Ground Response and Mining-Induced Stress in Longwall ...

Research ArticleGround Response and Mining-Induced Stress in LongwallPanel of a Kilometer-Deep Coal Mine

Zhaohui Wang 1 Shengli Yang 2 Guoliang Xu 1 and Zhijie Wen 3

1School of Energy and Mining Engineering China University of Mining and Technology Beijing 100083 China2Coal Industry Engineering Research Center of Top Coal Caving Mining China University of Mining and TechnologyBeijing 100083 China3Key Laboratory of Mining Disaster Prevention and Control Shandong University of Science and TechnologyQingdao 266590 China

Correspondence should be addressed to Zhijie Wen 1301591557qqcom

Received 29 December 2020 Accepted 16 March 2021 Published 27 March 2021

Academic Editor Traian Mazilu

Copyright copy 2021 ZhaohuiWang et al-is is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

In order to improve ground control of the longwall mining ground response and mining-induced stress in the longwall panel of akilometer-deep coal mine are investigated in this study Field measurements on abutment stress roof displacement and fracturedevelopment indicate that the region influenced by the longwall mining reaches 150m ahead of the longwall face Failure scope ofthe coal seam where mining-induced fractures are well developed ranges from 10 to 13m inward the face line Vertical stressconcentration coefficient reaches 22 Based on the field measurements a numerical model is moreover developed and utilized toexamine the response of the principal stress to the longwall mining -e concentration coefficient peak point location andinfluence scope of the principal stress gradually become stable with an increase in face advancement Regarding the majorprincipal stress the concentration coefficient and influence scope are 24 and 152m respectively and the peak point locates 13minward the face line which are consistent with the field measurements With respect to the minor principal stress the referredcoefficient and scope are 15 and 172m respectively and its peak point location is 21m ahead of the face line -emajor principalstress in the coal seam rotates from vertical to horizontal direction in the vertical plane parallel with face advance direction -emaximum rotation angle reaches 20deg -e minor principal stress first rotates into the referred vertical plane and then it rotatesfrom horizontal to vertical direction at the same speed with the major principal stress in the same plane Rotation angle of theprincipal stress in roof strata is greatly enlarged the rotation trace of which is influenced by the longwall mining and verticaldistance above the seam Based on the relation between rotation trace of the principal stress and face advance direction theinfluence of stress rotation on the stability of roof structure is discussed

1 Introduction

In the past decade the depth of underground coal miningincreased by 10 to 20m per year in China [1 2] As a resultthe mining-induced stress becomes increasingly large andthe stability of the surrounding rock is dramatically dete-riorated [3] Such changes in mining conditions causecontinuous growth in the frequency of mining accidentssuch as face fall roof fall support failure or even dynamicdisasters [4ndash6] It is widely accepted that serious groundresponses are closely related to the stress environmentwhich is important to the design of the ground control

system in the longwall mining -us the distribution ofmining-induced stress and associated ground responseshave been extensively studied with many methods in deepcoal mines

Field measurement serves as the most fundamentalmethod of examining ground response and mining-inducedstress Xie et al investigated mining-induced stress distri-bution in the longwall panel of a kilometer-deep coal minewith borehole stress monitoring method [7] -e resultsindicated that the influence scope of front abutment stressreaches about 100m and peak point of the abutment stresslocates about 10 to 20m inward the face line Chang et al

HindawiShock and VibrationVolume 2021 Article ID 6634509 14 pageshttpsdoiorg10115520216634509

studied the influence of the adjacent gob on the distributionof the abutment stress [8] Peak stress in the unmined coalseam showed an increasing trend with decrease in thedistance from the gob edge Song et al proposed an elec-tromagnetic radiation-based method in assessing the min-ing-induced stress [9] -e intensity of the electromagneticsignal was positively related to the stress magnitude Withvibrating wire stress meters mining-induced stress distri-bution in coal pillars was monitored in sixteen undergroundcoal mines It was revealed that the influence region andpeak value of the vertical stress are positively related to thecover depth and hardness of the overburden [10] By in-stalling stress sensors in the inclined borehole Guo et alfound that the influence scope of the longwall miningreached 300m [11] -e vertical stress experienced a peakpoint at a distance of 28m ahead of the face line whilehorizontal stress maintained a continuous decreasing trendWith the specifically designed stress cells the stress pathundergone by the surrounding rock at Winston Lake Minein Canada has been monitored During the mining processthe principal stresses in different directions experienced aconcentration phenomenon consistently Based on the stresspath two types of failure modes in the surrounding rockwere recognized at the mine site [12] Recently the compactconical ended borehole monitoring system has been ex-tensively used in long-term investigation of full stress tensorchanges in cooperation with the seismicity monitoringsystem A positive relation between the stress level andseismic signal frequency has been achieved [13 14]According to such a relation it was found that the mining-induced stress showed an increasing trend with the facelength [15] With respect to the relationship between thestress magnitude and cover depth the mining-induced stressincreased linearly and the ratio between horizontal andvertical stresses decreased nonlinearly from a large value of35 to 10 with an increase in the cover depth [16 17]

Based on the field measurements a series of empiricalmodels have been established to predict the distribution ofmining-induced stress By simplifying the coal into infiniteelastic isotropic and homogeneous material Salamon firstproposed an empirical equation for stress distribution at theedge of the longwall panel [18] However this empiricalequation could not match subsequent field measurementswhich were illustrated in a nomograph by Whittaker andSingh [19] -e nomograph proved to be accurate for thestress distribution around the longwall panel but it wasdifficult to be mathematically expressed Wilson separatedthe unmined coal seam ahead of the longwall face into twozones namely the failure and intact zones [20] Accordinglythe distribution curve for vertical stress was also divided intotwo pieces where the cover depth mining height andmaterial properties of the surrounding rock were taken intoaccount Recently Xue et al discussed the mining-induceddiscontinuous stress drop observed in deep coal mines [21]A damage index was defined for the failure zone based onfracture development in the unmined coal seam -e peakstress in the failure zone decreased with the damage indexcausing onset of the stress drop at the boundary between thefailure and intact zones ahead of the longwall face

-e field measurement and theoretical equation areapplicable in analyzing the stress distribution along specificlines However quantitative evaluation of mining-inducedstress in three dimensions is of more interest in dealing withsurrounding rock stability Numerical simulation providesan effective way for analyzing the mining-induced stressmore precisely Shabanimashcool and Li proposed amethodology for simulating progressive undergroundmining with continuum-based software where the move-ment of overburden strata was considered [22 23] -us themining-induced stress could be realistically reproduced Juet al adopted a continuum-based discrete element methodto simulate the evolution of mining-induced stress in alongwall panel [24] -is method was capable of charac-terizing the heterogeneity of coal measure rocks beddingseparation dislocation as well as caving of the multilayeredrock strata Basarir et al simulated the stress distributionaround the gate during longwall top-coal caving [25] Itturned out that the maximum magnitude of the abutmentstress increased to about 3 times the initial value Besides theorientation of the principal stress changed frequently Byincluding a discontinuous interface in the numerical modelthe dynamic disaster resulting from mining-induced stressand tectonic stress was investigated and the mining ar-rangement at the near-fault region was optimized [26]Numerical modeling of 3D stress rotation ahead of an ad-vancing tunnel face indicates that the rotation angle of theprincipal stress was so large that its influence on the stabilityof the surrounding rock should be underlined [27 28] -erotation phenomenon led to a more complex propagationpath of mining-induced fractures Such stress rotation alsooccurred in the top coal during longwall top-coal caving-erotation trace depended heavily on the panel layout whichinfluenced top-coal cavability to a large extent [29ndash31] Indeep coal mines stress-relief measures were commonlycarried out to control problems related to mining-inducedstress Kang et al greatly decreased the front abutment stressby using the hydraulic fracturing method [32]

-e ground response and mining-induced stress oflongwall mining has been extensively investigated Howeverthe study associated with longwall mining in kilometer deepcoal mine is limited In such a longwall panel ground re-sponse and stress distribution become more complex Inorder to improve the surrounding rock control groundresponse and mining-induced stress in a longwall panel withcover depth of more than 1000m is thoroughly analyzed inthe present study -e emphasis is placed on the rotation ofthe principal stress and potential influences provided by thestress rotation

2 Engineering Background

21 Geological and Mining Conditions -e Kouzidong coalmine is located in Huainan city of Anhui province China-e high reach single pass longwall mining method is uti-lized to extract a thick coal seam -e target panel of thisstudy is 121304 as shown in Figure 1(a) On its right side is agob remaining after the extraction of panel 121303 -eother side is unmined coal seam Several faults exist within

2 Shock and Vibration

the region between panel 121303 and panel 121304 -us alarge coal pillar 100m wide is left between the referred twopanels to protect the tail gate of the target panel -elongwall face 350m in length is installed at the face startline and advances toward the mains until the face stop line isreached -e longwall face advances in the direction ofS30degW -e extracted coal seam is 56m thick with averagecover depth of 1080m It is a flat seam with dip angle smallerthan 8deg According to rock core logging a simple geologicalcolumn is obtained and illustrated in Figure 1(b) In panel121304 the immediate roof is composed of mudstone andsiltstone which are thin and soft-emain roof is composedof siltstone and sandstone which are thick and hard -eimmediate roof caves timely behind the longwall face so thatthe gob left behind is partially backfilled -e main roof is sohard that it fails in the form of bending rupture Periodicrupture of the main roof leads to roof weighting in thelongwall face

22 Initial Ground Stress and Rock Properties Initial groundstress has been in situ measured at the Kouzidong coal mine-e results reveal that the major principal stress is ap-proximately in the vertical direction whose magnitude isequal to 24MPa -e angle between the major principal andvertical directions is smaller than 12deg -e intermediateprincipal stress 212MPa in magnitude is parallel withN30degW-S30degE direction in the horizontal plane -e minorprincipal stress with the magnitude of 15MPa is perpen-dicular to the immediate principal stress in the horizontalplane -ere is an angle equal to 30deg existing between theminor principal and face advance directions (seeFigure 1(a))

According to Figure 1(b) the overburden strata of panel121304 are mainly composed of mudstone siltstone andsandstone Accordingly both cylindrical and disk samples ofthe coal and roof rocks are prepared for compression andBrazilian tests Rock properties are deduced from the ex-perimental results and listed in Table 1 where E and v areelastic modulus and Poisson ratio respectively

23 Problems in the Surrounding Rock Control Under thegeological and mining conditions of the Kouzidong minesurrounding rocks in both longwall face and gate way ex-perience serious damage after influenced by the longwallmining A series of rock instabilities observed at the minesite are presented in Figure 2 A large coal block falls to thefloor from the longwall face in Figure 2(a) Large-scale facefall causes subsequent roof fall and even instability of thehydraulic support (Figure 2(b)) Both roof fall and face falldrastically threaten the safety of the mining environment inthe longwall face In addition the gateway presents largedeformation characteristics Initial cross section of the tailgate of panel 121304 reaches 126m2 After being influencedby the longwall mining the cross section decreases to about21m2 (Figure 2(c)) Material transport and panel ventilationare dramatically influenced by the large deformation Inorder to prevent such bad influences both rib expanding andfloor dinting are executed in the tail gate of panel 121304(Figures 2(d)ndash2(e)) In fact surrounding rock instabilitiespresented in Figure 2 are attributed to the stress redistri-bution resulting from the longwall mining -us mining-induced stress should be investigated for optimizing theground control

Gob of panel 12130330deg

35deg

25deg

Gas drainage gate

The mains

Coal pillar

The fault

Tail gate

Head gate

Face start line

North

Face stop line

Panel 121304

Face stop line

Tail gate

head gate

Face start line

Gas drainage gate

σ2 σ3

(a)

550m Mudstone

Column Rock

Coal

Mudstone

Siltstone

Mudstone

Sandstone

Siltstone

Sandstone

Mudstone

Thickness

518m

440m

325m

270m

220m

556m

410m

310m Roofstrata

Mainroof

Immediateroof

Coal seam

Floorstrata

Roof

(b)

Figure 1 Mining conditions (a) Plane view of the panel (b) Geological column

Shock and Vibration 3

3 Field Investigations on the Ground Response

31 Monitoring Method In order to achieve ground re-sponse characteristics a series of field measurements havebeen carried out at the Kouzidong coal mine -ree mon-itoring stations are installed in the head gate -e location ofthe first station is 150m ahead of the longwall face -einterval between different monitoring stations is 20m At thestation one horizontal and two vertical boreholes are drilledin the side rib and roof strata of the gate which are 15m indepth -e borehole stress sensor which is widely used toinvestigate vertical stress distribution in underground coalmine is buried at the bottom of the horizontal borehole Anoil pump connected to the stress sensor with flexiblepipeline is left outside of the borehole (Figure 3(a)) Hy-draulic oil is injected into the stress sensor through the oilpump to make the sensor contact tightly with the boreholewall Note the largest oil pressure that the stress sensor canbear commonly ranges from 20 to 25MPa In the longwallmining the concentration coefficient of the vertical stress atits peak point commonly falls between 2 and 4 -us theinitial value of the stress sensor should not be larger than aquarter of the largest value -e load-bearing capacity of the

borehole stress sensor used in this study is 20MPa -einitial pressure of the injected oil is accordingly set to be5MPa which is much smaller than the initial ground stress(24MPa) One vertical borehole is utilized to install thedisplacement sensor (Figure 3(b)) which in fact is the two-point extensometer -e extensometer is composed of twoanchors One anchor (A) is fixed at the bottom of theborehole which locates in the main roof -e other anchor(B) is fixed at the location 6m inside the borehole whichlocates in the immediate roof Borehole camera detection iscarried out in another vertical borehole -e data from thestress sensor and displacement sensor are recorded with thesame data collector

32 Vertical Stress Distribution Vertical stress distributionobtained from the borehole stress sensor is presented inFigure 4 -e vertical stress at three monitoring stationsshows a similar evolution process Initial magnitude is equalto the initial pressure of the hydraulic oil injected into thestress sensor which ranges from 5 to 6MPa At the location150m ahead of the face line the vertical stress shows anincreasing trend -at means the coal in the vicinity of the

Table 1 Rock properties from the experimental test

Rock type E (GPa) v Cohesion (MPa) Friction (deg) Tensile strength (MPa) Uniaxial compressive strength (MPa)Coal 36 020 52 33 18 20Mudstone 186 025 8 35 32 31Siltstone 206 025 12 40 64 52Sandstone 320 016 18 42 112 80

(a) (b) (c)

15m

(d)

10m

(e)

Figure 2 Surrounding rock instabilities (a) Face fall (b) Roof fall (c) Large deformation of the gate (d) Rib expanding (e) Floor dinting

4 Shock and Vibration

monitoring station is influenced by mining operations at thelongwall face Shrink of the borehole wall causes an increasein the oil pressure During the growing process the verticalstress experiences local fluctuation due to frequent crushingof the borehole wall Such an increasing stage ends at thelocation 11 to 13m inward the face line where a peak pointis reached by the vertical stress -e peak point arrives at 10to 12MPa and the stress concentration coefficient is ap-proximately equal to 22 After the peak point the coal failsDevelopment of mining-induced fractures leads to contin-uous loss of the load-bearing capacity in the coal -us thevertical stress starts to decrease At the face line verticalstress decreases to about 25 to 35MPa which is consistentwith residual strength of the coal in the uniaxial compressiontest According to the vertical stress distribution the regionof the coal seam influenced by mining operations expands to150m ahead of the face line Due to the large cover depth thewidth of fracturing area reaches 11 to 13m much larger thanthat in the longwall panel with a small cover depth [33 34]Note due to the small installation pressure (5 to 6MPa) thevertical stress obtained from the borehole stress sensor ismuch lower than the real value However the evolutiontrend is reliable and extensively utilized to evaluate the

distribution of mining-induced stress -e vertical stress iscommonly named as the abutment stress Its evolution trendis consistent with the abutment stress distribution in situobserved by Chang [8]

33 Roof Displacement Evolution of the roof displacementis shown in Figure 5 Note anchor A in the main rooflocates 15m above the coal seam and anchor B in theimmediate roof is 6m above the coal seam According toFigure 5 vertical displacement of the immediate roof islarger than that of the main roof -e immediate roof startsto subside at about 150m ahead of the face line -e initialsubsidence position of the main roof locates 10m behindthat of the immediate roof After that an increasing rate ofthe vertical displacement in the immediate roof is larger thanthat in the main roof In addition an increasing rate of thevertical displacement in the immediate roof keeps relativelystable in the monitoring process However an increasingrate of the vertical displacement in the main roof changescontinuously -at is attributed to relatively low sensitivityof the main roof to the longwall mining At 1 and 3monitoring stations the main roof subsides abruptly at thelocation 5m ahead of the face line Such a quick increase invertical displacement is attributed to bending rupture of themain roof-e abrupt increase in the vertical displacement isnot experienced by the anchor at station 2 which means thatthe main roof does not fracture when the longwall faceapproaches to this station At the face line vertical dis-placements in the immediate roof are 110mm 116mm and130mm at 1 2 and 3 monitoring stations respectivelyIn contrast at the referred stations vertical displacementsunderwent by the anchors installed in the main roof are62mm 70mm and 75mm -at means the bedding sep-arations between immediate and main roof strata reach48mm 46mm and 55mm respectively Such an evolutionin the vertical displacement is consistent with that observedby Xie [2]

34 Fracture Development in Roof Strata Fracture devel-opment in roof strata is presented in Figure 6 When themonitoring station is 50m ahead of the face line the coal

0 50 100 1502

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Figure 4 Vertical stress distribution

(a) (b)

Figure 3 Installation of the monitoring apparatus (a) Stress sensor (b) Displacement sensor

Shock and Vibration 5

located 12m inside the borehole is intact where a thin layerof coal seam is observed At the depth of 6m in the boreholea close fracture intersects with the borehole wall the dip

angle of which reaches about 75deg In the vicinity of theborehole end a series of horizontal fractures are formedBut the fracture size is small Such fractures are a result of

0 50 100 1500

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Figure 5 Vertical displacement of roof strata (a) 1 station (b) 2 station (c) 3 station

Depth = 12m Depth = 6m Depth = 12m

(a)

Depth = 12m Depth = 6m Depth = 12m

(b)

Depth = 12m Depth = 6m Depth = 12m

(c)

Figure 6 Fracture development in roof strata (a) 50m ahead of the face line (b) 20m ahead of face the line (c) 10m ahead of the face line

6 Shock and Vibration

gate extraction As the stations move to a position 20minward the face line small horizontal fractures initiate in thevicinity of the thin coal seam at the location 12m inside theborehole But the borehole wall still remains intact As thelocation moves to 6m inside the borehole the close fracturestarts to open But the aperture is small as shown inFigure 6(b) At the depth of 12m the small horizontalfractures coalesce into a large fracture leading to beddingseparation of roof strata When the station locates within thefailure region of the seam roof strata become stronglyinfluenced by the longwall mining As shown in Figure 6(c)bedding separation and small fractures appear in the vicinityof the thin coal seam -e aperture of the fracture located6m inside of the borehole reaches about 5mm At theborehole end the immediate roof is sheared into smallfragments Without bolt support immediate roof within theregion 12m above the coal seam would cave under its owngravity

4 Numerical Modeling of the Mining-Induced Stress

41 Model Configuration -e information on the stressdistribution covered by the field measurements is limited Inorder to reveal more information of the mining-inducedstress a large-scale numerical model is developed in thissection which is presented in Figure 7 -e model is 1080min length and width and its height is 130m-e bottom andfour sides of the numerical model are fixed displacementboundaries -e top surface is set to be stress boundarythrough applying a compressive load of 225 MPa -is loadis equal to the gravity of rock strata which are not included inthe numerical model -e initial ground stress is assigned tothe model in accordance to the field measurements -emajor intermediate and minor principal directions areparallel with z y and x axes respectively -ere are twolongwall panels being included in the model which areseparated by a 100m wide coal pillar -e longwall face withthe length of 350m is installed at the face start line -ere isan angle of 30deg existing between face advance and minorprincipal directions which is consistent with the miningcondition plotted in Figure 1 In the numerical model panel121303 is first mined followed by panel 121304 -us theinfluence of the gob left by the extraction of panel 121303 istaken into account Note that the thick coal seam is extractedby 2m per step With face advancement a monitoring line isinstalled in the coal seam as shown in Figure 7

In order to simulate mechanical behavior of the sur-rounding rock in a realistic way the constitutive modeldeveloped by Wang et al is assigned to the coal measurerocks included in the numerical model [35] Rock massproperties as listed in Table 2 are estimated from the intactrock properties (listed in Table 1) by using the geologicalstrength index system proposed by Hoek and Brown [36] InTable 2 parameters m n and k are strain-softening indexesof the referred constitutive model which are determinedaccording to method proposed by Wang et al [35] In orderto simulate the influence of roof rupture on stress distri-bution the largest tensile stress of the main roof is tracked in

the modeling process If the largest tensile stress reaches thetensile strength of the main roof namely the zone fails intension both tensile strength and cohesive strength of thiszone are set to be zero In this way the failure zone plays asimilar role with real discontinuous fracture in cutting offthe transfer path of mining-induced stress -us fracturingbehavior of the main roof can be simulated implicitly and itsinfluence on mining-induced stress is taken into accountWith respect to cavingmaterials in the gob area double yieldcriterion is utilized to simulate the consolidation behavior[37 38] -e properties for gob materials are determined bycomparing the predicted data with the empirical equationproposed by Salamon [18] which are listed in Table 3 Notethat evolution of the cap pressure composed of the double-yield model is controlled with the model proposed by Wanget al [31]

p a ebεpsm minus 11113874 1113875 + cεps

m (1)

where p is the cap pressure εpsm is the volumetric plastic strain

of caved materials and a b and c are the cap pressure modelparameters

42 Model Validation Spatial distribution of the verticalstress in panel 121304 is extracted from the numerical modeland displayed in Figure 8 It is obviously revealed that thevertical stress in the vicinity of the gob area is significantlyincreased due to the influence provided by the longwallmining Two peak values appear on two sides of the face wallahead of the longwall face -at is attributed to the roadwayson two sides of the longwall panel In the gob area thevertical stress is drastically released because the load-bearingcapacity of the caving materials is small But the cavingmaterials are gradually compacted by roof strata movementwith enlargement in the face advancement -us verticalstress shows increasing trend with the growth in the distancebetween the longwall face and gob materials As the longwallface advances 300m from the face start line the largestvertical stress increases to about 20MPa in the gob areawhich means about 67 of the initial ground stress is re-covered Such distribution of the vertical stress is consistentwith the field measurements illustrated in Figure 4 In orderto conduct quantitative comparison vertical stress along themonitoring line is extracted from Figure 8 and presented inFigure 4 which shows a similar evolution trend with the in-situ data -e peak point locates about 13m ahead of thelongwall face and the influence range of the vertical stressreaches 150m Such data from the numerical model agreewell with the field measurements indicating that the de-veloped model is reliable

438eEvolution of the Principal Stress -e evolution of themajor principal stress along the monitoring line with faceadvancement is plotted in Figure 9(a) which shows a similartrend with the vertical stress Initial value of the majorprincipal stress in the coal seam uninfluenced by thelongwall mining is about 24MPa equal to the field mea-surement -e concentration and recovery phenomenon are

Shock and Vibration 7

also experienced by the major principal stress in front and atthe rear of the longwall face respectively -e major prin-cipal stress recovers to about 18MPa in the gob when faceadvancement reaches 280m reaching 75 of the initialvalue Besides main roof rupture leads to local decrease inthe major principal stress -e region influenced by thelongwall mining ahead of the longwall face reaches 152mwhich is approximately equal to that influenced by thevertical stress

-e evolution of the minor principal stress on themonitoring line with face advancement is plotted in

Figure 9(b) -e stress concentration and stress recoveryphenomenon also occur to the minor principal stress underthe influence of the longwall mining In the unmined coalseam uninfluenced by the longwall mining initial value ofthe minor principal stress is equal to 15MPa which isconsistent with the field measurement After being influ-enced by mining operations the concentration degree of theminor principal stress is lower than that of the majorprincipal stress However the influence provided by themain roof rupture on the minor principal stress is moreobvious than that on the major principal stress Besides therecovery ratio of the minor principal stress in the gob area issmaller than that of the major principal stress When thelongwall face advances 280 m from face start line the minorprincipal stress only recovers to about 6 MPa in the gob areaaccounting for 40 of the initial value Local decrease is alsoexperienced by the minor principal stress due to periodicrupture of the main roof Regarding the minor principalstress the region ahead of the longwall face influenced by thelongwall mining increases to 172m

Based on Figure 9 the variation in the peak stressconcentration coefficient and peak point location with faceadvancement is moreover achieved and displayed in Fig-ure 10 According to Figure 10(a) the concentration coef-ficients of the major and minor principal stresses present asimilar increasing trend in the advancing process -e

Panel 121304

Panel 121303

Tail gate

Head gateTail gate

Head gate

Coal pillar

Monitoring line

30

Point O

130m

1080m

1080

m

Face start line

ZY

X

Figure 7 Numerical model including two longwall panels

Table 2 Rock mass properties

Rock mass E (GPa) v Cohesion (MPa) Friction (deg) Tensile strength (MPa) m n kCoal 28 020 13 30 02 00035 040 270Mudstone 147 025 24 32 06 00021 056 320Siltstone 175 025 50 33 13 00015 065 400Sandstone 212 016 100 38 20 00010 070 750

Table 3 Gob material properties

Property Density Bulk modulus Shear modulus Cohesion Friction Tensile strength a b c(kgm3) (GPa) (GPa) (MPa) (deg) (MPa) (MPa) mdash (MPa)

Value 2000 12 06 0 30 0 60 15 20

60

50

40

30

20

10

0 (MPa)

Advancedirection Gob area

Figure 8 Spatial distribution of the vertical stress in panel 121304

8 Shock and Vibration

increasing speed declines gradually with enlargement in theadvancing distance As the longwall face advances about240m from the face start line the stress coefficient becomesinsusceptible to the face advancement Regarding the majorand minor principal stresses the stable coefficient magni-tudes are 24 and 15 respectively -e value correspondingto the major principal stress agrees well with that of thevertical stress in situ monitored Periodic rupture of themain roof leads to local decease in the concentrationcoefficient

-e peak point location means that the distance from thepeak point to the face line as shown in Figure 10(b) Regardingthe major and minor principal stresses this distance shows anopposite evolution trend With face advancement the peakpoint of themajor principal stressmoves far away from the faceline In contrast the peak point of the minor principal stressmoves closer to the face line As the longwall face advancesabout 140m from the face start line the peak point locationbecomes insusceptible to the face advancement-e peak pointof the major principal stress locates about 13m inward the face

ndash25 0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 4250

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Figure 9 -e evolution of the principal stress with face advancement (a) Major principal stress (b) Minor principal stress

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40 80 120 160 200 240 2808

101214161820222426

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catio

n (m

)

(b)

Figure 10 -e variation in (a) stress concentration coefficient and (b) peak point location with face advancement

Shock and Vibration 9

line which is in accordance to the field measurement -estable peak point of the minor principal stress stays 21m aheadof the face line significantly larger than that of the majorprincipal stress Besides periodic rupture of themain roof leadsto the enlargement in the distance between the face line and thepeak point of the principal stress However with respect to themajor principal stress this distance becomes insusceptible toroof rupture as the face advancement increases to 140m

44 Principal Stress Rotation In order to investigate therotation process experienced by the principal stress in the coalseam ahead of the face line the stress data on the monitoringline are extracted from the numerical model -e principalstress the stress in the initial principal direction namelyvertical stress (σv) and the stress in x-axial direction (σx) andtheir differences are presented in Figures 11(a) and 11(b)-eprincipal direction is presented in the stereonet inFigures 11(c) and 11(d) Note that the lower hemisphereprojection method is utilized to create the stereonet in thisstudy -e y-axial and x-axial directions in the numericalmodel orientate to 0deg and 90deg on the stereonet -at meansthat the longwall face advances from 60deg to 240deg on thestereonet According to Figures 11(a) and 11(b) the coal seamis initially uninfluenced by the longwall mining -us themajor and minor principal stresses stay in vertical and x-axialdirections respectively at point O As shown in Figure 11(a)from point O to point A the difference between the majorprincipal and vertical stresses shows an increasing trend It iseasy to understand that the increase in the difference betweenthe principal stress and the vertical stress implies the deviationof the principal stress from its initial direction -us themajor principal stress gradually deviates from the verticaldirection and tilts toward the gob in this stage At point A thedip angle of the major principal stress decreases to 75deg Afterthat the stress difference starts to decrease until point B isapproached In this process the major principal stress rotatesreversely and nearly goes back to the vertical direction at pointB From point B to point C the stress difference increasesagain which means an increase in the rotation angle of themajor principal stress But the major principal stress rotatestoward the face advance direction in this stage At point C theangle between the major principal stress and face advancedirections decreases to 83deg -en the major principal stressrotates back toward the vertical stress once again At point Dthe difference between the major principal and verticalstresses vanishes As a result point D and point O coincide onthe stereonet From point D to the face line (point E) thestress difference presents an increasing trend and the majorprincipal stress tilts toward the gob area again At point E dipangle of the major principal stress decreases to 70deg Rotationprocess of the major principal stress nearly remains in thevertical plane parallel with the face advance direction

Regarding the minor principal stress in Figures 11(c) and11(d) its difference with x-axial stress also shows an increasingtrend from point O to point J In this process the minorprincipal stress rotates toward the vertical direction in thevertical plane parallel with the x-axial direction Dip angle ofthe minor principal stress increases to 10deg at point J -e

increasing rate of the stress difference rises abruptly from pointJ to point K and then to point F In the referred process theminor principal stress deviates from the vertical plane parallelwith x-axial direction and it rotates gradually toward thevertical plane parallel with the face advance direction Frompoint J to point K rotation angle experienced by the minorprincipal stress in the horizontal plane reaches 20deg Howeverthe dip angle of theminor principal stress decreases to 0deg Frompoint K to point F the angle between the minor principal andx-axial directions and the dip angle of the minor principalstress show an increasing trend consistently Within the regionfrom point F to the face line (point N) the difference betweenthe minor principal and x-axial stresses drops quickly Frompoint F to point M the angle between the minor principal andx-axial directions is enlarged while the dip angle of the minorprincipal stress decreases to 0deg again At point M the minorprincipal stress rotates into the vertical plane parallel with theface advance direction After that rotation process of theminorprincipal stress remains in this plane From pointM to point Nthe dip angle of theminor principal stress increases to 20deg at thesame speed with the major principal stress

In addition to the principal stress in the coal seam therotation phenomenon is also experienced by the principalstress in roof strata As shown in Figure 12 the principalstress orientation along three lines parallel with the moni-toring line are presented which are 15m (line 1) 60m (line2) and 100m (line 3) above the coal seam in roof stratarespectively In comparison with the principal stress in thecoal seam rotation angle of the principal stress in higheroverburden strata is greatly enlarged -at means theprincipal stress orientation in roof strata is more sensitive tothe longwall mining -us the major and minor principalstresses deviate from vertical and x-axial directions at pointO far away from the face line With a decrease in the distancefrom the face line the major principal stress tilts to the gobarea in the vertical plane parallel with 45deg minus225deg directiondeviating at an angle of 15deg from the face advance directiondue to the influence of the gob left by extraction of panel121303 At the face line the rotation angle experienced bythe major principal stress on line 1 is relatively larger thanthat on lines 2 and 3 -e rotation traces of the majorprincipal stress along three lines are similar except for localdifferences Regarding the minor principal stress it rotatesgradually from horizontal to vertical direction At the faceline the dip angle of the minor principal stress on three linesincreases to about 45deg equal to the rotation angle of themajor principal stress -ough the minor principal stressrotates toward the vertical plane parallel with 45degndash225degdirection consistently in the horizontal plane rotation angleexperienced by the minor principal stress along three linesvaries a lot leading to different rotation traces on thestereonet Rotation angle experienced by the minor principalstress on line 1 in horizontal plane is much larger than thatexperienced by the minor principal stress on line 2 whilesuch rotation angle of the minor principal stress on line 3becomes negligible-e difference in the rotation trace of theminor principal stress on three lines is attributed to verticaldistances from the coal seam -e larger distance from thecoal seam leads to the weaker influence of the longwall

10 Shock and Vibration

mining As a result rotation angle of the minor principalstress in the horizontal plane shows a decreasing trend fromline 1 to line 3 Besides in the vertical direction the rotationangle experienced by the minor principal stress on line 1 isrelatively larger than that on lines 2 and 3

5 Influence of Stress Rotation onRoof Structure

In the longwall mining mining-induced fractures tend topropagate in the direction perpendicular to the minorprincipal stress direction which means the bending ruptureplane is closely related to rotation trace of the minorprincipal stress Besides the orientation of the fractureformed by the rupture of the main roof provides great

influence on roof structure stability According to voussoirbeam theory proposed by Qian [39] broken blocks of themain roof form a balance structure above the longwall facewhich is defined as the voussoir beam structure (Figure 13)-e broken blocks are composed of the structure contact atpoints A B and C and a temporal balance state is achieved-e structure greatly weakens the roof load applied on thehydraulic support in the longwall face According to thevoussoir beam theory the structure remains stability when

R

Tle tan(φ + θ) (2)

where R and T are shear and normal forces at the contactpoint on the fracture plane θ is the angle between thefracture plane and the vertical direction

0 20 40 60 80 1000

10

20

30

40

50

60

00

03

06

09

12

15

18

Distance from the face line (m)

Stre

ss d

iffer

ence

(MPa

)

σ1σvσ1 ndash σv

A

B

C

D

E

O

Stre

ss (M

Pa)

(a)

A

B

C

D

E

O

300

315

330345 0 15

30

45

60

75

90

105

120

135

150165180195

210

225

240

255

270

285

(b)

0

5

10

15

20

25

30

0 20 40 60 80 10000

05

10

15

20

25

30St

ress

diff

eren

ce (M

Pa)

Distance from the face line (m)

J

K

F

M

N

O

σ3σxσx ndash σ3

Stre

ss (M

Pa)

(c)

J

K

K F

M

M

NO

300

315

330345 0 15

30

45

60

75

90

105

120

135

150165180195

210

225

240

255

270

285

(d)

Figure 11 Stress rotation along the monitoring line (a) Difference between the major principal and vertical stresses (b) Rotation trace ofthe major principal stress (c) Difference between the minor principal and x-axial stresses (d) Rotation trace of the minor principal stress

Shock and Vibration 11

If the fracture plane tilts toward the face advance di-rection as shown in Figure 13 the value of θ is positive andthe stability of the voussoir beam structure is greatly im-proved However if the fracture plane tilts toward the gobarea the value of θ is negative indicating a bad roofstructure condition In both scenarios the stability of theroof structure is closely related to the angle θ In fact angleθ is equal to the final dip angle of the minor principal stress-us if the rotation trace of the principal stress can bedetermined the roof structure stability can be evaluated-ough the rotation trace of the minor principal stress ismore complex than that of the major principal stress it isalso influenced by face advance direction For the minorprincipal stress in roof strata its rotation trace is alsorelated to the vertical distance above the coal seam Basedon such an understanding fracture orientation in the mainroof can be determined Moreover the stability of thevoussoir beam structure above the longwall face can beevaluated to guarantee the stability of the hydraulicsupport

6 Conclusions

With an increase in the cover depth of the longwall miningthe difficulty in the surrounding rock control is greatly

improved -e distribution of mining-induced stress isimportant to the surrounding rock stability -us in thepresent study stress analysis is carried out in a longwallpanel with a face length of 350m and a cover depth morethan 1000m Based on the field measurements and nu-merical modelling results the following conclusions aredrawn

(1) Borehole stress monitoring indicates that the verticalstress shows an increasing trend at about 150mahead of the longwall face -e peak point is reachedat the location 10 to 13m inward the face line Afterthat the coal seam fails and the vertical stress shows adecreasing trend At the face line vertical stressdrops to 25 to 35MPa Multipoint displacementmeter-based roof subsidence measurement revealsthat immediate roof and main roof start to subside atabout 150 m and 140 m ahead of the longwall facerespectively Borehole camera detection shows thatroof fracture development increases rapidly withinthe failure region of the coal seam

(2) -e modelling results show that the magnitude andinfluence scope of the principal stress show a risingtrend with the enlargement in face advancement-ey become stable when the advancement reaches240m -e stable concentration coefficients of themajor and minor principal stresses are 24 and 15respectively Corresponding influence scopes are 152and 172m ahead of the face line -e distancesbetween the face line and peak points of the majorandminor principal stresses initially show increasingand decreasing trends and then they become stablewith the advancement of 140m-e stable values are13 and 21m respectively

(3) Longwall mining results in stress rotation whichinfluences the stability of the roof voussoir beam

Line 1Line 2Line 3

O

Face line

300

315330

345 0 1530

45

60

75

90

105

120

135150

165180195210

225

240

255

270

285

(a)

Line 1Line 2Line 3

O

Face

line

300

315330

345 0 1530

45

60

75

90

105

120

135150

165180195210

225

240

255

270

285

(b)

Figure 12 Principal stress rotation in roof strata (a) Major principal stress (b) Minor principal stress

Main roof

Immediate roof

Coal seam

Floor strata SupportGob area

θ

A

BC

Figure 13 -e voussoir beam structure

12 Shock and Vibration

structure In the coal seam the major principal stressrotates from vertical to horizontal direction -emaximum rotation angle reaches 20deg and the rotationtrace remains in the vertical plane (β) parallel withthe face advance direction -e minor principalstress first rotates into plane β and then it rotatesconsistently with the major principal stress Its ro-tation angle in the horizontal plane is equal to theangle between face advance and initial minorprincipal directions Rotation angle of the principalstress in roof strata is greatly increased and its ro-tation trace is influenced by the longwall miningadjacent gob area and the distance above the coalseam

Data Availability

-e data used to support main conclusions of this study areincluded within the paper -e processed data are availablefrom the corresponding author upon request

Conflicts of Interest

-e authors declare that they have no conflicts of interest

Acknowledgments

-is study was sponsored by the National Key RampDProgramof China (Grant No 2017YFC0603002) and Key Laboratoryof Mining Disaster Prevention and Control (Grant NoMDPC201906) It was also supported by the FundamentalResearch Funds for the Central Universities -e authors aregrateful for their support

References

[1] H Xie H Zhou D Xue H Wang R Zhang and F GaoldquoResearch and consideration on deep coal mining and criticalmining depthrdquo Journal of China Coal Society vol 37 no 4pp 535ndash542 2012

[2] H Xie ldquoResearch review of the state key research develop-ment program of China deep rock mechanics and miningtheoryrdquo Journal of China Coal Society vol 44 no 5pp 1283ndash1305 2019

[3] Q Wang B Jiang R Pan et al ldquoFailure mechanism ofsurrounding rock with high stress and confined concretesupport systemrdquo International Journal of Rock Mechanics andMining Sciences vol 102 pp 89ndash100 2018

[4] H Kang H Lv F Gao X Meng and Y Feng ldquoUnder-standing mechanisms of destressing mining-induced stressesusing hydraulic fracturingrdquo International Journal of CoalGeology vol 196 pp 19ndash28 2018

[5] P Konicek K Soucek L Stas and R Singh ldquoLong-holedestress blasting for rockburst control during deep under-ground coal miningrdquo International Journal of Rock Mechanicsand Mining Sciences vol 61 pp 141ndash153 2013

[6] J Wang S Yang B Yang et al ldquoRoof sub-regional fracturingand support resistance distribution in deep longwall face withultra-large lengthrdquo Journal of China Coal Society vol 44no 1 pp 54ndash63 2019

[7] J Xie J Xu and FWang ldquoMining-induced stress distributionof the working face in a kilometer-deep coal mine-a case study

in Tangshan coal minerdquo Journal of Geophysics and Engi-neering vol 15 no 5 pp 2060ndash2070 2018

[8] J-c Chang ldquoDistribution laws of abutment pressure aroundfully mechanized top-coal caving face by in-situ measure-mentrdquo Journal of Coal Science and Engineering (China)vol 17 no 1 pp 1ndash5 2011

[9] D Song E Wang X He et al ldquoUse of electromagnetic ra-diation from fractures for mining-induced stress field as-sessmentrdquo Journal of Geophysics and Engineering vol 15no 4 pp 1093ndash1103 2018

[10] A K Singh R Singh J Maiti R Kumar and P K MandalldquoAssessment of mining induced stress development over coalpillars during depillaringrdquo International Journal of RockMechanics and Mining Sciences vol 48 no 5 pp 805ndash8182011

[11] H Guo L Yuan B Shen Q Qu and J Xue ldquoMining-inducedstrata stress changes fractures and gas flow dynamics inmulti-seam longwall miningrdquo International Journal of RockMechanics and Mining Sciences vol 54 pp 129ndash139 2012

[12] P Kaiser S Yazici and S Maloney ldquoMining-induced stresschange and consequences of stress path on excavation sta-bility-a case studyrdquo International Journal of Rock Mechanicsand Mining Sciences vol 38 pp 167ndash180 2011

[13] P Konicek and P Waclawik ldquoStress changes and seismicitymonitoring of hard coal longwall mining in high rockburstrisk areasrdquo Tunnelling and Underground Space Technologyvol 81 pp 237ndash251 2018

[14] J Liu Y Li and S Xu ldquoRelationship between microseismicactivities and mining parameters during deep mining pro-cessrdquo Journal of Applied Geophysics vol 159 pp 814ndash8232018

[15] S Wang and Q Huang ldquoStudy on roof weighting of 400mfully-mechanized mining face in shallow coal seamrdquo Inter-national Journal of Coal Science and Technology vol 46no S1 pp 75ndash80 2018

[16] B Lund and M D Zoback ldquoOrientation and magnitude of insitu stress to 65 km depth in the baltic shieldrdquo InternationalJournal of Rock Mechanics and Mining Sciences vol 36 no 2pp 169ndash190 1999

[17] A G Corkum B Damjanac and T Lam ldquoVariation ofhorizontal in situ stress with depth for long-term performanceevaluation of the deep geological repository project accessshaftrdquo International Journal of Rock Mechanics and MiningSciences vol 107 pp 75ndash85 2018

[18] M Salamon ldquoElastic analysis of displacements and stressinduced by the mining of seam or reef deposits part IIrdquoJournal of the Southern African Institute of Mining andMetallurgy vol 64 no 6 pp 197ndash218 1964

[19] B Whittaker and R Singh ldquoDesign and stability of pillars inlongwall miningrdquo Mining Engineering vol 139 pp 59ndash701979

[20] A H Wilson ldquo-e stability of underground workings in thesoft rocks of the Coal Measuresrdquo International Journal ofMining Engineering vol 1 no 2 pp 91ndash187 1983

[21] D Xue J Wang Y Zhao and H Zhou ldquoQuantitative de-termination of mining-induced discontinuous stress drop incoalrdquo International Journal of Rock Mechanics and MiningSciences vol 111 pp 1ndash11 2018

[22] M Shabanimashcool and C C Li ldquoNumerical modelling oflongwall mining and stability analysis of the gates in a coalminerdquo International Journal of Rock Mechanics and MiningSciences vol 51 pp 24ndash34 2012

[23] M Shabanimashcool and C C Li ldquoA numerical study ofstress changes in barrier pillars and a border area in a longwall

Shock and Vibration 13

coal minerdquo International Journal of Coal Geology vol 106pp 39ndash47 2013

[24] Y Ju Y Wang C Su D Zhang and Z Ren ldquoNumericalanalysis of the dynamic evolution of mining-induced stressesand fractures in multilayered rock strata using continuum-based discrete element methodsrdquo International Journal ofRock Mechanics and Mining Sciences vol 113 pp 191ndash2102019

[25] H Basarir I Ferid Oge and O Aydin ldquoPrediction of thestresses around main and tail gates during top coal caving by3D numerical analysisrdquo International Journal of Rock Me-chanics and Mining Sciences vol 76 pp 88ndash97 2015

[26] H G Ji H S Ma J A Wang Y H Zhang and H CaoldquoMining disturbance effect and mining arrangements analysisof near-fault mining in high tectonic stress regionrdquo SafetyScience vol 50 no 4 pp 649ndash654 2012

[27] E Eberhardt ldquoNumerical modelling of three-dimension stressrotation ahead of an advancing tunnel facerdquo InternationalJournal of Rock Mechanics and Mining Sciences vol 38 no 4pp 499ndash518 2001

[28] M S Diederichs P K Kaiser and E Eberhardt ldquoDamageinitiation and propagation in hard rock during tunnelling andthe influence of near-face stress rotationrdquo InternationalJournal of Rock Mechanics and Mining Sciences vol 41 no 5pp 785ndash812 2004

[29] J Wang and Z Wang ldquoPropagating mechanism of top-coalfracture in longwall top-coal caving miningrdquo Journal of ChinaCoal Society vol 43 no 9 pp 2400ndash2413 2018

[30] J Wang Z Wang and Y Li ldquoLongwall top coal cavingmechanisms in the fractured thick coal seamrdquo InternationalJournal of Geomechanics vol 20 no 8 2020

[31] J C Wang Z H Wang and S L Yang ldquoStress analysis oflongwall top-coal caving face adjacent to the gobrdquo Interna-tional Journal of Mining Reclamation and Environmentvol 34 no 7 pp 476ndash497 2020

[32] H Kang G Wang P Jiang et al ldquoConception for stratacontrol and intelligent mining technology in deep coal mineswith depth more than 1000 mrdquo Journal of China Coal Societyvol 43 no 7 pp 1789ndash1800 2018

[33] S R Islavath D Deb and H Kumar ldquoNumerical analysis of alongwall mining cycle and development of a compositelongwall indexrdquo International Journal of Rock Mechanics andMining Sciences vol 89 pp 43ndash54 2016

[34] H Yavuz ldquoAn estimation method for cover pressure re-es-tablishment distance and pressure distribution in the goaf oflongwall coal minesrdquo International Journal of Rock Mechanicsand Mining Sciences vol 41 no 2 pp 193ndash205 2004

[35] J Wang Z Wang and S Yang ldquoA coupled macro- and meso-mechanical model for heterogeneous coalrdquo InternationalJournal of Rock Mechanics and Mining Sciences vol 94pp 64ndash81 2017

[36] E Hoek and E T Brown ldquoPractical estimates of rock massstrengthrdquo International Journal of Rock Mechanics andMining Sciences vol 34 no 8 pp 1165ndash1186 1997

[37] G C Zhang Z J Wen S J Liang et al ldquoGround response of agob-side entry in a longwall panel extracting 17 m-thick coalseam a case studyrdquo Rock Mechanics and Rock Engineeringvol 53 no 2 pp 497ndash516 2020

[38] L Jiang P Zhang L Chen et al ldquoNumerical approach forgoaf-side entry layout and yield pillar design in fracturedground conditionsrdquo Rock Mechanics and Rock Engineeringvol 50 no 11 pp 3049ndash3071 2017

[39] M G Qian Strata Control and Sustainable Coal MiningChina University of Mining and Technology Press XuzhouChina 2011

14 Shock and Vibration