CASE STUDY: Exploitation of new reserves is key to a prosperous mining operation. Towards this end, mine operators are looking more to the tunnelling industry for rapid and economic methods of excavating access roadways and shafts for new projects. In the mid-1980s, a full face TBM was used for the first time by the mining industry in Canada to develop a vast coal deposit extending out under the Atlantic Ocean off Cape Breton Island, Nova Scotia.
SITE VISIT REPORT: MAY 1985
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f the traditional heavy industries, as manpower has become more expensive, the future of deep seam coal mines worldwide depends heavily on rapid, economic development of new deposits. This has forced, rather than encouraged, the introduction of labour saving methods, which in turn has radically changed the design and operation of new developments. On Cape Breton Island, Nova Scotia, unemployment, in a once major coal producing area, is high. Developing a vast coal deposit that dips in a 7-8% declining wedge out under the sea, was one positive way of solving the problem and increasing the productivity of the community. It was decided to drive two 3.5km-long drifts to intersect with the seams at approximately 200m below sea level and 150m beneath the sea bed. To excavate these in the shortest possible time, the operators, the Cape Breton Development Corporation (CBDC), established by the Canadian Government, looked for the most economic tunnelling methods. After visiting and examining various European coal mine expansion projects, particularly the modern Selby development in Britain, CBDC was convinced of the inherent advantages of driving circular roadways with a full face TBM, and decided to apply a similar concept. This ambitious project, named the Donkin-Morien Mine Development, is stratigraphically situated within a unit that consists of 425m of interbedded sandstones, siltstones, mudstones and shales within which are located nine major coal seams, four of economic importance.
lnterbedded sediments
Difficult site investigation, requiring an offshore drilling programme, indicated that mixed ground would be encountered, with compressive strengths varying between 10MPa and 100MPa. The tunnels are designed to follow the dip of a sandstone layer before levelling out at -176m for intersection with the Harbour Seam at -200m. Site investigation data suggested a homogeneous sandstone formation with no faults. It was the drive through the interbedded layers, after leaving the sandstone that could pose problems due to possible faulting. A TBM capable of handling all eventualities and rock types was required. As a government-funded project, CBDC wanted to keep it a wholly Canadian operation and it was with the cooperation of the National Research Council of Canada (NRCC), the Department of Supply and Services (DSS), and Lovat Tunnel Equipment of Toronto, that the first Canadian-built full face TBM for medium and hard rock drilling was designed. Development costs of more than Can$l0 million were shared by the NRCC, DSS and CBDC. In July 1981, CBDC invited tenders for development of the Donkin-Morien Mine and awarded the contract to Beaver Underground Structures of Canada. Beaver was already the owner of a Lovat 4m diameter soft ground TBM, and it was agreed that this would be used as an experimental 60
TBM treatment for machine. The cutterhead was modified by Lovat and following successful field trials carried out on the site by Beaver, the 7.6m diameter prototype rock machine was officially ordered by CBDC. As Lovat is a designer and manufacturer of fully shielded soft ground TBMs, the rock machine is quite different from the more familiar hard rock TBM grippers. The prototype, Model M-300, is a fully shielded, open face machine equipped with 45 interchangeable roller disc cutters and 55 carbide ripper teeth. Since either type of cutter can be used, the TBM can excavate hard rock or softer ground. To control shield rotation, it is equipped with antiroll devices and the cutterhead can rotate at variable speeds clockwise or anti-clockwise. The excavated rock is directed by a series of scrapers through the open face of the cutterhead where it drops onto the central primary conveyor which extends through the bulkhead of the machine. A rubber curtain and controlled ventilation system prevents dust entering the crew’s work area. The operator can watch the removal of the rock from his position on the right side of the conveyor, and has quick access to the face if necessary. The cutterhead is thrust forward by hydraulic cylinders which can be retracted from the face. Forward thrust of the 6.3m long x 350 tonne The Lovat rock TBM is fitted with either 45 discs or 55 carbide teeth with a steel set expander and probe drill in the rear
