THE IMPACTS OF MINE SUBSIDENCE ON CREEKS, RIVER VALLEYS AND GORGES DUE TO UNDERGROUND COAL MINING OPERATIONS

ABSTRACT: Measured subsidence profiles above coal mining operations regularly show less than expected subsidence at creeks, river valleys and gorges. Horizontal measurements across such surface notches indicate that they reduce in width as mining occurs. The reduction in subsidence is referred to as ‘upsidence ’ and the reduction in width is referred to as valley closure. The upsidence and closure movements tend to increase in amplitude as the size of the valley increases, and as the magnitude of subsidence increases. The movements are greatest when the insitu horizontal stresses are high and when the valleys are fully undermined. The upsidence is a combination of anticlinal valley bulging and buckling or shearing of the surface and near-surface strata. As Longwalls 8 and 10, at Tower Colliery, in the Southern Coalfield of New South Wales, were mined beneath the Cataract River Gorge, the incremental upsidence in the base of the gorge, due to mining each longwall, was approximately 360 mm, resulting in the base of the gorge being uplifted as much as 250 mm above its original level. At the same time, the width of the gorge was reduced by approximately 280 mm. Cracking and buckling of the strata, within the base of the gorge, resulted in a loss of water from some of the natural ponds in the bed o