A model study of the application of roof bolts under unsymmetrical loading conditions

When coal is extracted by the long wall mining method, two gate-roads on either side of the working face are driven with the advancing operation. In the surroundings of these openings the state of stress which existed prior to mining is not only disturbed by the drifting process of the gate-roads but also by the coal extraction in the long wall, resulting in an unsymmetrical stress distribution. Varying incremental pressure components and the high complexity of strata movements make it extremely difficult to control the rock behavior under unsymmetrical conditions by conventional support. Application of roof bolts as an auxiliary support can improve the control of the strata surrounding long wall gate-roads. To indicate the qualitative effect of roof bolts on the stress distribution around underground openings in stratified rock under unsymmetrical loading conditions, composite models made of photo-elastic material were tested in a 6 ft. diameter centrifuge, providing proper loading conditions. Stress distributions were permanently frozen into the models while under load. Test results indicate that the high shearing stresses which occur near the rigid abutment of the long wall gate-road and the bending stresses in the roof beams can be reduced considerably by the application of roof bolts in proper pattern arrangements. Since this study was concerned with the development of an experimental technique and simple structural conditions were chosen, an extension of this investigation to more complicated conditions is possible --Abstract, page i