Abstract. The longwall face supports are provided to prevent the caving of the immediate roof at the working face. The thickness of the immediate roof may be obtained from the working height and the average bulking factor. A strong bed, capable of withstanding maximum bending moment among the beds within the above roof, can be identified. The load on the strong bed may be obtained by finding out the overlying beds that will rest on the former during sagging. The limiting span, L, for this unsupported loaded bed, as a beam with built-in ends, can be calculated. The support density required at the supported span (face to goal edge), a, during the first fall, is now estimated with a view to have a safety factor 2 at the face end, when bending failure occurs at the barrier end. However, the above value is inadequate to provide necessary safety margin against bending failure at the face end, during the subsequent carings as cantilever, for L/a less than 20.6. For the latter case a separate expression is derived.
INTRODUCTION
The success of a longwall working largely depends on the adequacy of the face support provided. Several methods of estimating the support requirement are already available in the literature (Birön and Arioglu, 1983; Borisov et al. , 1972; Kravchenko, 1975; Peng, 1978; Sarkar and Singh, 1985). However, "there still is no uniform method for estimating support requirements" (Peng, 1978) , and this gives a scope for further investigation.
APPROACH
The primary objective of the face support in longwall mining is to provide a safe working condition, preventing the caving of the immediate roof at the work- ing place. This means that the immediate roof above the supported span, i.e., the distance between the face to the goaf edge, should remain undamaged by the stresses, induced into it, during the roof failure by bending beyond the goaf line. This is required to be achieved by the face supports. Experiences of longwall mining with strong immediate roof have shown that the first roof fall takes place only when the face advance considerably from the barrier pillar. Immediately before the fall, the face supports are subjected to heavy 'weighting'. Inadequate support capacity, in such cases, may lead to the damage of the supports and/or collapse of longwall face (Peng 1978; Sarkar and Singh, 1985). It becomes reasonable, therefore, to estimate the minimum support requirement to withstand the first fall at the beginning and then to find its suitability for the subsequent caving.
SUPPORT REQUIREMENT TO WITHSTAND TO FIRST FALL
Immediate Roof
The stratum or strata above the working thickness, which tend to separate from the overlying strata on mining excavation and fill up the void created completely on caving due to the volume expansion on fragmentation, are termed as immediate roof. Its thickness is obtained by the well known formula (mathematical equation)
However, if a part of a stratum comes within the above thickness, the full thickness of the former is to be included in the immediate roof unless the same fragments into flakes during caving.
