Pre-wetting of the solAd coal is now widely employed in underground mines, the water usually being pumped in via long boreholes; this method has numerous advantages over other wetting methods.The moisture taken up by the coal causes a marked change in its properties [1], which inevitably leads to a difference between the rock pressure phenomena occurring during the working of pre-wetted seams and those observed in nonwetted seams under similar conditions, Two questions are of particular practical interest, namely stress distribution in the solid coal ahead of the mine workings, and the behavior of the surrounding rock within the supported waste area.In our theoretical examination of these questions we used Barenblatt and Khristianovich's solution of the problem of stress distribution in the solid coal during extraction operations [2]. We also studied Kuznetsov's papers [3,4], which give a closer examination of this problem.
The most reliable strength characteristics are found by testing rocks and coals in a state of bulk stress [1, 2]. At the All-Union Scientific-Research Mine Survey Institute (VNIMI) special devices have been developed for this purpose -"stabilometers," which enable us to study both the strength and the deformation characteristics of rocks and coals for various types of load over a wide range of lateral loadings [2]. Despite their advantages, all the devices are quite complicated and require special apparatus to create the lateral pressure. To make tests under comparaavely small loadings, it is therefore necessary to create simpler and more convenient devices.At the VostNlI Institute we have designed and built one such device [3]. The axial load on the specimen is exerted by a press, and the lateral load by pressure of liquid (oil) in a flask. This, however, had certain shortcomings associated with the operation of the valve which regulates the liquid pressure in the flask. Owing to its inertia there are marked pressure oscillations of the liquid, and consequently large deviations of the lateral loading from the intended value during the test.In an improved device, shown in Fig. 1, the liquid pressure is created by compressed air. There are no dif~-culties in regulating the pressure. The device consists of a cylinder 1 with a spherical foot 2 on which the specimen is set up, and a piston 3 with rod 4. To pack the gap between the cylinder and piston, the latter is fitted with a ring of oil-resistant rubber. To the piston is welded connecting tube 5 for feeding the air, with screw 6 for releasing it. Compressed air is fed from cylinder 7 via copper tube 8 to the cylinder. The air pressure in the cylinder is measured by manometer 10.A coal specimen, 40 mm in diameter, previously covered with two coats of BF-6 glue, is placed on the spherical foot and oil is poured in. The piston is then fitted in the cylinder, and the device is placed between the plates of a press. Compressed air is fed into the cylinder to create a given lateral pressure, which is read off on the manometer. The axial load on the specimen is then increased to breaking point.
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