Representativeness of physical and mechanical characteristics of rocks surrounding coal seams, and methods of estimating them

Abstract

In recent work on planning pits and coal-winning technology, increasing use has been made of the results of laboratory research on the physical and mechanical properties of the surrounding country rocks, in particular their mechanical strengths. The effectiveness of engineering solutions largely depends on the representativeness of the parameters under investigation--their error of measurement and spatial variability within the regions of solid rock to which the results of tests on individual samples are to be extended. In estimation of the strength of rocks, information is usually provided by drilling cores. Of course, during drilling there is partial crushing of the core, especially in fissured and mechanically weak strata. Owing to this selective crushing of the rock material extracted from the borehole and brought to the laboratory, it does not entirely represent the drilling solid rock; it is biased toward samples with high strength. Preparation of samples for laboratory tests involves separating the core into specimens of geometrically regular or semiregular shape. Specimens which meet requirements (exact correspondence to the standard shape and size, and absence of cracks, surface flaking, and other defects) can usually be obtained only from the strongest unbroken fragments. Thus during divison of the core there ismore » a further reduction in the representativeness of the samples.Furthermore, the values of the breaking loads created by laboratory presses and used to calculate the compressive breaking strength, tensile strength, etc., depend to some extent on the care with which the specimens are prepared, the skill of the laboratory workers, and other subjective factors.« less