Seafloor mining subsidence in the Sydney coalfield : D. J. Forrester, R. C. Courtney, S. V. Forgeron, J. McG. Stewart & S. T. Hart, CIM Bulletin, 89(996), 1996, pp 55–59

Abstract

The operation of any mine under large bodies of water brings with it the need for careful design of extraction operations. This is especially true of coal mines, that are characterized by extensive areas of extraction beneath weak, stratified rock and overburden. Longwall coal production in the Sydney Coalfield, Nova Scotia has, for many years, been concentrated in longwall mine workings under the Atlantic Ocean. Ground movement above the caved area behind the longwalls is typically characterized on the overlying ocean floor by shallow trough-shaped subsidence depressions. Precise knowledge of the magnitude and extent ofsuch subsidence is critical to the optimization of mine design to ensure safe and successful operations. Since 1983, the Cape Breton Coal Research Laboratory (CBCRL) and the Cape Breton Development Corporation (CBDC) have jointly researched the measurement of this seafloor subsidence. By observing seafloor topography before and after mining, comparisons are made to reveal the subsidence troughs. Many profiles have now been identified, first using a precision echosounder, but more recently, using swath and sweep bathymetry together with the Geological Survey of Canada-Atlantic (GSCA) and the Canadian Hydrographic Survey (CHS). Despite the relatively poor accuracy (±30%) of these approaches compared to the more usual geodetic land surveying methods (where land access is possible), much progress has been made in this key area of improving mine design. The paper summarizes the findings over both Phalen and Prince Collieries, giving the principal characteristics of observed transverse subsidence troughs. For the first time, exciting new developments obtained by swath bathymetry in 1994 over Prince Colliery are presented. These indicate that in certain circumstances and using the latest imaging techniques, postmining surveys alone may suffice to identify seafloor subsidence.