processes Article
A Numerical Study of Stress Distribution and Fracture Development above a Protective Coal Seam in Longwall Mining Chunlei Zhang 1,2,3, *, Lei Yu 1 , Ruimin Feng 4 , Yong Zhang 2 and Guojun Zhang 2 1 2 3 4
*
ID
International Engineering Company of China Coal Technology and Engineering Group, Beijing 100013, China;
[email protected] College of Resources and Safety Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China;
[email protected] (Y.Z.);
[email protected] (G.Z.) Department of Mining and Mineral Resources Engineering, Southern Illinois University, Carbondale, IL 62901, USA Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada;
[email protected] Correspondence:
[email protected]; Tel.: +86-10-156-5294-0306
Received: 7 August 2018; Accepted: 17 August 2018; Published: 1 September 2018
Abstract: Coal and gas outbursts are serious safety concerns in the Chinese coal industry. Mining of the upper or lower protective coal seams has been widely used to minimize this problem. This paper presents new findings from longwall mining-induced fractures, stress distribution changes in roof strata, strata movement and gas flow dynamics after the lower protective coal seam is extracted in a deep underground coal mine in Jincheng, China. Two Flac3D models with varying gob loading characteristics as a function of face advance were analyzed to assess the effect of gob behavior on stress relief in the protected coal seam. The gob behavior in the models is incorporated by applying variable force to the floor and roof behind the longwall face to simulate gob loading characteristics in the field. The influence of mining height on the stress-relief in protected coal seam is also incorporated. The stress relief coefficient and relief angle were introduced as two essential parameters to evaluate the stress relief effect in different regions of protected coal seam. The results showed that the rock mass above the protective coal seam can be divided into five zones in the horizontal direction, i.e. pre-mining zone, compression zone, expansion zone, recovery zone and re-compacted zone. The volume expansion or the dilation zone with high gas concentration is the best location to drill boreholes for gas drainage in both the protected coal seam and the protective coal seam. The research results are helpful to understand the gas flow mechanism around the coal seam and guide industry people to optimize borehole layouts in order to eliminate the coal and gas outburst hazard. The gas drainage programs are provided in the final section. Keywords: longwall mining; gob behaviors; stress relief; permeability; gas drainage
1. Introduction and Background Coal as a major energy source accounted for 62% of Chinese energy consumption in 2016. It has remained the most important fuel in China’s energy mix for the last few decades [1]. In recent years, the safety situation of Chinese coal industry has improved, but gas and coal outbursts still seriously threaten coal mine safety due to the fact that the mining depth increases at a rate of 30–50 m/year in China [2]. According to statistical data, the death toll of coal miners reached up to 608 in 76 gas accidents from 2012 to 2015 [3]. Additional effective measures and research, hence, should be carried out to reduce and avoid gas disasters in China. Processes 2018, 6, 146; doi:10.3390/pr6090146
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Stress redistribution and rock mass fracture are two dominant factors influencing rock mass permeability [4–6]. Coal rock mass fracture permeability increases duefactors to stress relief and mining Stress Stress redistribution redistribution and and rock rock mass mass fracture are are two two dominant dominant factors influencing influencing rock rock mass mass induced fracturing [5]. Itand hasrock beenmass reported by several researchers at stress both laboratory and field scales permeability [4–6]. Coal permeability increases due to relief and mining induced permeability [4–6]. Coal and rock mass permeability increases due to stress relief and mining [1–4,7]. A [5]. great dealbeen of previous work showed that changes inlaboratory coal permeability isscales induced by fracturing It has reported by several at both and field induced fracturing [5]. It has been reported by researchers several researchers at both laboratory and field[1–4,7]. scales changes in the stress, and found exponential relationship between permeability and A great A deal ofconfining previous work showed thatanchanges in coal permeability is induced by changes [1–4,7]. great deal of previous work showed that changes in coal permeability is induced by stress [8]. A confining stress change of 10 MParelationship may resultbetween in changes in permeability with in the confining stress, and found an exponential permeability and stress [8]. changes in the confining stress, and found an exponential relationship between permeability and approximately onechange order of of10 magnitude. Yang in et al. [9] provides a visible relationship between A confining MPa mayofresult in permeability with one stress [8]. Astress confining stress change 10 MPachanges may result in changes in approximately permeability with dimensionless permeability and multiples ofvisible the initial normalbetween stress (see Figure 1, permeability is initial order of magnitude. Yang et al. [9] provides a relationship dimensionless approximately one order of magnitude. Yang et al. [9] provides a visible relationship between normal stress,of the⁄ initialisnormal dimensionless multiples initial normal and and multiples stressof (seethe 1, σn0 is initialstress, normal stress, σnis/σ dimensionless permeability and multiples ofFigure the initial normal stress (see Figure 1,n0 is multiples is initial permeability). It shows permeability with permeability). increasing normal stress.permeability Therefore, the mining of the initial normal k f is decreases dimensionless It shows decreases ⁄ stress, normal stress, is dimensionless is and multiples of the initial normal stress, and of theincreasing protectivenormal coal seam is wildly used inthe themining multiple coal seams in China (Figure 2). As shown with stress. Therefore, of the protective coal seam is wildly used in permeability). It shows permeability decreases with increasing normal stress. Therefore, the mining Figure 2, the coal seam which overlies the target seam is designated as the upper protective seam, the multiple coalcoal seams in China (Figure As multiple shown incoal Figure 2, the coal seam which overlies of the protective seam is wildly used 2). in the seams in China (Figure 2). As shownthe in whereas theisunderlying coal layer is called the lower protective seam. Extraction oflayer a protective target seam designated as the upper protective seam, whereas the underlying coal is called Figure 2, the coal seam which overlies the target seam is designated as the upper protective seam, seam results in redistribution and relieving of some ofseam the stress around underlying or overlying the lower protective seam. of a protective resultsseam. in redistribution relieving whereas the underlying coalExtraction layer is called the lower protective Extraction ofand a protective rock mass, establishing new stressorequilibrium [10].mass, Reestablishment of a new stress state of some of thereby the around underlying overlying therebyunderlying establishing stress seam results in stress redistribution and relieving of some ofrock the stress around ornew overlying will inevitably lead to the changesofofastructure and properties of rocklead masses, which willofeventually equilibrium [10]. Reestablishment new stress state will inevitably to the changes structure rock mass, thereby establishing new stress equilibrium [10]. Reestablishment of a new stress state promote the desorption rate of which gas from matrix and considerably increaserate permeability the and of rock masses, willcoal eventually promote desorption ofwill gaseventually fromincoal will properties inevitably lead to the changes of structure and properties ofthe rock masses, which target coal seam or strataincrease with high gas content.in the target coal seam or strata with high gas content. matrix and permeability promote theconsiderably desorption rate of gas from coal matrix and considerably increase permeability in the target coal seam or strata with high gas content.
Figure between 1. Relationship betweenand permeability and [9] normal stress [9]. with permission Figure 1. Relationship permeability normal stress (Reproduced from [9], Copyright Elsevier, 2011). Figure 1. Relationship between permeability upper protected coal seam and normal stress [9].
b2
b1
α
β
b2
b1
upper a2 protected coal aseam 1 relief area a2 a1 relief gobarea gob β
γ
ε
γ
α
gob gob middle protective coal seam ε
middle protective coal seam relief area relief area lower protected coal seam lower protected coal seam Figure 2. Illustration of protective coal seam mining.
Figure 2. Illustration of protective coal seam mining. 2. Objective of Research Figure 2. Illustration of protective coal seam mining.
2. Objective of Stress relief coefficient and pressure relief angles are introduced by authors of this paper to 2. Objective of Research Research evaluate the influence of lower protective excavation on its upper protected coal seam. The location Stress relief relief coefficient coefficient and and pressure pressure relief relief angles are are introduced introduced by by authors of of this paper paper to to of theStress protective layer, the horizontal butt entry angles and the mining height are authors key factorsthis affecting the evaluate the influence of lower protective excavation on its upper protected coal seam. The location evaluate influence of seam lowermining protective excavation on its upper protected The location of safety of the protected coal [11]. This is because the relief area incoal theseam. protected coal seam of the protective layer, the horizontal butt entry and the mining height are key factors affecting the the protective theofhorizontal buttsuch entry thethickness mining height are key factors affecting the safety depends on a layer, variety parameters as:and The and strength of the strata between the safety of protected coalmining seam mining [11]. This is because the relief area in the protected coal seam of protected coal seam [11]. This is because the relief area in the protected coal seam depends protected coal seam and the protective coal seam, the mining thickness, gob loading behavior, face depends on a variety of parameters suchthickness as: The thickness andofstrength of the stratathe between the on a variety parameters as: The strength the strata advance rate,ofpanel width, such and pillar width of theand extracted protective coal between seam [11]. Allprotected of these protected coal seam and the protective coal seam, the mining thickness, gob loading behavior, face factors should be taken into consideration to insure the safety of mining and the protected coal advance rate, panel width, and pillar width of the extracted protective coal seam [11]. All of these factors should be taken into consideration to insure the safety of mining and the protected coal
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coal seam and the protective coal seam, the mining thickness, gob loading behavior, face advance rate, 3 of 19 panel width, and pillar width of the extracted protective coal seam [11]. All of these factors should be taken into consideration to insure the safety of mining andcharacteristics the protected coal seam. In this the seam. In this paper, the mining height and gob behavior of protective coalpaper, seam are mining height and gob behavior characteristics of protective coal seam are chosen as the two main chosen as the two main factors affecting the results from protected coal seam excavation. As the factorsofaffecting thefactors resultson from coal seam As the effect of these two Therefore, factors on effect these two the protected safety of mining the excavation. protected coal seam is still not clear. thein-depth safety of investigation mining the protected is still not clear. Therefore, investigation an on thesecoal twoseam factors is necessitated in order an to in-depth acquire the relief area on in these two factors is necessitated in order to acquire the relief area in the protected coal seam and insure the protected coal seam and insure engineering safety. The final goal of this paper is to determine engineering safety. The finaland goalpressure of this paper to determine thestrike stress and reliefincline coefficient and pressure the stress relief coefficient reliefisangles along the of the protective relief angles along the strike and incline of the protective coal seam in order to locate the protection coal seam in order to locate the protection region (which is used for drilling boreholes) in the region (which is used for drilling boreholes) in the protected coal seam. protected coal seam. In this attempt to develop a 3Damodel of two of adjacent longwalllongwall faces in a faces physically In this paper, paper,authors authors attempt to develop 3D model two adjacent in a realistic way with a consideration of gob loading behavior behind the face to improve understanding physically realistic way with a consideration of gob loading behavior behind the face to improve of stress distribution and distribution stress relief not around coal seam but also incoal the seam protected understanding of stress andonly stress reliefthe notprotective only around the protective but coal seam after the excavation of protective coal seam (Figure 3). The stress distribution in the also in the protected coal seam after the excavation of protective coal seam (Figure 3). The stress gob depends on mining depth, mining speed, excavation height, bulking factor and compressive distribution in the gob depends on mining depth, mining speed, excavation height, bulking factor strength. Since increasing mining speed may the speed time for the delay stress recovery gob, it is and compressive strength.theSince increasing thedelay mining may the time in forthethe stress worthwhile to do some research with a consideration of different gob loading behaviors behind the recovery in the gob, it is worthwhile to do some research with a consideration of different gob face. The longwall mining method has been widely used around the world in recent years because of loading behaviors behind the face. The longwall mining method has been widely used around the its mostinimportant advantage high [12]. A mine in Shanxi province (China) with world recent years becauseofof its efficiency most important advantage of high efficiency [12]. A longwall mine in mining method is chosen for the research in this paper. Shanxi province (China) with longwall mining method is chosen for the research in this paper. Processes 2018, 6, x FOR PEER REVIEW
Y 5
300 20
5
Tailgate
Headgate
Panel 1 Shield Support
Pillar
200
γH Cover Stress Distance
λγH
γH
135
Panel 2
Gob Area
X γH
γH Cover Stress Distance λγH
Figure 3. A schematic plan view of two adjacent longwall faces in the numerical model, showing the Figure 3. A schematic plan view of two adjacent longwall faces in the numerical model, showing the geometry of the longwall faces. Both panel 1 and panel 2 represent half of original panel width in the geometry of the longwall faces. Both panel 1 and panel 2 represent half of original panel width in the studied studied mine mine site. site.
3. Literature Review on Gob Behaviors in Longwall Mining 3. Literature Review on Gob Behaviors in Longwall Mining Numerical modeling of protective coal seam mining’s influence on the protected coal seam has Numerical modeling of protective coal seam mining’s influence on the protected coal seam has been extensively performed in the previous research [1, 11]. However, gob loading characteristics been extensively performed in the previous research [1,11]. However, gob loading characteristics have been widely ignored in their models. The authors believe that incorporating the gob loading have been widely ignored in their models. The authors believe that incorporating the gob loading characteristics into the model is essential for obtaining meaningful and realistic results. This is characteristics into the model is essential for obtaining meaningful and realistic results. This is because because the waste in the gob area can transfer the load from overburden to floor after the the waste in the gob area can transfer the load from overburden to floor after the excavation of the coal excavation of the coal seam. Ignoring load carried by the gob material will tend to increase stress concentrations in the face area and cause different stress distribution around excavation. As the gob is inaccessible and direct measurements cannot be easily taken, it is very difficult to determine rock mass properties and stress distribution in the gob. A few techniques and research had been used and reported by previous researchers [13–16]. Figure 4 shows the strata pressure redistribution in the plane view of the seam around a longwall face after extraction. It shows that the
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seam. Ignoring load carried by the gob material will tend to increase stress concentrations in the face area and cause different stress distribution around excavation. As the gob is inaccessible and direct measurements cannot be easily taken, it is very difficult to determine rock mass properties and stress distribution in the gob. A few techniques and research had been used and reported by previous researchers [13–16]. Figure 4 shows the strata pressure Processes 2018, 6, x FOR PEER REVIEW 4 of 19 redistribution in the plane view of the seam around a longwall face after extraction. It shows that theabutment abutmentstress stressinincoal coalribs ribsgradually graduallyrecovers recoverstotothe theoriginal originalstress stressstate statewith withthe theincreasing increasing distance from thethe rib-edge. The startsto topick pickup upthe theoverburden overburden distance from rib-edge. Theloading loadingininthe thewaste waste area area gradually gradually starts loading, and recovers at aadistance distancefrom from the face after material can the take loading, and recoversitsitsoriginal original stress stress at the face after the the gobgob material can take theload loadfrom fromthe theoverburden. overburden.Peng Pengetetal.al.[13] [13]studied studiedthe thesupporting supportingrole roleofofthe thegob gobmaterial materialbyby performing three-dimensionalelement elementanalysis. analysis. They They divided performing three-dimensional divided the thegob gobinto intothree threedifferent differentzones: zones: Loosely packed zone, packed zone,and andwell wellpacked packedzone. zone. The side Loosely packed zone, packed zone, side and and front frontabutment abutmentstresses stresseswill will decrease considerably becauseofofthe thesupport supportoffered offeredby by the the compacted compacted gob. decrease considerably because gob.Whittaker Whittaker[14] [14]claimed claimed cover pressure re-establishmentdistance distance isis 0.3–0.4 0.3–0.4 times times the thatthat thethe cover pressure re-establishment the coal coalseam seamdepth depth(H) (H)from fromthe the solid abutment (Figure 4). Wilson et al. [15] found that the vertical stress in the gob changes linearly, solid abutment (Figure 4). Wilson et al. [15] found that the vertical stress in the gob changes linearly, increasing from 0 at theoriginal originalstress stressat ataa distance distance of depth. increasing from 0 at thethe ribrib totothe of 0.2–0.3 0.2–0.3times timesthe theoverburden overburden depth. Whittaker and Singh [16] assumed that the gob began to take the load at a distance of 45 m from the Whittaker and Singh [16] assumed that the gob began to take the load at a distance of 45 m from the face, and whether the gob can recover its original stress only depended on the width of the panel. face, and whether the gob can recover its original stress only depended on the width of the panel. Campoli et al. [17] investigated the longwall gob behavior in No. 3 coal seam of Pocahontas by field Campoli et al. [17] investigated the longwall gob behavior in No. 3 coal seam of Pocahontas by field measurements under varying sets of geological conditions, and they found that the stress recovery measurements under varying sets of geological conditions, and they found that the stress recovery distance in the gob was approximately 0.2 times of coal seam depth (360 m). distance in the gob was approximately 0.2 times of coal seam depth (360 m). Z
Y
X1X1 P
X1 X2
X X2X2 P 1.3m 0.3-0.4H
X1
YY P X2
P
Y
Figure 4. Strata pressure redistribution in the plane of the seam around a longwall face [16] (Reproduced Figure 4. Strata pressure redistribution in the plane of the seam around a longwall face [16] (adapted with permission from [16], Copyright Elsevier, 1979). from Whittaker B.N., 1974).
Modeling Longwall miningwith withconsideration consideration of of gob gob behaviors behaviors has Modeling of of Longwall mining hasalso alsobeen beendocumented documented in previous studies [18–20]. In these studies, the “double yield” elements were incorporated in previous studies [18–20]. In these studies, the “double yield” elements were incorporatedinintheir their simulation model the was gob regarded was regarded as a strain-hardening material. The stress-strain simulation model andand the gob as a strain-hardening material. The stress-strain response response of was gob obtained material through was obtained through uniaxialtests compression tests and displacementin of gob material uniaxial compression and displacement measurements measurements in a laboratory test [17]. Li et al. [18] evaluated the stress distribution in the yield a laboratory test [17]. Li et al. [18] evaluated the stress distribution in the yield pillar and the other pillar and the other rib of the entry in order to find the principle for yield pillar design by Flac3D rib of the entry in order to find the principle for yield pillar design by Flac3D numerical modeling. numerical modeling. Esterhuizen et al. [19] examined the interaction between the surrounding rock Esterhuizen et al. [19] examined the interaction between the surrounding rock mass and typical pillar mass and typical pillar systems for different geological conditions at various spans and depths of2D systems for different geological conditions at various spans and depths of cover by establishing Flac cover by establishing Flac2D numerical models. A method was proposed by H. Yavuz [20] to estimate numerical models. A method was proposed by H. Yavuz [20] to estimate the pressure distribution the pressure distribution in the gob of flat-lying longwall panels and cover pressure distance, and in the of flat-lying longwall cover pressure distance, and this method was thisgob method was verified withpanels curvesand obtained through numerical models. However, allverified these with curves obtained through numerical models. However, all these numerical longwall models with numerical longwall models with the gob behaviors were about pillar design and stability of entries, thebut gobnot behaviors were about design and stability of entries, but not for the protective coal for the protective coalpillar seam mining. seam mining. Generally, gob modeling can follow two approaches, explicit model and implicit model [21]. Generally, gob modeling can follow two the approaches, explicit model andonimplicit model [21]. The explicit model needs to explicitly model gob formation process based the study of roof Thefracturing, explicit model needsand to explicitly model the gob formation processwhile based onimplicit the study of roof roof caving gob development in response to coal mining, the simulates fracturing, caving gob development in response coalentries mining, while the making implicitsure simulates the effectroof of the gob and on the stability of surrounding coalto mine and pillars, that both redistribution to the of surrounding rockcoal andmine the large-scale overburden defection thethe effect ofload the gob on the stability surrounding entries and pillars, making sureand that are correct. Both and the goblarge-scale loading model belong todefection the second thesubsidence both load redistribution to the the double-yield surroundingmodel rock and overburden and approach [4]. This paper addresses the second method. Alternatively, setting up a large-scale model is capable of avoiding simulating the complex material behavior that corresponds the response curve, and equivalent gob stress distribution can be created by simply following the gob response curve. The gob load model has been used in Abbasi et al. [22] work with Flac3D. They estimated the overall gob mechanical behavior by comparing model outputs with field measurements at several
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subsidence are correct. Both the double-yield model and gob loading model belong to the second approach [4]. This paper addresses the second method. Alternatively, setting up a large-scale model is capable of avoiding simulating the complex material behavior that corresponds the response curve, and equivalent gob stress distribution can be created by simply following the gob response curve. The gob load model has been used in Abbasi et al. [22] work with Flac3D . They estimated the overall gob mechanical behavior by comparing model outputs with field measurements at several points around a longwall face in a coal mine of Illinois. The gob achieved pre-mining vertical stress at approximately 55 m behind the back end of the shields, and the gob load installed in the mine-out area was estimated by field measurements. The gob load varied both along and cross the face advance directions. The gob load model not only characterizes the effect of the gob on the surrounding entries and coal pillars but also simplifies the process of calculation, thus the model size was reduced with finer mesh at the area of interest. As such, in this paper, the gob load model was adopted. 4. Materials and Methods 4.1. Description of Case Coal Mine The Changping coal mine, with the annual production capability of 5 Mt, is located in the northwestern of Gaoping City, Shanxi, China. The thickness of the coal-bearing geological section ranges from 125 m to 174 m. It contains about sixteen coal seams. The four key minable seams include seams 2#, 3#, 8# and 15#, only portions of 2# and 8# coal seams can be mined. The position relation of Changping coal mine is shown in Figure 5. The thickness of 2# and 3# coal seams are 0–2.95 m and 4.67–6.58 m, respectively. Currently the mine is extracting seam 3# which is classified as a low permeability seam with a methane content of 6.5–15.09 m3 /t. The gas content affected by the geological structure and mining depth is extremely unbalanced; it increases gradually from the eastern part to the western part of the coal field. As the coal mining turns to the west, the gas content and emission quantity increases constantly. The gas dynamic phenomenon and gas outbursts were found in the roadway of Faces 4304 and 4306, which largely decreased the roadway excavation efficiency and brought potential dangers to the production. To avoid the coal and gas outbursts, 8# coal seam was taken as the lower protective coal seam for 3# coal seam.
low permeability seam with a methane content of 6.5–15.09 m3/t. The gas content affected by the geological structure and mining depth is extremely unbalanced; it increases gradually from the eastern part to the western part of the coal field. As the coal mining turns to the west, the gas content and emission quantity increases constantly. The gas dynamic phenomenon and gas outbursts were found in the roadway of Faces 4304 and 4306, which largely decreased the roadway excavation Processes 2018, 6, 146 efficiency and brought potential dangers to the production. To avoid the coal and gas outbursts, 8#6 of 21 coal seam was taken as the lower protective coal seam for 3# coal seam. stratum column
stratum thickness/m 0.00-1.70 0.67 4.40-25.69 20.68
lithology No.2 coal seam mudstone siltstone sandy mudstone
4.67-6.58 5.64
No.3 coal seam
30.57-41.09 37.13
mudstone sandy mudstone fine sandstone siltstone
0.00-2.95 1.22
No.8 coal seam
31.04-79.6 50.12
mudstone sandy mudstone limestone
2.20-5.75 4.02
lithology
Figure 5. Position relation of coal seams.
Figure 5. Position relation of coal seams.
The basic parameters of coal seam 3# (No. 3 coal seam) are listed in Table 1. It shows the characteristics of coal seam Low permeability, gas seam) contentare andlisted low hardness. The panel the The basic parameters of 3#: coal seam 3# (No. high 3 coal in Table 1. 4306 It shows is in seam of 3#,coal its dip angle 0–6°. Average can be regarded as 0°. TheThe coal4306 seampanel characteristics seam 3#:ranges Low from permeability, highdip gasangle content and low hardness. has an average thickness of 5.23 m, with overburden depth ranging between 500–550 m. The panel ◦ ◦ is in seam 3#, its dip angle ranges from 0–6 . Average dip angle can be regarded as 0 . The coal seam advance length is about 1100 m and the panel width is about 270 m. The 84306 panel is in seam 8#, its has an average thickness of 5.23 m, with overburden depth ranging between 500–550 m. The panel advance length is about 1100 m and the panel width is about 270 m. The 84306 panel is in seam 8#, its gas content is 4–6 m3 /t, there is no danger of coal and gas outburst in 8# coal seam, and the 84306 panel is mined first as a protective seam for 4306 panel. Table 1. Basic parameters of No. 3 coal seam. Coal Seam No. 3 No. 8
Coal Face 4306 protected longwall face 84306 protective longwall face
Mining Thickness/m
Gas Content/m3 /t
Original Gas Pressure/MPa
Coal Stiffness
Outburst Risk
5.23
3.5–15.09
0.38–0.55
0.44–0.56
Yes
1–3
2–3
None
0.51–1.12
No
4.2. Description of Numerical Models Flac3D , an explicit finite-difference program for engineering mechanics computation, is used to model the excavations of protective coal seam in this paper, which has been widely used in mining engineering [23]. In this paper, Flac3D was employed to investigate the displacement and stress distribution in the overlying strata (including the No. 3 coal seam) of coal seam 8# (No. 8 coal seam—see Figure 6). In order to eliminate the effect of boundary conditions to the simulation results, both the downside boundaries and the upper coal pillar were confirmed by taking the movement and deformation of the cover rock into account [24]. Therefore, the size of the model is 300 m × 100 m × 200 m. The working face advances in the Y direction, and the surrounding displacement boundary is restricted horizontally; the top of the model is free in Z direction, while the vertical displacement of bottom is restricted. A load of 11.25 MPa was added in the upside boundary of the model for representing an overlying strata of 450 m. The lateral pressure stress was added
seam—see Figure 6). In order to eliminate the effect of boundary conditions to the simulation results, both the downside boundaries and the upper coal pillar were confirmed by taking the movement and deformation of the cover rock into account [24]. Therefore, the size of the model is 300 m × 100 m × 200 m. The working face advances in the Y direction, and the surrounding displacement boundary Processes 2018, 6, 146 7 of 21 is restricted horizontally; the top of the model is free in Z direction, while the vertical displacement of bottom is restricted. A load of 11.25 MPa was added in the upside boundary of the model for representing an overlying strata of 450 m. The lateral pressure stress was added according to the according to the measured data of the original in-situ stress of this mine. The horizontal stress in measured data of the original in-situ stress of this mine. The horizontal stress in the X and Y the X and Y direction was approximately confirmed as 14.56 MPa and 13.26 MPa, respectively in the direction was approximately confirmed as 14.56 MPa and 13.26 MPa, respectively in the numerical numerical models by comparing calculated values with measured values. The dimension of the 8# models by comparing calculated values with measured values. The dimension of the 8# coal seam is coal seam is 1 m × 1 m × 1 m, extraction of the coalbed was modeled by removing elements over the 1 m × 1 m × 1 m, extraction of the coalbed was modeled by removing elements over the height of the height of the coalbed by 1 element height, 2 element heights, 3 element heights, respectively. coalbed by 1 element height, 2 element heights, 3 element heights, respectively.
Figure 6. Flac3D model. Figure 6. Flac3D model.
There are 2 monitoring lines that are totally installed in the model: (1 m above the No. 8 coal 2 monitoring areNo. totally installed in the model: (1 mdirection). above the No. 8 coal seamThere along are mining direction),lines (2 mthat above 3 coal seam floor along mining seamRock alongmass mining direction), (2 m above No. 3 coal seam floor along mining direction). engineering properties for different lithologies were described in Table 2. These Rock mass engineering properties for different lithologies wereindex described Table 2. These values values were calculated through estimated the geological strength (GSI)inrock mass system and were calculated through estimated the geological strength index (GSI)Hoek-Brown rock mass system Hoek-Brown failure criterion [25]. (Equations (1)–(7)). The generalized criterionand is Hoek-Brown failure criterion [25]. (Equations (1)–(7)). The generalized Hoek-Brown criterion is expressed as: expressed as: a σ30 0 = 0+ (1) + +s (1) σ1 = σ3 + σci mb σci where σ10 and σ30 are principle stresses stresses at at failure, failure, σci is the uniaxial uniaxial are the the major major and and minor effective principle where compressive strength (UCS) of the intact rock material, m , s and a are material constants. where mb is compressive the intact rock material, b , s and are material constants. a reduced value of the material mi and is given byand is given by where is a reduced value ofconstant the material constant GSI − 100 (2) mb = mi exp 28 − 14D s and a are constants for the rock mass, and we have the following relationships: GSI − 100 s = exp 9 − 3D
a=
1 −GSI/15 + e − e−20/3 2
(3)
where D is a factor depending upon the degree of disturbance to which the rock mass has been subjected by blast damage and stress relaxation. It varies from 0 for undisturbed in situ rock mass to 1 for very disturbed rock masses. D = 0 is assumed in this paper. The estimation of rock masses strength are as follows: The UCS is: σc = σci s a when σ30 = 0 (4) The tensile strength is: σt = −
sσci by setting σ10 = σ30 = σt mb
(5)
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The rock mass modulus of deformation (GPa) is: 1−
Em =
D 2
r
Em =
1−
σci ·10((GSI −10)/40) for σci ≤ 100 MPa 100
(6)
D ·10((GSI −10)/40) for σci > 100 MPa 2
(7)
Table 2. Rock mass properties, GSI and Hoek-Brown parameters used in the numerical modeling. Lithology
ν
σci/MPa
GSI
mi
mb
s
a
Em /MPa
3# Coal seam Fine grained sandstone Medium grained sandstone Mudstone Sandstone Sandy mudstone Limestone 8# Coal seam
0.33 0.19 0.20 0.28 0.24 0.26 0.19 0.29
6 90 73 16 40 35 75 6.4
75 90 88 80 86 85 90 75
11 16 15 12 13 13 10 11
4.504 11.195 9.772 5.874 7.885 7.084 6.997 4.504
0.0622 0.3292 0.2636 0.1084 0.2111 0.1512 0.3292 0.0622
0.501 0.500 0.500 0.501 0.500 0.500 0.500 0.201
2938.86 6566.53 5020.52 3433.36 4203.05 4095.07 9682.03 3020.50
4.3. Gob Behaviors in the Model Numerical modeling of gob behavior in longwall mining is a challenge. In order to make a better understanding of the stress recover distance in the gob, some researchers use double yield model to simulate the gob behavior [18–20], H. Yavuz et al. [20] proposes a methodology for estimating the re-establishment distance of the cover pressure and stress distribution in the gob by analyzing a large quantity of field data from British longwall coal mine over several years. He introduced a three-parameter power function, in which the independent variables are excavation height, mining depth, bulking factor and compressive strength of the rock fragments. The cover pressure re-establishment distance in the gob is estimated via Equation (8). His conclusion was finally verified with curves obtained through numerical models. As the bulking factor of the rock pile increases with mining height increasing, hence this results in increase in cover pressure re-establishment distance. According to the mining condition of the case coal mine, the parameters obtained for the case mine are given as: b = 1.2–1.4, the bulking factor of the caved roof; σc = 30 MPa, the compressive strength of the rock pieces; H = 480–530 m, the mining depth; h = 1–3 m, coal thickness; γ = 0.025 MN/m3 , the unite weight of overburden. As a result, the cover pressure re-establishment distance for the example working face were found to be 120 m–140 m from Equation (8). Xcd = 0.2H 0.9 6γHb
8.7 / [10.39σ1.042 ( b −1)+ γHb8.7 ] c
(8)
In this paper, gob behaviors were regarded as linear recovery, two gob behaviors (cover pressure re-establishment distance for Gob 1 and Gob 2 at 120 m and 140 m respectively) were incorporated in the 4 Flac3D numerical models with different mining height (Table 3). Table 3. Parameters of each Flac3D model. Model Parameters
Cover Pressure Re-Establishment Distance/m
Mining Height/m
Model 1 Model 2 Model 3 Model 4
Gob1-120 Gob1-120 Gob1-120 Gob2-140
1 2 3 3
Model Parameters Model 1 Model 2 Model 3 Processes 2018, 6, 146 Model 4
Cover Pressure Re-Establishment Distance/m
Mining Height/m
Gob1-120 Gob1-120 Gob1-120 Gob2-140
1 2 3 3
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5. 5. Results Results and and Discussion Discussion 5.1. Stress Distribution around the Protective Coal Seam 5.1.1. Stress Stress Distribution Distribution in in the the Gob Gob 5.1.1. The lower seam is is extracted. The weight of The lower roof roof will willmove movedownward downwardand andcollapse collapseafter afterthe thecoal coal seam extracted. The weight thethe upper strata will will be supported by both the panel the lower fallstrata into the of upper strata be supported by sides both of sides of theas panel as thestrata lower fallextracted into the space. The stress in the gob area will be redistributed due to coal seam extraction (Figure 7), as the extracted space. The stress in the gob area will be redistributed due to coal seam extraction (Figure overburden strata broken condition “O” shape, al. [26] Lin andet Qian et al.and [27] Qian characterized it 7), as the overburden strata brokenlikes condition likesLin “O”et shape, al. [26] et al. [27] as “Ring” circle and “O” circle, respectively. The pressure arch will develop across the solid coal and characterized it as “Ring” circle and “O” circle, respectively. The pressure arch will develop across the destressed zonethe willdestressed be formedzone abovewill the be gob. As theabove longwall the caving process the solid coal and formed the face gob.retreats As the and longwall face retreats continues, the caved material comes contact with the roof takeswith loadthe from upper and the caving process continues, theinto caved material comes intoand contact roofthe and takesstrata load due to the combined influence of floor heave, roof sag, and waste rock bulking. And then the stress in from the upper strata due to the combined influence of floor heave, roof sag, and waste rock bulking. the gob will make a further redistribution. And then the stress in the gob will make a further redistribution. Tailgate "O" Circle (Macrofractures) Compaction area (Microfractures)
Headgate Figure Figure 7. 7. The The plane plane schematic schematic diagram diagram of of goaf goaf mining mining fissure. fissure.
Figure 8 shows the vertical stress distribution in the roof of gob area after the excavation of No. Figure 8 shows the vertical stress distribution in the roof of gob area after the excavation of No. 8 8 protective coal seam (results of model 3). It can be seen from the figure that the vertical stress is protective coal seam (results of model 3). It can be seen from the figure that the vertical stress is small small in the area close to the face and pillar, but the stress values increase when moving toward the in the area close to the face and pillar, but the stress values increase when moving toward the center center of gob. The results are consistent with the “O” circle theory mentioned above. This is because of gob. The results are consistent with the “O” circle theory mentioned above. This is because there there is less support near the face and pillar area, the roof therefore gets stress relief. These areas is less support near the face and pillar area, the roof therefore gets stress relief. These areas typically typically make up the main gas flow fissure in protective coal seam. Processes 2018, 6, main x FOR PEER REVIEW 9 of 19 make up the gas flow fissure in protective coal seam.
Figure 8. Vertical stress distribution in the roof of gob area. Figure 8. Vertical stress distribution in the roof of gob area.
5.1.2. The Zones in the Horizontal Direction of Protective Coal Seam 5.1.2. The Zones in the Horizontal Direction of Protective Coal Seam Figure 9 shows the stress and displacement distribution in the immediate roof of protective coal Figure 9 shows the stress and displacement distribution in the immediate roof of protective coal seam after 130 m of face advance (results from Model 3), the face position is at 0 m in the horizontal seam after 130 m of face advance (results from Model 3), the face position is at 0 m in the horizontal coordinate, as can be seen from Figure 9, and the overburden can be divided into 5 zones according coordinate, as can be seen from Figure 9, and the overburden can be divided into 5 zones according to to the stress and displacement change: The original zone, compression zone, expansion zone, the stress and displacement change: The original zone, compression zone, expansion zone, recovering recovering zone and re-compacted zone [9]. zone and re-compacted zone [9]. The original zone was 40 m beyond the coal face, this area was far away from the coal face, the stress was almost initial stress and the displacement decreased to 0, very little deformation occurred, the pr-existing fracture was isolated, showing that the mining activity had little effect on this area, the gas extraction therefore is difficult in this area. The compression zone was from 5 m to 40 m ahead of the coal face, it can also be called the
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The original zone was 40 m beyond the coal face, this area was far away from the coal face, the stress was almost initial stress and the displacement decreased to 0, very little deformation occurred, the pr-existing fracture was isolated, showing that the mining activity had little effect on this area, the gas extraction therefore is difficult in this area. The compression zone was from 5 m to 40 m ahead of the coal face, it can also be called the abutment stress area, the pre-existing cracks in the coal and rock mass of this area was closed due to the high abutment stress, new cracks developed and expanded. The cracks were gradually connected to the network especially from the 5 m to the peak abutment stress and the gas emission and transportation fissure was gradually formed. The expansion zone extended from 5 m ahead of the coal face to −60 m behind the coal face, including the stress relief area ahead of the coal face and part of the gob area. As plastic failure occurred in the stress relief area of the coal, it reduced the ability to support the overburden. In the working face area, the overburden was broken due to the shield supporting, and more and more new cracks developed and connected in this area. The roof displacement increased gradually from coal face to gob, which means the expansion of the rock mass. The cracks in this area were open and connected into the network, which made the rock mass permeability increasing and the methane much easier to be extracted. This area was the best place for gas drainage of the protected coal seam. The recovering zone extended from −60 m to −120 m within the gob. The vertical stress gradually increased with the increasing distance from the working face, and the rock mass in the gob was gradually compacted. The gob material could take some load from overburden, which resulted in the gradual closure of cracks in the roof and gob area. The rock mass permeability and the gas extraction quantity in this area decreased slowly. The re-compacted zone was located at the back of the gob starting from −70 m behind the coal face. The vertical stress gradually recovered to its original stress. At the same time, the rock mass deformation, roof displacement and permeability became stable in this area. The expansion cracks shrunk 2018, and 6,most ofPEER the cracks Processes x FOR REVIEWwere closed. Gas extraction in this area was therefore very difficult. 10 of 19 Gob
0
solid coal
Z stress/MPa
30
-0.2 re-compacted zone
recovering zone
compression zone original zone
expansion zone
20
-0.4
10
0 -140
vertical stress vertical displacement
-120
-100
-80
-60
-40
-20
Location away from coal face
0
20
40
-0.6
vertical displacement/m
40
-0.8 60
Figure Figure 9. 9. Five Five zones zones above above the the protective protective coal coal seam seam along along mining mining direction. direction.
5.2. Protected Coal Coal Seam Seam 5.2. Factors Factors Affecting Affecting Pressure Pressure Relief Relief and and Deformation Deformation of of Protected 5.2.1. of Protected Protected Coal Coal Seam Seam 5.2.1. Stress Stress Relief Relief Coefficient Coefficient and and Relief Relief Angle Angle of After After the the extraction extraction of of the the protective protective coal coal seam, seam, the the stress stress around around the the opening opening will will redistribute. redistribute. Figure 10 shows that the stress added to the surrounding rib of the gob and the working Figure 10 shows that the stress added to the surrounding rib of the gob and the working face face is is concentrated at both ends of protective coal seam. At the same time, the roof and floor at the back concentrated at both ends of protective coal seam. At the same time, the roof and floor at the back of of the gob were relieved. This is due to the influence of the abutment pressure of two-way rib of the gob, which lead to a certain pressure relief of the protected coal seam. The extent of the pressure relief can be described with pressure relief coefficient [28], which is defined as Equation (9).
=
−
(9)
5.2. Factors Affecting Pressure Relief and Deformation of Protected Coal Seam 5.2.1. Stress Relief Coefficient and Relief Angle of Protected Coal Seam Processes 2018,the 6, 146 After extraction
11 of 21 of the protective coal seam, the stress around the opening will redistribute. Figure 10 shows that the stress added to the surrounding rib of the gob and the working face is concentrated at both ends of protective coal seam. At the same time, the roof and floor at the back of the gob were relieved. This is due to the influence of the abutment pressure of two-way rib of the gob, the gob were relieved. This is due to the influence of the abutment pressure of two-way rib of the which lead to a certain pressure relief of the protected coal seam. The extent of the pressure relief can gob, which lead to a certain pressure relief of the protected coal seam. The extent of the pressure be described with pressure relief coefficient [28], which is defined as Equation (9). relief can be described with pressure relief coefficient [28], which is defined as Equation (9).
k=
=
σ0 − −σ σ0
(9) (9)
where k is stress relief coefficient, σ0 is the initial stress of the coal seam (MPa), σ is the stress in where is stress relief coefficient, is the initial stress of the coal seam (MPa), σ is the stress in the the protected coal seam after extraction of the protective coal seam (MPa). When k > 0, σ0 > σ, it > , it means protected coal seam after extraction of the protective coal seam (MPa). When > 0, means pressure relief occurred in the protected coal seam; when k < 0, σ0 < σ, it means pressure pressure relief occurred in the protected coal seam; when 0, , it means pressure concentration occurred in the protected coal seam; when k = 0, σ0 = σ, it means there is no stress concentration occurred in the protected coal seam; when = 0, = , it means there is no stress change in the protected coal seam. change in the protected coal seam.
Figure 10. Contour of vertical stress along incline in the gob for 120 m face advance. Figure 10. Contour of vertical stress along incline in the gob for 120 m face advance.
The protection criteria taking mining stresses into account is adopted in our study [29]. The protection criteria taking mining intocritical account is adopted in ouras study [29]. The relief reliefThe angle can be then evaluated by the stresses following stress relief value, given in Equation angle can be then evaluated by the following critical stress relief value, as given in Equation (10). (10). ≤ cos2 α + + λsin2 α γH (10) (10) ||σzc || ≤ where σzc is the vertical stress when the coal seam angle is 0◦ . σzc is the Z-stress. α is the coal seam angle. λ is the lateral pressure coefficient. γ is the bulk density and H is the initial depth of the outburst. According to the conditions of Jincheng coal mine, the followings are taken: γ = 25 kN/m3 , H = 470 m, α = 0, then |σzc | ≤ 2500 × 470 = 11.7 MPa. Therefore, when the stress decreases to 11.7 MPa, a gas outburst will not occur. Therefore, 11.7 MPa is taken as the critical value, the cross point of the critical value line and the stress curves are critical points. By substitution of value 11.7 MPa into Equation (9), the critical value of stress relief coefficient (0.1) could be obtained. 5.2.2. Abutment Stress Changes for Different Gob Behaviors and Mining Height in Protective Coal Seam Figure 11 shows the vertical stress concentration factor (or VSCF) in the immediate roof of 84306 working face with different gob behaviors after 120 m face advance at the mining height of 3 m. The coal face is at 0 point on the horizontal coordinate. As can be seen from Figure 11, the vertical stress concentration factor varies in the gob and ahead of the coal face. In the back of the coal face, the value gradually decreases from 1 at the coal face to almost 0 at approximately 10 m back of the coal face, then it gradually increases as it is far away from the coal face. While in front of the coal face, the VSCF gradually increases until it reaches its peak value at around 7 m ahead of the coal face. Comparing the cures of Gob 1 with Gob 2, the VSCF value is almost the same from 7 m behind the coal face to 4m ahead of the coal face, and this means the length of yield zones in ahead of the coal face is the same, while the peak value of the VSCF in ahead of coal face and affecting range is different, the affecting range of Gob 2 is larger than Gob 1, and the peak value of VSCF of gob 2 is 3.1 compared
value gradually decreases from 1 at the coal face to almost 0 at approximately 10 m back of the coal face, then it gradually increases as it is far away from the coal face. While in front of the coal face, the VSCF gradually increases until it reaches its peak value at around 7 m ahead of the coal face. Comparing the cures of Gob 1 with Gob 2, the VSCF value is almost the same from 7 m behind the coal face to 4m ahead of the coal face, and this means the length of yield zones in ahead of the coal Processes 2018, 6, 146 12 of 21 face is the same, while the peak value of the VSCF in ahead of coal face and affecting range is different, the affecting range of Gob 2 is larger than Gob 1, and the peak value of VSCF of gob 2 is 3.1 with 2.8 forwith Gob 2.8 1 model. This1 ismodel. because the is gob 2 has larger recovery to pre-mining stress compared for Gob This because the gob 2 has distance larger recovery distance to value and the coal face need to take more overlying strata load than gob 1. pre-mining stress value and the coal face need to take more overlying strata load than gob 1.
Figure 11. Vertical stress concentration factor in the face area of protected coal seam. Figure 11. Vertical stress concentration factor in the face area of protected coal seam.
Figure 12 shows the VSCF curves with gob 1 for different mining height of 1 m, 2 m, and 3 m, Figure 12It shows curves withisgob 1 for height 1 m, 2 m, and 3 m, respectively. shows the thatVSCF the VSCF value equal at different the back mining of the coal faceoffor different mining respectively. It shows that behaviors. the VSCF value equal the back of theahead coal face forcoal different mining heights with the same gob Whileisthe peakatvalue of VSCF of the face and their heights with the same gob behaviors. While the peak value of VSCF ahead of the coal face and affecting range is different with mining height changing. The peak values of VSCF for 1 m, 2 m,their and range is different changing. The peakvalue valuesofofVSCF VSCFisfor 1 m, 2inm, and 3affecting m mining height are 3 m,with 2.8 mmining and 2.7height m, respectively; the peak located 4 m, 6 3 m8 mining height are 3 m,of2.8 mcoal andface. 2.7 m, respectively; thearea peakofvalue of VSCF is located inwith 4 m, m, m respectively ahead the And the influence abutment stress increases 6 m,mining 8 m respectively ahead of face. And the influence area of abutment increases the height. According tothe the coal above analysis, a negative relationship is foundstress between seam with the mining height. According to the above analysis, a negative relationship is found between height and the VSCF values ahead of the coal face. This is because the stiffness of the coal seam seam height and the VSCF values ahead of the coal face. This is because the stiffness of the coal decreases with increasing coal seam height. The coal seam deforms more both laterallyseam and decreases with increasing coal seam coal more distresses both laterally and vertically. vertically. Larger deformation on theheight. face orThe even faceseam fallsdeforms in the return, the solid coal and Larger deformation the face even facehand, falls inthinner the return, distresses the solid coal andbecause decreases decreases the VSCFon values. Onorthe other seams can sustain more load of the VSCF values. On the other hand, thinner seams can sustain more load because of higher stiffness. higher stiffness. Processes 2018, 6, x FOR PEER REVIEW
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Figure 12. 12. Vertical Vertical stress stress concentration concentration factor factorwith withdifferent differentmining miningheight heightfor forGob Gob1.1. Figure
5.2.3. The Effect of Gob Behaviors to Protected Coal Seam 5.2.3. The Effect of Gob Behaviors to Protected Coal Seam Figure 13 shows the pressure relief coefficient of the protected coal seam with different gob Figure 13 shows the pressure relief coefficient of the protected coal seam with different gob behaviors for different face advance. The starting line lies 20 m of the horizontal coordinate. As behaviors for different face advance. The starting line lies 20 m of the horizontal coordinate. As shown shown in Figure 13, overall, the pressure relief coefficient of the protected coal seam has same in Figure 13, overall, the pressure relief coefficient of the protected coal seam has same changing trends changing trends under the condition of two different gob behaviors in the protective coal seam, under the condition of two different gob behaviors in the protective coal seam, stress concentration stress concentration occurs both ahead of the coal face and behind the set-up room. The pressure relief changes regularly during the mining process. Taking the gob 1 for example, it increases gradually at the beginning of the mining process. It then reaches the peak value at approximately 0.3 when the protective coal seam is mined 50 m; and later, the peak value gradually decreases and levels off at approximately 0.26. While it also shows some difference in terms of magnitude and
5.2.3. The Effect of Gob Behaviors to Protected Coal Seam Figure 13 shows the pressure relief coefficient of the protected coal seam with different gob behaviors for different face advance. The starting line lies 20 m of the horizontal coordinate. As shown 2018, in Figure Processes 6, 146 13, overall, the pressure relief coefficient of the protected coal seam has13same of 21 changing trends under the condition of two different gob behaviors in the protective coal seam, stress concentration occurs both ahead of the coal face and behind the set-up room. The pressure occurs both ahead of the coal face the andmining behind the set-upTaking room. The pressure changes relief changes regularly during process. the gob 1 forrelief example, it regularly increases during the mining process. Taking the gob 1 for example, it increases gradually at the beginning of the gradually at the beginning of the mining process. It then reaches the peak value at approximately 0.3 mining process. It then reaches the peak value at approximately 0.3 when the protective coal seam when the protective coal seam is mined 50 m; and later, the peak value gradually decreases and is mined and later, the0.26. peakWhile valueitgradually decreases and levels at approximately levels off 50 at m; approximately also shows some difference inoff terms of magnitude 0.26. and While it also shows some difference in terms of magnitude and scope for different face advance with scope for different face advance with different gob behaviors. It does not show the difference until different gob behaviors. does 50 notm, show difference until the face advance is larger m, the face advance is largerIt than this the shows these two kinds of gob behaviors havethan the 50 same this shows these two kinds of gob behaviors have the same effect to the protected coal seam when effect to the protected coal seam when the protective coal seam is mined within 50 m. When the the protective is mined within 50 m. When protective coal seam is mined 100tom, the protective coalcoal seamseam is mined 100 m, the magnitude andthe scope of protected coal seam begins show magnitude and scope of protected coal seam begins to show difference with gob 1 and gob 2. The peak difference with gob 1 and gob 2. The peak value of pressure relief coefficient of gob 1 is about 0.28, value relief coefficient of gob is about which is approximately m behind thewhile coal whichofispressure approximately 34 m behind the1coal face, 0.28, and the pressure relief scope34 is about 74 m, face, and the pressure relief about values of gobby 2 are 0.33 m, the corresponding values ofscope gob 2isare 0.3374 m,m, 37while m andthe 79corresponding m, respectively, increasing 18%, 8.8% 37 m and 79 m, respectively, increasing by 18%, 8.8% and 6.8%, respectively. and 6.8%, respectively.
(a) Gob 1 Figure 13. Cont. Processes 2018, 6, x FOR PEER REVIEW
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(b) Gob 2 Figure 13. Pressure relief coefficient of protected coal seam with different gob behaviors for different Figure 13. Pressure relief coefficient of protected coal seam with different gob behaviors for different face advance. face advance.
The relief angle in Figure 2, is one of the major parameters used to describe the relief effect after Thearelief angle incoal Figure 2, is Relief one of the major parameters used to describe ⁄ mining protective seam. angles α and β can be expressed as the=relief effect after mining a protective coal seam. Relief angles α and β can be expressed as α = arctan b /a ( ) 1 1 ⁄ and = [1]. The relief area of the protected coal seam presents the range where and the β = arctan b /a [1]. The relief area of the A protected coalrelief seamangle presents the rangetowhere therelief coal ( ) 1 coal and gas 1outburst danger is eliminated. greater the corresponds a larger and gas danger is eliminated. greater the angle in corresponds a larger relief area. area. As outburst stress distribution in the gob isA different, it relief will result a different to relief angle. Table 4 As stress distribution in the gob is different, it will result in a different relief angle. Table 4 shows the shows the relief angles change along strike with different gob behaviors for different face advance. relief angles change alonginstrike with different gobthan behaviors forthe different advance. the Overall, the relief angles the coal face are larger those in set-up face room side. thisOverall, is because the broken rock mass in the gob can pick up some load from overlying strata while there is only a shield support in the face area which can only take a little load, and the pressure of overlying strata above the face area transfers to the solid coal, which result in larger pressure relief above face area than gob side. In the starting line side, the relief angles gradually decrease during the mining process, decreasing from 82° for 30 m face advance to 43.1° (gob 1) and 55.2° (gob 2) for 120 m face
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relief angles in the coal face are larger than those in the set-up room side. this is because the broken rock mass in the gob can pick up some load from overlying strata while there is only a shield support in the face area which can only take a little load, and the pressure of overlying strata above the face area transfers to the solid coal, which result in larger pressure relief above face area than gob side. In the starting line side, the relief angles gradually decrease during the mining process, decreasing from 82◦ for 30 m face advance to 43.1◦ (gob 1) and 55.2◦ (gob 2) for 120 m face advance, and the relief angle in the set up room side become different for gob 1 and gob 2 after the coal seam is mined 100 m, gob 2 has larger relief angle than gob 1 in gob side, this is because gob 2 has larger stress recover distance than gob 1, the relief angle for gob 1 and gob 2 is 55.2◦ and 61.0◦ , respectively, when the coal seam is mined 100 m. In the coal face side, the relief angles experience the increase, decrease, and stabilization process; it decreases from 82◦ for 30 m face advance to 76◦ for 100 m face advance and stabilizes at approximately 76◦ . The relief angle is almost the same in the face area for different gob behaviors; it means the gob behaviors have little effect on the pressure relief of the face area. Table 4. Relief angles change along strike with different gob behaviors for different face advance. Face Advance Gob Set up room/◦ Coal face/◦
30 m 1 82.0 82.0
2 82.0 82.0
50 m 1 74.5 80.6
2 74.5 80.6
100 m 1 55.2 76.0
2 61.0 76.0
120 m 1 41.3 76.0
2 55.2 76.0
5.2.4. The Effect of Mining Height to Protected Coal Seam Figure 14 shows the pressure relief coefficient of protected coal seam with different mining heights as a function of face advance. It is the results of Models 1 and 3. The peak value of pressure relief coefficient is 0.12 and 0.14 respectively for 1 m mining height and 3 m when the coal seam is mined 30 m from the start position. The value of 3 m mining height increases by 17% than that of 1 m mining height, and both of the peak values are at 17 m behind the coal face. The peak values are 0.21 and 0.28 respectively for 1 m and 3 m mining height when the coal seam is mined 50 m, the value of 3 m mining height increases by 33%, while these values become 0.23%, 0.24%, and 4.3% respectively when the coal seam is mined 70 m, and the peak values are equal for 1 m and 3 m mining height when the coal face in mined 120 m. According to the above data, we can draw the following conclusion: the mining height of protective coal seam mainly has an effect to the protected coal seam at the beginning of the mining process, the effect to protected coal seam decreases and even becomes very little when the coal seam is mined a certain distance.
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conclusion: the mining height of protective coal seam mainly has an effect to the protected coal seam Processes 2018, 6, 146 of the mining process, the effect to protected coal seam decreases and even becomes 15 of 21 at the beginning very little when the coal seam is mined a certain distance.
Figure 14. Pressure relief coefficient of protected coal seam with different mining height for different Figure 14. Pressure relief coefficient of protected coal seam with different mining height for different face face advance. advance.
In order to further quantitatively determine the regularities of mining height of protective coal to further quantitatively determine regularities mining height of protective coal seamIn to order the stress relief and range of protected coalthe seam, the reliefofangles along strike with different seam toheights the stress and face range of protected coal seam, relief angles along strike with different mining forrelief different advances are obtained bythe Model 1 and Model 3 (Table 3). As can be mining heights for different face advances are obtained by Model 1 and Model 3 (Table 3). can seen from Table 5, the relief angles decrease with face advance. In the set-up room side, theAs relief be seendecrease from Table 5, the decrease with face advance. the advance set-up room side, relief angles from 82°◦relief for 30angles m face advance to 41.3° for 120 mInface when thethe mining ◦ for 120 m face advance when the mining angles decrease from 82 for 30 m face advance to 41.3 height is 3 m, and the relief angles are a little larger compared with 1 m mining height at the same height is 3The m, and the relief anglesless are aand little larger with 1 m mining height at theofsame condition. difference becomes less withcompared further face advance. The relief angles two condition. The difference becomes less andthe lesscoal with further face advance. Theinrelief angles two different mining heights are the same when seam is mined 120 m; while the coal faceofside, different mining heights are the same when the coal seam is mined 120 m; while in the coal face side, the relief angle of 3 m mining height is always larger than 1 m mining height with face advancing. the relief angle of 3 m mining height is always larger than 1 m mining height with face advancing. Table 5. Relief angles along strike with different mining height for different face advance. Table 5. Relief angles along strike with different mining height for different face advance. Face Advance 30 m 50 m 70 m 120 m Ming 3 30 m 1 3 50 m 1 370 m 1 3120 m 1 Faceheight/m Advance Set-upMing room/° height/m Coal face/° Set-up room/◦ Coal face/◦
82.0 3 82.0 82.0 82.0
74.5 1 77.5 74.5 77.5
74.5 3 80.6 74.5 80.6
5.2.5. Pressure Relief Scope in Protected Coal Seam
71.6 1 76.0 71.6 76.0
64.7 3 77.5 64.7 77.5
63.5 1 74.5 63.5 74.5
41.3 41.3 3 1 76.0 68.1 41.3 41.3 76.0 68.1
the lower protective coal seamCoal (No. 8 seam) mining will generate stress relief in low 5.2.5.As Pressure Relief Scope in Protected Seam permeability protected coal seams (No. 3 seam), the adsorbed gas will desorb and flow into the As the lower protective coal seam (No. 8 seam) mining will generate stress relief in low fracture network as the effective stress decreases and new fracture system growth occurs. The permeability protected coal seams (No. 3 seam), the adsorbed gas will desorb and flow into the fracture cross-measure boreholes are constructed as an artificial channel because the desorbed gas cannot network as the effective stress decreases and new fracture system growth occurs. The cross-measure flow out of the stress relief coal seams independently. A good understanding of stress-relief area boreholes are constructed as an artificial channel because the desorbed gas cannot flow out of the stress with protective coal seam mining therefore is the key point for high efficiency gas drainage. The relief coal seams independently. A good understanding of stress-relief area with protective coal seam simulation results are capable of helping us identify the stress relief area of target coal seam prior to mining therefore is the key point for high efficiency gas drainage. The simulation results are capable of mining operation. According to the results of the four Flac3D models, the pressure relief scope in helping us identify the stress relief area of target coal seam prior to mining operation. According to the protected coal seam can be calculated (Table 6), both the pressure relief scope and location in results of the four Flac3D models, the pressure relief scope in protected coal seam can be calculated protected coal seam changes with the excavation of protective coal seam, so the boreholes can be (Table 6), both the pressure relief scope and location in protected coal seam changes with the excavation dynamically connected and disconnected from the gas drainage system to get high-concentration of protective coal seam, so the boreholes can be dynamically connected and disconnected from the gas gas. drainage system to get high-concentration gas.
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Table 6. Pressure relief scope in protected coal seam. Processes 2018, 6, x FOR PEER REVIEW
Face Advance
30 m
50 m
70 m
100 m
120 m
Table 6. Pressure relief scope in protected coal seam. Ming 3 1 3 1 3 1 3 1 height/m Face Advance 30 m 50 m 70 m 100 m Gob 1/m 27 19 41 36 52 49 73 – Ming height/m 3 1 3 1 3 1 3 1 Gob 2/m 27 – 41 – – – 78 – Gob 1/m Gob 2/m
27 27
19 –
41 41
36 –
52 –
49 –
73 78
3
15 of 19
1
120 m 77 72 3 1 93 – – 77 72 – 93 –
5.3. Gas Drainage Program 5.3. Gas Drainage Program
A field study was carried in protected longwall face 4306 and protective longwall face 84306 A field study was carried protected longwall face 84306 for face for the drainage of desorbed gasinand elimination offace coal4306 andand gasprotective outburstlongwall risks. 4036 working the drainage of desorbed gas and elimination of coal and gas outburst risks. 4036 working face has has three roadways (Figure 15): 43061, 43062 and 43063 (original 43042). As 43063 is the original three roadways (Figure 15): 43061, 43062 and 43063 (original 43042). As 43063 is the original roadway of 4304 working face, 43061 and the 43042 roadway still need to be excavated. According to roadway of 4304 working face, 43061 and the 43042 roadway still need to be excavated. According to 3 the field data,data, 4306 working face has of 7.66–13.25 7.66–13.25mm methane emission 3/t. /t. the field 4306 working face hasa amethane methane content content of TheThe methane emission 3 content during the excavation of 43061 and 43062 roadways is 2.25–6.32 m /min and 2.35–6.34 m3 /min, 3 content during the excavation of 43061 and 43062 roadways is 2.25–6.32 m /min and 2.35–6.34 respectively. The prediction methaneof emission in content 4306 face about m3 /min. m3/min, respectively. Theofprediction methanecontent emission in area 4306 is face area21.2–37.38 is about 21.2– 37.38 m3/min. combination gas drainage program of gas drainage while driving drilling the roadway, A combination gasAdrainage program of gas drainage while driving the roadway, boreholes boreholes along coal seam gasTunnel drainage by floor Tunnel 2 after stress-relief are usedoutburst to alongdrilling coal seam and gas drainage byand floor 2 after stress-relief are used to eliminate eliminate outburstface. risk The in 4306 working face. and The boreholes roadway layout and boreholes parameters are15. risk in 4306 working roadway layout parameters are shown in Figure shown in Figure 15.
Figure 15. Illustration of gas drainage during and after roadway excavation.
Figure 15. Illustration of gas drainage during and after roadway excavation.
5.3.1. Gas Drainage While Driving the Roadway and Drilling Boreholes along Coal Seam
5.3.1. Gas Drainage While Driving the Roadway and Drilling Boreholes along Coal Seam
Areas A and B in the Figure 15 represent the process of gas drainage while driving the roadway and drilling seam, the respectively. The of floor Tunnelthe 1 and Areas A andboreholes B in the along Figurethe 15 coal represent process of gasconstruction drainage while driving roadway cross-measure boreholes was completed prior to gas drainage. The drilling of the borehole can1 and and drilling boreholes along the coal seam, respectively. The construction of floor Tunnel increase plasticity of coal mass, reduce outburst prevention indexes effectively, and make sure cross-measure boreholes was completed prior to gas drainage. The drilling of the borehole can the increase roadway tunnel safely and effectively.
plasticity of coal mass, reduce outburst prevention indexes effectively, and make sure the roadway tunnel safely and effectively.
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The results of gas drainage by floor Tunnel 1 are shown in Table 7. It is proven through field application that draining methane while driving cannot only effectively prevent gas and coal outburst from occurring and reduce methane emission in the roadway, but also greatly reduce the frequency to adopt technical measures against outburst and raise the advancing speed. Table 7. The results after gas drainage by floor tunnel 1. No.
Parameters
1 2
Average gas content/m3 /t Maximum gas emission during roadway excavation/m3 /min Average value Gas outburst prevention index K1 Exceed or not Excavation footage/m Roadway excavation speed Weather gas affect roadway excavation or not
3 4
Before Gas Drainage
After Gas Drainage
12.41 7.5 0.4–0.42 Y 2.4
7.68 3.48 0.32–0.33 N 5–7
Y
N
5.3.2. Gas Drainage by Floor Tunnel 2 after Stress-Relief As the high initial stress, low permeability and low hardness condition of protected coal seam, the gas drainage by borehole along coal seam cannot meet the safety mining requirements of protected coal seam. Further gas drainage must be done before excavating the protected coal seam. The floor tunnel is therefore driven for further gas drainage during the excavation of the lower protective coal seam (longwall face 84306). The floor tunnel is located in the K7 fine sandstone of No. 3 coal seam floor at a depth of 6 m. The cross-measure boreholes are drilled through the full thickness of the No. 3 coal seam. The horizontal and vertical cross-sectional views of the protected longwall face are shown in Figure 16. The spacing of adjacent holes and rows is 10 m × 5 m in length and width. Each borehole is attached to the gas drainage system immediately after drilling. The range of boreholes is 140 m × 900 m in width and length. There are a total of 18 gas drainage units in 4306 longwall face, every unit has 10 groups of boreholes, the quantity of boreholes is 15 and 14 respectively for single array and double array (Figure 16a). The borehole parameters of 4306 floor tunnel 2 are listed in Table 8. According to the results of Table 6, for different mining height and face advance of lower protective coal seam, the pressure relief scope in protected coal seam is different. As such, the results of Table 6 can be used to guide the gas drainage scope along the strike. This will highly improve gas drainage efficiency and reduce unnecessary work.
respectively for single array and double array (Figure 16a). The borehole parameters of 4306 floor tunnel 2 are listed in Table 8. According to the results of Table 6, for different mining height and face advance of lower protective coal seam, the pressure relief scope in protected coal seam is different. As such, the results Processes 2018, 6, 146 18 ofgas 21 of Table 6 can be used to guide the gas drainage scope along the strike. This will highly improve drainage efficiency and reduce unnecessary work.
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(a)
(b) Figure 16. Diagram Diagramofof spatial configuration the protected longwall and stress-relief gas Figure 16. thethe spatial configuration of theofprotected longwall and stress-relief gas drainage. drainage. (a) Horizontal plane view; (b) vertical section view. (a) Horizontal plane view; (b) vertical section view. Table 8. The parameters of of 4306 4306 floor floor tunnel tunnel 2. 2. Table 8. The borehole borehole parameters
No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Singular Group Group No. Singular Dip/i Azimuth/z Depth/m Dip/i Depth/m 1 14 Azimuth/z 270 70.5 214 16 60.5 270270 70.5 270270 60.5 316 19 51 270270 51 419 23 41.5 270270 41.5 523 29 32.5 29 270 32.5 6 41 270 24.5 41 270 24.5 761 61 18.5 270270 18.5 857 57 19 177177 19 959 59 18.5 9090 18.5 9090 25 1040 40 25 9090 32.5 1129 29 32.5 23 90 41.5 12 23 90 41.5 18 90 51 1316 18 51 9090 60.5 1414 16 60.5 9090 70.5 15 14 90 70.5
Even Group Even Group No. Dip/i Azimuth/z Depth/m 1No. 15 Dip/i270Azimuth/z 65.5 Depth/m 2 1 3 2 4 3 5 4 5 6 6 7 7 8 8 9 9 1010 1111 12 12 13 1314 14
18 21 26 34 51 56 55 48 34 26 20 17 15
15 18 21 26 34 51 56 55 48 34 26 20 17 15
270 270 270 270 270 200 156 90 90 90 90 90 90
270 270 270 270 270 270 200 156 90 90 90 90 90 90
56 46.5 37 28 20.5 19.5 19.5 21.5 28.5 37 46.5 56 65.5
65.5 56 46.5 37 28 20.5 19.5 19.5 21.5 28.5 37 46.5 56 65.5
6. Conclusions 6. Conclusions The mining of protective coal seam is wildly used in the multiple coal seams in China to eliminate The mining of protective coal seam is wildly used in the multiple coal seams in China to or at least mitigate the coal and gas outburst. In this study, a 3D numerical model has been established eliminate or at least mitigate the coal and gas outburst. In this study, a 3D numerical model has been with a consideration of the gob loading characteristics, to make a better understanding of the mining established with a consideration of the gob loading characteristics, to make a better understanding of the mining influence of lower protective coal seam to the protected coal seam. The gob characteristics are developed by H. Yavuz’s method and calibrated by the realistic VSCF value ahead of coal face. The model can be easily changed for specific coal mine gob loading behaviors based on field measurements and has the potential to realistically model longwall mining. The stress recovering distance in the gob and mining height of protected coal seam were selected as the
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influence of lower protective coal seam to the protected coal seam. The gob characteristics are developed by H. Yavuz’s method and calibrated by the realistic VSCF value ahead of coal face. The model can be easily changed for specific coal mine gob loading behaviors based on field measurements and has the potential to realistically model longwall mining. The stress recovering distance in the gob and mining height of protected coal seam were selected as the important variables to assess their influence on the stress relief of the protected coal seam. The stress relief coefficient and relief angle were introduced as two important parameters to evaluate the stress relief effect in different regions of the protected coal seam. Finally, the reasonable gas drainage programs were provided. The main results of this study are summarized below. (1) “O” circle was found in the gob after the mining of protective coal seam. The area closes to pillar and coal face made up of the main macro-fractures for gas flow. (2) The overburden of protective coal seam can be divided into 5 zones according to the distribution of stress and displacement in the mining horizon: the original zone, compression zone, expansion zone, recovering zone, and re-compacted zone; the expansion zone had largest permeability compared with other areas and was the best place for gas drainage. (3) The peak value of VSCF ahead of coal face increased with the increase of the stress recover distance; it decreased with mining height. (4) For the same mining height with different gob behaviors of protective coal seam, the relief angles experienced the increase, decrease and stabilization process with face advancing in the coal face side; the relief angle was almost the same in the face area for different gob behaviors, indicating that it has little effect to the pressure relief of the face area. (5) For the same gob behaviors with different mining height of protective coal seam, the mining height of protective coal seam mainly has an effect to the protected coal seam at the beginning of the mining process. This effect decreases and becomes very little when the coal seam is mined a certain distance. And the relief angle of 3 m mining height is always larger than 1m mining height with face advancing on the face side. (6) Pressure relief scope in protected coal seam was acquired under specific mining condition. A combination gas drainage programs were provided for the case working face. The gas content reduced from 12.41 m3/t to 7.68 m3/t in 15 m range of roadway. It effectively prevented gas and coal outburst from occurring and reduced methane emission in roadway, and raised the advancing speed at the same time. And the research results can be used to guide the scope of gas drainage and highly improve gas drainage efficiency and reduce unnecessary work. More field data need to be collected for further research. Author Contributions: C.Z. and Y.Z. conceived the main idea of the paper and designed the numerical model; C.Z. performed the numerical model; C.Z. and L.Y. analyzed the data; Y.Z. and R.F. contributed analysis tools and theoretical analysis; C.Z. wrote the paper; R.F. and G.Z. did a lot work to modify figures and proofread the revised version. Funding: This paper was supported by the State Key Research Development Program of China (Grant No. 2016YFC0600708), CUMTB fund for creative PHD Student Project (00-800015z685), Tiandi Technology Co., Ltd. Technology Innovation Fund, China Scholarship Council (CSC), Zhongguancun Ten Hundred Thousand Special Project-7m High Mining Support Technology and Equipment (KJ-2017-GJGC-01). The authors would like to cordially acknowledge excellent guidance of Y.P. Chugh in Southern Illinois University Carbondale and G. Song in North China University of technology. The authors would also like to express special thanks to the editor and anonymous reviewers for their professional and constructive suggestion. In addition, C.Z. wants to thank, in particular, the patience, support and love from Y.Z. over the past years. Conflicts of Interest: The authors declare no conflict of interest.
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