Analysis on Backfill Mining of Under Three Coal in Zhouyuanshan Mine and Strata Movement Law

Transcription

1 Research Journal o Applied Sciences, Engineering and Technology 6(17): , 2013 ISSN: ; e-issn: Maxwell Scientiic Organization, 2013 Submitted: January 10, 2013 Accepted: January 31, 2013 Published: September 20, 2013 Analysis on Backill Mining o Under Three Coal in Zhouyuanshan Mine and Strata Movement Law 1 Weijian Yu, 2 Bo Xu, 1 Tao Feng and 1 Xinyuan Chen 1 School o Energy and Saety Engineering, Hunan Provincial Key Laboratory o Sae Mining Techniques o Coal Mines, Hunan University o Science and Technology, Xiangtan, Hunan , China 2 China International Trust and Investment Corp (CITIC) Mining Technology Development Co., Ltd, Beijing, , China Abstract: In order to solve the problem o "under three" (under railways, buildings and water bodies) coal pillar mining, analysis on backill mining and strata movement law is carried on in 24 mining district o Zhouyuanshan coal mine. First o all, according to the engineering geology and strata occurrence condition in 24 mining district o Zhouyuanshan coal mine, FLAC2D sotware be used to establish a two-dimensional numerical model and to analyze and calculate the ull caving method, strip method and backill method and then gaining that backill mining method is beneicial to improve the protection level o surace buildings and acilities. Then, using the theory o strata control and method o related mechanics to analyze the strata movement law and strata control principle o backill mining, considering that supporting role o backill body is mainly on lateral reinorcement o coal pillar and vertical supporting role o overlying strata, orming a cooperative control system o "bearing strata + coal pillar + backill body" and deducing the equilibrium equations when it is in steady state. At last, using the numerical analysis method, respectively analyzing the surace subsidence o the corresponding important buildings o the three proiles o C-8 exploration line,c-6 exploration line and A-A (cross section o the proile o C-8 exploration line) ater using backill coal mining in 24 mining district. The results show that: the surace subsidence and horizontal deormation basically control within 30 mm and the surace deormation curvature o buildings generally in /km in 24 mining district o Zhouyuanshan coal mine, which accord with the relevant standards and requirements. Keywords: Backill mining, coal mining, strata movement, under three coal (under buildings, railways and water bodies) INTRODUCTION At present, China's coal reserves o "under three" coal is about billion tons, including the coal which under buildings is 8.76 billion tons, accounting or 63.5% o the entire amount (Fa-kui and Feng-ming, 2005). Especially or the southern provinces and cities that the coal resources badly lacking in, the coal has been nearly exhausted this easily mined. However, there is still a considerable part o coal resources that was let in "under three" (under water bodies, buildings and railways). According to incomplete statistics, only in dierent areas o Hunan province the coal reserves o "under three" coal amounts to 650 million tons (Weijian et al., 2010), most coal mines are aced or will be aced with "under three" mining, the amount o coal reserves not only brings huge loss to national resources, but also brings diiculties to enterprise's production and management. However, or coal mining o "under three", both considering the recovery o mineral resources and ocusing on the surace subsidence, deormation and environmental protection etc. On coal pillar mining o "under three", backill mining technology is used mainly currently (Xie-Xing and Ming-Gao, 2009). With the large demand o coal resources and enhancement o environmental protection consciousness, it should say that coal backill mining technology is an important part o the green mining technology, especially that it is a kind o eective method to mine coal resources and important guarantee to coal sustainable development, now which is gradually becoming the key technology o the coal industry. Thereore, this study main research on "under three" coal mining technology in 24 mining district o Zhouyuanshan coal mine, including demonstration o the backill mining scheme and analysis on strata movement law. GENERAL SITUATION The 24 mining district o Zhouyuanshan coal mine is located in the north wing o Zhouyuanshan mine ield, Old pingan ault to the north, Baoli ault to the south, the elevation between -370 and -650 m, which Corresponding Author: Weijian Yu, School o Energy and Saety Engineering, Hunan University o Science and Technology, Xiangtan, Hunan , China 3110

2 Table 1: Each stratum and its description Stratum Lithology description Average thickness /m Cumulative thickness/m Surace soil layer Brown, pale-yellow clay, ine sand with a little gravel Sandy mudstone The upper part is gray to grayish green and purple, mottled sandy mudstone, level or gently wavy bedding, gray thin-bedded sandy ; middle part is gray to grayish, greenish to grayish green, thick bedded quartz eldspar ine in major, intercalated between gray to gray green mudstone; the lower part is gray green to dark grey sandy mudstone, mudstone with ine Medium grained Sandy mudstone Medium grained Sandy mudstone Sandstone Medium grained. Light gray to grayish white, consist o quartz in irst place, eldspar secondly, enormous thick layered, cross-bedding, several layers o sandy mudstone in the middle-upper section. Gray to dark lamellar, ine o thin bed locally, common development. Gray to light gray, consists o quartz in irst place, eldspar secondly, several layers o dark lamellar sandy mudstone. Dark gray to black sandy mudstone, mudstone, with several layers o ine, containing iron concretion and lamellibranch ossils; the lower 13 m is quartz eldspar ine. Light gray to gray white, consist o quartz in irst place, eldspar secondly, coarse particles upward tapering, sandwiched between the sandy mudstone, mudstone. Gray to light gray, consist o quartz in irst place, eldspar secondly, sandwiched several layers o dark gray lamellar mudstone coal In bulk, single structure, dim type Sandy mudstone Gray to dark gray, medium thickness layered in major, with Ling irony structure and negative 1 coal. Fine Gray to gray white, thin to medium thickness layered, grit stone locally. Sandy mudstone Gray to dark gray, thin to medium thickness layered, containing plant ossils, occasionally sandwiched irony structure. 2 coal Complex structure, upper layer with bright, good quality in major, lower layer with carbonaceous mudstone, laky, poor quality, not minable in most part o the district Sandy mudstone Gray to dark gray, thin to medium thickness layered, jointing development, jointing surace is smooth and like a "mirror". 3 coal Flaky, complex structure, penumbra, poor quality Sandy mudstone Gray to dark gray, thin to medium thickness layered, jointing development. 4 coal In bulk, single structure, semi-bright, high quality Sandy mudstone Dark gray, thin to medium thickness layered, the lower sandwiched some layers o ine sand layer belongs to the second level in deep mining. The area corresponding to south and east o the surace is mountainous region, Baoyuan River, Zhangjialong reservoir, coking coal washing plant, Baoyuan railway, highway o mountain to the north, gangue power plant and other buildings to the west. The 1, 3, 4 coal are minable coal seams in the district and the total amount o "under three" coal is 4.1 million tons, accounting or 85.8% o the entire amount o 24 mining district. At present, the 2411 and 2441 work aces are close to the mining scope o "under three", the 2412 work ace which to be arranged belongs to the mining scope o "under three". The 1, 3, 4 coal are minable coal seams in 24 district and 1 coal seam is a single structure, in bulk, with dull coal in irst place, including bright coal and mirror coal strip, average thickness o 1.75 m, average dip angle o 19 ; 3 coal seam is a complex structure, scaly, by bright coal strip, dull coal and carbonaceous shale interblended, generally with 1~2 layers o gangue, average thickness o 1.6 m, the average dip angle o 19, with 26.5 m away rom 1 coal, 2 coal which between 1 coal and 3 coal cannot be minable; 4 coal seam is a single structure, in bulk, the type ormed by bright and dull coal, average thickness o 1.07 m, the average angle o 19, with 26.6 m away rom 3 coal, deputy 3 coal which between 3 and 4 coal cannot be minable. According to geological data in 24 mining district o Zhouyuanshan coal mines, the each stratum and its description shown in Table 1. DEMONSTRATION OF BACKFILL MINING SCHEME Basic calculation parameters and calculation model: Here, using the numerical simulation method to demonstrate the backill mining scheme and calculating sotware is FLAC 2D program. The Mohr-Coulomb yield criterion is used or the model o rock mass. According to the geological report, the mechanical parameters o

3 Table 2: The mechanical parameters o each stratum Surrounding rock (rom Bulk density Uniaxial compressive Uniaxial tensile Cohesion Angle o internal Elastic modulus Poisson above down) γ/g/cm 3 strength R/MPa strength R t /MPa C/MPa riction Φ/ E/GPa ratio μ 1 Surace soil layer Sandy mudstone Medium grained Sandy mudstone Medium grained Sandy mudstone Sandstone Medium grained coal Sandy mudstone Fine Sandy mudstone coal Sandy mudstone coal Sandy mudstone coal Sandy mudstone Table 3: The mechanical parameters o backill body Degree o compaction D r Initial void ratio e 0 Bulk density γ/g/cm 3 Compressive strength R/MPa Tensile strength R t /MPa Cohesion C/MPa Angle o internal riction Φ/ Elastic modulus E/GPa Poisson ratio μ ore body and surrounding rock determined by the mechanical and deormation parameters in Table 2, but in the simulation o initial stress ield, the higher rock mass parameters or elastic module can be used or calculation, to avoid parameters o rock mass too low that produce plastic region in the simulation o initial stress ield (Wei-jian et al., 2009). The degree o backill compaction is one o the key quotas to backill construction quality detection o "under three" coal seam mining, characterizing the condition o density ater compaction. Namely: the higher o the degree o compaction, the bigger o the density, the better overall perormance o backill material. From the view o mechanical concept, backill compactness means compacted degree o backill body under the action o external orce and the speciic calculation method is the ratio o actually that reached the dry density and test o the maximum dry density o indoor standard compaction. In order to analyze problems conveniently, the backill mechanics strength parameters o compaction degree o 0.9 bases on the experimental study are shown in Table 3. According to occurrence conditions o the 1, 3, 4 coal which are minable in Zhouyuanshan mine coal and considering the inluence o mining area and surace construction in 24 district, to identiy calculate range o the model as m, thereore, the grid o model is divided into The let and bottom o the computational model take displacement boundary and the upper takes stress boundary, the stress that is gravity stress o the overlying strata. In addition, owing to the 3112 horizontal tectonic stress caused by tectonic movement is small, so the side pressure coeicient (λ) sets to 1. Comparative analysis o the three kinds o mining schemes: In order to prove the rationality o backill mining in Zhouyuanshan coal mine, now aiming at ull caving method, strip method and backill method or calculation and analysis and carrying on the contrast analysis and the calculated results are shown in Fig. 1 to 4, knowing that: From the perspective o overlying strata movement law o the whole mining area, when using ull caving method or coal seam, roo strata produce large-scale caving ater mining and strata displacement is big, the maximum can reach a ew meters, equivalent to the thickness o mining coal seam. And range o strata stress release is large, making a wide range o strata lose bearing capacity. When using strip mining method or coal seam, the coal pillar has a deinite unction to the overlying strata, but due to gob area more and depth o burial is big, the coal pillar occurs some disruption under the high stress and it lost supporting ability, leading to the movement o overlying strata is big and spread to the surace. In addition, rom the view o the stress state, the stress release range o roo strata is big, but near the coal pillar appears the stress concentration phenomenon, which is very adverse or the stability o gob, once the instability

4 Fig. 1: The situation o strata movement and stress redistribution caused by ull caving mining Fig. 2: The situation o strata movement and stress redistribution caused by strip mining Fig. 3: The situation o strata movement and stress redistribution caused by backill mining Settlement displacement /mm Model length/m ull caving method backill mining method strip mining method Fig. 4: Contrast in the curves o surace subsidence caused by three kinds mining methods occurs that it is bound to aect the surace, causing damage o surace buildings. When the coal picked out and taking backill body to ill the mined area, as the backill body plays a 3113 deinite supporting role, eectively inhibiting the subsidence o overlying strata which is urther, reducing the release degree and scope, so in this mining method the strata movement is in low level,

5 contributing to the control o surace subsidence, providing basis to ensure the surace construction. Based on the analysis o the results, the maximum value o surace subsidence caused by ull caving mining is nearly 2.5 m; the maximum value o surace subsidence caused by strip mining is nearly 0.6 m; but the maximum value o surace subsidence caused by backill mining is less than m. Moreover rom the view o subsidence curve, the in homogeneity o surace subsidence caused by ull caving mining and strip mining become more obvious, while the surace subsidence value caused by backill mining not only small, but also homogeneous. Thereore, backill mining is a kind o green mining methods, suitable or coal seam mining o "under three" and conducive to the protection o surace buildings and acilities. THEORY OF THE CONTROL OF BACKFILL MINING AND PREDICTION OF BUILDING SUBSIDENCE Fig. 6: System cooperative control principle o the supporting body o "bearing strata + coal pillar + backill body" F c : Backill body generates lateral supporting pressure on coal pillar; F cr : Coal pillar generates supporting orce on overlying strata; F r : Backill body generates supporting orce on overlying strata System cooperative control principle o strata movement in backill mining: In order to pick out the coal resources more, the width o coal pillar much reduced in the mining o "under three" coal, such as there are important buildings or objects to be protected on the surace, backill mining is needed. Generally speaking, when using backill mining, overlying strata and backill body orm new bearing community, this "bearing strata + coal pillar + backill body" that orm supporting body whose stability is the basic requirements to guarantee o overlying strata to achieve stable situation and their interactions as shown in Fig. 5. The mechanical mechanism is mainly maniested as ollows: Lateral reinorcement eect o backill body on coal pillar: Under the pressure o overlying strata, due to both sides o gob, making the pillar stress by threedimensional to two-dimensional state, pillar near gob side will generate lateral expansion (deormation displacement). When gob is backilled, backill body is in contact with coal pillar. Backill body generates lateral stress on coal pillar and making the twodimensional stress state variable or the threedimensional stress state again, inhibition o the coal pillar o lateral displacement to increase urther that resulting instability. From another perspective, strengthening coal pillar, improving the stability o coal pillar, making the coal pillar get balance in the threedimensional pressure, which is beneicial to improve the overlying strata supporting orce. The mechanism o action as shown in Fig. 5: Both sides o backill body generate lateral pressure F c on the coal pillar, inhibition o the lateral expansion deormation o coal pillar, meanwhile the coal pillar generates support pressure F cr on overlying strata Fig. 5: Mechanical model o roo strata Backill body generates vertical supporting role on bearing strata: When backill body reaches roo strata, in process o overlying strata continuing to sink and compress, the strength o backill body sustaining to play out, generating supporting orce opposite to strata pressure o overlying strata, inhibition o overlying strata to sink urther delection, to avoid the collapse and destruction o strata. Its mechanism o action as shown in Fig. 6: In process o overlying rock sinking, backill body is gradually compacted and plays a greater upward supporting orce F cr to resist the overlying strata continue to sink, which is conducive to maintain the integrity o the strata. According to Fig. 6 shows, the overlying strata rely on backill body, coal pillar and bearing strata (including key layer) to maintain stability. I the backill body destroyed, it will certainly inluence the stability o coal pillar; eventually leading to the sinking displacement o overlying strata is larger and cause larger surace subsidence. Thereore, it is a system engineering o supporting body o "bearing strata + coal pillar + backill body, which is a coordinated unction process and its stability is also the comprehensive perormance results o whole system. Here, in order to the need o analysis, roo strata can be viewed as simply supported beam. The mechanical model is shown in Fig. 5.

6 Thereore, to get the roo subsidence this moment is the delection o simply supported beam, i.e.: Res. J. Appl. Sci. Eng. Technol., 6(17): , 2013 qx S1 = ( b 2bx + x ) (1) 24EI In the above ormula: E = The modulus o elasticity or roo strata, GPa I = The section moment or roo strata, m 4 q = The concentrated load that role in the roo strata, nn its value is q = ii=1 γγ ii h i, MPa b = The distance o two midpoints o the two coal pillars which let in strip mining, m x = The arbitrary distance o the midpoint o the let coal pillar and roo o gob, m (a) Proile o C-8 prospecting line (b) Proile o C-6 prospecting line Figure 7 shows that: When x = b/2, S 1 gets the maximum, i.e.: S 1max 4 5qb = (2) 384EI In addition, to use the void ratio o backill body to represent degree o compaction, thereore, the amount o compaction d 1 which generates ater backill compaction can be obtained through the ollowing ormula: 1+ e0 d1 = H (1 + e) (3) In the above ormula: H : The height o gangue backill body, m e 0 : The initial void ratio o gangue backill body e : The void ratio o gangue backill body ater compaction According to the reality o backilling, the ratio o the height o gob and backill body can be as backill ratio, namely the calculative ormula o backill rate is the ollowing: H F = 100% (4) H c In the above ormula: F = Backill rate, % H c = The height o gob, m H = The height o backill body, m Thus, can get the height o gob which is not backilled: ( 1 ) H = F H (5) n c In the above ormula, H n is the height o not backilled, m 3115 (c) A-A proile Fig. 7: The numerical models o corresponding proile o 24 mining district According to theory o the strata movement and principle o beams, subsidence movement value o overlying strata should not be greater than the maximum delection, so as to saeguard the stability, thereore, according to the backill body is compressed and subsidence o overlying strata and alliance with the ormula (2), (3), (4) and (5), can get the stability conditions o backill body which supporting the overlying rock, as below: ( 1 F ) H 4 1+ e 5qb H(1 e) 384EI 0 c + + (6) The expectation o subsidence o buildings caused by backill mining: Now to select three proiles that calculated respectively and getting the value o subsidence o the main buildings. Selecting proile o C-8 prospecting line, proile o C-6 prospecting line and proile A-A (cross section o the proile o C-8 prospecting line) o 24 mining district. These proiles establish numerical calculation models shown in Fig. 7. In order to analyze the problem and calculate conveniently, the calculations on proile o C-8 prospecting line and proile o C-6 prospecting line, the seams don't take into account except 24 mining district and the important buildings are located in the let o 24 mining district, thereore, only to choose the scope away rom 1250 m o amilies district to establish the models. For A-A proile, due to the important buildings are located in the middle centre above 24 mining district, thereore, choosing the scope away rom 1400 m o amilies district to establish the model. The strata

7 Table 4: The maximum deormation value o the corresponding important buildings o the proile o C-8 prospecting line o 24 mining district Total deormation o surace Unit length deormation o surace The name o the buildings Horizontal displacement d/mm Subsidence displacement s/mm Horizontal distortion ε (mm/m) Tilt deormation i (mm/m) Curvature K (10-3 /km) Mine amily village Gangue power plant Reservoir Table 5: The maximum deormation value o the corresponding important buildings o the proile o C-6 prospecting line o 24 mining district Total deormation o surace Unit length deormation o surace The name o the buildings Horizontal displacement d/mm Subsidence displacement s/mm Horizontal distortion ε (mm/m) Tilt deormation i (mm/m) Curvature K (10-3 /km) Gold trade city Endowment bureau II Coal washing plant house Oice building o coking plant Site area o coking plant Gangue brick residence Fig. 8: The calculation results o the proile o C-8 prospecting line o 24 mining district Fig. 9: The calculation results o the proile o C-6 prospecting line o 24 mining district displacement and stress distribution can be obtained ater calculating the model established by proile o C-8 prospecting line which is shown in Fig. 8, the maximum subsidence value to the corresponding structure shown in Table 4; The strata displacement and stress distribution can be obtained ater calculating the model established by proile o C-6 prospecting line which is shown in Fig. 9, the maximum subsidence value to the corresponding structure shown in Table 5; The strata displacement and stress distribution can be obtained ater calculating the model established by A-A proile which is shown in Fig. 10, the maximum subsidence value to the corresponding structure shown in Table According to the deormation calculation results o important buildings in above, two conclusions can be obtained: From the situation o strata movement, each proile carries on backilling ater mining, improving the stability o stope or supporting system which ormed by overlying strata, coal pillar and backill body and getting better control o strata moving in large range, greatly reducing the degree o subsidence. The results show that: the subsidence o surace and horizontal deormation basically control within 30 mm, subsidence displacement o a ew buildings beyond 30 mm; the deormation

8 Table 6: The maximum deormation value o the corresponding important buildings o A-A proile (cross section o the proile o C-8 prospecting line) o 24 mining district Total deormation o surace Unit length deormation o surace The name o the buildings Horizontal displacement d/mm Subsidence displacement s/mm Horizontal distortion ε (mm/m) Tilt deormation i (mm/m) Curvature K (10-3 /km) Treasure source highway Treasure source Railway Oice building o coking plant Dormitory o bureau Oice building o power plant Reservoir o zhang ridge Fig. 10: The calculation results o A-A proile o 24 mining district curvature o surace buildings in /km, the maximum no more than /km, thereore, the backill mining in accordance with the relevant standards and requirements. From the characteristics o strata stress distribution caused by backill mining, due to the dip angle o coal seam is 19, the range o stress release in the head entry o working ace is most obvious and the range o stress release is relatively moderate, indicating the backill body, coal pillar and overlying strata has ormed a more coordinated mechanics system. The phenomenon o stress concentration exists mainly in the troughs o working ace; thereore, it should strengthen the mine pressure monitoring and roadway supporting. CONCLUSION For the calculation and analysis on ull caving method, strip method and backill method, the results show that the maximum value o surace subsidence caused by ull caving mining nearly 2.5 m; the maximum value o surace subsidence caused by strip mining nearly 0.6 m and the maximum value o surace subsidence caused by backill mining is less than m. Furthermore the surace subsidence caused by backill mining is relatively uniorm, this kind o mining method is suit or coal mining o "under three", beneicial to protect the important buildings and acilities o surace. The supporting role o backill body mainly in these two aspects or reinorcement o coal pillar 3117 and vertical supporting role on load-bearing strata. The lateral stress role o backill body on coal pillar, can eectively inhibit the swelling deormation o coal pillar, playing the role o reinorcing coal pillar, enhancing the stability o coal pillar, urther improving supporting capacity on the overlying rock; in addition, backill body generating supporting orce on overlying strata, inhibition o displacement to increase or overlying strata subsidence, avoiding the strata caving and instability, maintaining the integrity o the strata. To make the cooperative control system o "bearing strata + coal pillar + backill body" be stable, lateral stress o backill body on coal pillar should reach a certain value, the value o stress not only relevant with the own strength o coal pillar, but also relevant with the pressure o overlying strata; In addition, also considering the relationship among subsidence o roo bearing strata, backill rate o gob and the compression between backill body and coal pillar. Prediction is carrying on the backill mining, rom the view o calculated results o three proiles o C- 8 exploration line, C-6 exploration line and A-A (cross section o the proile o C-8 prospecting line), owing to each section were illing ater mining, surace subsidence and horizontal deormation basically control within the 30 mm; the deormation curvature o surace building generally in /km, in line with the relevant standards and requirements.

9 ACKNOWLEDGMENT This study was inancially supported by Project supported by the National Natural Science Foundation o China ( ; ; ) and Scientiic Research Fund o Hunan Provincial Education Department (12cy013). REFERENCES Fa-Kui, X. and L. Feng-Ming, Brie analysis on some problems o mining "under three" coal in China. Coal Econ. Res., 5: Wei-Jian, Y., Z. Shu-Hai and G. Qian, Stability evaluation indexes o deep stope pillar and roadway surrounding rock. Disaster Adv., 5(4): Wei-Jian, Y., F. Tao, W. Wei-Jun, et al., Coordination support systems in mining with illing and mechanical behavior. Chinese J. Rock Mech. Eng., 31(Supp1): Xie-Xing, M. and Q. Ming-Gao, Research on green mining o coal resources in China: Current status and uture prospects. J. Min. Saety Eng., 26(1):