Hanging Wall Pressure Relief Mechanism of Horizontal Section Top-Coal Caving Face and Its Application A Case Study of the Urumqi Coalfield, China

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1 energies Article Hanging Wall Pressure Relief Mechanism Horizontal Section Top-Coal Caving Face Its Application A Case Study Urumqi Coalfield, China Jinshuai Guo 1, Liqiang Ma 2,3, * ID, Ye Wang 4 Fangtian Wang 2 1 State Key Laboratory for Geomechanics & Deep Underground Engineering, School Mechanics Civil Engineering, China University Mining Technology, Xuzhou , China; gjscumt@163.com 2 State Key Laboratory Coal Resources Safe Mining, School Mines, China University Mining Technology, Xuzhou , China; wangfangtian111@163.com 3 College Earth Mineral Sciences, Pennsylvania State University, University Park, PA 16802, USA 4 Jiangou Coal Mine, Shenxin Energy Company, Urumqi , China; wangye1976@sohu.com * Correspondence: ckma@cumt.edu.cn; Tel.: Received: 2 August 2017; Accepted: 1 September 2017; Published: 10 September 2017 Abstract: Abundant steeply-dipping thick seams (SDTCS) have been found in Xinjiang, China, y are mined largely by horizontal section top- caving (HSTCC) method. wall HSTCC face is nearly vertical does not fracture easily after underlying is extracted. As a result, tends to concentrate in wall lower-section working face (LSWF) n induce dynamic disasters. In this study, a mechanical model a HSTCC face s wall in steeply-dipping seams constructed to study characteristics wall deformation. mechanism wall pressure relief by deep-hole blasting (DHB) analyzed effectiveness DHB investigated by simulation using LS-DYNA stware. Based on se studies, parameters relevant to pressure relief by DHB were determined n DHB applied to 4301 working face in Jiangou mine. results show that average pressure measured at 4301 working face decreased about 34% from those at 4501 face where wall not blasted. Accidents related to dynamic rock pressure, such as support crushing large-scale rib fall, did not occur at 4301 working face throughout mining process. Additionally, in order to constrain surface V -shaped collapsed grooves induced by repeated mining HSTCC faces prevent subsequent failure surrounding rock on sides collapsed grooves, loess used to fill in grooves to provide constraint dynamic control on surrounding rock. two complementary technologies proposed in this study provide a guide on how to control wall SDTCS in similar conditions. Keywords: steeply-dipping thick seams (SDTCS); horizontal section top- caving (HSTCC); mechanical structure wall; pressure relief by blasting; loess filling 1. Introduction SDTCS is widely distributed in -producing areas worldwide, such as Urumqi field in China, Donbass field in Ukraine, Karaga field in Kazakhstan, Ruhr field in Germany, Lorraine field in French, West Virginia field in America [1]. In particular, SDTCS in Urumqi field have reserves amounting to over 30% world s reserves in similar seams [2]. In thick seams that dip steeply at 45 90, working faces usually extend horizontally as it is difficult to advance along dip direction. Such seams are typically divided into several horizontal sections roughly parallel to strike is first mined from bottom, allowing Energies 2017, 10, 1371; doi: /en

2 Energies 2017, 10, top to cave behind face. This mining method is referred to as HSTCC can improve extraction efficiency [3 5]. However during mining, wall does not fracture cave in time, leading to concentration in wall LSWF that can n induce dynamic disasters. Meanwhile, due to high intensity mining, large number sections, short distances between faces, fully-mechanized caving can repeatedly disturb overlying goaf thus leading to surface V -shaped collapsed grooves [6,7]. refore, it is necessary to study deformation behavior HSTCC face s wall develop effective technologies for controlling wall under such conditions. oretical studies have been conducted into structure wall over SDTCS. For example, Kulakov [8,9] researched how rock pressure varied with different depth steep bed pillar width. Klishin et al. [10,11] developed a sublevel caving technology, analyzed laws top movement during caving effects different caving methods or factors on top movement. Shi Zhang [12] proposed a oretical structure called arch spanning strata analyzed sliding instability structure instability this structure as well as two instability modes on strata behavior at working face. In a study SDTCS, Lai et al. [13] discovered a noticeably asymmetric distribution vertical displacement overburden at HSTCC face. He suggested that top overlying residual gangue toger formed an asymmetrical arch structure, which n developed into a typical ellipsoidal structure. According to research on wall stability control for SDTCS, hydraulic fracturing blasting are two well-established technologies currently used to relieve in wall. Hanging wall weakening by hydraulic fracturing requires strata to be fairly water-absorbent is too inefficient to ensure a normal rate face advance [14 16]. In contrast, blasting is subject to fewer technical limitations, highly efficient effective in weakening strata, thus more widely adopted [17 19]. At present, technology pressure relief by blasting is applied primarily to gently dipping seams, but its application to SDTCS is rarely reported. Currently, surface collapse over goaf areas is controlled mainly by goaf backfilling, grouting bed separations in overburden, surface treatment [20 22]. Goaf backfilling is obviously unsuitable for SDTCS that are mined with HSTCC method, because upper goaf area in SDTCS is susceptible to repeated disturbances caused by mining at LSWF [23,24]. As wall over steep bed spans only one side face, it plays a limited role in supporting upper goaf. This means that grouting bed separations in overburden is ineffective for such seams [25,26]. Since loess is widely distributed easy availability from surface Urumqi field, it is relatively practical feasible to fill in V -shaped collapsed grooves above SDTCS with loess. effectiveness this method has been demonstrated by practical application in surface collapse control over past years [5]. This study analyzed deformation behavior HSTCC face wall, with example Jiangou mine in Urumqi field. technology pressure relief by DHB proposed as a way to facilitate fracturing wall above goaf so as to prevent dynamic disasters induced by concentration in wall LSWF. This technology applied to 4301 working face. Furrmore, method loess filling employed to repair surface collapsed grooves in order to minimize surface damage caused by SDTCS mining. 2. HSTCC Method 2.1. Geology Conditions Urumqi field is representative Chinese mining areas that work with SDTCS, account for over 1/4 country s SDTCS in terms reserves [27]. With a total 33 seams, Middle Jurassic Xishanyao Formation is main -bearing formation in this field. se seams range from 63 to 88 in average dip ir total workable thickness is between m. refore, y are regarded as a steeply-dipping closely-spaced thick seams group. Located in central part

3 Energies 2017, 10, Urumqi field (Figure 1), Jiangou mine is a highly productive efficient mine with an annual production 1.8 million tons. main seams being mined are 43# (B3 6 ) 45# (B1 2 )Energies seam2017, (Figure 2). 10, Energies 2017, 10, Urumqi Urumqi Xinjiang Xinjiang F1 F1 F3 F3 Tiechanggou Tiechanggou F2 F2 Dahonggou Dahonggou Xiaohonggou Xiaohonggou Weihuliang Weihuliang Liudaowan Liudaowan Jiangou Jiangou F1 F1 F3 F3 F2 F2 F1 F1 F2 F F32 F3 Legend Legendanticline Qidaogou Qidaogou anticline Badaogou anticline Badaogou anticline Baiyangfu anticline Baiyangfu anticline Figure 1. Distribution Urumqi mines. Figure 1. Distribution field field major major mines. Figure 1. Distribution Urumqi Urumqi field major mines. Figure 2. Distribution seams in Jiangou mine. (B1 2 B3 6 in figure represent 45# Figure 2. Distribution seams in Jiangou mine. (B1 2 B3 6 in figure represent 45# Figure 2.43# Distribution seams in Jiangou mine. (B1 2 B3 6 in figure represent 45# seam respectively). 43# seam respectively). 43# seam respectively) Mining Method 2.2. Mining Method 2.2. Mining Method conventional method sublevel caving, which divides a seam along its thickness into conventional method sublevel caving, which divides a seam along its thickness into sections, is not applicable to steeply-dipping seams. refore, Jiangou mine adopts conventional method sublevel caving, divides a seam along its adopts thickness sections, is not applicable to steeply-dipping seams.which refore, Jiangou mine into HSTCC method. At HSTCC face, seam is divided along its dip direction into several HSTCC method. At to HSTCC face, seams. seam is refore, divided along its dip direction into several sections, is not applicable steeply-dipping Jiangou mine adopts horizontal sections certain heights is first cut from bottom working face, leaving horizontal sections certain heights is first from bottom working face, into leaving HSTCC method. face, seam iscut divided along itsratio, dip direction several top to At collapse.hstcc With multiple advantages such as lower tunneling higher output top to collapse. With multiple advantages such as lower tunneling ratio, higher output horizontal sections certain heights is first cut from bottom in working economic efficiency, HSTCC has become principal method for mining SDTCS Xinjiangface, [28].leaving economic efficiency, HSTCC has become principal method for mining SDTCS in Xinjiang [28]. to mining sequence HSTCC method is: shearer tunneling advancing hydraulic supports top collapse. Withmultiple advantages suchcutting as lower ratio, higher output mining sequence HSTCC method is: shearer cutting advancing hydraulic supports advancing front scraperhas conveyor caving top method pulling scraper conveyor. caving economic efficiency, HSTCC become principal forrear mining SDTCS in Xinjiang [28]. advancing front scraper conveyor caving top pulling rear scraper conveyor. caving method HSTCC is same as full-mechanized longwall top caving face in gently dipped mining sequence HSTCC method is: shearer cutting advancing hydraulic supports method HSTCC is same as full-mechanized longwall top caving face in gently dipped seams. However, due to conveyor working alongtop dip direction, whichrear is equal to thickness advancing front scraper face caving pulling scraper conveyor. caving seams. However, due to working face along dip direction, which is equal to thickness seam, is relatively shorter than normal length, a relatively faster mining speed can be method HSTCC is same as full-mechanized caving in gently seam, is relatively shorter than normallongwall length, atop relatively faster face mining speed dipped can be achieved along strike direction working face. seams. However, direction workingface along dip direction, which is equal to thickness achieved alongdue to strike working face. seam, is relatively shorter than normal length, a relatively faster mining speed can be achieved Mining Conditions Miningdirection Conditions along strike working face working face Jiangou mine is buried at depths m measures 4301 working face Jiangou mine is buried at depths m measures 56.5 m wide 1000 m long in its direction advance. mining section height is 24 m; cutting Mining Conditions m wide 1000 m long in its direction advance. mining section height is 24 m; cutting height is 3.2 m caving height is 20.8 m, with caving ratio being about 1:7. shearer has a height is 3.2 m caving height is 20.8 m, with caving ratio being about 1:7. shearer has a 4301 working Jiangou mine buried depths m measures cutting depth 0.6 face m face advances 0.6 misduring a at mining cycle. interval distance cutting depth 0.6 m face advances 0.6 m during a mining cycle. interval distance between caving cycles is 1.2inm.its Figure 3 shows stratigraphic column this height face. wall 56.5 m wide 1000 m long direction advance. mining section is 24 m; cutting between caving cycles is 1.2 m. Figure 3 shows stratigraphic column this face. wall

4 Energies 2017, 10, height is 3.2 m caving height is 20.8 m, with caving ratio being about 1:7. shearer has a cutting depth 0.6 m face advances 0.6 m during a mining cycle. interval distance Energies Energies 2017, 2017, 10, 10, between caving cycles is 1.2 m. Figure 3 shows stratigraphic column this face. wall is is composed is composed primarily primarily siltstone siltstone dips dips dips at at at 85 ; 85it it ; has has it has a compressive compressive a strength strength strength MPa 25.6 MPa MPa tensile strength tensile strength 3.1 MPa. 3.1 MPa. tensile strength 3.1 MPa. Lithological Lithological column column Sy Lithology 45# seam Siltstone 43# seam Siltstone Siltstone Sy mudstone Lithology 45# seam Siltstone 43# seam Siltstone Siltstone Average mudstone thickness/m Average thickness/m Figure Figure 3. Stratigraphic 3. column 4301 working face. face. Figure 3. Stratigraphic column 4301 working face Face Layout Face Layout Face As shown Layout in Figure 4, HSTCC face is perpendicular to strike seam. headentry As shown tailentry in Figure along 4, sides HSTCC face, facedriven is perpendicular in same level, to are strike close to seam. wall headentry As shown in Figure 4, HSTCC face is perpendicular to strike seam. headentry tailentry footwall, along respectively. sides face, driven in same level, are close to wall tailentry along sides face, driven in same level, are close to wall footwall, footwall, respectively. respectively. Residual gangue upper-sections Residual gangue upper-sections Footwall V -shaped collapsed grooves V -shaped collapsed grooves Hanging Hanging wall-2 Hanging Footwall Goaf Hanging wall-2 Goaf Tailentry Headentry Tailentry 4301working face (a) (a) Residual gangue upper-sections Residual gangue upper-sections Headentry Goaf 4301working face Lower-section working face Goaf Lower-section working face (b) Figure 4. Layout HSTCC face. (a) Along-strike prile; (b) Along-dip prile. (b) 3. Deformation Behavior HSTCC Face s Hanging Wall Figure 4. Layout HSTCC face. (a) Along-strike prile; (b) Along-dip prile. Figure 4. Layout HSTCC face. (a) Along-strike prile; (b) Along-dip prile. 3. Deformation Behavior HSTCC Face s Hanging Wall

5 Energies 2017, 10, Numerical Simulation HSTCC Energies 2017, 10, 1371UDEC 4.0 stware used to simulate mining process HSTCC face. To facilitate 5 20 modeling, strata thickness set at an integer. A numerical simulation model with three horizontal sections dimensions 150 m 200 m constructed. To monitor laws 3. Deformation change in Behavior wall HSTCC during Face s mining, Hanging a monitoring Wall line arranged in wall, 8 m above seam (Figure 5) Numerical Simulation failure criterion HSTCC used in numerical analyses is Mohr Coulomb model. In order to calibrate input parameters modeling, Mohr Coulomb model used to simulate UDEC uniaxial 4.0 compression stwaretest used results to simulate are compared mining with process experimental data. HSTCC parameters face. To facilitate modeling, this strata model are thickness used in following set atsimulations, an integer. until A numerical simulation results model with three strain characteristics are in good agreement with that experimental data. mechanical horizontal sections dimensions 150 m 200 m constructed. To monitor laws parameters models are shown in Tables 1 2. Both left right boundaries model change in were fixed by displacement, wall during limited mining, displacement a monitoring in line x direction. arranged bottom in boundary wall, 8 m above fixed by seam displacement, (Figure 5). limited displacement in y direction. Figure 5. model. Figure 5. HSTCC numerical simulation model. Table 1. Rock physical mechanical parameters (block). failure criterion used in numerical analyses is Mohr Coulomb model. In order to calibrate Thickness Density Bulk Modulus Shear Modulus Cohesion Friction Tensile Strength Number input parameters Strata (m) modeling, (kg/m 3 ) Mohr Coulomb (GPa) (GPa) model (MPa) used to Angle simulate ( ) (MPa) uniaxial compression 1 test Footwall results 30 are2530 compared 10.8 with experimental data. 38 parameters 3.1 this 2 43# seam model 3 are used Hanging in following 8 simulations, 2530 until 10.8 numerical 8.1 simulation 2.8 results 38 strain Hanging wall characteristics are in good agreement with that experimental data. mechanical parameters models are shown in Tables 1 2. Both left right boundaries model were fixed Table 2. Rock physical mechanical parameters (contact). by displacement, limited displacement in x direction. bottom boundary fixed by Normal Shear Stiffness Cohesion Friction Tensile displacement, Number limited Strata displacement Thickness (m) Stiffness in y(gpa) direction. (GPa) (MPa) Angle ( ) Strength (MPa) 1 Footwall # seam Table 1. Rock 56 physical15 mechanical 8 parameters 1.0 (block) Hanging Hanging wall Number Strata Bulk Shear Friction Tensile Thickness Density Cohesion (m) (kg/m 3 Modulus Modulus Angle Strength ) (MPa) (GPa) (GPa) ( ) (MPa) 1 Footwall # seam Hanging Hanging wall Table 2. Rock physical mechanical parameters (contact). Number Strata Thickness (m) Normal Stiffness (GPa) Shear Stiffness (GPa) Cohesion (MPa) Friction Angle ( ) Tensile Strength (MPa) 1 Footwall # seam Hanging Hanging wall

6 Energies 2017, 10, Energies 2017, 10, Due to particular geological conditions strata characteristics HSTCC method, Due to particular geological conditions strata characteristics HSTCC method, HSTCC face located underneath upper-sections goaf, above working face lay HSTCC face located underneath upper-sections goaf, above working face lay top top residual gangue left behind after upper-sections were mined, rar than immediate residual gangue left behind after upper-sections were mined, rar than immediate ro ro main ro. After top drawn by caving behind face, overlying gangue main ro. After top drawn by caving behind face, overlying gangue tended tended to collapse. Since width HSTCC face about 50 m generally, disturbance along to collapse. Since width HSTCC face about 50 m generally, disturbance along dip dip caused by mining too limited to fracture wall easily [29], as illustrated in Figure 6. caused by mining too limited to fracture wall easily [29], as illustrated in Figure 6. (a) (b) (c) Figure 6. HSTCC numerical simulation results. (a) After section 1 mined; (b) After Figure 6. HSTCC numerical simulation results. (a) After section 1 mined; (b) After section 2 section 2 mined; (c) After section 3 mined. mined; (c) After section 3 mined. laws horizontal vertical change during mining were analyzed (Figure 7). results laws show horizontal that after section vertical 1 mined, change during mining were LSWF analyzed (Figure subject 7). to an average results show horizontal that after section 11.9 MPa, 1 up mined, 10.2% from initial 10.8 LSWF MPa. After subject to section an average 2 horizontal mined, average 11.9 horizontal MPa, up 10.2% in from initial 10.8 MPa. LSWF After reached section 13.6 MPa, 2 9.7% mined, higher than average horizontal initial 12.4 in MPa. mining LSWF section reached 3 led 13.6 to 10.1% MPa, 9.7% increase higher in that than initial from 13.9 MPa 12.4 to 15.3 MPa. MPa. mining section 3 led HSTCC to 10.1% face underwent increase in that an overall decrease from 13.9 in MPa horizontal to 15.3 MPa. throughout mining process, HSTCC primarily face underwent because an overall caving decrease top in horizontal resulted in loss throughout support for mining process, wall primarily reby because relieved caving top in resulted in wall. loss support for wall reby relieved in wall. After section 1 mined, average vertical in LSWF 9.6 MPa, 9.1% increase from initial 8.8 MPa. After section 2 mined, average vertical in 10.2 MPa, 13.3% higher than initial 9.0 MPa.

7 After section 3 mined, average vertical in LSWF s reached up to 10.8 MPa after 16.1% increase from 9.3 MPa. It follows that during mining process, vertical in LSWF increased rate increase grew as mining level downwards [29]. This finding suggests that if wall HSTCC face is not preconditioned, Energies 2017, 10, wall 1371 LSWF would tend to concentrate during mining n induce 7 20 dynamic disasters. (a) (b) Figure 7. Distribution during mining. (a) Horizontal distribution; (b) Figure 7. Distribution during mining. (a) Horizontal distribution; Vertical distribution. (b) Vertical distribution Mechanical Model HSTCC Face s Hanging Wall After section 1 mined, average vertical in LSWF 9.6 MPa, After 9.1% increase top from drawn initial from HSTCC 8.8 MPa. face, After overlying section 2 gangue mined, collapsed average accumulated vertical above in LSWF MPa, wall 13.3% higher LSWF than exhibited initial bending deformation, 9.0 MPa. because After section gangue 3 accumulation mined, average too loose vertical to impose ina sufficient LSWF sconstraint on it. After reached up to beneath 10.8 MPa after extracted, 16.1% increase from 9.3 MPa. It follows HSTCC that face, during made up mining 8-m-thick process, sstone, vertical did not fracture in easily thus provided LSWF increased support for overlying rate increase strata grew (Figure as 8a). mining refore level investigating downwards [29]. This deformation finding suggests characteristics that if wall HSTCCcan facereveal is not preconditioned, mechanical behavior wall HSTCC face s LSWF wouldwall. tend to concentrate during mining n induce dynamic disasters Mechanical Model HSTCC Face s Hanging Wall After top drawn from HSTCC face, overlying gangue collapsed accumulated above LSWF. wall LSWF exhibited bending deformation, because gangue accumulation too loose to impose a sufficient constraint on it. After beneath extracted, HSTCC face, made up 8-m-thick sstone, did not fracture

8 Based on field conditions in Jiangou Coal Mine, following parameters can be obtained for equations mentioned above: θ = 85, Lj = 64.1 m, λ = 0.3 [36], k = 10 5 kn/m 3 [33]. n q0 over a unit length along strike q0 = γhlj = = kn/m. distribution bending moment in, denoted Mx, calculated for three different goaf heights: L = 24 m, 48 m, 72 m, i.e., goaf heights respectively after section 1, Energies 2017, 10, easily thus provided support for overlying strata (Figure 8a). refore investigating deformation characteristics can reveal mechanical behavior HSTCC face s wall. Energies 2017, 10, Hanging O y O y Footwall Hanging wall-2 x x L q 2 q 1 Sections Fz θ (a) (b) (c) Figure 8. Mechanical model HSTCC face s. (a) Rock pressure model; (b) Figure 8. Mechanical model HSTCC face s. (a) Rock pressure model; Mechanical mode; (c) Bending moment distribution. (b) Mechanical mode; (c) Bending moment distribution. face simplified to a clamped-clamped elastic beam [30 35], as shown in Figure 8b. Since face strata were simplified nearly-vertical, to a clamped-clamped relative elastic motions beam between [30 35], strata as shown were not in Figure taken 8b. into Since consideration strata were in nearly-vertical, analysis. n relative motions wall-2 exerted between only strata a normal were not load taken on into consideration. in Meanwhile, analysis. n magnitude wall-2 supporting exerted only force a normal exerted load on on. by Meanwhile, caved gangue magnitude depended on supporting gangue s force strength exerted on compactness. We can by reasonably caved assume gangue that depended this supporting on gangue s force increased strength linearly compactness. downward We can reasonably [36,37]. assume that forces this acting supporting on force increased linearly can be calculated downward using following equations: [36,37]. forces acting on can be calculated using following equations: q1 = ( q0 + γ Ljxsinθ)( cosθ + λsin θ), q2 = Lj( q0 + γ xsinθ) sinθ (1) q 1 = ( q 0 + γl j x sin θ ) (cos θ + λ sin θ), q 2 = L j (q 0 + γx sin θ) sin θ (1) Fz = kx (2) F z = kx (2) where q0 is load on upper clamped end (kn/m 3 ); where q1 q 0 is normal load onload upper exerted clamped on end by (kn/m wall-2 3 ); (kn/m 3 ); q2 is tangential load on (kn/m 3 ); q 1 is normal load exerted on by wall-2 (kn/m γ is bulk density wall strata (kn/m 3 ); 3 ); q 2 θ is is tangential dip load on wall strata; (kn/m 3 ); γ is Lj is bulk total density thickness wall strata (kn/m 3 ); wall-2 (m); θ is Fz is dipsupporting force wallexerted strata; on by caved gangue (kn); L j Ld is is total height thickness goaf after several sections were mined wall-2 (m); (m); F z λ is is lateral supporting pressure force coefficient; exerted on by caved gangue (kn); L d k is is height distribution goaf factor after several supporting sections force wereapplied mined to (m); by caved λ gangue is lateral(kn/m pressure 3 ). coefficient; k is distribution factor supporting force applied to by caved Deformation Caused by Normal Load gangue (kn/m 3 ). According to principle force superposition under small deformation, deformation Deformation Caused can be decomposed by Normal into Load two components, one caused by q1 or by Fz. n According bending tomoment principle force superposition can be under expressed smallin deformation, following equation: deformation 2 can be decomposed2 into two components, ( ) 2 one caused ( ) 2 by q 3 ql sin cos sin 0 cosθ + λsinθ x 6x γlxl j θ θ + λ θ kxl x 1 or by F x z. n M xbending = moment in can be expressed in following equation: 3 (3) 12 L L 60 L L

9 Energies 2017, 10, ( M x = q 0L 2 (cos θ+λ sin θ) 12 6 L x 6x2 1 L 2 ) + γl jxl 2 sin θ(cos θ+λ sin θ) kxl 2 60 ( ) 9 L x x L 3 (3) Based on field conditions in Jiangou Coal Mine, following parameters can be obtained for equations mentioned above: θ = 85, L j = 64.1 m, λ = 0.3 [36], k = 10 5 kn/m 3 [33]. n q 0 over a unit length along strike q 0 = γhl j = = kn/m. distribution bending moment in, denoted M x, calculated for Energies three different 2017, 10, 1371 goaf heights: L = 24 m, 48 m, 72 m, i.e., goaf heights respectively after 9 20 sections 1, 2, 3 mined. During During mining, mining, deformed deformed under under action action normal normal load load maximum maximum bending bending moment moment occurred occurred at at its its lower lower clamped clamped end. end. bending bending moment moment exerted exerted an effect an effect primarily primarily upon upon top top wall wall LSWF, LSWF, resulting resulting in rock in rock pressure pressure increase increase significant deformation rock surrounding wall-side entry. After section 1 significant deformation rock surrounding wall-side entry. After section 1 mined, maximum bending moment 61.6 kn m. After section 2 mined, that value mined, maximum bending moment 61.6 kn m. After section 2 mined, that value kn m, representing a 8.5-fold increase from that observed after section 1 mined kn m, representing 8.5-fold increase from that observed after section mined. After section 3 mined, maximum bending moment reached up to kn m, a After section 3 mined, maximum bending moment reached up to kn m, a 29.1-fold fold increase from that observed after section 1 mined. se suggest that, as mining level increase from that observed after section 1 mined. se suggest that, as mining level downwards, bending moment in increased by orders magnitude its downwards, bending moment in increased by orders magnitude its effect on LSWF grew gradually, as shown in Figure 9. effect on LSWF grew gradually, as shown in Figure After section 1 mined After section 2 mined After section 3 mined lower clamped end section 3 x=72m, M x =1790.6kN m Bending moment/kn m lower clamped end section 1 x=24m, M x =61.6kN m lower clamped end section 2 x=48m, M x =521.1kN m Length /m Figure 9. Bending moment distribution Deformation Caused by Tangential Load As nearly-vertical, its its lower lower clamped end end subjected subjected to a greater to a greater load than load than or or parts. parts. tangential load load q2 q 2 transferred through to LSWF [38]. It It is is clear from from Equation (1) (1) that that q 2 isq2 directly is directly proportional to to length length behind behind face, denoted face, denoted x, tends x, to tends increase to increase mining as mining level downwards. level downwards. analysis analysis above above concludes concludes that that HSTCC HSTCC face does face not does fracture not fracture easily, easily, which which poses aposes threat a threat to to safe production. safe production. It is refore It is refore necessary necessary to artificially to artificially cut f cut f walk walk behind behind face face by by DHB, DHB, in order in order to reduce to reduce energy energy stored stored in in wall wall in in LSWF. LSWF. 4. DHB Technology for Pressure Relief 4.1. Mechanism Pressure Relief by DHB 4.1. Mechanism Pressure Relief by DHB re are two types wall blasting: shallow-hole blasting (blasting hole length < 5 m) re are two types wall blasting: shallow-hole blasting (blasting hole length m) deep-hole blasting (blasting hole length 5 m). In shallow-hole blasting, detonation deep-hole blasting (blasting hole length m). In shallow-hole blasting, detonation explosives creates shallow cracks in wall tensile n arise from se cracks during bending induce wall fracture. In contrast, DHB directly uses explosion to fracture wall reduce energy it accumulates [39,40]. Given total thickness wall-2 over 43# seam 64.1 m, blasting holes should be longer than 5

10 Energies 2017, 10, explosives creates shallow cracks in wall tensile n arise from se cracks during bending induce wall fracture. In contrast, DHB directly uses explosion to fracture wall reduce energy it accumulates [39,40]. Given total thickness wall-2 over 43# seam 64.1 m, blasting holes should be longer than 5 m in order to ensure effectiveness blasting in relieving. refore, technology wall pressure relief discussed in this study accounts to DHB. nearly-vertical wall did not fracture easily after mined, which led to concentration in wall, deformation LSWF surrounding rock. Implementing DHB technology for HSTCC face s wall can reduce bearing capacity surrounding rock thus weaken wall. On one h, explosion can cut f wall behind face reduce bending moment produced by normal load on wall. On or h, fractured wall can disperse tangential load on wall act as a cushion against its transfer to LSWF oretical Analysis DHB Numerical Simulation oretical Analysis DHB After explosive in blasting holes explodes, rock surrounding blasting holes can be roughly divided into three zones: crushed zone, fractured zone elastic zone. Rock masses in both crushed fractured zones have been broken, while that in elastic zone, which is subject to slight disturbance, remains intact. refore, effective damage zone around blasting holes normally refers to crushed zone fractured zone toger. radii two zones can be calculated using following equations [40 42]: ( ρ0 D R c = vnk 2 2γ ) l c B 1 8 a r b (4) 2σ cd in which: A = B = a = 2 µ d b = R ρ = 2ρC p ρc p +ρ 0 D v ( σcd σ td ) 1 β Rc (5) (1 + b) 2 + (1 + b 2 ) 2µ d (1 b) 2 1 µ µ d d 1 µ d β = 2 3µ d 1 µ d where R c R ρ are radii crushed zone fractured zone, respectively (m); r b r c are radii blasting holes explosive charge, respectively (m); ρ ρ 0 are wall rock density explosive density, respectively (kg/m 3 ); C p D v are speed sound in rock blasted detonation velocity (m/s); a is attenuation coefficient load; b is lateral coefficient; u d is dynamic Poisson s ratio rock, u d = 0.8u; in which u is Poisson s ratio rock; K is radial decoupling coefficient charge, K = r b /r c ; l c is axial decoupling coefficient charge, l c = 1; n is coefficient pressure on blasting holes wall due to impact exping detonation products, normally n = 10; γ is adiabatic expansion coefficient detonation products, usually set at 3;

11 Energies 2017, 10, σ cd σ td are dynamic compressive strength tensile strength rock, respectively; Energies σ cd 2017, = ε 1/3 10, σ1371 c σ td = ε 1/3 σ d, in which σ c σ d are compressive strength tensile strength rock, respectively; ε is strain rate load (s 1 ), normally ε = 10 s 1. Based on field conditions in Jiangou Coal Mine results laboratory experiments, Based Energies 2017, on 10, 1371 field conditions in Jiangou Coal Mine results laboratory experiments, following parameters can be obtained for equations mentioned above: rb = 0.05 m; rc = m; ρ following = 2550 parameters can be obtained for equations mentioned above: r b = 0.05 m; r c = m; Based kg/m on 3 ; ρ = 2550 kg/m 3 ρ0 field = 1000 conditions kg/m 3 ; ; ρ 0 = 1000 kg/m 3 Dv in Jiangou = 3600 m/s; Coal Cp Mine = 3500 m/s; results u = 0.25; laboratory σc = 25.6 MPa experiments, σd = 3.1 MPa. following radii parameters crushed can be obtained fractured ; D v = 3600 for zones m/s; equations were C p n = mentioned 3500 calculated m/s; above: u = using 0.25; σ c = 25.6 MPa rb = 0.05 equations m; rc = above: σ d 3.1 MPa. radii crushed fractured zones were n calculated using equations Rc = m; ρ = m 2550 kg/m Rρ 3 ; = ρ = 1000 m. kg/m So 3 ; Dv = effective 3600 m/s; damage Cp = 3500 zone m/s; u had = 0.25; a radius σc = 25.6 MPa m. σd In = 3.1 ory, above: refore, MPa. R c = space radii m between Rcrushed ρ = blasting m. fractured holes So should effective zones not were damage exceed n calculated zone had m. using a radius equations m. above: In ory, refore, Rc = space m between Rρ = blasting m. So holes effective shoulddamage not exceed zone had a m. radius m. In ory, refore, Numerical Simulation space between blasting DHB holes should not exceed m Numerical Simulation DHB LS-DYNA Numerical 970, Simulation a stware for DHB dynamic analysis, used to simulate process DHB. LS-DYNA 970, a stware for dynamic analysis, used to simulate process surrounding DHB. LS-DYNA rock surrounding 970, a modeled stware with rock for dynamic MAT_PLASTIC_KINEMATIC, modeled analysis, with MAT_PLASTIC_KINEMATIC, used to simulate a kinematic process hardening DHB. a kinematic plastic model surrounding provided rock by this modeled stware, with MAT_PLASTIC_KINEMATIC, explosive material a kinematic hardening modeled plastic with hardening plastic model provided by this stware, explosive material modeled with MAT_HIGH_EXPLOSIVE_BURN. model provided by this stware, Jones Wilkens Lee explosive (JWL) material state equation modeled used to with describe MAT_HIGH_EXPLOSIVE_BURN. Jones Wilkens Lee (JWL) state equation used to describe pressure-volume MAT_HIGH_EXPLOSIVE_BURN. relationship for Jones Wilkens Lee products detonation (JWL) state a equation high explosive used [41,42]. to describe Based on pressure-volume relationship for products detonation a high explosive [41,42]. Based actual pressure-volume conditions relationship 4301 working for products face, blasting detonation holes a high space explosive determined [41,42]. Based to on be 6 m. on actual actual conditions 4301 working workingface, face, blasting blasting holes holes space space determined determined to be to 6 m. be 6 m. Blasting holes with a diameter 0.1 m were made wall model with dimensions Blasting Blasting holes holes with with a diameter m were made in wall wall model model with with dimensions dimensions 20 m 8 m. A non-radioactive boundary applied to model. three monitoring points in m m. 8 m. A non-radioactive boundary applied to to model. three three monitoring points points in model were 1 m, 2 m, 3 m from blasting holes, respectively, as shown in Figure 10. model model were were m, 1 m, m, 2 m, 3 3 m from blasting holes, respectively, as as shown in Figure in Figure Figure DHB numerical simulation model. Figure 10. DHB numerical simulation model. Figure 11 implies that effective waves generated by explosion propagated evenly Figure Figure in all directions implies implies As that shown that in effective effective figure, 119 μs waves waves after generated detonation, generated by by boundary explosion explosion propagated propagated region affected evenly evenly in all in all directions. by directions. effective As As shown shown in waves in figure, figure, m 119 from µs μs after after detonation, blasting detonation, holes, roughly boundary boundary equal to region radius region affected affected by by effective crushed effective zone around waves waves blasting 0.6 holes. m0.6 from m from affected blasting region blasting holes, extended roughly holes, 3.2 roughly equal m beyond toequal radius to blasting radius holes, crushed zone crushed including around zone blasting around aforementioned holes. blasting holes. crushed affected region affected fractured extended region zones, extended m μs beyond after 3.2 detonation. m beyond blasting holes, blasting including holes, including aforementioned aforementioned crushed crushed fractured zones, fractured 1380 zones, µs after 1380 detonation. μs after detonation. (a) (a) Figure 11. Cont.

12 Energies 2017, 10, Energies 2017, 10, Figure 11. Cont. (b) (c) (d) Figure 11. Effective contours after blasting. (a) 1.19E+02 μs; (b) 5.39E+02 μs; (c) 9.00E+02 μs; (d) Figure 11. Effective contours after blasting. (a) 1.19E+02 µs; (b) 5.39E+02 µs; (c) 9.00E+02 µs; 1.38E+03 μs. (d) 1.38E+03 µs. Figure 12 illustrates distribution effective around blasting holes. It is found that effective Figure 12 illustrates produced distribution by blasting decreased effective with increasing around distance blasting from holes. It blasting is found holes. that effective peak effective produced monitoring by blasting point decreased 1 with increasing MPa, occurred distance at 425 from μs after blasting detonation. holes. peak peak effective effective monitoring monitoring point point MPa, MPa, occurred occurred at at μs µs after after detonation. detonation. peak peak effective effective monitoring monitoring point point 3 2 were MPa MPa, occurred 75.1 MPa at 900 successively, µs after detonation. occurred at 1250 peak μs effective 1390 us after monitoring detonation. point This peak 3 were effective 70.0 MPa 75.1 monitoring MPa successively, point 3 occurred occurred two at 1250 times, µs possibly 1390 because after detonation. superposition This peak effective effective waves monitoring from two point blasting 3 occurred holes. two As times, compressive possibly because strength superposition wall effective 4301 working waves from face, at 25.6 twompa, blasting holes. significantly As compressive lower than strength effective produced wall by DHB, 4301 working face, wall at 25.6 rock MPa, easily significantly failed by yielding lower than under such effective, produced fractured by DHB, zone developed. A wall comprehensive rock easily failed analysis by yielding results under such, oretical calculation fractured zone numerical developed. simulation A comprehensive suggests that analysis blasting holes results space 6 m oretical can ensure calculation effective weakening numerical simulation wall under suggests 4301 that working blasting holes face s space geological 6 mconditions. can ensure effective weakening wall under 4301 working face s geological conditions.

13 Energies 2017, 10, Energies 2017, 10, Effective /MPa Peak effectinve : 255.3MPa Peak effectinve : 125.5MPa Monitoring point 1 Monitoring point 2 Monitoring point 3 Superposition effectinve : 70.0MPa, 75.1MPa respectively Time/us 5. Engineering Application Figure 12. Variation in effective with distance from blasting hole Determination Blasting Parameters DHB technology implemented to weaken wall 4301 working face. Blasting holes were drilledinto into wall wall along along headentry headentry in a fan-shaped in a fan-shaped pattern. pattern. y were y loaded were loaded with emulsion-based with emulsion-based explosive; explosive; mass mass explosive explosive per meter per hole meter 7.5 holekg/m. n 7.5 kg/m. se holes n were seconnected holes werein connected series. As in series. face advanced, As face advanced, blasting site blasting set right site above set right rear scraper above conveyor, rear scraper as shown conveyor, in Figure as shown 13a. in details Figure 13a. blasting details site are shown blasting in site Figure are 14. shown in Figure 14. (1) Blasting holes diameter A ZDY-1900S drill drillrig rig employed employedto to drill drill blasting blasting holes holes with with a diameter a diameter 100 mm. 100 mm. explosive explosive packaged packaged in PVC in PVC tubes tubes n n placed placed into into blasting blasting holes. holes. se se PVC PVC tubes tubes had an hadouter an outer diameter diameter 90 mm 90 mm inner inner diameter diameter 86 mm, 86 mm, explosive explosive charge charge diameter diameter 70 mm 70 mm (Figure (Figure 13c). 13c). (2) Blasting holes length (2) Blasting holes length Experiences from previous projects suggest that blasting hole length should be more than half Experiences wall s from thickness previous to projects be blasted suggest [43]. that Since blasting total hole thickness length should be more than half wall-2 wall s thickness 4301 working to be face blasted [43]. about Since 64.1 m, total blasting thickness holes were designed to be 35 m long. wall-2 ratio sealing 4301 working length to face hole length about is normally 64.1 m, between blasting 25 holes were 30%. To designed ensure to adequate be 35 m sealing long. performance, ratio sealing ratio length to set hole at length 30% for is this normally face, between corresponding 25 30%. To sealing ensure length adequate 10.5 sealing m. performance, For convenient field ratio operation, set at 30% sealing for this length face, charge corresponding length were sealing determined length to be m m. For 25 convenient m, respectively. field operation, sealing length charge length were determined to be 10 m 25 m, respectively. (3) Blasting holes space (3) Blasting holes space As stated above, blasting holes space 6 m can ensure effective weakening wall. Given fan-shaped As statedpattern above, blasting holes space in this scheme, 6 m can ensure space between effectiveblasting weakening holes would vary with wall. different Given length, fan-shaped but it pattern only necessary blasting holes to ensure in this that scheme, bottoms space two between adjacent blasting holes were holes no would more than vary6 with m apart. different If length, angle between but it two only blasting necessary holes to is ensure 10, ir that bottoms bottoms are 6.1 m two apart. adjacent Though holes this space were no is slightly more than greater 6 mthan apart. 6 m, If it is angle acceptable, between because two blasting holes wall is 10 can, be ir furr bottoms weakened are 6.1by m apart. movement Though this overburden space is slightly driven by greater blasting. thanrefore, 6 m, it is acceptable, blasting holes because were designed to wall be at can angle be furr 15, weakened 25, 35, byrespectively, movement to horizontal overburden direction driven(figure by blasting. 13b). refore, blasting (4) Blasting interval

14 Energies 2017, 10, holes were designed to be at an angle 15, 25, 35, respectively, to horizontal direction (Figure 13b). (4) Blasting interval Energies 2017, 10, Energies 2017, 10, Experience Experience from from an an adjacent adjacent face face suggest suggest that that periodic periodic weighting weighting interval interval 4301 working 4301 face should Experience be approximately from an adjacent 22 m. face To follow suggest that principle periodic that blasting weighting interval must not 4301 working face should be approximately 22 m. To follow principle that blasting interval must exceed not working face should be approximately 22 m. To follow principle that blasting interval must not periodic exceed weighting periodic weighting step to step minimize to minimize amount amount work [44], work blasting [44], interval blasting interval determined exceed periodic weighting step to minimize amount work [44], blasting interval todetermined be 20 m for 4301 working face (Figure 13a). determined to to be be m m for for working working face face (Figure (Figure 13a). 13a). Rear scraper Rear scraper Front scraper Front scraper conveyor conveyor conveyor conveyor Tailentry Tailentry Footwall Hanging wall wall working face face Blasting Blasting hole hole 20m 20m 20m 20m 20m 20m (a) (a) Headentry Headentry Tailentry Tailentry Headentry Headentry (b) (b) m 3 6.1m m m 25m 25m Charge section Charge section 10m 10m Sealing section Sealing section (c) (c) Figure 13. Layout blasting hole. (a) Plan blasting hole; (b) Prile blasting hole; (c) Figure 13. Layout blasting hole. (a) Plan blasting hole; (b) Prile blasting hole; (c) Figure Detail 13. structure Layout blasting hole. (a) Plan blasting hole; (b) Prile blasting hole; (c) Detail structure blasting hole. Detail structure blasting hole. 100mm 100mm 90mm 90mm 86mm 86mm (a) (b) (c) Figure 14. (a) Practical application DHB at 4301 (b) working face. (a) Blasting hole drilling; (c) (b) Blasting hole charging; (c) Residual hole after blasting. Figure 14. Practical application DHB at 4301 working face. (a) Blasting hole drilling; (b) Blasting Figure 14. Practical application DHB at 4301 working face. (a) Blasting hole drilling; (b) Blasting 5.2. hole Effectiveness charging; (c) DHB Residual in Hanging hole after Wall blasting. hole charging; (c) Residual hole after blasting. Pressure Relief working face are located at same level, but 4501 working face mines 5.2. Effectiveness DHB in Hanging Wall Pressure Relief 5.2. Effectiveness 45# seam DHB (Figure in15). Hanging equipment, Wall Pressure mining Relief technology mining section height 4501 working 4501 face 4301 are same working as face 4301 are working located face. at 4501 same working level, face but is measured 4501 working 35 m wide face mines # 4501 m long seam in 4301 its (Figure direction working 15). face advance. are equipment, located se two at mining working same technology level, faces are but equipped mining 4501 working with section KJ377 face height type mines on-line # working seam monitoring (Figure face are system 15). same to as record equipment, 4301 working pressure mining face. technology hydraulic 4501 working supports mining face is legs. measured section average height 35 m wide pressure 4501 working 800 face m are long hydraulic same in its as legs direction in 4301 working working advance. face were face. se monitored 4501 two working while mining; face faces is measured are moreover, equipped 35 changes mwith wide KJ377 those type 800 values on-line m long in monitoring two working system faces, to from record open-cut pressure to advanced hydraulic 80 m, were supports compared legs. in this section. average pressure hydraulic legs in working face were monitored while mining; moreover, changes those values in two working faces, from open-cut to advanced 80 m, were compared in this section.

15 Energies 2017, 10, its direction advance. se two working faces are equipped with KJ377 type on-line monitoring system to record pressure hydraulic supports legs. average pressure hydraulic legs in working face were monitored while mining; moreover, changes those values in two working faces, from Energies open-cut 2017, 10, to1371 advanced 80 m, were compared in this section Energies 2017, 10, 1371 V -shaped collapsed V -shaped collapsed grooves grooves Footwall Footwall V -shaped collapsed grooves 4501 working face 4501 working face Middle wall Middle wall V -shaped collapsed grooves 4301 working face 4301 working face Hanging Hanging wall-2 Hanging Hanging wall-2 Figure Figure Location working working face. face. After DHB applied Figure to 15. Location 4301 working 4301 face for 4501 working wall face. pressure relief, pressure After hydraulic DHB legs applied along tace showed 4301 working a sharp increase face for every 20 m wall face pressure advancing, relief, n followed pressure hydraulic by a legs slowly After along DHB paced down. face applied This showed to is consistent a4301 sharp working with increase face blasting every for intervals 20 mwall designed. face pressure advancing, relief, average n pressure followed by a slowly measured paced hydraulic down. at legs 4301 along This working is consistent face face showed with 14.7 a sharp MPa, increase 34% blasting decrease every intervals 20 compared m face designed. to advancing, 22.3 MPa n average at followed 4501 pressure by working a slowly face, paced which down. is located This is in consistent same with mining blasting level intervals to which designed. DHB technology average pressure not measured at 4301 working face 14.7 MPa, 34% decrease compared to 22.3 MPa at measured applied (Figure at 16) working face 14.7 MPa, 34% decrease compared to 22.3 MPa at working face, which is located in same mining level to which DHB technology working face, which is located in same mining level to which DHB technology not not applied (Figure 40 applied (Figure 16). 16). Blasting 4501 working face 35 Blasting 4301 working face Blasting 40 Blasting 4501 working face Blasting Average 4301 working pressure face Blasting 4501 working face Average pressure working face Average pressure working face 5 10 Average pressure working face Advancing length working face /m 0 0 Figure Pressure hydraulic 25 30legs 35change 40 during 45 advance working 65 70face Advancing length working face /m Pressure Pressure hydraulic hydraulic legs/mpa legs/mpa When Figure 4501 working 16. Pressure face moved hydraulic forward legs change 100 m during from advance open-f cut, working supports face. at end headentry Figure 16. were Pressure crushed, hydraulic exit legs change face during tailentry advance were blocked working f. As face. distance between When face 4501 working open-f face moved cut reached forward m, m large-scale from open-f rib fall cut, took place supports along at 15 end m When headentry adentry 4501 section working were ahead crushed, face face, moved exit stopped forward face 100 face mproduction. from tailentry were During open-f blocked mining cut, f. As process, supports distance at 4501 face has seen a total 7 major accidents related to dynamic rock pressure exhibited noticeable end between headentry face were crushed, open-f cut reached exit 230 m, large-scale face rib fall tailentry took place were along blocked 15 f. m As headentry deformation section ahead rock surrounding face, roadways. stopped At face 4301 production. working face, During by comparison, mining process, no support distance between face open-f cut reached 230 m, large-scale rib fall took place along 4501 crushing, face has large-scale seen a total rib fall 7 major or accidents manifestations related to dynamic dynamic rock pressure rock pressure exhibited have noticeable occurred 15 mdeformation throughout headentry section mining rock process. surrounding aheadse roadways. demonstrate face, At that stopped 4301 energy working stored face face, in production. by comparison, nearly-vertical During no support mining process, crushing, wall 4501 effectively large-scale face has released rib seen fall by aor DHB. total or By far, manifestations 7 major technology accidents dynamic pressure related rock relief to pressure by dynamic DHB has have rock been occurred applied pressure exhibited throughout to five noticeable faces in mining deformation Jiangou process. mine. se 7.5 demonstrate million rock tons surrounding that energy have been roadways. stored mined safely, nearly-vertical At without 4301 occurrence working face, by comparison, wall accidents effectively no related support to released dynamic crushing, by DHB. rock pressure By large-scale far, throughout technology rib fall or mining pressure or processes. relief manifestations by DHB has been dynamic applied rock to five faces in Jiangou mine. 7.5 million tons have been mined safely, without occurrence pressure have occurred throughout mining process. se demonstrate that energy stored in accidents related to dynamic rock pressure throughout mining processes. nearly-vertical wall effectively released by DHB. By far, technology pressure

16 Energies 2017, 10, relief by DHB has been applied to five faces in Jiangou mine. 7.5 million tons have been mined safely, without occurrence accidents related to dynamic rock pressure throughout mining processes Loess Filling to Constrain Surface Collapsed Grooves Energies 2017, 10, After 5.3. Loess implementation Filling to Constrain Surface DHB, Collapsed residual Grooves gangue above face fractured wall tend to collapse, resulting in subsidence overburden. As vertical displacement After implementation DHB, residual gangue above face fractured overburden gradually increases during mining SDTCS, surface over face ultimately shows wall tend to collapse, resulting in subsidence overburden. As vertical displacement a V -shaped overburden collapsed gradually grooves. increases If during surface mining collapsed SDTCS, grooves surface were over not face properly ultimately treated shows in time, scope a V -shaped grooves collapsed would grooves. continueif to exp, surface both collapsed vertically grooves were horizontally. not properly This treated can cause in time, a series problems, scope such grooves as failure would continue rock surrounding to exp, both vertically grooves, lslide, horizontally. spontaneous This can cause combustion a, series damage problems, to local such ecological as failure system. rock Given surrounding easy availability grooves, lslide, loess across spontaneous Urumqi field, combustion loess filling, employed damage to to local control ecological surface system. V -shaped Given easy collapsed availability grooves loess induced across by mining Urumqi SDTCS. field, loess filling employed to control surface V -shaped collapsed grooves induced by mining SDTCS. Loess is granular mixture fine clay silt particles. With a certain bearing capacity, loess filling Loess is granular mixture fine clay silt particles. With a certain bearing capacity, loess in V -shaped collapsed grooves can apply constraint forces to lateral surrounding rock, filling in V -shaped collapsed grooves can apply constraint forces to lateral surrounding as shown in Figure 17. Due to its looseness low-strength, loess filled in grooves showed rock, as shown in Figure 17. Due to its looseness low-strength, loess filled in grooves significant showed deformation significant deformation at beginning. at beginning. deformation deformation enables enables loess loess to provide to provide a flexible a yielding flexible support yielding to support surrounding to surrounding rock while rock reducing while reducing its deformation. its deformation. Loess filling Footwall Hanging Horizontal section top- caving face Hanging wall-2 (a) Loess filling Footwall Hanging Hanging wall-2 Sections Horizontal section top- caving face (b) Figure 17. Schematic surface loess filling. (a) After shallow sections mined; (b) After deep sections mined. Figure 17. Schematic surface loess filling. (a) After shallow sections mined; (b) After deep sections mined. After a certain period time, loose loess consolidated into hard solid mass with improved bearing capacity. Meanwhile, mobility loess particles allows fill to slide down as grooves collapse, reby achieving dynamic control on surrounding rock [45,46]. photographs surface after application loess filling are shown in Figure 18.

17 Energies 2017, 10, After a certain period time, loose loess consolidated into hard solid mass with improved bearing capacity. Meanwhile, mobility loess particles allows fill to slide down as grooves collapse, reby achieving dynamic control on surrounding rock [45,46]. photographs surface after application loess filling are shown in Figure 18. Energies 2017, 10, (a) (b) (c) Figure 18. Practical application surface loess filling. (a) V -shaped collapsed grooves; (b) Surface Figure 18. Practical application surface loess filling. (a) V -shaped collapsed grooves; (b) Surface cracks; (c) Surface after loess filling. cracks; (c) Surface after loess filling. 6. Discussion 6. Discussion (1) steeply-dipping thick seams group in Urumqi field product local tectonic (1) steeply-dipping thick seams group in Urumqi field product local tectonic movements. Lateral tectonic movements exert a control role on formation in-situ. movements. Lateral tectonic movements exert a control role on formation in-situ. study found that rocks in this region are subject primarily to horizontal es. study found that rocks in this region are subject primarily to horizontal es. refore, refore, effect tectonic should be considered when analyzing deformation effect tectonic should be considered when analyzing deformation behavior HSTCC behavior HSTCC face s wall. In this study, a lateral pressure coefficient 0.3 face s wall. In this study, a lateral pressure coefficient 0.3 included in included in mechanical model presented above to account for effect tectonic [46]. mechanical model presented above to account for effect tectonic [46]. (2) Major factors influencing effectiveness DHB include blasting holes diameter, length, (2) Major factors influencing effectiveness DHB include blasting holes diameter, length, space, blasting interval, etc. While blasting holes diameter length blasting interval space, blasting interval, etc. While blasting holes diameter length blasting interval depend primarily on equipment available, geological conditions, actual production depend primarily on equipment available, geological conditions, actual production situation, blasting holes space is easily adjustable has a direct influence on effectiveness situation, blasting holes space is easily adjustable has a direct influence on effectiveness DHB in pressure relief. For this reason, numerical analysis effectiveness DHB DHB in pressure relief. For this reason, numerical analysis effectiveness DHB focused on influence blasting holes space. focused on influence blasting holes space. (3) blasting site set right above rear scraper conveyor behind face, meaning that (3) explosive blasting site detonated set when right above rear scraper rear scraper conveyor conveyor reached behind position face, under meaning blasting that holes. explosive In this way, detonated blasting when energy rear scraper largely conveyor released reached into goaf position behind under face, blasting reby reducing holes. In this influence way, blasting blasting energy on face largely as well released as into headentry goaf behind tailentry face, along reby it. (4) reducing DHB technology influence blasting intended on to relieve face as well as wall headentry arising tailentry from along it. wall (4) deformation DHB technology after working intended face to relieve mined reby wall ensure safe arising production. from loess wall filling deformation technology after working proposed face as a way mined to constrain reby surface ensure V -shaped safe production. collapsed grooves loess resulting filling technology from SDTCS mining proposed by HSTCC as a way method. to constrain It uses surface loess to V -shaped constrain collapsed surrounding grooves rock on resulting sides from SDTCS grooves mining area, byso HSTCC as to prevent method. rock It uses failure loess to constrain minimize ecological surrounding damage. rock on two sides technologies grooves can be area, used so in as combination to prevent rock to guarantee failure green minimize mining ecological SDTCS. damage. two technologies can be used in combination to guarantee green mining SDTCS. 7. Conclusions (1) mining process HSTCC simulated in this study. It found that wall HSTCC face nearly-vertical did not fracture easily after working face mined. HSTCC face modeled with a clamped-clamped elastic

18 Energies 2017, 10, Conclusions (1) mining process HSTCC simulated in this study. It found that wall HSTCC face nearly-vertical did not fracture easily after working face mined. HSTCC face modeled with a clamped-clamped elastic beam model to analyze its deformation behavior. results show that after section 2 mined, maximum bending moment in increased 8.5-fold compared with that observed after section 1 mined. After section 3 mined, maximum bending moment increased 29.1-fold from that observed after section 1 mined. se suggest that, as mining level downwards, bending moment in increased its effect on lower-section working face grew gradually. (2) pressure relief mechanism DHB oretically analyzed, its effectiveness examined by numerical simulation. results suggest that blasting holes space 6 m can ensure effective weakening wall. This technology n applied to 4301 working face Jiangou mine. average pressure hydraulic supports legs measured at this face decreased by about 34% compared to that measured at 4501 face, which DHB not applied. (3) loess filling technology proposed as a way to constrain surface V -shaped collapsed grooves resulting from repeated mining SDTCS by HSTCC. large deformation high mobility loess enable loess fill in grooves to provide constraint dynamic control on lateral surrounding rock. Meanwhile, this technology can be used to reduce ecological damage caused by mining steeply-dipping seams. Acknowledgments: This work supported by National Key Basic Research Program China (973 Program) (2015CB251600), State Key Laboratory Coal Resources Safe Mining(SKLCRSM13X03), Qing Lan Project, Priority Academic Program Development Jiangsu Higher Education Institutions. Author Contributions: Jinshuai Guo proposed innovative points conceived; Liqiang Ma established solved mechanical model; Ye Wang monitored engineering test results; Fangtian Wang performed simulation; Jinshuai Guo Liqiang Ma wrote paper. Conflicts Interest: authors declare no conflict interest. References 1. Wu, Y.P.; Yun, D.F.; Zhang, M.F. Study on elementary problems full-mechanized mining in greater pitching seam. J. China Coal Soc. 2000, 25, Shenxin Energy Company Has Achieve Rapid Development Extracting Steeply Inclined Thick Coal Seam. Available online: (accessed on 1 September 2017). 3. Onica, I.; Mihailescu, V.; Andrioni, F. Economical optimization mechanized longwall faces with top caving mining, in horizontal slices. Arch. Min. Sci. 2016, 61, [CrossRef] 4. Tu, H.S.; Tu, S.H.; Yuan, Y.; Wang, F.T.; Bai, Q.S. Present situation fully mechanized mining technology for steeply inclined seams in china. Arab. J. Geosci. 2015, 8, [CrossRef] 5. Shao, X.P.; Zhang, H.X.; Shi, P.W. Selection reasonable section heights during top- caving to steep seams. J. China Univ. Min. Technol. 2009, 38, Ju, W.J.; Li, W.Z. Fracture mechanical model main ro along inclined for fully-mechanized top- caving in steep extra-thick seam. J. China Coal Soc. 2008, 33, Ma, L.Q.; Zhang, Y.; Zhang, D.S.; Cao, X.Q.; Li, Q.Q.; Zhang, Y.B. Support stability mechanism in a face with large angles in both strike dip. J. S. Afr. Inst. Min. Metall. 2015, 115, [CrossRef] 8. Kulakov, V.N. Geomechanical conditions mining steep beds. J. Min. Sci. 1995, 31, [CrossRef] 9. Kulakov, V.N. Stress state in face region a steep bed. J. Min. Sci. 1995, 31, [CrossRef] 10. Klishin, V.I.; Klishin, S.V. Coal extraction from thick flat steep beds. J. Min. Sci. 2010, 46, [CrossRef] 11. Klishin, S.V.; Klishin, V.I.; Opruk, G.Y. Modeling discharge in mechanized steep thick mining. J. Min. Sci. 2013, 49, [CrossRef]

19 Energies 2017, 10, Shi, P.W.; Zhang, Y.Z. Structural analysis arch spanning strata top caving in steep seam. Chin. J. Rock Mech. Eng. 2006, 25, Lai, X.P.; Sui, H.; Shan, P.F.; Qiu, H.F. Overlying strata ellipsoid-style structure horizontal section top- caving in steeply inclined extra thick seam. J. Min. Saf. Eng. 2014, 31, Huang, B.X.; Liu, C.Y.; Fu, J.H.; Guan, H. Hydraulic fracturing after water pressure control blasting for increased fracturing. Int. J. Rock Mech. Min. Sci. 2011, 48, [CrossRef] 15. Hossain, M.M.; Rahman, M.K. Numerical simulation complex fracture growth during tight reservoir stimulation by hydraulic fracturing. J. Pet. Sci. Eng. 2008, 60, [CrossRef] 16. Gao, M.F. Numedcal simulation hard ro processing step. J. Laoning Tech. Univ. 2006, 25, Konicek, P.; Soucek, K.; Stas, L.; Singh, R. Long-hole de blasting for rockburst control during deep underground mining. Int. J. Rock Mech. Min. Sci. 2013, 61, [CrossRef] 18. Wang, K.; Kang, T.H.; Li, H.T.; Han, W.M. Study control caving methods reasonable ro length on hard ro. Chin. J. Rock Mech. Eng. 2009, 28, Zhang, Z.Z.; Bai, J.B.; Chen, Y.; Hao, S.P. Shallow-hole blasting mechanism its application for gob-side entry retaining with thick hard ro. Chin. J. Rock Mech. Eng. 2016, 35, Zhang, J.X.; Zhang, Q.; Sun, Q.; Gao, R.; Germain, D.; Abro, S. Surface subsidence control ory application to backfill mining technology. Environ. Earth Sci. 2015, 74, [CrossRef] 21. Sun, Q.; Zhang, J.X.; Zhang, Q.; Zhao, X. Analysis prevention geo-environmental hazards with high-intensive mining: A case study in China s western eco-environment frangible area. Energies 2017, 10, 786. [CrossRef] 22. Zhou, H.Q.; Hou, C.J.; Sun, X.K.; Qu, Q.D.; Chen, D.J. Solid te paste filling for none-village-relocation mining. J. China Univ. Min. Technol. 2004, 33, Qu, Q.D.; Yao, Q.L.; Li, X.H.; Rong, T.Y. Key factors affecting control surface subsidence in backfilling mining. J. Min. Saf. Eng. 2010, 26, Zhou, Z. Research on goaf filling methods with super high-water material. J. China Coal Soc. 2010, 35, Teng, H.; Xu, J.L.; Xuan, D.Y.; Wang, B.L. Surface subsidence characteristics grout injection into overburden: Case study Yuian No. 2 mine, China. Environ. Earth Sci. 2016, 75, 530. [CrossRef] 26. Zhu, W.B.; Xu, J.L.; Lai, W.Q.; Wang, Z.G. Research isolated section-grouting technology for overburden bed separation space to reduce subsidence. J. China Coal Soc. 2007, 32, Zhang, D.S.; Liu, H.L.; Fan, G.W.; Wang, X.F. Connotation prospection on scientific mining large Xinjiang base. J. Min. Saf. Eng. 2015, 32, Shao, X.P.; Shi, P.W. Strata behavior in large section face steep seams. J. Min. Saf. Eng. 2009, 26, Wang, N.B.; Zhang, N.; Cui, F.; Cao, J.T.; Lai, X.P. Characteristics stope migration roadway surrounding rock fracture for fully-mechanized top- caving face in steeply dipping extra-thick seam. J. China Coal Soc. 2013, 38, Lai, X.P.; Li, Y.P.; Wang, N.B.; Liu, Y.H.; Ran, P.J. Ro deformation characteristics with full-mechanized caving face based on beam structure in extremely inclined seam. J. Min. Saf. Eng. 2015, 32, Zhang, J.W.; Wang, J.A. Energy distribution characteristics rock burst control methods steeper inclined thick seam ro. J. China Coal Soc. 2014, 39, Shi, P.W.; Shao, X.P. Function destruction instability basic ro in top caving steep seams. J. Liaoning Tech. Univ. 2006, 25, Lai, X.P.; Sun, H.; Shan, P.F.; Cai, M.; Cao, J.T.; Cui, F. Structure instability forecasting analysis giant rock pillars in steeply dipping thick seams. Int. J. Miner. Metall. Mater. 2015, 22, [CrossRef] 34. Ma, L.Q.; Cao, X.Q.; Li, Q.Q.; Jia, J.L.; Fan, G.W. support stability mechanism in dip direction fully mechanised working face with big dip angle considering strike angle. Int. J. Oil Gas Coal Technol. 2015, 9, [CrossRef] 35. Huang, Y.L.; Li, J.M.; Song, T.Q.; Kong, G.Q.; Li, M. Analysis on filling ratio shield supporting pressure for overburden movement control in mining with compacted backfilling. Energies 2017, 10, 31. [CrossRef] 36. Chen, J.; Du, J.P.; Zhang, W.S.; Zhang, J.X. An elastic base beam model overlying strata movement during mining with gangue back-filling. J. China Univ. Min. Technol. 2012, 41, Yang, J.X.; Liu, C.Y.; Yang, P.J.; Yang, Y. Research on roadside packing technology for end zone steep inclined seam face. Rock Soil Mech. 2014, 35,

20 Energies 2017, 10, Cui, F.; Lai, X.P.; Cao, J.Y. Mining disturbance horizontal section full-mechanized caving face in steeply inclined seam. J. Min. Saf. Eng. 2015, 32, Sanchidrián, J.A.; Segarra, P.; López, L.M. Energy components in rock blasting. Int. J. Rock Mech. Min. Sci. 2007, 44, [CrossRef] 40. Wang, F.T.; Tu, S.H.; Yuan, Y.; Feng, Y.F.; Chen, F.; Tu, H.S. Deep-hole pre-split blasting mechanism its application for controlled ro caving in shallow depth seams. Int. J. Rock Mech. Min. Sci. 2013, 64, [CrossRef] 41. Shang, X.J.; Su, J.Y. Dynamic Analysis Method Engineering Example Ansys/Ls-Dyna; China Water Conservancy Hydropower Press: Beijing, China, Shi, S.Q.; Kang, J.G.; Wang, M.; Liu, Y.; Li, X.D. Engineering Applications Ansys/ls-Dyna in Field Explosion Impact; China Construction Industry Press: Beijing, China, Gao, K.; Liu, Z.G.; Liu, J.; Deng, D.S.; Gao, X.Y.; Kang, Y.; Huang, K.F. Application deep borehole blasting to gob-side entry retaining forced ro caving in hard compound ro deep well. Chin. J. Rock Mech. Eng. 2013, 32, Liu, Z.; Cao, A.; Zhu, G.; Wang, C. Numerical simulation engineering practice for optimal parameters deep-hole blasting in sidewalls roadway. Arab. J. Sci. Eng. 2017, 42, [CrossRef] 45. Shao, X.P.; Shi, P.W. Stability research surrounding rock at gob area using large section mining in steep seam. J. Liaoning Tech. Univ. (Nat. Sci.) 2010, 29, Lai, X.-P.; Cai, M.-F.; Ren, F.-H.; Shan, P.-F.; Cui, F.; Cao, J.-T. Study on dynamic disaster in steeply deep rock mass condition in urumchi field. Shock Vib. 2015, 2015, [CrossRef] 2017 by authors. Licensee MDPI, Basel, Switzerl. This article is an open access article distributed under terms conditions Creative Commons Attribution (CC BY) license (