The operation of a photoelastic sensor outside the zone of influence of extraction operations can be represented as follows. After the drilling of an isolated borehole of small diameter and the installation of the sensor in it, the latter operates in combination with the rock, i.e., reacts to changes in its state of stress and deformation caused by the drilling of the borehole. If the rock has creep properties, the action of the sensor will be the same as the action of an elastic ring support in a circular working with total cohesion with the solid rock, as described in [1]. In this case, the stresses in the sensor due to the creep properties of the rock will take [1] the following form:%0=--'if-3 R' ---~--a-1 r 2 --3 2r' J (1--k) sln20.(1)The coefficients in (1) depend on the elastic constants ax, bt, a-l, b_l, as, b-s of the sensor material and rock, the ratio between the internal and external diameters of the sensor, the creep parameters of the rock, and the time.It is known that the stress components o r, o 0 , rr0 at any point in an elastic sensor are related as follows to the maximum tangential stresses Vma x by the elasticity relation:~lll aX --

No abstract

Photoelastic transducers in the form of cylinders with axial holes, made from optically sensiUve materials such as glass and used for measuring stresses in a rock mass, are installed in boreholes and glued on the outer side to the borehole wails at a considerable distance from the end face. In the planes normal to the borehole axis, the rock mass may be under conditions close ~o the state of plane stress (for example, near the periphery of the mine workings) or plane strain (further away from the periphery). The photoelastic transducers, which may have different lengths, depending on the required sensitivity and on their weak or firm attachment to the boreholes (which depends on the strength and quality of the glue), may also be in different mechanical states.If the transducer is not attached (or only weakly attached) to the borehole wails, it may take up only radial deformations of the borehole walls. In this case, deformations of the transducer along its axis are not impeded and, whatever the mechanical state of the rock, and for anylength of the transducer, the latter will be in a state of plane stress. In practical work, this is undesirable.If the transducer is completely attached to the borehole and the rock mass is in a state of plane strain or plane stress, the ~ansdueer will operate in a state of plane strain.If the photoelastic transducer is installed in the zone of influence of the end face of the borehole, where the stressed state of the rock is three-dimensional, it is even more difficult to determine the character of interaction of the transducer with the rock mass than in the first two cases, when the transducer is located far from the end face of the borehole.The stresses in the transducer are given by the formulas [2]

Using plane models of optically active material ED6M, the authors have simulated the following: a) an active working in a seam with thickness 10 m and angle of dip 75 ~ driven through the whole height of the panel (80 m) and at different depths from the surface ( Fig. la, b, c); b) two active workings in the same seam, driven in two different panels (Fig. ld).In contrast to [1-4], we simulated undeepened active workings of considerable extent to the dip (the ratio of the length of the working to the extractable thickness was 8 : 1), thus requiring allowance for the force due to the weight of the solid rock.We studied the case of mining a seam with a pillar at the surface. The solid rock was assumed to be elastic, continuous, homogeneousTand isotropic, and the active working free from caved rock, supports and stowage. The aim was to assess the elastic stress in the solid rock near active workings (including the abutment pressure zone) in relation to depth, and the effects of mining operations in the superincumbent panel and of different lateral thrusts,To establish in the model stresses corresponding to those due to the weight of the solid rock, we used the ~ freezing" principle in a centrifuge with R= 1.25 m. The centrifuging coefficient Kc: 50-58, corresponding to a rate of rotation of the centrifuge arm n = 220 rpm, was calculated by a known method based on obtaining a band order sufficient for the measurements (Fig. 2a). Lateral-thrust stresses were established in the models by subjecting them to a "triangularly" distributed load in an oven (Fig. 2b).In elastic models the relation between load and stress is linear, so if we determine the stresses in the model at any two values of )`. the coefficient of lateral thrust, the stresses can be calculated for any value of [5]. We simulated three values: ! ~o----0, ~t-~--and ),~=1, 3For all other possible cases the following formula is proposed:where o i is the stress in the model with )`z= 1/3; Oz is the stress in the model with )`z = 1; OniSthe stress with )`= )`n"Each of the four models was thus loaded and "frozen" in turn under the centrifugal force ()`0 = O) and two lateral thrusts )-t and k z. From the 12 band pictures thus obtained we calculated the stresses by graphical integration of the differential equations of equilibrium of the plane problem of elasticity theory. The overall stress distribution curves were obtained by the superposition principle.Analysis of the total distribution curves of vertical normal stress Oy and horizontal normal stress Ox show that, in the solid rock around a single undeepened active working, the horizontal stresses are concentrated in regions lying near the upper and lower parts of the working at the side of the roof and floor. However, for a working at depth 91

Complete information on the mechanical state of solid rock is given by a knowledge of its stress-strain state. However, experimental methods of measuring the stresses in solid rock are still in the development stage. Analyrical methods of attacking the problems of stress redistribution in rock due to mining operations can be solved in the general form, but as a rule it is impossible to use these methods at present to solve practical problems in particular geological and mining conditions. The stress pattern in the rock can be found by a method which combines the possibility of analytical solutions with the use of data on rock pressure phenomena.In mine observations the most widely used method of investigation is observation of movements of the side rocks due to mining operations. The experimental-analytical method of determining the stress pattern, developed in the Laboratory of Mining of the Institute of Mining of the Siberian Branch of the Academy of Sciences of the USSR, permits us to determine the changes of stress around mine workings from measurements of the movements of the roof while a coal seam is being worked.As a result of mining operations, there is a spatial redistribution of the stress-stain pattern in me rocks. We will consider this spatial pattern in cross sections, assuming that each such cross section obeys the conditions for a plane problem. In this case, the caiculated stress-strain pattern of the rock will usually be greater than the actual spatial redistribution of stresses.The redistribution of the stress-strain pattern due to mining operations will also vary with time (as well as space), owing to the gradual advance of the face and to the theological properties of the rock mass [l].