The Plains Region of Western Canada has coal-bearing strata whose strength is equal to or lesser than the coal. This is not common among coal basins throughout the world. The factors causing this are discussed.
Diagenesis, along with the compressive strength of rocks, increases with depth of burial. Diagenetic effects are limited or absent for the strata under consideration. The rocks consist of 90 percent very fine-grained sediments which have lower strength and elasticity than the remaining 10 percent of coarse-grained sediments.
Clay minerals decrease rock strength with the strength reduction depending on the amount and type of clay minerals, and the natural moisture content.
Stratification produces very large anisotropy because of the low bonding between individual rock layers. This, in particular, causes very low tensile strength of the rocks.
Understanding the basic factors which influence rock strength is very important for the design of strata control and mechanization in underground coal mines.
The Plains Region of Western Canada contains coal of low rank (high volatile bituminous, sub-bituminous and lignite), interlayered with weak and soft strata, very similar to conditions in the Moscow Coal Basin. The Plains coal deposits extend east into Saskatchewan and from northwest of Lesser Slave Lake southwards into the United States. On the west they are bounded by the Rocky Mountains (Fig. 1)(Jeremic, 1979). The coal-bearing strata in the Prairies underlie an area of about 256,000 km2
(Figure in full paper)
Coal-bearing strata are present in lateral zones of Upper Cretaceous rocks which dip gently southwest. Thus horizon mining can be considered for such lenticular coal seams of variable thickness which are hard and well cleated, and which have strength equal to or higher than adjacent rocks. The depths of coal seams in the Plains Region range from several metres to more than 650 m. Future large-scale underground mining is anticipated at depths ranging from 25 to 250m.
Numerous tests on mine sites and drill cores by geotechnical consulting firms and by researchers at the University of Alberta show the following rock strength characteristics.
The large majority of the rock samples tested exhibited very low uniaxial compressive strength (range 0.30 – 30.00 MN/m2).
Laboratory results suggest that strength increases with increased depth of burial. It is obvious that there is a relationship between diagenetic effects and rock strengths.
On the basis of rock strength three depth zones can be delineated. Strata buried to depths up to 75 m show rock strengths up to 6 MN/m2 (soil-like rock). Burial of 75 to 150 m produced very weak rock with strengths between 3 and 15 MN/m2. Finally strata buried more than 150 m (the majority of rocks) have strengths up to 30 MN/m2(Fig. 2).
(Figure in full paper)
The low intensity of diagenesis is caused by particular geological events (1)The coal-bearing strata were buried in shallow parts of the Cretaceous- Tertiary geosyncline at depths estimated at not more than 1000 m. The overburden pressure was not sufficient to cause appreciable rock induration (Anonymous, 1976).
