SYNOPSIS
The development of instrumentation for underground research into problems of Strata Control has been a major objective at the Strata Control Research Laboratory at King's College. The instruments required for an intensive study of the pressure distribution and strata movement over and around mining excavations, in British coal mining conditions, must be designed for use in gassy mines, thus severely limiting the scope of the designer. This requirement of operating conditions has, it is believed, resulted in simplicity of design, together with a reasonably acceptable accuracy.
The principal objective of the research is to produce the overall picture of pressure or load distribution from the solid coal in advance, to the consolidated waste behind the long wall face. British coal mining development in face mechanization in long wall mining has involved the use of supports having controlled load-yield characteristics. Associated with the hydraulic or yielding type friction props articulated, cantilever of bars are in use on prop free front faces. The need for systematic surveys of prop loads in various operational conditions has led to the development of the prop load cell. The prop load cell incorporates electrical wire resistance strain gages, the complete equipments capable of individual measurement 6f up to 40 prop loads by selection control. Surveys using this equipment have indicated the degree of load transference associated with the cycle of face operations and will provide design data for the face supports required in a particular seam or district.
Stress meters have been designed for use in coal and the softer coal measure rocks and for high pressure measurement in hard rock. The use of stress meters in advance of long wall f aces has provided a clear picture of the pressure distribution in the front abutment area, and the influence of face operations on the transferred load conditions. The stress meter designed for high pressures has successfully recorded the general distribution of the transferred load, in front of a stope face in the Champion Reefs Mine, Kolar Gold Field, South India, at a depth of over 9,000 feet.
Roof-floor convergence according has been developed to run concurrently with prop load and stress meter measurements. The pack load has also been measured behind the long wall f ace using hydraulic capsules buried in the waste packs.
A development which, it is hoped, will provide corroborative evidence of the results of the stress meter surveys in advance of the coal face, is the use of sonics. Equipment has been designed and used in naked-light mines, to measure the sonic velocity change over a path of known length, with increased transferred load, as the face advances. The sonic pick-up heads are set in boreholes in advance of the face. A pulse producing hammer and triggering device is used at the end of a parallel borehole to initiate the pulse which is recorded photographically on an oscillograph. With laboratory calibration tests in a triaxial loading device for rock cubes, it is hoped to provide an alternative method to study the pressure distribution in the solid in advance of and around excavations.
