Journal of Engineering Science and Technology Review 7 (4) (2014) 40-45

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1 Jestr Journal of Engineering Science and Tecnology Review 7 (4) (14) -45 JOURNAL OF Engineering Science and Tecnology Review Mecanics Evolution Caracteristics Analysis of in Fully-mecanized Mining Field S.R. Wang 1, 3*, N. Li 1, C.L. Li 1, 2 and P. Hagan 4 1 Scool of Civil Engineering and Mecanics, Yansan University, Qinuangdao 0604, Cina 2 Institute of Urban Construction, Hebei Normal University of Science & Tecnology, Qinuangdao 0604, Cina 3 Opening Laboratory for Deep Mine Construction, Henan Polytecnic University, Jiaozuo 4503, Cina 4 Scool of Mining Engineering, University of New Sout Wales, Sydney, 52, Australia Received 30 Marc 14; Accepted 10 September 14 Abstract Based on a practical engineering, te tree-dimension computational model was built using FLAC 3D under te fullymecanized mining condition. Considering four variation factors, suc as te distance of mining advancing, te strengt of te surrounding rock, te speed of mining advancing and te dip angle of te coal seam, te mecanics evolution caracteristics of te pressure-arc were analyzed. Te result sowed tat for te orizontal seam, te geometric sape of te pressure-arc varied from flat arco round arc gradually and te eigt and tickness of te pressure-arc also increased; te maximum principal stress in te skewback also increased wite working face advancing. Wite strengt of te surrounding rock from soft to ard, te arcickness reduced, and te arc loading decreased. To improve te mining speed can do some contributions to te stability of te pressure-arc in te mining field. Wite increase of dip angle of te seam, te pressure-arc displayed an asymmetric sape, te vault was tilted and moved to te upward direction. At te same time, te tickness of te pressure-arc increased, and te stress concentration in te skewback tended to be furter intensified. Keywords:, Evolution Caracteristics, Mining Field, Fully-mecanized Mining, Numerical Analysis 1. Introduction Wite underground coal mining out, te equilibrium of te original tree-dimensional stress of mining field is broken, te stress of te surrounding rock gradually transfers from te mining face to te deep of te rock, and te concentration area of compressive stress will generate witin a certain range in te surrounding rock, wic forms te pressure-arc in te mining field [1]. Te pressure-arc of te mining field is te result of te stress self-regulating. Te pressure-arc usually as mecanics properties of te arc structure, wic can sustain its loading and transfer te oter loads to te foot of te pressure-arc, and tis is of great significance to ensure te stability of te surrounding rock of te mining field [2]. Over te past years, many scolars ave done te relevant researces on te pressure-arc in te engineering rocks and ave made a lot of acievements. For example, W. Hack et al. put forward a pressure-arc ypotesis about te overlying strata movement of te mining field and made a preliminary explanation of te penomenon of te mining pressure [3]. Z.P. Huang et al. studied te formation mecanism of te natural arc for underground camber and made a preliminary conclusion [4]. B.A. Poulsen et al. conducted te coal pillar load calculation by pressure arc teory [5]. R. Trueman et al. researced te multiple drawzone interaction in block caving mines by means of a large * address: w_sr88@3.com ISSN: Kavala Institute of Tecnology. All rigts reserved. pysical model [6]. X.D. Liang et al. ad defined te pressure-arc of a tunnel, and te inner and outer boundaries of te pressure-arc were determined by te stress analysis, but te determination of outer boundary for te arc body is not clear enoug [7]. G.X. Xie et al. ad discovered te sell structure of macroscopic stress of te mining field, and studied te influence factors suc as mining eigt and oter factors on te mecanical caracteristics of te surrounding rock of te fully-mecanized caving face [8]. X.L. Du et al. discussed te stress variation of te mining field wite working face advancing, but te influence factors on te pressure-arc were not considered [9]. Y.M. Geng et al. simulated te variation of te stress arc in te overlying strata after coal mining out by using UDEC [10]. Y.Y. Xu et al. ad derived te deflection formulas of te rock slab based on te pressure-arceory [11], and oters studies [12]. In summary, tese studies on te geometry parameters and mecanics evolution caracteristics of te pressure-arc are still not deep enoug, and te discussions are mainly restricted to two-dimensional problem, so an in-dept study on te mecanics evolution caracteristics of te treedimensional pressure-arc in te fully-mecanized mining field is of important teoretical and practical significance. 2. Geometry Parameters As sown in Fig. 1, after te coal was mined out, te treedimensional stress balance of te mining field was broken,

2 S.R. Wang, N. Li, C.L. Li and P. Hagan/Journal of Engineering Science and Tecnology Review 7 (4) (14) - 45 ten te minimum principal stress at te mining free face became zero, and te stress value gradually increased from te free face to te deep of te surrounding rock until te original stress state. Meanwile, te maximum principal stress underwent te cange from small to te peak value and ten decreased to te original stress state too. Te pressure-arc formed as a result of te tree-dimensional stress self-regulating. Based on te previous researc results, te inner boundary of te pressure-arc are fixed at point A, were is te maximum principal stress (Fig. 1). Stress 1 σ Distance 2 σ B A O Mined-out areas Fig.1. Scematic boundaries of te pressure-arc Coal seam In order to determine te outer boundary of te pressurearc, te variable k is defined as σ1 σ 3 k = (1) σ 1 Were σ 1 is te maximum principal stress, and σ 3 is te minimum principal stress of te surrounding rock of te mining field. Wen k is equal to 10 %, te corresponding point B is determined, wic is at te outer boundary of te pressure-arc (Fig. 1). From te two points in te seam floor wic also are at bot laterals of te mining field as te starting points, two lines are drawn at te rock rupture angle and are extended upward respectively, until tey meet wite inner and outer boundaries of te pressure-arc, tus te lateral boundaries of pressure-arc are determined as sown in Fig. 1. Te rupture angle of te surrounding rock is defined as 45 0 ϕ = + (2) 2 Were φ is te inner friction angle of te surrounding rock. Te pressure-arc of te surrounding rock is not only in te roof of te mining field but also in bot sides and te floor of te mining field. In tis paper, te pressure-arc is determined mainly on te top of te mining field as sown in Fig. 1. For te purpose of analysis, te geometry parameters of pressure-arc are sown in Fig. 2. C 1 f1 B A B 1 2 m1 t Mined-out areas Fig.2. Geometry parameters of te pressure-arc m2 Coal seam f2 C 2 Taking C 1 line and C 2 line as te skewbacks, were te arcickness are and f2 and te peak value of te maximum principal stress are P f1 and P f2, respectively. And similarly, taking B 1 line and B 2 line as arc waist, were te arcickness are and m2 and te peak value of te maximum principal stress are and P m2, respectively. Te line A represents te vault, were te arcickness is and te peak value of maximum principal stress is. Te dotted line is te central axis of te pressure-arc, and it is called as te pressure-arc curve. As sown in Fig. 2, te smoot pressure-arc curve formed trouge median value of te lines between te inner and te outer boundary along te normal direction of te pressure-arc. 3. Tree-dimension Computational Model Taking a practical engineering in a mine as background, after rock group on te roof and floor of te coal seam being generalized, te tree-dimension computational model was built using FLAC 3D under te fully-mecanized mining condition. As sown in Fig. 3, te computational model was 0 m long, 0 m wide and 0 m ig, and te dimensions of te fully-mecanized mining was m long, m wide and 3.5 m ig in te x-, y-, and z-axis, respectively. Te fullymecanized mining was at te dept of 0 m under te ground, and it was in ydrostatic state of stress. Tere was 0 meter tickness rock above te top of te model, and te vertical load p converted from te rock weigt is 15 MPa, were te rock average weigt is 25 kn/m 3. Z Y X Roof Mining advance Coal seam Floor Fig.3. Scematic diagram of te computational model Te orizontal displacement of four lateral boundaries of te model were restricted, te top of te model was applied te vertical load p, and its bottom was fixed. Te material of te model was supposed to meet te Mor-Coulomb strengt criterion, and tree groups of te pysics and mecanics parameters were selected as listed in Table Calculation Results and Discussion 4.1 Distance Variation of te Mining Advancing Seen from Fig. 4 (a), for te orizontal seam, along te dip of te coal seam, te geometric sape of te pressure-arc P 41

3 S.R. Wang, N. Li, C.L. Li and P. Hagan/Journal of Engineering Science and Tecnology Review 7 (4) (14) - 45 varied from flat arco round arc gradually wite working face advancing. Te eigt of te pressure-arc increased, and te central axis of te pressure-arc gradually moved to te deep of te surrounding rock from te mining face. Seen from Fig. 4 (b), along te strike of te orizontal coal seam, te pressure-arc varied from te pointed arco well arced sape, te openings of te pressure-arc increased gradually, and te front foot of te pressure-arc constantly moved forward wit working face advancing. Also, te eigt of te pressure-arc gradually increased. For te orizontal coal seam, weter along te strike or te dip of te coal seam, te pressure-arc sapes in fullymecanized mining field were symmetrical, namely = f2, P f1 = P f2, = m2 and. Terefore, we can take alf of te pressure-arco study te variation caracteristics of arcickness and te maximum principal stress in te arc. Tab.1 Pysical and mecanical parameters of te computational model Classification Name Density (kg/m 3 ) Elasticity modulus (GPa) Poisson ratio Tensile strengt (MPa) Coesion (MPa) Friction angle ( ) Soft rock Roof Coal Floor Roof Coal Floor Roof Coal Floor Fig.4. Sape caracteristics of te central axis of te pressure-arc As sown in Fig. 5, wit working face advancing te tickness of skewback, arc waist and vault increased, but te vault tickness would no longer continue to increase until to a certain value, te overall trend was = f2 > = m2 >. Fig.5. Tickness variation caracteristics of te pressure-arc Seen from Fig. 6, te maximum principal stress produced in te foot of te pressure-arc. Wite working face advancing, te maximum principal stress in te skewback gradually increased, but in oter parts te maximum principal stress constantly decreases. Te results 42

4 S.R. Wang, N. Li, C.L. Li and P. Hagan/Journal of Engineering Science and Tecnology Review 7 (4) (14) - 45 sowed tat te pressure of te skewback increased due to loads passing, so te tickness of te skewback constantly increased wite increase of mining advance distance. Peak value of principal stress/mpa 21.0 P f1 = P f Peak value of principal stress/mpa 21.0 P f1 = P f Mining advance distance/m Mining advance distance/m Fig.6. Peak value of principal stress variation of te pressure-arc 4.2 Rock Strengt Variation of te Mining Field Seen from Fig. 7, wite strengt of te surrounding rock from soft to ard, along te dip of te seam, te pressurearc sape canged from flat-top arco round arc, and te eigt of te arc-pressure was getting away from te mining face. Along te strike of te seam, te pressure-arc sape canged from te flat-top arco te pointed arc gradually. Overall, wite strengt of te surrounding rock from soft to ard, except tat te vault tickness ad a sligt increase, te tickness of te oter parts of te pressure-arc decreased (See Fig. 8). Meanwile, as sown in Fig.9, te peak value of te maximum principal stress in te arc foot demonstrated a significant decreasing trend. Tus, te iger te rock strengt was, te more easily te pressure-arc formed. Te arcickness reducing sowed tat te arc loading was reducing, and te decrease of peak value of te maximum principal stress in te pressure-arc sowed tat te mining field was more stability. 4.3 Velocity Variation of te Mining Advancing Assumed tree mining speed was 5 m/week, 10 m/week and m/week, te calculation results were sown in Fig. 10. Seen from te Fig. 10, te slower te mining speed was, te more obvious stress te concentration penomenon of te surrounding rock was. Terefore, to improve te mining speed can significantly reduce te stress concentration in te surrounding rock in front of te working face, and also can do some contributions to te stability of te pressure-arc in te mining field. Z/m Z/m S oft rock Y/m S oft rock X/m Fig.7. Sape caracteristics of te central axis of te pressure-arc T icknes s of pres s ure- arc/m S oft roc k T icknes s of pres s ure- a rc/m S oft rock C la s s ific a tion C las s ification = m2 = f2 = m2 = f2 Fig.8. Tickness variation caracteristics of te pressure-arc 43

5 S.R. Wang, N. Li, C.L. Li and P. Hagan/Journal of Engineering Science and Tecnology Review 7 (4) (14) - 45 Peak value of pricipal stress/mpa Soft rock Classification 1 =P m2 1 = 2 As sown in Figs. 12, 13 and 14, wite dip angle of te coal seam canging from 0 º, 10 º, º to 30 º along te strike of te seam, te pressure-arc was symmetrical. But along te dip of te seam, te pressure-arc displayed an asymmetric sape, and te vault was tilted and moved to te upward direction. Te greater te dip angle was, te more apparent te inclination of te pressure-arc was. Te arc tickness and peak stress is not asymmetric, tat is f2, P f1 P f2, m2, P m2. Peak value of principal stress/mpa 1 = 2 (a) 0 Soft rock Classification (b) 10 Fig.9. Peak value of principal stress variation of te pressure-arc Maximum principal stress/mp a m/week 10 m/week m/week Distance from te mining face/m Fig.10. Maximum stress variation wit mining speed 4.4 Dip Angle Variation of te Coal Seam Fig. 11 demonstrates te geometry parameters of te pressure-arc wit dip α of te coal seam. C1 f1 α B1 m1 Fig.11. Asymmetric sape of pressure-arc A t m2 B2 C2 f2 (c) (d) 30 Fig.12. sape caracteristics wit different dips Tickness of pressure- arc/m 1 f2 m

6 S.R. Wang, N. Li, C.L. Li and P. Hagan/Journal of Engineering Science and Tecnology Review 7 (4) (14) - 45 Tickness of pressure- arc/m 1 = f2 = m Fig.13. Tickness variation of te pressure-arc wit different dips Maximum principal stress/mp a Maximum principal stress/mp a P f2 P m2 P f P f1 = P f Seen from Figs. 12 and 13, wite dip angle of te coal seam increased, te arcickness and te maximum principal stress also increased, and te increasing trend of te tickness was > f2 > > m2 > in general. Meanwile, te skewback located in te low position of te pressure-arc would bear more loading, and te stress concentration tended to be furter intensified (Fig. 14). 5. Conclusion For te orizontal seam, wite working face advancing, te geometric sape of te pressure-arc varied from flat arco round arc gradually in te fully-mecanized mining field. Te eigt of te pressure-arc increased, and te tickness of skewback, arc waist and vault also increased. Wite strengt increase of te surrounding rock from soft to ard rock, te tickness and te loading of pressurearc decreased. To improve te mining speed can significantly reduce te stress concentration in te surrounding rock in front of te working face, and tat can do some contributions to te stability of te pressure-arc in te mining field. Wite increase of dip angle of te coal seam, along te dip of te seam, te pressure-arc displayed an asymmetric sape, and te vault was tilted and moved to te upward direction. Meanwile, te stress concentration tended to be furter intensified, and te skewback located in te low position of te pressure-arc would bear more load tan te oter one. Acknowledgments Tis work was financially supported by te National Natural Science Foundation of Cina (514748; ; ), te Natural Science Foundation of Hebei Province of Cina (E143012) and Program for Taiang Scolars, all tese are gratefully acknowledged. Fig.14. Peak value of principal stress variation wit different dips References [1]. Wu Y.-P., Wang H.-W. and Xie P.-S., Analysis of surrounding rock macro-stress arc-sell of long-wall face in steeply dipping seam mining, Journal of Cina Coal Society, 37 (4), 12, pp (in Cinese) [2]. Wang S.-R., Wang H. and Hu B.-W., Analysis of catastrope evolution caracteristics of te stratified rock roof in sallow mined-out Areas, Disaster Advances, 6 (S 1), 13, pp [3]. Mark B., Analysis of ground support metods at te kristinederg mine in Sweden, Proc. t Canadian Rock Mecanics Symposium and te International symposium on Rock support, Canada, Rotterdam, 1992, pp [4]. Huang Z.-P., Broc E. and Lu M., Cavern roof stabilitymecanism of arcing and stabilization by rock bolting, Tunnelling and Underground Space Tecnology, 17 (3), 02, pp [5]. Poulsen B.-A., Coal pillar load calculation by pressure arc teory and near field extraction ratio, International Journal of Rock Mecanics and Mining Sciences, 47 (7), 10, pp (in Cinese) [6]. Trueman R., Castro R. and Halim A., Study of multiple drawzone interaction in block caving mines by means of a large 3D pysical model, International Journal of Rock Mecanics and Mining Sciences, 45 (7), 08, pp [7]. Liang X.-D., Zao J. and Song H.-W., Experimental and numerical analysis on te arcing action from stress adjusting in surrounding rocks, Journal of Engineering Geology, (1), 12, pp (in Cinese) [8]. Xie G.-X., Influence of mining tickness on mecanical caracteristics of working face and surrounding rock stress sell, Journal of Cina Coal Society, 31 (1), 06, pp (in Cinese) [9]. Du X.-L. Song H.-W. and Cen J., Numerical simulation of te evolution of te pressure-arc during coal mining, Journal of Cina University of Mining & Tecnology, (6), 11, pp (in Cinese) [10]. Geng Y.-M., Study on evolution law of stress arc of overlying strata in mines, Journal of Sandong University of Science and Tecnology (Natural Science), 28 (4), 09, pp (in Cinese) [11]. Xu Y.-Y., Yu G.-M., Li C.-J. Xu J.-F. Zang M.-P. and Meng D., Study on te laminated board - unloading arc model of te Mining overburden, Mining Safety & Environmental Protection, 33 (3), 06, pp [12]. Xie Z., Luan T. and He N., Numerical simulation of air leakage in goaf of fully mecanized face wit ig dip and ard roof, Computer Modelling and New Tecnologies,17 (2), 13, pp