Research on rock pressure phenomena in flat-lying seams with mechanized supports has shown that during extraction strong roofs of coal seams can form cantilevered slabs, usually formed from stacks of strata with different physical properties. Under gravity and the load due to the superincumbent strata, these slabs break up into blocks with different ratios between the lengths to the strike (L) and the heights (h). If the roof contains thick strata (over 2 m), this ratio is usually less than 0.2, but for other roofs is may reach 1-2.Many problems in calculating the stability of strata of sedimentary rocks reduce to determining the stresses arising above the edge of the extraction face; the strata are taken to be uniform, and the distribution of normal stresses in the cantilevered stacks is assumed to be linear over the height of the cross section. At the same time there are a good many papers which limit the possibility of determining the normal stresses by formulas of the theory of elasticity and strength of materials to the ratio L/h > 3-4.Analytical solutions cannot take account of the great variety of factors influencing the processes of fracture and displacement of the strata in the immediate and main roofs above an extraction working.On the basis of these considerations, at the Laboratory of Rock Mechanics of the All-Union Scientific-Research Mine Surveying Institute (VNIMI) we have carried out research on the stress distribution in homogeneous and stratified cantilevered beams with various ratios L/h. Investigation of a homogeneous cantilever means elucidating the character of the vertical stress distribution across the beam, the regions of influence of the point of support (root) for various values of L/h, the limits of possible application of the analytical solution for the stress components, etc. Owing to the complexity of field conditions, the investigations were performed with successive approximations of the experimental conditions to those in nature.We will give the derivation of formulas for the component stresses and deformations for the simplest casean elastic cantilevered slab loaded on its uppermost surface by a uniformly distributed load and by its own weight (Fig. 1) -and the results of an experimental investigation of the distribution of normal stresses o x in cross sections at the roots of beams of various lengths L and constant thickness h. The cantilevers were made from SD-8 optically active material and were studied by the optical polarization method.The mathematical methods used here can be used to derive formulas for the component stresses and deformations in a muhilayer roof.

In assessing the stability of mine workings we need reliable indices of the strength of the solid rock in situ. Determination of these indices is one of the most complex problems in rock mechanics owing to the presence of many types of defects in the rocks. Pocks, being of geolo~cal origin, differ fundamentally from other solids in the arrangement' development, and dimensions of their defects. We can distinguish three categories of defects in terms of the methods of analyzing their influence on the strength properties and of their geometrical parameters.The first catagory includes micro-and macrmiefects more or less uniformly scattered throughout the rock. Such defects are found in all solids. They include lattice defects, microfissures, various inclusions, etc. We will refer to these defects as dispersed or random. The second category of defects is characterized by the natural fissuring. Here we will consider only those properties of the fissuring which influence the strength properties of the rock. We distinguish between endogenic and exogenic fissuring. Endogenic fissures form three mutually perpendicular systems, the fissures of one of these systems are approximately parallel to the stratification. The number of systems of exogenic fissures depends on the number of cycles of action of the tectonic forces. ~ogenic fissure systems lie at oblique angles to the plane of stratification. In sedimentary rocks endogenic fissures are almost ubiquitous, but exogenic fissures are found only in regions subjected to tectonic processes.

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...

Recent years have seen much discussion [ 1-~] of the development of reliable methods for predicting the behavior of rocks and the selection of the parameters of supports for mining-out faces. The authors of these reports concluded that the rocks above workings usually begin to break up when there is a loss of continuity along various natural weakened points induced by shear or cleavage.The distinguishing feature of sedimentary rocks is the presence of weak links along the beds, which are largely responsible for the onset of the development of disintegration; in igneous and metamorphic rocks the weakened sites are block contacts, cracks, and various other defects.From the practical point of view it is important to know not only the sites of possible break-up of the rocks but also the sequence of the processes of further development of these disturbances. The latter must be used in calculating the support capacity of the roof rocks (delineation of the beds and bands, determination of the stresses induced by the additional load imparted by the superincumbent roof rocks, and determination of the interbed friction forces), in selecting the characteristics of the support and the mining system to be used for underground mining operations.For the case of a single room of rectangular cross section, in [ 5] it was shown analytically for the first time that the loss of continuity of weakened interbed rock contacts by shear begins above the wall of the room. Similar results have been obtained for cases of several contiguous rectangular workings [7, 8]. Nevertheless, there is still no method enabling us to investigate by the use of engineering-geologic data the discrete discontinuity of the solid rock and to predict by calculation the stages of development of bed separation in a stratum at weakened contacts and positions.This article deals with the criterial postulates for such a method.We examine a system of contiguous rectangular workings (rooms) (Fig. 1), for which the geometric dimensions (pillar, room spans), the characteristics of the geological cross section of the layered roof, and the mechanical indices of the contact are known. As a result of the calculation we need to determine the sequence of inception and development of bed separation cracks in the roof rocks.

622.831 : 622.284-523.3 and V. T. GorokhovThe All-Union Scientific-Research Mine Survey Institute (VNIMI) has been conducting experiments since 1964 on powered supports in the Kuzbass in order to examine thoroughly the reaction between these supports and the surrounding rock. From the engineering point of view, the object of the experiment was to define the optimum calculated, working, and initial setting loads for hydraulic supports. The experiments involved a program of continuous variation of working and setting load; caved rock was suspended on the restricting parts of the powered support section.OMKT units [1][2][3][4] were used for the investigations. This paper gives the results of an investigation into the force reaction of the supports and rock in face 22 of the Nadbaikaimskii-1 seam at the 7th November Colliery in the Leninugol' area of the Kuzbassugol' group, with a sequential variation of forward and trailing support load settings in an MK face equipment. The face length was 60rn, the working pillar, 400 m. The face was divided by a 6-8 m wide pillar starting on the top at the worked-out pillar and was bounded at the bottom by face entry 24. There were no faults in the face area; the gradient was 2-3 ~ total seam thickness, 2.6-2.7 m, and the worked thickness, 1.75 m. Two 0.1 and 0.4 m argillite bands in the lower part of the seam were not mined. The coal was of average hardness. The working depth was 70 m. Argilltte lies 5-7 m above the seam, and is overlain by sandstone.Holes were drilled to examine the structure and mechanical properties of the roof. Examination of the core showed that the argillite contained a number of weak inter-bed interfaces (with slide surfaces), coal bands and plant residue. Table 1 gives breaking strength perpendicular to stratification for various interfaces in the argtllite.To break down the immediate roof into packs and to evaluate the fluctuating thickness of the beds, it is assumed that bed separation occurs along interfaces with average strength (0.04-0.06 kg/cmZ). Fairly thick beds are present only in the IV and VI bands; the overhang in these beds can reach as much as 4.5-5.0 m (Table 2, graphs 9 and 10). Thicker beds are to he expected above pack VI, with a caving interval correspondingly larger. The MK equipment has been described in [5].To carry out measurements on eight supportsecti0ns in the center of the face (from 25 to 32), a measuring station was instrumented (57 sections in the whole face). Pressure gauges were arranged on all the props in these sections, and the 32nd section was fitted out with self-recording gauges designed by the A. A. Skochinskii Mining Institute. The period of observation can be broken down into three stages; the first, with a setting load in the props of 40 tons, and of 80 tons in the section; a second period with pressures of 80 and 160 tons, respectively, and a third period with 54 and 108 tons, respectively (the prop valves were changed over on the measuring station only in the second and third periods).At the beginning of obse...