The qualitative and quantitative indices of the external loading of supports in mining-out workings are determined by the laws of deformation, fracture, and displacement of mul,dlayer sedimentary strata. The external loading indices of supports usually vary as the face advances. The greater their range of variation, the severer the conditions under which the supports must operate. The variation of these indices as the face advances is due to periodic fracture and displacement of the thicker and stronger layers in the immediate and main roofs. In absence of such layers, the changes in rock pressure as the face advances are less marked.It has been assumed that the layers of the immediate and main roofs break up as result of flexure. Investigations by the All-Union Scientific-Research Mine Surveying Institute (VNIMI) in mines [1, 2 J have shown that the ratio of the length of the blocks to the thickness of the layer is often 0.25-0.40, or even less than 0.1; i.e., in mines the disintegration of thick layers into blocks, the length of which is close m the thickness of the layer or less, evidently takes place under stresses different from those on which the basic condition of limiting stress is based in bending calculations. In these cases fracture may be due either to shear deformations (where the layers above the face are compressed) or breaking-away (above the area around the face, under the effect of bending moments). Bearing in mind that rocks are brittle, Kuznetsov [3] suggested that cleavage calculations should be performed for thick cantilever beams. The condition of the limiting state during cleavage has been described only approximately. Fisenko [4] assumes that the bending moment due to external forces and gravity is balanced by the internal forces in the rock which resist cleavage and are distributed over the whole cross section of the beam in accordance with a linear law. This method of calculating cleavage must be regarded as approximate, because the actual stress distribution in short beams has never been experimentally investigated and strict analytical solutions are absent. Hence the conditions of limiting stress, on which calculations of cantilevers of rock layers are based, correspond to two types of stress distribution patterns in unsafe cross sections-bending and shear. When brittle rocks are bent, fracture begins also in the regions of tension, i.e., as a result of cleavage.Calculations on real thick layers of rocks are complicated by the fact that in many cases they consist of a stack of mutually cohesive Iayers of different thicknesses, with different moduli of eIasticity and different coefficients of lateral deformation. The greater the thickness of the layer being calculated, the more likely is it to be nonuniform. Furthermore, in a mine this layer is wedged above supports and interacts in a fairly comrJlicated way with the superincumbent layers. In theoretical investigations, it is difficult to take account of whole range of factors on which the sequence of development of fracture of the r...

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Development of modelling as a way of investigating rock pressure phenomena largely involves the solution of a set of purely methodological problems. In particular, experience in the construction of models from equivalent materials [i] has shown that the technology of their preparation has a very important influence on the reliabilityand representativeness of the tests. Failure to follow the established technology, even in one single stage, may completely distort the process reproduced in the model and lead to erroneous conclusions.

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