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1、英文原文Exploitation of developed coal mine pillars by shortwall mininga case exampleA. Kushwaha, G. BanerjeeCentral Mining Research Institute, Barwa Road, Dhanbad 826001, Jharkhand, IndiaAbstract: The shortwall mining technique is similar to longwall mining but with shorter face lengths, ranging betwee
2、n 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 three-dimensional numerical modelling studies have been conducted for the longwall panel extraction of
3、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 asse
4、ss 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,
5、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; Geo-mining
6、; Obliquity; Block contours; Main fall; Advance gallery1. IntroductionIn India, underground coal production is mostly dependent upon the conventional bord and pillar (room and pillar) method of mining, although the overall output per man shift (OMS) through this method is generally not more than 1 t
7、on in any of the mines 1. Large areas in all the subsidiaries of Coal India Limited and even in Singarani Coal Companies Limited (India) have been developed via bord and pillar workings. There is a need to search for a new method of mining for the faster exploitation of these developed pillars to im
8、prove productivity.In this paper, the authors have conducted different numerical modelling studies using Fast Lagrangian Analysis of Continua (FLAC) software, to assess the cavability of the overlying strata of the Passang seam in Balrampur Mine of SECL (India) over developed bord and pillar working
9、s along with support requirements at the face and in the advance gallery. This study is based on eld observations of a longwall panel and laboratory tested data of the overlying roof rocks as the input parameter for the modelling. The caving nature of the overlying rocks characterized by the main fa
10、ll span is predicted for varying face length, strata condition and depth of cover. Further, optimal obliquity of the face was also estimated for faster and safe exploitation of the developed pillars by shortwall mining to improve the productivity.2. Shortwall miningBord and pillar mining will not be
11、 a suitable option for developed pillars at higher depth cover in terms of productivity, safety and percentage of recovery. The technology of shortwall mining overcomes most of the limitations experienced in operating longwall mining. In the Indian context, a face length of about 90m is economically
12、 optimum with a moderately priced shearer and earning per man shift (EMS) as observed in the Indian longwall mining faces 2. Shortwall mining of the developed bord and pillar workings would be a good option to overcome the limitations of the conventional bord and pillar method of mining.3. Status of
13、 the Passang seam of Balrampur mineThe geo-mining parameters of the proposed area of the Balrampur mine for shortwall mining panel are given below.Thickness of the seam2.4mProposed height of extraction2.4mDepth of proposed panel37-50mExisting overlying/underlying workout areasNilExisting mining patt
14、ernDeveloped on bord and pillar workings Pillar size20m20m(center to center)Gallery width4mVarious boreholes have been drilled over the longwall panels P-1 and P-2 of Passang seam of Balrampur mine of the same area with results as given in Table 1. The average hard cover in panels P-1 and P-2 were 2
15、9 and 39m, respectively, and the depths of the seam were 50 and 53.1m, respectively. Based on the eld observations of the caving nature of the overlying strata of the longwall panel P-1, the overlying strata have been divided into six major beds overlying the coal seam. Based on engineering judgemen
16、t and giving a higher weight to the borehole lithology in panel P-1, estimated RQD and the intact average compressive 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 Bed-I and Bed-III are weak beds, with RQD
17、of 40% and 43%, respectively. Bed-II and Bed-IV are relatively strong with RQD of 78% and 75%, respectively. It is expected that these two strong beds will pose difculty for caving. Bed-V and Bed-VI consist of fractured/weathered rock and alluvial soil.Table 1 The borehole details over longwall pane
18、ls P-1 and P-2 at theBalrampur mineBorehole no.Depth of seam(m)Seam thickness(m)Hard cover(m)BIX-145 (behind panel P-1)47.502.3226.30BIX-146(middle of panel P-1)49.302.2824.30BIX-144 (at the end of panel P-1)48.902.5038.55BHP-2.1 (over panel P-2)51.252.9033.00BHP-2.3 (behind panel P-2)52.4939.39BHP-
19、2.3 (over panel P-2)53.62.6041.00BHP-2.4 (over panel P-2)54.701.5042.004. Field experiences of alreadyextracted longwall panelLongwall panel P-1 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
20、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 o
21、bserved. 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 7980m from the barrier. This loading caused extensive damage to the powered supports installed at the face and subsidence was observed on the surf
22、ace. This was recorded as the rst main fall.Table 2 Representative lithology above the Passang seam, plus their intact propertiesBed No.Run up wards(m)Rock typesThickness(m)RQD(%)Compressive strength(MPa)Tensile strength(MPa)Bed-0.5-5.51Coal Medium grained sandstone, laminated with shale2.45.51-4023
23、.810.962.51.5Bed-5.51-12.17Coarse grained to medium grained sandstone6.667817.11.4Bed-12.17-16.02Very coarse grained sand stone3.854313.921.4Bed-16.02-30.00Medium grained sand stone13.987514.52.2Bed-30.00-41.00Weathered rock11.00-Bed-41.00-50.00Sandy soil9.00-5. Cavability analyses of the overlying
24、strataNumerical modelling for shortwall mining of devel-oped 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 v
25、ariation 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 seam2.4mDepth of cover50 and 40mHard cover/Alluvia
26、l soil for 50m depth cover30m/20mHard cover/Alluvial soil for 40m depth cover20m/20m and 30m/10mPillar size (Center-to-Center)20m20mWidth of gallery4mFace length84m/104mIn the absence of the in situ measurements of stress values, theoretical values were calculated using the following equation:(1)whe
27、re and are the vertical and horizontal stresses (Mpa), E, Youngs modulus of in seam values (2000Mpa), v; Poissons ratio (0.25), the coefcient of thermal expansion (), G the geothermal gradient () and H the depth of cover, (m).This equation shows that the mean in situ horizontal stress (mean of major
28、 and minor horizontal stresses) depends on the elastic constants (E,V) of in-seam values, the coefcient of thermal expansion () and the geothermal gradient (G).The vertical stress can be taken for coal measure rocks as:(2)Then, substituting for in Eq. (1), we obtain the mean horizontal stress as : M
29、Pa(3)The strength of the rock mass has been estimated using an empirical criterion proposed by Sheorey et al. for the present study. This criterion is expressed as:(4)where is the major principal stress required for failure of rock mass when the minor principal stress is :(5)(6)(7)where ; are the co
30、mpressive strengths of intactrock and rock mass, respectively (Mpa), ; the tensile strengths of intact rock and rock mass, respectively (Mpa), , the exponent in failure criterion of intact rock and rock mass, respectively and RMR the Rock Mass Rating proposed by Bieniawski.To estimate the stability
31、or instability of the rock mass, safety factors are evaluated for all the elements of the numerical model. The safety factor (F) is dened as:(8)except when (9)Where and are the major and minor induced stresses from the numerical model output.The sign convention followed here is that compressive stre
32、sses are taken as positive.待添加的隐藏文字内容25.1. Calibration of the modelTo standardise the tested properties in the laboratory given in Table 2, a three-dimensional FLAC-3D model of the already extracted longwall panel P-1 (Fig. 1) was made and run with the above-tested properties for different face adva
33、ncement. The value of RMR for the modelling has been assumed bed-wise, tentatively giving weight to the RQD and intact strength of the beds. Block contours of safety factors over a quarter portion of the model for different face advancement are shown in Fig. 2(a)(c). Elements which show a factor of
34、safety less than 1 are liable to fail. It was observed that up to 60m of face advance in the model, only the immediate roof of about 56m height exhibited a factor of safety less than 1, indicating caving of the immediate roof only. At the same time, both the strong beds such as Bed-II and Bed-IV, sh
35、ow a factor of safety more than 1, indicating they would not cave (Fig. 2a). After a face advance of 70m, Bed-II has caved, while Bed-IV still remains intact (Fig. 2b). At 80m of face advance, the Bed-IV caves (Fig. 2c), which corresponds to the main fall. These results were obtained after some adju
36、stment in the actual tested properties to suit the caving behavior of already extracted panel P-1 by longwall mining. These standardised properties, as given in Table 3, have been utilised for the further modelling of shortwall panels.Fig. 1. Three dimensional model with grids/elements showing coal
37、seam and different beds for Balrampur mine of SECL (India).Fig. 2. Block contours of safety factor over quarter portion of Panel P-1 of Balrampur mine for: (a) 60m face advance; (b) 70m face advance; (c) 80m face advance.5.2. Modelling for main fall predictionFour main three-dimensional models (Mode
38、l-A to Model-D) were studied in detail, covering different combinations of the existing geo-mining conditions at the Balrampur mine, to predict the main fall position from the starting of the panel. Model-A: Models for extraction width of 5 pillars (104m) at a depth cover of 50m (30m hard cover+20m
39、alluvial soil).Table 3 Calibrated propertiesModulus of elasticity(Gpa)Poissons ratioRMRRock mass compressivestrength (Mpa)Rock mass tensilestrength (Mpa)caol1.80.25501.950.393Bed-1.80.25551.1550.283Bed-3.00.25652.9720.38Bed-2.00.25551.4670.264Bed-3.00.25703.160.724Bed-&0.30.10-Model-B: Models for ex
40、traction width of 4 pillars (84m) at a depth cover of 50m (30m hard cover+20m alluvial soil). Model-C: Models for extraction width of 4 pillars (84m) at a depth cover of 40m (30m hard cover+10m alluvial soil).Model-D: Models for extraction width of 4 pillars (84m) at a depth cover of 40m (20m hard c
41、over+20m alluvial soil).The ndings of the above different predictive models have been summarized in Table 4.6. Modelling for estimation of appropriate face orientation and rib stabilityOrientation of the face should be oblique; otherwise, when the face approaches the advance gallery, a thin long rib
42、 will be formed and a sudden collapse may take place. Such a collapse would expose a wide span in front of the powered supports throughout the face length instanteously. To overcome this problem, obliquity of the face will minimise the exposed area and provide easier access to cross the advance gall
43、ery, from one end of the face to the other. With an oblique face crossing the advance gallery, triangular ribs will be formed and their stability is important during the shortwall mining. In order to estimate the safety factor of the pillar or triangular rib, its strength and the load on it must be
44、determined. The strength of pillar/rib (S) is estimated using the CMRI pillar strength formula as given.MPa, (10)Where is the uniaxial compressive strength of a 25mm cube coal sample (23.8Mpa), H the depth of cover (50m), h the extraction height (2.4m), the (4A/Cp), (m), A the area of the rib and Cp
45、 the periphery of the triangular rib (m).Table 4 Findings of the predictive modelsModel no.Face advancementCondition of the different bedsMain fall predictionModel-A80mBed-I, Bed-II and Bed-III lying over the extracted area failed completely, while Bed-IV is intact throughout the excavation span.Mai
46、n fall reaching up to surface is not expected.90mDue to further face advancement up to 90m for the excavation span of 104m wide face length, Bed-IV also fails partially and the fall may reach the surface.Main fall can be expected.100mThe overlying strata failure zone increases over the excavated are
47、a as the Bed-IV fails completely.Main fall reached up to the surface.Conclusion: The strong Bed-II is expected to cave before 80m of face advance, while the main fall is expected to occur between 90 and 100m of face advancement. At the main fall, Bed-IV is expected to fail and subsidence will reach
48、up to surface.Model-B100mBed-II overlying the extracted area failed completely. Bed III caves in along with Bed II, while Bed-IV is intact throughout span.Minor weightings will be experienced at the face.110mAt the face advancement of 110 m, there will be partial caving of Bed-IV in the goaf but still overha
