Risk Assessment and Control Strategy of Residual Coal Pillar in Room Mining: Case Study in Ecologically Fragile Mining Areas, China

sustainability

Article Risk Assessment and Control Strategy of Residual Coal Pillar in Room Mining: Case Study in Ecologically Fragile Mining Areas, China

Hengfeng Liu 1, Qiang Sun 1,2,*, Nan Zhou 1 and Zhongya Wu 1

1 State Key Laboratory of Coal Resources and Safe Mining, School of Mines, China University of Mining & Technology, Xuzhou 221116, China; [email protected] (H.L.); [email protected] (N.Z.); [email protected] (Z.W.) 2 State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining & Technology, Xuzhou 221116, China * Correspondence: [email protected]; Tel.: +86-152-6201-0542

Abstract: Gradual instability of coal pillars left behind underground with room mining is one of the main reasons for sudden roof caving in the gob, surface subsidence, and other significant hazards. Moreover, room mining implies great losses of coal resources. In this paper, the main failure mode and room mining coal pillar process were analyzed according to the coalfield regional engineering geological and hydrogeological conditions. A numerical model was adopted to study the effect of different sizes of coal mining pillars and progressive instability failure of coal pillar on the plastic zone’s evolution characteristics and stress field of coal pillars in the stope. The proposed technologies of cemented paste backfilling and reinforcement of residual coal pillars are applied, and a numerical simulation model is established to study the strata movement characteristics and analyze the stability degree of residual coal pillar and key aquiclude strata in the Pliocene series of Neogene. Consequently, Citation: Liu, H.; Sun, Q.; Zhou, N.; the performance and application prospect were evaluated. The results obtained substantiate a new Wu, Z. Risk Assessment and Control method for the long-term stability control of coal pillars in room mining and protecting the ecological Strategy of Residual Coal Pillar in environment in China’s western eco-environmental frangible area. Room Mining: Case Study in Ecologically Fragile Mining Areas, Keywords: room mining; coal pillar stability; numerical modeling; coal pillar reinforcement; envi- China. Sustainability 2021, 13, 2712. ronmental protection https://doi.org/10.3390/su13052712

Academic Editor: Francesco Faccini

Received: 28 January 2021 1. Introduction Accepted: 26 February 2021 The water resources of mining areas in western China are extremely short [1]. The Published: 3 March 2021 degree of land desertification is high, the surface is covered with thick aeolian sand, the ecological environment system is very fragile, and soil erosion is very serious [2]. At the Publisher’s Note: MDPI stays neutral end of the 20th century, the room and pillar (R&P) mining method was widely used in the with regard to jurisdictional claims in western coal resources, and many coal pillars remained in the goaf [3,4]. With the mining published maps and institutional affil- area’s extension and passage of time, the remaining coal pillars’ fracture and instability iations. will cause the goaf roof to collapse in a large area, inducing mine tremors and shock waves. The goaf surface has a large number of subsidence areas. At the same time, it is easy to cause the mining fractures of overburden rock to expand from the bottom to the top and directly communicate with the surface, resulting in the leakage of shallow Copyright: © 2021 by the authors. water and groundwater [5]. This would destroy the vegetation on the surface and imply Licensee MDPI, Basel, Switzerland. land desertification and other environmental hazards, deteriorating the originally fragile This article is an open access article ecological environment system [6–8] (Figure1). The long-term stability control of coal distributed under the terms and pillars is one of the main challenges in room mining [9]. Therefore, this issue must be conditions of the Creative Commons addressed to protect the economic and social development of western China. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Sustainability 2021, 13, 2712. https://doi.org/10.3390/su13052712 https://www.mdpi.com/journal/sustainability Sustainability 2021,, 13,, 2712x FOR PEER REVIEW 2 of 13

Figure 1. SchematicSchematic diagram e environmentalnvironmental destructions and problems caused by room mining.mining.

InIn the the past past decades, decades, the the b backfillackfill mining mining technology, technology, which which offers offers economic economic and and envi- en- ronmentalvironmental benefits, benefits, has has been been used used extensively extensively in in China China’s’s underground underground mines. mines. An An [[10]10] presented the framework and detailed method to retreat room mining standing pillars with consolidated consolidated solid solid backfilling backfilling mining mining (SBM). (SBM) Alternatively,. Alternatively, Fang Fang [11 ][11] proposed proposed a new a newmethod method to extract to extract the remaining the remain coaling pillar coal by pillar replacing by replacing it with anit artificialwith an pillarartificial based pillar on basedSBM technology. on SBM technology. This approach This envisaged approach a comprehensive envisaged a comprehensive account of cemented account material’s of ce- mentedmechanical material characteristics,’s mechanical the characteristic developments, the of an development overlying strata of an supportoverlying system, strata sup- and portstrata system, behavior and under strata different behavior sizes under and different intervals size ofs artificial and interval pillars,s of which artificial should pillars be, whichvalidated should by laboratory be validated tests, by theoretical laboratory analysis, tests, theoretical and numerical analysis simulation, and numerical results. Finally, simu- lationit implied results the. Finally engineering, it implied design the of engineering recovering roomdesign coal of pillarsrecovering based room on thecoal western pillars basedChina on mining the western area’s China geological mining conditions. area’s geological A previous conditions. study, A which previous included study, two which co- includedauthors of two the co present-authors paper of the [12 present], investigated paper [12] the, plasticinvestigated zone and the stressplastic evolution zone and of stress coal evolutionpillar in stope of coal during pillar short-stripin stope during coal short pillar-strip recovery coal pillar with cementedrecovery with paste cemented backfill (CPB)paste backfilland proposed (CPB) and to utilize proposed the remainingto utilize the coal remaining pillar with coal aeolian pillar sandwith asaeolian the main sand backfill as the mainmaterial. backfill Aiming material at the. Aiming treatment at the of permanent treatment of ground permanent fractures gro causedund fractures by mining, caused based by mining,on the test based of physical on the test and of mechanical physical and properties mechanical of ultra-high-water properties of ultra materials,-high-water the fieldma- terials,filling technologythe field filling and fillingtechnology system and of ultra-high-waterfilling system of materialsultra-high were-water also materials developed. were A alsothree-step developed. method A three of ground-step method fissure treatmentof ground of fissure “deep treatment filling, surface of “deep soil filling, covering, surface and soilvegetation covering, greening” and vegetation was put greening forward” by was Liu put et al.forward [13]. by Liu et al. [13]. However, because of the difference between the backfill materials’ and filling costs, no However, because of the difference between the backfill materials’ and filling costs, rapid backfilling coal mining method suitable for wide use in China’s local coal mines has no rapid backfilling coal mining method suitable for wide use in China’s local coal mines been proposed and validated yet. Backfill materials (tailings and gangues) are not very has been proposed and validated yet. Backfill materials (tailings and gangues) are not abundant because of the special mining and geological conditions in western China. The very abundant because of the special mining and geological conditions in western China. resultant high cost of backfill greatly limits the SBM application. Because aeolian sand is The resultant high cost of backfill greatly limits the SBM application. Because aeolian sand found abundantly in the local mining area, it is considered quite expedient to use CPB is found abundantly in the local mining area, it is considered quite expedient to use CPB materials associated with aeolian sand to control residual coal pillars’ long-term stability. materials associated with aeolian sand to control residual coal pillars’ long-term stability. In this study, the influencing factors of coal pillar stability in room mining are analyzed. In this study, the influencing factors of coal pillar stability in room mining are analyzed. The evolution patterns of stresses and plastic zone of coal pillars in stope are studied. The The evolution patterns of stresses and plastic zone of coal pillars in stope are studied. The regional reinforcement method of coal pillar is proposed, and a relevant case study proves regionalits feasibility. reinforcement method of coal pillar is proposed, and a relevant case study proves its feasibility. 2. Background 2.2.1. Background The Coal Mine Case Study 2.1. TThehe Coal case Mine study Case is Study performed for the coal mine located in the Dongsheng Coalfield, InnerThe Mongolia case study Autonomous is performed Region, for the China, coal mine which located belongs in tothe the Dongsheng North China Coalfield, strati- Innergraphic Mongolia region and Autonomous Ordos stratigraphic Region, division.China, which Most ofbelongs the coalfield to the is North covered China by aeolian strati- graphicsand and region loess. and According Ordos stratigraphic to the ground division. geology Most and drillingof the coalfield data, the is stratacovered in theby aeo- area lian sand and loess. According to the ground geology and drilling data, the strata in the Sustainability 2021, 13, x FOR PEER REVIEW 3 of 13

Sustainability 2021, 13, 2712 3 of 13

area are, from old to new: Upper Triassic Yanchang Formation (T3y), Middle-Lower Juras-

sic Yan’an formationare, from (J1-2y old), Middle to new: Jurassic Upper Zhiluo Triassic Formation Yanchang (J Formation2Z), Middle(T Jurassic3y), Middle-Lower And- Jurassic ing formation (JYan’an2A), Upper formation Jurassic (J Lower1-2y), Middle Cretaceous Jurassic Zhidan Zhiluo group Formation (J3-K1zh), (J T2Zertiary), Middle (N2) Jurassic, Anding 2 and Quaternaryformation (Q), respectively (J2A), Upper. The Jurassiccoalfield Lowerarea is Cretaceous20.96 km , 6.54 Zhidan km grouplong from (J3-K east1zh), Tertiary (N2), to west and 4.665and km Quaternary wide from (Q), north respectively. to south. The coalfield area is 20.96 km2, 6.54 km long from east to The main westcoal seam and 4.665 is located km wide in the from lower north part to south. of the middle and lower Jurassic Yan’an formation (J1The-2y). mainThe coal coal seam seam thickness is located is in 5.3 the5– lower6.95 m, part with of the an middleaverage and of 6. lower0 m. Jurassic Yan’an The coal seam structureformation is (J simple,1-2y). The without coal seam gangue thickness or with is 1 5.35–6.95–2 layers m,of withgangue. an averageThe li- of 6.0 m. The thology is generallycoal seam mudstone structure and is carbonaceous simple, without mudstone. gangue orThe with coal 1–2 seam layers’s roof of gangue.lithol- The lithology ogy is sandy mudstone,is generally mudstone, mudstone fine and sandstone, carbonaceous and siltstone mudstone.. The The floor coal lithology seam’s is roof lithology is siltstone, mudstone,sandy and mudstone, sandy mudstone. mudstone, The fine compressive sandstone, and strength siltstone. of the The minable floor lithology coal is siltstone, seam’s immediatemudstone, roof is between and sandy 17.4 mudstone.and 19.2 MPa, The and compressive that of the strengthfloor ranges of the between minable coal seam’s 16.1 and 17.2 MPa.immediate Although roof the is betweenmain soft 17.4 rock and is soft, 19.2 the MPa, structural and that plane of the is floornot devel- ranges between 16.1 oped, and the androck 17.2quality MPa. designation Although the(RQD main) value soft rockis between is soft, the50 and structural 90%. The plane rock is not developed, quality is good,and and the the rock rock quality mass is designation relatively complete. (RQD) value The is room between coal 50mining and 90%. method The rock quality is was adopted forgood, coal andproduction the rock and mass the is coal relatively pillar complete.support method The room was coal utilized mining for method roof was adopted management. Afterfor coal the productioncoal mine under and the study coal was pillar put support into operation, method was8.21 utilizedmillion tons for roof of management. raw coal have beenAfter mined, the coal and mine the under recovery study rate was was put only into 31%. operation, Furthermore, 8.21 million large tons coal of raw coal have pillars were leftbeen behind mined, in the and mining the recovery area, some rate of was them only being 31%. unstabl Furthermore,e at the largeearly coalmin- pillars were left ing stage, and thusbehind, they in failed the mining after long area,-term some loading of them. In being some unstable areas, the at roof the earlyhad even mining stage, and caved into the gob.thus, Given they failedthis, it afteris critical long-term to ensure loading. the coal In somepillar’ areas,s stability the roofin the had mined even- caved into the out area to achievegob. sustainable Given this, production it is critical of to the ensure mine the, protect coal pillar’slocal water stability resources in the, and mined-out area to satisfy other environmentalachieve sustainable requirements production. The of panel the mine, layout protect of the local study water area resources,is depicted and satisfy other in Figure 2. environmental requirements. The panel layout of the study area is depicted in Figure2.

N Legend Fresh air Boundary line No. Columnar Lithology Thickness Description （m） Dirty air Coal pillar Grayish yellow, mainly loam, 7 Topsoil 24.00 containing a small amount of silt

Fine Grayish yellow fine-grained 6 sandstone 8.00 sandstone, containing mica fragments and dark minerals, mainly quartz

Sandy Gray siltstone with thin layer of fine 5 mudstone 15.00 sandstone, horizontal bedding and Study area small cross bedding Gray white thin-layer fine-grained Fine 4 5.00 feldspathic quartz sandstone, mainly sandstone quartz, followed by feldspar quartz partition coal pillar

Gray, dark gray thin bedded siltstone 3 Siltstone 20.00 with fine sandstone, horizontal bedding and small cross bedding

Fine Gray white sandstone, mainly quartz, 2 sandstone 12.00 followed by feldspar quartz, contains mica fragments and dark minerals

Gray black, call a small amount of 1 Mudstone 2.00 coal line and carbon chips

0 Coal seam 6.00 Black, mainly dark coal, light coal next, more peat

1 Siltstone 8.00 Gray, easily weathered, with closed fissures and calcite

Gray white sandstone, mainly quartz, Medium 2 22.00 contains mica fragments and dark

209 panel roadway sandstone 210 panel roadway 208 panel roadway minerals (a) (b) FigureFigure 2. 2.The The panelpanel layoutlayout ofof thethe studystudy area: area:( a(a)) plane plane view; view; ( b(b)) the the generalized generalized stratigraphic stratigraphic column. column. 2.2. Failure Mode and Process of Room Mining Coal Pillar 2.2. Failure Mode and Process of Room Mining Coal Pillar With the coal seam’s continuous room mining, the coal pillar’s lateral stress is gradu- With the coalally seam relieved,’s continuous and the pressure room mining, of the the overlying coal pillar strata’s lateral is gradually stress is transferred grad- to the coal ually relieved, andpillar, the increasing pressure of compressive the overlying stresses strata andis gradually strains. transferred The coal pillar’s to the overallcoal response to pillar, increasingthe compre stressssive redistribution stresses and in roomstrains mining. The coal depends pillar’s on overall the dimensions response to (length, the width, and stress redistributionheight), in shape, room and mining internal depends structure on ofthe the dimensions coal pillar. (length, According width, to the and ﬁeld investigation SustainabilitySustainability 2021 2021, 13, 13, x, FORx FOR PEER PEER REVIEW REVIEW 4 of4 of13 13 Sustainability 2021, 13, 2712 4 of 13

height),height), shape, shape, and and internal internal structure structure of of the the coal coal pillar. pillar. According According to to the the field field investigation investigation resultsresults,results, there, there are are mainly mainly six six kinds kinds of of failure failure modes modes of of room room coal coal pillars pillars [14], [[14],14 ],as asas shown shownshown in inin FigureFigure 33. 3..

FigureFigure 3. 3. Main MainMain failure failure failure modes modes modes of of coal ofcoal coalpillars: pillars: pillars: (a )( al)ateral l (aterala) lateral spalling; spalling; spalling; (b ()b c)ompression compression (b) compression-shear-shear-shear failure; failure; failure;(c )( c) splitting(scplitting) splitting inside inside inside pillar; pillar; pillar; (d ()d joint) (jdoint) jointfailure; failure; failure; (e )( ef)lexural (felexural) ﬂexural failure; failure; failure; (f )( fc)ollapse (cfollapse) collapse damage damage damage.. .

TheThe room room room mining mining mining coal coal coal pillar pillar pillar’s’s ’instabilitys instability instability and and andfailure failure failure occur occur occur as as a agradual as gradual a gradual process process process,, de-, de- picteddepictedpicted in in Figure Figure in Figure 4. 4According. According4. According to to the the to distribution distribution the distribution pattern pattern pattern of of stress stress of (vertical stress (vertical (vertical direction) direction) direction) in in the the coalincoal thepillar, pillar, coal a pillar,atoroidal toroidal a toroidaltype type (Figure (Figure type (Figure4b, 4b,c) c)is 4isab,c) atypical typical is a typicalsign sign of of signthe the coal of coal the pillar pillar coal’s pillar’s ’stables stable state. stable state. Whenstate.When the Whenthe toroidal toroidal the toroidaltype type is istransformed typetransformed is transformed into into the the central into central the platform platform central type platform type (Figure (Figure type 4d), 4d), (Figure the the coal 4coald), pillarthepillar coalenters enters pillar the the accelerated enters accelerated the accelerated failure failure stage, stage, failure making making stage, the the makingcentral central platform the platform central type type platform an an important important type an turningimportantturning point point turning in in the the cr point criticalitical in state thestate of critical of instability. instability. state of With With instability. a graduala gradual With reduction reduction a gradual of of the reduction the elastic elastic core of core the areaelasticarea of of the corethe coal coal area pillar, pillar, of thethe the coallatter latter pillar, undergoes undergoes the latter five five undergoesstates states from from ﬁvethe the initial states initial one from one to to theinstability instability initial one[15] [15], to , whichinstabilitywhich features features [15 the], the which corresponding corresponding features thefive five corresponding stress stress distribution distribution ﬁve patterns: stress patterns: distribution wide wide toroidal toroidal patterns: type, type, nar- wide nar- rowtoroidalrow toroidal toroidal type, type, type, narrow central central toroidal platform platform type, type, type, central sharp sharp platform peak peak type type,type (Figure sharp(Figure peak4e), 4e), and typeand blunt (Figureblunt peak peak4e), type andtype (Figureblunt(Figure peak4f). 4f). type (Figure4f).

FigureFigure 4. 4.Failure Failure process processprocess of of ofroom roomroom mining mining mining coal coal coal pillar pillar: pillar: ( (a: a)() ainitial initial) initial state state, state, ( (b, ())b wide ) wide toroidal toroidal type,type type, ((cc,) )( narrowcnar-) narrow toroidal type, (d) central platform type, (e) sharp peak type, (f) blunt peak type. rowtoroidal toroidal type, type (d,) ( centrald) central platform platform type, type (e,) ( sharpe) sharp peak peak type, type (f), ( bluntf) blunt peak peak type. type.

SomeSome researchers researchers [16 [[1616,17,171]7 reported] reported that that the the coal coal pillar pillarpillar under underunder the thethe toroidal toroidaltoroidal type typetype load loadload modemode has has has high high high stability, stability, stability, and and and the the the central central central platform platform platform type type type load load load form form form is isthe the is turning theturning turning point point pointbe- be- tweenbetweentween the the coal the coal coalpillar’s pillar’s pillar’s stability stability stability and and andinstability. instability. instability. A Arelatively Arelatively relatively stable stable stable state state state of of the ofthe coal the coalcoal pillar pillar pillar is is thenisthen then violated, violated, violated, and and andan an accelerated an accelerated accelerated instability instability instability stage stage stage is isobserved. isobserved. observed. Sustainability 2021, 13, x FOR PEER REVIEW 5 of 13

3. Stability Analysis of the Residual Coal Pillar in Room Mining 3.1. Global Model and Simulation Plans The FLAC3D 5.0 software package, developed by Itasca Consulting Group, Inc. (Min- neapolis, MN, USA) and widely used in mining engineering, was implemented in this study using the conventional Mohr-Coulomb model of coal and rock damage [18–20]. Sustainability 2021, 13, 2712 Based on the case study’s geological conditions, the numerical model assumed a length5 of 13of 281 m in the dip direction, a width of 185 m in the strike direction, and a height of 122 m area (Figure 5a). The horizontal displacements of four vertical planes of the model were restricted in the normal direction, and the vertical displacement at the base of the model 3. Stability Analysis of the Residual Coal Pillar in Room Mining was set to zero. The elastic and shear moduli could be derived via the stress-strain rela- 3.1.tionship. Global Different Model and parameters Simulation were Plans employed, adjusted, and scaled in a laboratory to de- 3D termineThe rock FLAC and 5.0coal software samples package,’ mechanical developed properties. by Itasca The final Consulting physical Group, and mechanical Inc. (Min- neapolis,parameters MN, of rock USA) in and numerical widely simulations used in mining are presented engineering, in Table was implemented1. in this studyThe using numerical the conventional simulation Mohr-Coulomb process can be modelreduced of to coal (i) andcalculat rocking damage the initial [18 –state20]. Basedinduced on by the gravity case study’s and (ii) geological simulating conditions, the room the’s excavation. numerical This model paper assumed focuses a length on the ofinfluence 281 min of thedifferent dip direction, sizes of coal a width mining of pillars 185 m and in the progressive strike direction, instability and failure a height of coal of 122pillar m areaon strata (Figure movement5a). The in horizontal the stope. displacements The first mining of fourscheme vertical covers planes mining of theroom models with weresizes restrictedof 7 m × 7 inm, the 9 m normal × 9 m, direction,and 11 m × and 11 m the; a verticalroom pillar displacement of 9 m × 9 atm, the 7 m base × 7 m of, theand model5 m × 5 was m sizes set to is zero. left behind The elastic underground. and shear moduliThe second could mining be derived scheme via i themplies stress-strain a mining relationship.room with a Differentsize of 9 m parameters × 9 m, while were the employed,room pillar adjusted, has the initial and scaled size of in 7 am laboratory × 7 m, which to determineis then reduced rock and to 5 coal m × samples’ 5 m from mechanical the stope properties.middle to the The boundary ﬁnal physical. The andspecific mechanical process parametersof excavation of rocksimulation in numerical is depicted simulations in Figure are 5b. presented in Table1.

(a)

122m

A A

185m Z Y 281m X

(b) 209 Headgate 209 Headgate 209 Headgate B B B

9m 7m 7m B 9m 5m 11m

185m

185m 185m

A A A A A A

Coal pillar size (9m*9m) ACoal pillar size (7m*7m) A Coal pillar size (5m*5m)

Monitoring Monitoring line Monitoring line Monitoring Monitoring line B B B 209 Tailgate 209 TailgateB 209 Tailgate 281m 281m 281m (c) 209 Headgate 209 Headgate 209 Headgate B B B

7m 7m 7m 7m 7m 7m

185m

185m 185m

A A A A A A

Coal pillar size (5m*5m) Coal pillar size (5m*5m) Coal pillar size (5m*5m)

Monitoring Monitoring line Monitoring Monitoring line B B Monitoring line B 209 Tailgate 209 Tailgate 209 Tailgate 281m 281m 281m

FigureFigure 5.5.Numerical Numerical model model and and the the excavation excavation simulation simulation process: process (a: )(a model) model size, size, (b )( theb) the ﬁrst first mining mining scheme, (c) the second mining scheme. scheme, (c) the second mining scheme. Sustainability 2021, 13, 2712 6 of 13

Table 1. Physical and mechanical parameters of rocks in numerical simulations.

Bulk Modulus Shear Modulus Cohesion Tensile Strength Internal Friction Density Strata (GPa) (GPa) (MPa) (MPa) Angle (Degrees) (kg·m−3) Topsoil 0.04 0.02 0.35 0.5 26 1900 Fine sandstone 1.8 1.4 1.2 0.6 22 1600 Sandy mudstone 0.3 0.2 0.5 0.05 30 1880 Fine sandstone 1.8 1.4 1.2 0.6 22 1600 Siltstone 4.5 3.2 2.5 1.1 30 2600 Fine sandstone 5.8 3.8 3.2 2.7 33 2360 Mudstone 4.2 2.6 1.5 2.1 28 2280 Coal seam 3.2 1.8 0.8 1.7 25 1400 Siltstone 4.2 2.6 1.5 2.1 32 2180 Medium Sandstone 6.5 4.8 2.5 2.5 36 2600

The numerical simulation process can be reduced to (i) calculating the initial state induced by gravity and (ii) simulating the room’s excavation. This paper focuses on the influence of different sizes of coal mining pillars and progressive instability failure of coal pillar on strata movement in the stope. The first mining scheme covers mining rooms with sizes of 7 m × 7 m, 9 m × 9 m, and 11 m × 11 m; a room pillar of 9 m × 9 m, 7 m × 7 m, and 5 m × 5 m sizes is left behind underground. The second mining scheme implies a mining room with a size of 9 m × 9 m, while the room pillar has the initial size of 7 m × 7 m, which is then reduced to 5 m × 5 m from the stope middle to the boundary. The specific process of excavation simulation is depicted in Figure5b.

3.2. Numerical Simulation Results The characteristics of the plastic zone and stress distribution in stope with different room pillar sizes of 9 m × 9 m, 7 m × 7 m, and 5 m × 5 m were obtained through numerical simulation, as shown in Figure6. The middle proﬁle stresses in left-behind coal pillars were selected to analyze room mining development and stability, as shown in Figure7. The plastic zone distribution during coal pillar size evolution from 9 m × 9 m to 5 m × 5 m is depicted in Figure8. The numerical simulation of room mining shows that the coal pillar’s failure gradually occurs from the outside to the inside. The coal pillar’s peak stress increases from 6.7 to 10.5 MPa with a size reduction from 9 m × 9 m to 7 m × 7 m. Then, the peak stress drops to 8.0 MPa at the pillar size of 5 m × 5 m. The boundary coal pillar’s peak stress changes from 4.1 to 5.9 MPa and then increases to 7.0 MPa. The reason is that the coal pillar area decreases, and the overlying load increases. When the coal pillar fails, the overlying load is transferred to the boundary pillar. Moreover, compared with the middle part of the coal pillars, the boundary pillars’ failure time is delayed. The characteristics of the plastic zone and stress distribution in stope with the gradual increase of the scope of small-sized coal pillars (5 m × 5 m) were obtained through numerical simulation, as shown in Figure9. The middle proﬁle of left-behind coal pillars’ stress was selected to analyze room mining development and stability, as shown in Figure 10. The plastic zone distribution of boundary coal pillars during the gradual increase of small-size coal pillars’ scope is shown in Figure 11. Sustainability 2021, 13, x FOR PEER REVIEW 6 of 13

Table 1. Physical and mechanical parameters of rocks in numerical simulations.

Sustainability 2021, 13, x FOR PEER REVIEW 7 of 13

to 8.0 MPa at the pillar size of 5 m × 5 m. The boundary coal pillar’s peak stress changes from 4.1 to 5.9 MPa and then increases to 7.0 MPa. The reason is that the coal pillar area FigureFigure 6. Characteristics 6. Characteristics of the of plasticthe plastic zone zone and and stress stress distribution distribution in in the the stopestope with different room room pillar pillar sizes: sizes: (a) ( a9) m 9 × m 9 × 9 m; m; (b) 7 m × 7 m; (c) 5 m × decreases,5 m. and the overlying load increases. When the coal pillar fails, the overlying load (b) 7 m × 7 m; (c) 5 m × 5 m. is transferred to the boundary pillar. Moreover, compared with the middle part of the coal pillars, the boundary pillars’ failure time is delayed.

Peak stress of residual pillar A-A Peak stress of residual pillar B-B

Peak stress of boundary pillar Peak stress of boundary pillar

9m*9m 9m*9m Initial stress Initial stress

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #1 #2 #3 #4 #5 #6 #7 #8

(a)

Peak stress of residual pillar A-A Peak stress of residual pillar B-B

Peak stress of boundary pillar Peak stress of boundary pillar

7m*7m 7m*7m Initial stress Initial stress

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #1 #2 #3 #4 #5 #6 #7 #8

(b)

Peak stress of boundary pillar Peak stress of residual pillar A-A B-B

Peak stress of residual pillar Peak stress of boundary pillar 5m*5m 5m*5m Initial stress Initial stress

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #1 #2 #3 #4 #5 #6 #7 #8

(c)

FigureFigure 7. Proﬁle 7. Profile of stress of stress distribution distribution characteristics characteristics of of coalcoal pillarspillars in the the middle middle of of stope stope with with different different sizes: sizes: (a) 9 (a m) 9× m9 × 9 m; (b) 7m m; (×b)7 7 m;m × ( c7) m 5; m (c)× 5 5mm. × 5 m.

(a) 10 (b) 14 #1 #7

9 12

m 6

6 10

6 m

6 7m 7 m

7m 9m 6

9m m 6

ZZ Stress/MPa ZZ 5m

ZZ Stress/MPa 5m 5m 5m 8 9m 9m 6 7m 7m

5 6 4 5 6 7 8 9 10 4 5 6 7 8 9 10 Coal pillar size/m Coal pillar size/m Figure 8. Plastic zone distribution during the coal pillar size reduction: (a) #1 coal pillar; (b) #7 coal pillar.

The characteristics of the plastic zone and stress distribution in stope with the gradual increase of the scope of small-sized coal pillars (5 m × 5 m) were obtained through numerical simulation, as shown in Figure 9. The middle profile of left-behind coal pillars’ stress was selected to analyze room mining development and stability, as shown in Figure Sustainability 2021, 13, x FOR PEER REVIEW 7 of 13

The numerical simulation of room mining shows that the coal pillar’s failure gradually occurs from the outside to the inside. The coal pillar’s peak stress increases from 6.7 to 10.5 MPa with a size reduction from 9 m × 9 m to 7 m × 7 m. Then, the peak stress drops to 8.0 MPa at the pillar size of 5 m × 5 m. The boundary coal pillar’s peak stress changes from 4.1 to 5.9 MPa and then increases to 7.0 MPa. The reason is that the coal pillar area decreases, and the overlying load increases. When the coal pillar fails, the overlying load is transferred to the boundary pillar. Moreover, compared with the middle part of the coal pillars, the boundary pillars’ failure time is delayed.

Peak stress of residual pillar A-A Peak stress of residual pillar B-B

Peak stress of boundary pillar Peak stress of boundary pillar

9m*9m 9m*9m Initial stress Initial stress

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #1 #2 #3 #4 #5 #6 #7 #8

(a)

Peak stress of residual pillar A-A Peak stress of residual pillar B-B

Peak stress of boundary pillar Peak stress of boundary pillar

7m*7m 7m*7m Initial stress Initial stress

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #1 #2 #3 #4 #5 #6 #7 #8

(b)

Peak stress of boundary pillar Peak stress of residual pillar A-A B-B

Peak stress of residual pillar Peak stress of boundary pillar 5m*5m 5m*5m Initial stress Initial stress

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #1 #2 #3 #4 #5 #6 #7 #8

(c)

Sustainability 2021, 13, 2712 8 of 13 Figure 7. Profile of stress distribution characteristics of coal pillars in the middle of stope with different sizes: (a) 9 m × 9 m; (b) 7 m × 7 m; (c) 5 m × 5 m.

(a) 10 (b) 14 #1 #7

9 12

m 6

6 10

6 m

6 7m 7 m

7m 9m 6

9m m 6

ZZ Stress/MPa ZZ 5m

ZZ Stress/MPa 5m 5m 5m 8 9m 9m 6 7m 7m

Sustainability 2021, 13, x FOR PEER REVIEW 8 of 13 5 6 4 5 6 7 8 9 10 4 5 6 7 8 9 10 Coal pillar size/m Coal pillar size/m FigureFigure 8. 8. PPlasticlastic zone zone10. distribution distribution The plastic during during zone the thedistribution coal coal pillar size of boundary reduction reduction:: coal( (aa)) #1 #1 pillars coal coal pillar; pillar; during ( (b)) #7 #7the coal coal gradual pillar pillar.. increase of small-size coal pillars’ scope is shown in Figure 11. The characteristics of the plastic zone and stress distribution in stope with the gradual increase of the scope of small-sized coal pillars (5 m × 5 m) were obtained through numerical simulation, as shown in Figure 9. The middle profile of left-behind coal pillars’ stress was selected to analyze room mining development and stability, as shown in Figure

Figure 9.9. Characteristics ofof thethe plastic plastic zone zone and and stress stress distribution distribution in in stope stope with with a gradual a gradual increase increase in the in scopethe scope of small-sized of small- coalsize pillars:d coal (pillarsa) 4 coal: (a pillars) 4 coal withpillars 5 m with× 5 5 m; m (×b 5) 16m;coal (b) 16 pillars coal withpillars 5 mwith× 55 m;m × (c 5) 36m; coal(c) 36 pillars coal pillars with 5 with m × 55 m m. × 5 m.

Peak stress of residual pillar A-A Peak stress of residual pillar B-B