【正文】 f 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。 Block contours。 Geomining。中文 6660字 出處： International journal of rock mechanics and mining sciences, 2020, 42(1): 127136 英文原文 Exploitation of developed coal mine pillars by shortwall mining— a case example A. Kushwaha, G. Banerjee Central Mining Research Institute, Barwa Road, Dhanbad 826001, Jharkhand, India Abstract: The shortwall mining technique is similar to longwall mining but with shorter face lengths, ranging between 40 and 90m, with the aim of controlling the caving nature of the overlying upper strata, the load on support and the overall operation of the supports applied at the face. Field observations and threedimensional numerical modelling studies have been conducted for the longwall panel extraction of the Passang seam at Balrampur Mine of SECL to understand the caving behavior of the overlying upper strata. A large area of the Passang seam adjacent to the longwall panels has already been developed via bord and pillar workings. In this paper, numerical modelling studies have been conducted to assess the cavability of the overlying strata of the Passang seam in the mine over developed bord and pillar workings along with the support requirement at the face and in the advance gallery. The caving nature of the overlying rocks characterized by the main fall is predicted for varying face lengths, strata condition and depths of cover. The support resistance required at the face, the load in the advance gallery and its optimal obliquity were estimated for faster exploitation of the developed pillars in the Balrampur mine by shortwall mining. Keywords: Exploitation。 Shortwall mining。 Obliquity。 Main fall。 Poisson’s ratio (), ? the coef?cient of thermal expansion ( C?/103 5?? ), G the geothermal gradient ( mC/ ? ) and H the depth of cover, (m). This equation shows that the mean in situ horizontal stress (mean of major and minor horizontal stresses) depends on the elastic constants (E,V) of inseam values, the coef?cient of thermal expansion (? ) and the geothermal gradient (G). The vertical stress can be taken for coal measure rocks as: Hv ?? (2) Then, substituting for v? in Eq. (1), we obtain the mean horizontal stress as : Hh ??? MPa (3) The