Year

1984

Degree Name

Doctor of Philosophy

Department

Department of Civil and Mining Engineering

Abstract

SUPPLEMENT : RESTRICTED (not available on-line)

Coal is being mined from seams which lie beneath urban areas around the City of Newcastle, New South Wales, under nearby Lake Macquarie and the Pacific Ocean. Subsidence occurs as a result of pillar extraction or longwall mining and homes and other structures or surface features can be affected.

Detailed field surveys commenced in the late 1960's in the coalfields north and south of Sydney. This work was developed and extended by the author in order to provide the basic information which was used to develop a method of subsidence prediction. Because of the large quantity of results, several computer programmes were designed to handle the calculations, filing and data manipulation from the field booking sheets to the presentation of calculated results from the computer.

Detailed analyses of the results of these surveys have enabled the relationship between the geometry of mine workings and the subsidence at the surface to be established for the particular geological environments. This research has shown that the massive and strong conglomerates of the Newcastle area have a significant effect on the value of the maximum subsidence. For the lower range of widths of extraction, the investigations showed that the nature of the caving of the roof strata over the mined seam influenced the magnitude of the surface subsidence.

Subsidence in Newcastle was shown to be significantly less than what would be experienced in areas of mainly argillaceous strata such as in the United Kingdom because of their different caving properties and strata deformation characteristics.

On the basis of a theoretical subsidence prediction method developed for the Newcastle area by the author using the survey, geological and mining information, panel and pillar mining layouts are now designed for maximum coal recovery consistent with small values of maximum subsidence. With the aid of these locally established guidelines, longwall extraction is now taking place beneath the Pacific Ocean, Lake Macquarie and areas of surface development along their shorelines.

The research showed that, in the Newcastle District, subsidence develops in four stages as the ratio of the width of extraction w to the depth of cover h increases for panels of critical or supercritical length.

Extraction layouts with subcritical w/h values up to 0.55 result in a slight undulation of the surface and it is within this range that panel and pillar extraction layouts have been designed for protection of structures on the surface. Where the w/h ratio is greater than 0.55 and less than 0.65, a pronounced subcritical subsidence trough develops.

For extraction layouts where the w/h ratio is greater than 0.65 the maximum subsidence increases rapidly as the conglomerate within the strata fails to support itself over the increasingly wide panel. The subsidence reaches its maximum possible value at a critical w/h value of around 1.3. In the supercritical range above 1.3 the maximum subsidence is 0.65 of the seam height mined.

The research also revealed the factors which influenced the maximum subsidence such as the recovery of coal from the seam mined, the caving of the roof strata, whether other seams have been mined, the presence of significant faulting and the stability of pillars which remain between or adjacent to extracted panels.

Other features of subsidence profiles in Newcastle studied in relation to the mine geometry were the shape of the subsidence profile, the various relationships between subsidence, slope change, curvature and strain, and the time-subsidence relationships. It was also discovered that the travelling slopes and strains above an advancing face are significantly less than the final static values over the end of the panel. Strain triangles were used to investigate the magnitudes and directions of the maximum and minimum principal strains.

The principles developed as a result of the author's research work are unique to the Newcastle district north of Sydney where they are now used as a predictive tool in the control of mine subsidence.

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