The 3rd issue of the journal, Vol.2, N. 1, January-June 2013 ... - ISRM

28 ISRM (India) JournalInitially, 50 m x 60 m pillars were divided into three halvesby driving 4.8 m x 3 m level split galleries. The size ofstooks after splitting was 15.2 m x 50 m. The splittingof pillars was restricted to 2 pillars ahead of the line ofextraction. A diagonal line of face was maintained atabout 60 0 .3.1 Safety Factor of the PillarsUnder normal pillar extraction condition, the localmine stiffness remains more or less constant while thereduction in pillar strength with the reduction in pillar sizegenerates a situation conducive to violent failure. Here,increasing the extraction ratio by splitting/stoking of pillarsahead of the face may be dangerous to the working area.Laboratory scale specimens become elasto-plastic whenwidth/height ratios approach 8 (Das 1986). In accordancewith the field data available for the actual failure of apillars (Sheorey 1993), the post failure modulus of a pillarwith width/height ratio about 4 becomes positive and itsbehaviour changes from strain-softening to elasto-plastic.Fortunately, the BG does not involve the process ofsplitting/stoking ahead of the face, except for splitting andslicing the pillars being extracted. Under these conditions,the chance of the violent failure of a pillar becomes lowand is limited to the pillar being extracted. The safetyfactor of the pillars of panel-3B remains nearly 2.33 fornormal height (3 m) pillar extraction, while the same forfull-height working of the seam by the BG method (11 m)with double splitting goes below one.Table 1: Pillar size variation as per regulation 99 ofIndian Coal Mining Regulations, 1957 (CMR 1957).3.2 Strata MonitoringField instrumentation was carried out in panel-3B tostudy stress distribution in the panel, load on hydraulicprops in galleries, convergence in galleries, roof layersmovement and stress change in the panel. Details of fieldinstrumentation programme are given here (Fig. 7):• Stress measurements in BG workings. Five vibratingwire stress cells, S 1at 66BLN/39D, S 2at 67LS/39D, S 3at 67ALS/39D, S 4at 68ALS/39D and S 5at 68BLS/39D,were installed in the BG panel.• Load over O.C. props in galleries. Vibrating wire loadcells were installed on the O.C. hydraulic props, till10 m ahead of working face in galleries. Recordingsfrom load cells were obtained at every 10 m interval.Load cells were shifted to new stations after every 10m of face retreat in the galleries.• Convergence in gate roads. Roof to floor convergencewas monitored at every 10 m interval in galleries usingtelescopic convergence indicators.• Immediate roof layers movement. Three Tell-taleextensometers, T1 at 66L, T2 at 67L and T3 at 68L,were installed in the junctions of the BG panel.Depth cover(m)Pillar size (center to center) for differentgallery widths (m)3.0 m 3.6 m 4.2 m 4.8 m360 39.0 42.0 45.0 48.0Pillars formed (Table 1) in accordance with the CoalMining Regulations possess adequate safety factorsand a high w/h ratio which sustain the various miningconditions resulting from pillar extraction at normalheights. However, there is an indirect increase in theheight of pillars due to the winning of the roof coal by theBG method (Singh et al., 2007). The process of roof coalwinning affects the stability of pillars designed for normalheight workings developed in accordance with the CoalMining Regulations.Fig. 7 : Location of different monitoring instruments.4. Results and discussion4.1 Stress Variation Over the PillarsThe measurements obtained from vibrating wire stresscells at 66BLN/39D (S 1), 67LS/39D (S 2), 67ALS/39D(S 3), 68ALS/39D (S 4) and 68BLS/39D (S 5) are presentedVolume 2 No. 1 January 2013