Western Canada produces 70 to 80 per cent of total Canadian coal production but of this less than 10 per cent is by underground methods. (Jeremic, 1978a) Table 1 indicates the present estimated reserves and of these approximately 90 per cent of total recoverable reserves are mineable by underground methods and the remainder by open pit. Since reserves accessible by surface mining methods are rapidly being depleted, underground reserves and consequently underground methods will become more important.
Up to seventy per cent of the valuable coking coal seams of Western Alberta and Northeast British Columbia are contained in the steeply dipping seams of the foothills and mountain region. This region is characterized by a high degree of tectonic disturbance leading to shear zones in the coal, slickensided surfaces in the adjacent strata and thrust faults at low angles to bedding throughout the coal series. The seams tend to be unstable and consist of bands of hard and soft coal; this banding is due in part to the petrographic composition and also to the tectonic history of the seam.
Traditional coal mining methods such as longwall or room and pillar mining cannot be readily applied to these steeply dipping seams so sublevel methods have been investigated and developed. At the Michel Colliery of Kaiser Resources Limited in Sparwood, British Columbia, a sublevel hydraulic mining system has been developed and used in the Balmer seam since 1970. (Parkes and Grimley, 1975) The nature of the coal allows hydraulic cutting and breaking and the mine layout is such that hydraulic transport by flumes and pipes can be utilised. This has proved to be a safe, efficient and productive method (55% overall recovery) at the current working depths. However as reserves are depleted, seams will be extracted that are both deeper and possibly more steeply dipping. Research will be required to investigate the stress distributions and modes of failure at depth so that support systems and strata control methods can be developed to maximise recovery. This paper describes the research approach adopted by the authors and compares their results with those obtained from current mining operations.
The idealized coal series was constructed as in figure 1 and consists of a massive sandstone bed containing a dipping coal seam. This situation represents an over simplification of any actual geological structure but is based on the geology of the foothills region in which the coal bearing strata consists of massive sandstones and siltstones with thin interbeds of coal and mudstones.
For analysis purposes it is assumed that the seam has been mined to a depth of 150 metres by underground methods and that the sandstone in the hanging wall will cave in the manner indicated in figures 1 and 2. This assumption is based on results of investigations presented by Hoek (1974) who derived relationships between the angle of break, ?b, and the mining depth, H. The resulting angle of break is 68º.
