and in position, a diversion valve is then activated to allow the fracturing tool to inject high water pressure to the desired location. Thus, given the complexity of predicting verticality of the hydro fracture in the interburden, a UIS borehole was drilled in the lower section of the GM seam as proving hole to check the effectiveness of hydro fracture.
The main benefit of the proactive interburden fracturing process is having the ability to reduce the likelihood of exposing development coal mine workers at the face to high methane levels.