THE CHARACTERISTICS OF SUBSIDENCE DUE TO UNDERGROUND COAL MINING AT NEWCASTLE, NEW SOUTH WALES

Ground movements occur when fluids or minerals are mined from beneath the surface and a subsidence bowl or trough subsequently forms over the extraction. The characteristics of all subsidence bowls and troughs include the vertical and horizontal displacements of points on the surface, the change in the slope of the surface and the tensile and compressive strains. The magnitudes and development o £ these elements vary with the depth of cover, the geological environment and with the extent and rate of mineral extraction. Subsidence surveys are being carried out over areas of underground coal extraction in the Newcastle district, 160 km north of Sydney. The investigations in one area showed the development of the subsidence, slope and strain at the surface as the extracted area increased. The maximum principal strains were determined at various stages of extraction. The characteristics of the subsidence trough over coal extraction were similar to those of the subsidence bowl over extraction of oil at Long Beach, even though the maximum subsidence at Long Beach was 8 times greater