Pergamon zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 6,t. J. Rock Mm-h. Min. SC;. & Geonwlz. Ah~rr Vol 3 1, No. 8. pp. 7.57.-768. 1996 Copynght t 1996 Elsevrer Science Ltd PII: SO148-9062(%)00036-8 Printed in Great Bntain. All rights reserved 0148-9062~96 $15.00 + 0.00 Coal Pillar Loading in Shallow Mining Conditions RAJENDRA SJ NGHt; T. N. SINGHT BHARAT B. DHARt zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Monitoring of the nature and amount of mining-induced stresses over pillars was undertaken at d@ erent coal seams with shallow depth corer and varying overlying strata and goaf treatment. The development @ abutment loading on the coal pillars facing goaf line is visualised by analwis of the in situ observations at difSerent sites with varying geomining conditions. The rock properties and depositional conditions of the immediate roqf’strata at each site were studied and incorporated in terms of a cavability index. Considering the relevant geomining parameters, which influenced the pillar loading, empirical relations are developed to estimate the range of inj?uence and the value qf’ ultimate induced stress (vertical) over the coal pillars with shallows depth cover. Copyright 0 1996 Elsevier Science Ltd 1. INTRODUCTION A coal seam is loaded by the weight of overburden. An opening, made by underground extraction of the coal, disturbs the natural state of stress equilibrium. The vertical load of rock strata, directly above the opening, shifts on to surrounding pillars leaving a distress zone in the roof strata. Depending on the nature and characteristics of the overlying strata [l], the immediate roof falls, at different opening dimensions at different sites, with an increase of the area of the extraction and acquires a new state of equilibrium. Generally, the surrounding pillars experience a maximum amount of abutment loading just before the main fall of the roof. At this juncture, the value of induced stress over the pillars depends on the location of the self-supporting strata in the roof. Due to further dimensional increase of the opening, abutment stresses are created by the immediate roof strata cantilevering over the goaf areas (Fig. 1) and their magnitudes depend, mainly, on the length and thickness of the roof that immediately overhangs the goaf area. The stability of the natural support-pillars, which are left to support rooms or the entries of underground mines, depends upon the amount and extent of the abutment loading created due to the extraction of the coal. Violent roof failure due to the presence of massive overlying strata has always been a threat to the safety tC.M.R.1. (formerly CMRS), Dhanbad, (Bihar) India. $Presently working in the Dept. of Photogrammetry and Surv., University College London, Gower Street, London WClE 6BT. U.K. and efficiency of Indian coal mines [2]. A large number of the country’s mines are facing this problem, where the average depth cover is only around 100 m. The total collapse of a longwall (powered support) face of Churcha colliery (technically supported by U.K.) and the overriding of pillars of a gallery blasting face at East-Katras colliery (technically supported by cdf, France) are some of the latest examples of casualties experienced by the country’s coal mining industry. In particular, the failure of chock-shields (4 x 680 tons) at Churcha colliery [3] is considered more seriously, because the face of this project comprised the country’s highest ever support density of around 1.0 MPa. The failure of these mechanised coal faces hampered the trend and pace of mechanisation of underground coal mines which is supposed to be the best way to achieve the future plan of increased targeted output of coal in the country. Considering the large amount of failed and stable cases of different sites, an empirical relation among different geomining parameters of the coal pillars has been developed [4,5] by CMRI (formerly CMRS) for the estimation of pillar strength, but lack of field data of mining-induced stresses, in and around the extraction face under the existing massive overlying strata of shallow coal mines, directly affected mining feasibility. Calculation of the actual load on a pillar is more complex than the determination of the pillar strength. Both the theory of elasticity and the principles of mechanics do not apparently completely explain the behaviour of coal measure formation and hence, a reliable simulation of the geomining condition of a field