【正文】 r mine The geomining parameters of the proposed area of the Balrampur mine for shortwall mining panel are given below. Thickness of the seam Proposed height of extraction Depth of proposed panel 3750m Existing overlying/underlying workout areas Nil Existing mining pattern Developed on bord and pillar workings Pillar size 20m20m(center to center) Gallery width 4m Various boreholes have been drilled over the longwall panels P1 and P2 of Passang seam of Balrampur mine of the same area with results as given in Table 1. The average hard cover in panels P1 and P2 were 29 and 39m, respectively, and the depths of the seam were 50 and , respectively. Based on the ?eld observations of the caving nature of the overlying strata of the longwall panel P1, the overlying strata have been divided into six major beds overlying the coal seam. Based on engineering judgement and giving a higher weight to the borehole lithology in panel P1, estimated RQD and the intact average pressive and tensile strengths of different bed rocks tested in the laboratory [3] are given in Table 2. From the borehole details, it is evident that BedI and BedIII are weak beds, with RQD of 40% and 43%, respectively. BedII and BedIV are relatively strong with RQD of 78% and 75%, respectively. It is expected that these two strong beds will pose dif?culty for caving. BedV and BedVI consist of fractured/weathered rock and alluvial soil. Table 1 The borehole details over longwall panels P1 and P2 at theBalrampur mine Borehole no. Depth of seam(m) Seam thickness(m) Hard cover(m) BIX145 (behind panel P1) BIX146(middle of panel P1) BIX144 (at the end of panel P1) (over panel P2) (behind panel P2) — (over panel P2) (over panel P2) 4. Field experiences of alreadyextracted longwall panel Longwall panel P1 with a face length of 156m, situated at an average depth cover of 50m at the Balrampur mine was extracted with the help of the ?rst Chinese powered support in 1998. In this panel, local falls had started taking place at regular intervals after a face advance of 25m, involving the immediate roof fall of around 5m height, ?lling approximately 60% of the void in the goaf. On 26th May, 1998, when the face advance was 67m, a fall of considerable extent was observed. It appeared to be the main fall but no subsidence was recorded at the surface. Later, the main fall took place on 28th May 1998 at a distance of 79–80m from the barrier. This loading caused extensive damage to the powered supports installed at the face and subsidence was observed on the surface. This was recorded as the ?rst main fall. Table 2 Representative lithology above the Passang seam, plus their intact properties Bed No. Run up wards(m) Rock types Thickness (m) RQD(%) Compressive strength (MPa) Tensile strength(MPa) BedⅠ Coal Medium grained sandstone, laminated with shale 40 BedⅡ Coarse grained to medium grained sandstone 78 BedⅢ Very coarse grained sand stone 43 BedⅣ Medium grained sand stone 75 BedⅣ Weathered rock BedⅥ Sandy soil 5. Cavability analyses of the overlying strata Numerical modelling for shortwall mining of developed bord and pillar workings was conducted using FLAC 3D software with the tested and calibrated rock mass properties. This model study was undertaken with a face length equivalent to four pillars (84m) and ?ve pillars (104m) wide, along with variation in depth and hard cover, to understand the cavability of the roof strata. The main fall position during the shortwall mining with varying face length and depth of cover was predicted. The following geometry was modelled: Average thickness of seam Depth of cover 50 and 40m Hard cover/Alluvial soil for 50m depth cover 30m/20m Hard cover/Alluvial soil for 40m depth cover 20m/20m and 30m/10m Pillar size (CentertoCenter) 20m20m Width of gallery 4m Face length 84m/104m In the absence of the in situ measurements of stress values, theoretical values were calculated using the following equation: )1000()1(1 ????? HEGvh ???? ?? (1) where v? and h? are the vertical and horizontal stresses (Mpa), E, Young’s modulus of in seam values (2020Mpa), v。 (b) 70m face advance。 in other words, they may be treated as indestructible pillars. Factor of safety=1–2 Short term stability . it may fail within few years. Factor of safety≥ Stable for few days. The following input data were taken for the 3D BESOL modelling: Material Young’s Modulus Poisson’s ratio Rock 3Gpa Coal The in situ stress ?eld is taken as given in Eqs. (2) and (3). The pillar size was kept as 20m20m (centre to centre) and gallery width 4m. This exercise has been done for a face length of 84m . 4 pillars wide. A panel extraction of four pillars wide (84m) with 91 face inclination and triangular rib is shown in Fig. 3. This approximation of the triangular rib by square elements will not affect the results and conclusions drawn for the actual rib condition. G OAFFAC EA DV AN CINGBARRIER PILLARSFACE LINEB ARR IER PILLARSB ARR IER PILLARSBARRIER PILLARS Fig. 3. Four pillars wide panel extraction with 9o face inclination To study the effect of obliquity of the face on the stability of the triangular ribs, 3D BESOL models with face obliquity of 176。with a triangular rib of thickness 4m are shown in Fig. 4. The safety factor of the pillar/rib is calculated as per Eq. (11): Safety FactorPS? (11) The results of the above study for different face obliquities are given in Table 5. The strength of the triangular rib is found to be increasing with decrease in the obliquity of the face, while the average stress concentration is decreasing with reduction of the obliquity from