A simple method of estimating far field movements associated with longwall mining

Traditional methods of predicting surface subsidence movements above areas of total extraction, in near horizontal coal seams, are based on work by the British National Coal Board. The methods predict vertical settlements of less than 20 mm beyond an ‘angle of draw’, which is typically about 25 to 30 degrees from the vertical, measured from the edges of an extraction area. The methods also predict insignificant lateral movements outside this draw angle. This traditional approach is satisfactory in geological environments where the natural horizontal stresses are less than, or about equal to overburden pressure. However, in areas such as the Southern Coalfields of the Sydney Basin, where horizontal stresses are much higher than overburden pressure, significant lateral movements at the ground surface occur well outside the traditional subsidence zone. These movements were first noted at the Cataract Dam some 30 years ago but, at that time, were dismissed by much of the profession as aberrations. Subsequently, more field data showed movements several kilometres away from groups of longwall panels and these became accepted as real and were termed ‘far field displacements’. Collation of many such measurements provided some empirical guidelines for the probable magnitudes of lateral movements, but with no idea of how they may be expected to vary in compass direction from a new longwall, or how they may be affected by successive longwalls.

This paper provides details of a simple method developed since 2009 in conjunction with mining at the Appin Colliery beneath the Hume Highway and within distances of important bridges that could be affected by far field movements. Comparison with field measurements, that followed predictions made using this method, have given confidence that the method is a useful design tool.