Research ArticleGround Stress Distribution and Dynamic PressureDevelopment of Shallow Buried Coal Seam Underlying AdjacentRoom Gobs

Ming Zhang 1 Chen Cao 12 and Bingjie Huo1

1College of Mining Liaoning Technical University Fuxin 123000 China2School of Civil Mining and Environmental Engineering University of Wollongong Wollongong NSW 2522 Australia

Correspondence should be addressed to Chen Cao 2065214306qqcom

Received 6 September 2020 Revised 19 January 2021 Accepted 26 July 2021 Published 9 August 2021

Academic Editor Luca Landi

Copyright copy 2021 Ming Zhang 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

+e condition of the coal pillars remained in the room-and-pillar gobs is complicated +e stresses loaded on the pillar floor maybe transmitted and overlapped It changes the stress environment of the lower coal seam roof leading abnormal periodicweighting In the procedure of coal seam 3minus1 mining in the Huoluowan Coal Mine the ground stress is high while the working facepassing through the room pillars of overlying coal seam 2minus2 leading to hydraulic shield being broken In this paper theoreticalanalysis numerical calculation and similar material simulation were used to analyse the stress environment of lower seam and theeffect of coal pillars remained in close-distanced upper seam+e stress transfer model was established for the room pillars of coalseam 2minus2 and the stress distribution of underlying strata was obtained based on theoretical analysis +e joint action of dynamicpressure of high stress-coal pillar withmovement of overlying rock strata in the working face 3minus1 under the coal pillar was revealed+e results showed that the horizontal stress and vertical stress under the large coal pillar of the room gob in coal seam 2minus2 werehigh being from 97 to 153MPa +e influencing depth of vertical stress ranged from 42m to 58m +e influencing depth ofhorizontal stress ranged from 10 to 23m+e influencing range of the shear stress was from 25 to 50mWhen the working face 3minus1

was mined below the coal pillar of 20m or 50m abutment pressure was relatively high +e stress concentration coefficientreached 444ndash500 +e dynamic pressure of the working face was induced by the stress overlying of the upper and lower coalseams instability of the inverted trapezoid rock pillar above the coal pillar and collapsing movement of the roof +e studyingresults were beneficial for guiding the safety mining of the coal seam 3minus1 in the Huoluowan Coal Mine

1 Introduction

In the shallow buried coal seam mining the overlying stratahave one single key stratum +e main roof can hardly forma stable voussoir beam structure With the increase of theoverlying strata thickness two key strata structure could beformed In this case the periodic weighting generated by onekey strata caving is not large but the periodic weightinginduced by the simultaneous caving of two key strata may begreat which generates great pressure on the hydraulicshields in the working face +at is roof caving behaviourhas great influence on the ground subsidence and the sta-bility of the hydraulic supports [1 2] It should be noted thatin multiple coal seams mining the interval between the coal

seams has great influence on the integrity of lower coal seamroof and magnitude of the mining induced dynamic stress[3 4]

In close-distanced multiple coal seams mining the roomgob was formed in upper coal seam when the room retreatmining method was used It has great influence on the stresscondition of overlying strata of the lower coal seam +egeo-conditions and mechanical properties of overlyingstrata dominate the deformational behaviour surroundingrock [5ndash9] In the mining procedure the underlying stratamay experience stress concentration unloading or circularloading which leads to various deformations or evenfracturing of the lower seam [10ndash13] +e stress change oflower seam may cause gas emission and absorption

HindawiShock and VibrationVolume 2021 Article ID 8812933 11 pageshttpsdoiorg10115520218812933

affecting gas migration [14ndash17] which may induce dynamicdisasters [18 19] +e remaining coal pillars with differentdimensions lead to different the stress concentration factorson the floor which generates the variation of the stressenvironment in the lower coal seam [20] Since the residualstress of the room gob is quite complicated previous studieson multi-seam mining commonly adopted semitheoreticaland semiempirical approaches and most of them are casebased Jie [21] proposed that a composite beam structurewas existed in the upper overlying rock strata above theroom and pillar gob +e different caving periods of theupper and lower cantilever beams would induce differentperiodic distances and abutment pressures which is themain reason for the overloading of the shield supports at theworking face +e caving behaviour of the overlying rockstrata is related to the elastic energy accumulation within therock mass which affect the strata deformation groundsubsidence and mining-induced dynamic pressure [22ndash24]Yang [25] believed that the roof caving was mainly theslippage instability between the strata around the workingface Li et al [26] proposed that the main reason in leadingto support shield crash was the abutment pressure inducedby the suspended roof behind the working face Zhao et al[27] monitored the support shield working resistance underthe intact coal room gob and room pillars with differentdimensions of upper seam For the stress transfer rule of thecoal pillar floor the influence of the remained coal pillars inthe gob on the floor stress and energy distribution werestudied based on theoretical analysis and numerical simu-lation methods [28ndash31] Among them Zhang et al [30] usedthe improved composite structure mechanics method andnumerical simulation to study the load transfer rule of thecoal pillar group and the roof structure Numerical methodwas also employed to study the stress field of the remainingcoal pillar floor [32ndash34] and the coupling effect of theoverlapped coal pillar and mining activities were studiedMen et al [35] analysed the rock mass movement of theinclined coal seam along the longwall face dip directionusing theoretical analysis numerical simulation and in situobservation the relationship between the coal seam dipangle and the coal pillar stress was obtained

However less research has been conducted on the stressinfluence on the lower coal seam and the floor stresstransferring with different coal pillar sizes in the room gobMoreover the mining-induced dynamic pressure at theworking face when crossing through large coal pillar stillneeds to be further studied In this paper the longwall face incoal seam 3minus1 with adjacent room gob in the coal seam 2minus2 inthe Huoluowan Coal Mine was studied using theoreticalanalysis numerical simulation and similar material simu-lation methods

2 Engineering Background

+e average buried depth of the coal seam 3minus1 in theHuoluowan Coal Mine was 178m and with average thick-ness of 386m It belonged to the medium thick to thick coalseam +e dip angle was from 0deg to 3deg +e variation of thecoal seam thickness was small +e coal seam structure was

simple and stable +e mining method is longwall retreatmining along the strike direction

+e room gob in coal seam 2minus2 was located 30m abovethe coal seam 3minus1 +e width of the room pillar was 6m andthe width of the coal pillar was 8m+e average thickness ofthe 2minus2 coal seam was 536m To avoid large-area roofcollapse the mining area was isolated by pillars to 90msubareas +e isolating coal pillars were 20m or 50maccording to the roof conditions as shown in Figure 1 +emechanical parameters of coal and rock were measured inthe geological exploration summarised in Table 1

3 Influence of Coal Pillars on the UpperSeam Gob

31 Stability of the Room Pillars and the Isolating Coal PillarsWhen the load of the coal pillars is beyond its strength pillarfailure occurs Its instability judgement was [36]

Kgeσp

σc10778 + 0222

w

h1113874 1113875 (1)

where K is the safety coefficient σp is the average bearingstress of the coal pillar MPa σc1 is the uniaxial compressivestrength (UCS) of the coal which equals 15MPa for coalseam 2minus2 w is the room pillar width m and h is the height ofthe coal pillar m

+e average stress of the room pillar ldquo6times 8rdquo is

σ6times8 cH(w + a)(a + l)

wl 35cH (2)

where c was the average unit weight of the coal and rockstrata around 24 gcm3Hwas the buried depth of the roompillar 148m awas the width of the coal room 6m and lwasthe length of the room pillar 6m Substituting the corre-sponding values into equation (1) led to the safety coefficientK 141

When Kgt 15 the abutment pressure is mainly con-centrated at the centre of the coal pillar +e coal pillar canbear the load from the overlying rock strata to maintainlong-term stabilityWithK decreases the maximum stress ofthe coal pillar gradually moved from the centre of the coalpillar +e coal pillar generates plastic deformation or evenfailure from the outside to inside of the pillar It finally leadsto the instability of the whole coal pillar +e coal pillarcannot maintain the long-term stability if Klt 15 +ereforeit can be inferred that the room pillar of 8m would collapsefinally

+e load of the isolated coal pillar was induced by theweight of the overlying strata and abutment pressure Forsimplifying plane analysis was used to avoid 3D spaceproblem A homogeneous overlying rock strata was assumedto simply the complicated nonhomogeneous and anisotropicrock strata +e influence of the stress concentration at themargin of the coal pillar the movement of the overlying rockstrata and the abutment pressure of the coal pillar wasneglected

+e room pillar of 8m was instability +e fractured coalmass was rushed into the gob Fractured coal mass has

2 Shock and Vibration

bulking effect the gob was fully filled by coal mass finallyand then strata become stable +e load bearing capacity offractured coal mass in the gob was weak +erefore theremaining coal pillar would bear the weight of the rock stratain the collapsing area Coal pillars of 20m and 50m bearmajority load of the overlying rock strata

According to the above analysis it was assumed that theoriginal height of the room pillar is h After the room pillarfractured the height is h1 +en the following equation canbe acquired

h minus h1( 1113857a times η times m h1 times w times n (3)

where h was the original height of the room pillar m h1 wasthe height after the room pillar fractured m η was the bulkexpansion coefficient of the coal mass n was the number ofcoal rooms m and m was the number of coal pillars +ebulk expansion coefficient of coal usually ranged from 13 to15 In this calculation 14 was used+en the parameters ofthe Huoluowan Coal Mine were substituted into equation(3) It can be acquired that h1 235m that is the shorteningof the coal pillar is around 3m +e load that the coal pillarsof 20m and 50m beared was

σi nw + ma + wi( 1113857 times H minus 025(nw + ma)

2 cot δwic

(4)

where wi was the width of the coal pillar in the miningsection and the isolated coal pillar m and δ was the col-lapsing angle of the rock strata above the coal room It can becalculated that the load of the 20m and 50m coal pillars is153MPa and 97MPa respectively Substituting them intoequation (1) it was easy to acquire that the coal pillars of20m and 50m can maintain long-term stability

32 Stress Distribution under Upper Coal Pillars Due to thelong-term stable equilibrium the load above the coal pillarswas almost uniformly distributed To conveniently calculatethe influence of the coal pillar width a uniform load for eachsegment was assumed +e loading model of the underlyingstrata of the coal pillars with different sizes in coal seam 2minus2

in the Huoluowan Coal Mine was established as shown inFigure 2

+e origin of the coordinate was set at the rightboundary of the coal pillarM1 andM8 indicate the isolatingcoal pillars of 20m and 50m in coal seam 2minus2 respectively+e loads of coal pillars in M1 and M8 were λ1q0 and λ2q0where λ1 and λ2 were stress concentration coefficient and q0was the intact stress of the coal seam +e widths were L1and L4 +e load of room pillars afterM2ndashM7 failure is λ3q0And the width was L3 +e coal room width was L2According to the loading condition and geometry of the

Mining section I

Coal pillar of 20 m

Mining boundary

Rooms 6 times 8Coal seam 3ndash1

Coal seam 2ndash2Mining section II

Coal pillar of 50 m

Mining section IIIMining boundary

Figure 1 Characters of coal pillars in the room gob in coal seam 2minus2

Table 1 Mechanical parameters of the coal seam 2minus2-3minus1 in the Huoluowan Coal Mine

Rock strata Property +ickness(m)

Volume force (kNm3)

Tensile strength(MPa)

Shear modulus(GPa)

Bulk modulus(GPa)

Fine sandstone Key rock strata 2026 2353 195 397 602Sandymudstone mdash 151 233 218 421 612

Coal seam 2minus2 mdash 536 147 082 067 160Mudstone mdash 421 238 158 273 435Sandymudstone mdash 209 233 218 421 612

Fine sandstone mdash 254 2353 195 397 602Sandymudstone mdash 47 233 218 421 612

Fine sandstone Second key rockstrata 1171 2353 195 397 602

Siltstone mdash 786 2209 424 346 526Sandymudstone mdash 188 233 218 421 612

Coal seam 3minus1 mdash 416 147 082 067 160Mudstone mdash 194 238 218 273 435

Shock and Vibration 3

rock strata in different zones in the coal seam the load ofdifferent zones in coal seam 2minus2 the distance between twosides of each zone and the coordinate origin (a and b) areshown in Table 2

+e coal pillar floor stress transfer equations under theuniform loading condition are [37]

σx minusq0

2π2 θ2 minus θ1( 1113857 + sin 2θ2 minus sin 2θ1( 11138571113858 1113859 (5)

σy minusq0

2π2 θ2 minus θ1( 1113857 minus sin 2θ2 minus sin 2θ1( 11138571113858 1113859 (6)

τxy minusq0

2πcos 2θ2 minus cos 2θ1( 1113857 (7)

θ1 arctanyA minus a

xA

θ2 arctanyA minus b

xA

(8)

where σx was the horizontal stress of an arbitrary point A inthe rock strata below the coal pillar σy was the vertical stressof point A τxy was the shear stress of point A θ1 and θ2 werethe vertical intersection angles between point A and the twoboundary positions of the coal pillar XA and XB were thevertical and horizontal coordinates of the point A and a andb were the distances between the coordinate origin and theright and left boundaries of the coal pillar

According to equations (5)ndash(8) the vertical stress ap-plied to point A by each coal pillar in the lower coal seam canbe calculated +rough superposition the vertical stress thatall coal pillars applied on it can be calculated+e dimensionof the instable room pillar in the room gob in the coal seam2minus2 was 8m +e dimension of the isolated coal pillars was20m and 50m +en L1 20m L2 6m L3 8m andL4 50m According to the above theoretical calculation thesoftware of MATLAB was applied +en the stress distri-bution contour was acquired as shown in Figure 3

According to Figure 3(a) the vertical stress in the middleof the gob in the mining section II was relatively small +estress disturbing under room pillars of 8m was relativelysmall +e influencing depth is from 5m to 8m Around the

coal pillars of 20m and 50m the vertical stress was high Itinfluences the lower rock strata up to 42m and 58m re-spectively According to Figure 3(b) the influence of theroom pillar of 8m was relatively small up to 4m Aroundthe coal pillars of 20m and 50m the horizontal stress hasinfluence on the lower rock strata in the range of 10ndash18mand 16ndash23m According to Figure 3(c) the coal pillar of 8mhas already been damaged It cannot transfer shear stress+e shear stress mainly concentrated at the boundary of twosides of the coal pillars of 20m and 50m +eir influencingdepths on the lower rock strata were 25ndash30m and 30ndash50mrespectively +e shear stress below the coal pillar shows thereversed symmetric distribution pattern It suggests thatshear failure was likely occurred when mining the coal seam3minus1 under coal pillars of 20m and 50m

+e vertical stress and horizontal stress under coal pillarsof 20m and 50m were relatively high It accumulates largeamount of elastic-plastic energy Furthermore the influencingrange of the vertical stress was from 42m to 58m howeverthe distance between two coal seams was only 30m Miningactivity was influenced by the stress concentration of coalpillars of 20m and 50m+e influencing range of shear stressof coal pillars 20m and 50m was relatively larger Howeverthe magnitude was relatively small It suggests that when theworking face coal seam of 3minus1 crossed the above room gobhigh ground stress could appear

33 Roof Structure andDynamicMechanism Much researchhas been conducted on the dynamic pressure Most of themregard the instability of the three-hinged structure of the keyblock However according to the stability equation of thevoussoir beam and the geological condition in the Huo-luowan Coal Mine the key block of the main roof can easilyform two structures around the edge of the large coal pillarOne of them is the voussoir beam whose rupture line isinside of the coal pillar It was believed that when the ruptureline is inside of the coal pillar due to the supporting effect ofthe lower rock strata the load would not completely transferto the main roof above the lower coal seam when thestructural block lost stability In this case the stress at theworking face would be weak [38]

y A

0

λ1q0

M1 M2 M3 M4 M5 M6 M7 M8

L4L2L2L2L2

L2L2L2L120 m L3

L3L3L3L3L3

x

λ3q0 λ3q0 λ3q0 λ3q0 λ3q0 λ3q0

λ2q0

Figure 2 Loading model of the floor rock strata of the coal seam 2minus2

4 Shock and Vibration

+e dynamic pressure phenomenon was different in theHuoluowan Coal Mine It means that the key block along thelarge coal pillar side has already slipped and lost stability inadvance +ere was no acting force on the key block abovethe coal pillar and the key block along two sides So the mainkey block had relatively large rotation space as shown inFigure 4

+e working face stress in the coal seam 3minus1 below thelarge coal pillar was concentrated +e stress of two coalseam layers was overlaid +e second key strata below the

coal pillar was easy caving behind working face +eoverlying strata between coal seams caved +e pressure atworking face was relatively violent and frequent

When the coal under the pillar was gradually exploitedthe upper seam coal pillar was not able to steadily supportthe weight of the overlying rock strata Furthermore it waseasy to be cut off along the edge of the coal pillar influencingrange +en dynamic impact was applied on the second keyrock strata At this time the dynamic mine pressure wasdifferent from the previous Not only did it have an impact

Table 2 Loading and geometric parameters of different zones in the gob of the coal seam 2minus2

Different coal pillars M1 M2 M3 M4 M5 M6 M7 M8

q λ1q0 λ2q0 λ2q0 λ2q0 λ2q0 λ2q0 λ2q0 λ1q0a 20 20 + L1 + L2 20 + L1 + 7L2 + 6L3b 20 + L1 20 + L1 + L2 + L3 20 + L1 + 7L2 + 6L3 + L4

020 40 60 80 100 120 140 160 180 200

y (m)

10

20

30

40

50

60

x (m

)

14121086420

MPa

(a)

20 40 60 80 100 120 140 160 180 200y (m)

0

10

20

30

40

50

60

x (m

)

14121086420

MPa

(b)

20 40 60 80 100 120 140 160 180 200y (m)

10

20

30

40

50

60

0

x (m

)

3210ndash1ndash2ndash3ndash4

MPa

(c)

Figure 3 Stress contour under the upper seam floor (a) Vertical stress (b) Horizontal stress (c) Shear stress

Shock and Vibration 5

on the working face but also it caused damage to the retreatroadway to a certain distance

4 Numerical Simulation

41 Stress Field of the Coal Seam 3minus1 +e working faces incoal seams 2minus2 and 3minus1 in the Huoluowan Coal Mine werestudied According to the practical status and consideringthe boundary effect isolating pillars of 50m were set as twoboundaries +e model dimension was 500mtimes 400mtimes 200m For the numerical model the boundary conditionand loading state were explained as follows for the boundaryof two sides along the direction of X the displacement alongthe direction of X was fixed It means that for the boundarythe displacement along the direction ofXwas zero Similarlythe displacement of the two sides along the direction of Ywas fixed +e bottom boundary of the model was fixed Atthe top of the model it was free Along the direction of Z theself-weight capacity was applied According to the measuredstress data in the mine site a stress of 462MPa was appliedalong the direction of X Along the direction of Y a stress of22MPa was applied

After the coal seam of 2minus2 was exploited stress con-centration occurred in the coal pillar +e vertical stressdistributions before and after the room pillars were damagedare shown in Figure 5 Before the coal pillars was damagedthe maximum vertical stress was 146MPa and the stressconcentration coefficient was 43 All stresses were in thecentral area of the coal pillar indicating that two sides of thecoal pillar have already entered the plastic zone +e rela-tively high stress is transferred to the internal area of the coalpillar after they lost the bearing capacity +ey did not in-tersect in the zone where the stress concentration wasmaximal in the room pillar In fact they were located alongtwo sides of the centre +e vertical stress in the room pillarcentre were lower than that at two sides +is indicates thatin the room pillar there was still the elastic core which hascertain loading capacity After the coal pillar was damaged

the stress in the coal pillar decreased to 82MPa higher thanthe intact stress +is indicates that after the room pillar wasdamaged it still had residual load bearing capacity

After the coal seam 2minus2 was mined the stress field in therock strata is shown in Figure 6 In the figure the blackbreak line shows the stress distribution along the per-pendicular direction of the coal seam 3minus1 According to therelative position relationship it can be known that the coalseam 3minus1 below the gob was in pressure-relieving area afterthe coal seam 2minus2 was mined +e vertical stress is from36MPa to 43MPa decreasing by 44ndash20 +e accu-mulated elastic energy in the coal seam 3minus1 was effectivelyreleased +e possibility of dynamic disaster decreasedwhich was beneficial for the exploiting of the coal seam 3minus1However below the coal pillars of 20m and 50m thepressure increases Among them in the coal seam 3minus1 thevertical stresses were 59MPa and 56MPa respectively+e stress concentration coefficients were 131 and 124respectively

42 FailureCharacteristic of the SurroundingRock afterUpperSeam Mining For the horizontally bedded coal seam afterthe roadway was excavated the stress distribution of thesurrounding rock was relatively uniform +ey often dis-tribute symmetrically Under the influence of the high stressthe surrounding rock masses in the working face generatedplastic deformation +e mining section II was regarded asan example It was analysed that the failure characteristic ofthe surrounding rock masses is shown in Figure 7 after thecoal seam 2minus2 was mined It can be known that the failurezones of the surrounding rock in the working face showed asymmetric distribution For the surrounding rock aroundthe coal rooms the main failure mode was tensile andshearing For the upper strata it was mainly of shearingfailure +e room pillar of 8m lost stability For the floorbelow the coal pillar there was no failure And this wasconsistent with the theoretical analysis

Main key rock strata

Second key rock strata

Figure 4 +e roof structure when coal pillars were mined

6 Shock and Vibration

43 Mining-Induced Stress under Upper Coal PillarsWhen the working face advanced to 100m the stressdistribution in the surrounding rock is shown inFigure 8(a)+e influencing range of the abutment pressurewas around 52m in front of the working face+e abutmentpressure was around 45MPa to 213MPa +e maximumabutment pressure occurred in front of the working face6m +e stress concentration coefficient was around 473+e working face was below the coal pillar of 20m

When the working face advanced to 200m the stressdistribution in the surrounding rock was shown inFigure 8(b) +e influencing range of the abutment pressurewas around 58m in front of the working face +e abutment

pressure was around 45MPa to 200MPa Among them themaximum stress occurred in front of the working face 50m+e stress concentration coefficient was around 444 +eworking face started entering the coal pillar of 50m

When the working face arrived at 240m the stressdistribution in the surrounding rock is shown in Figure 8(c)+e influencing range of the abutment pressure was in frontof the working face around 39m+e abutment pressure wasaround 45MPa to 225MPa Among them the maximumstress occurred in front of the working face 40m +e stressconcentration coefficient was around 500 At this point theright boundary of the coal pillar of 50m was located in frontof the working face 10m

3ndash1 coal seam

Distance (times100)

ZZ-s

tres

s (times1

0^6)

500

450

400

00 05 10 15 20 25 30 35 40

Figure 6 Vertical stress distribution of the coal seam 3minus1 under the mining influence of coal seam 2minus2

NoneShear-n shear-pShear-n shear-p tension-pShear-pShear-p tension-pTension-p

ZonePlane active onColor by state-average

Figure 7 +e plastic zone distribution in the surrounding rock

Contour of ZZ-stress34509E + 0400000E + 00ndash10000E + 06ndash20000E + 06ndash30000E + 06ndash40000E + 06ndash50000E + 06ndash60000E + 06ndash70000E + 06ndash80000E + 06ndash90000E + 06ndash10000E + 07ndash12000E + 07ndash13000E + 07ndash14000E + 07ndash14656E + 07

(a)

Contour of ZZ-stress

ndash14443E + 04ndash10000E + 06ndash20000E + 06ndash30000E + 06ndash40000E + 06ndash50000E + 06ndash60000E + 06ndash70000E + 06ndash80000E + 06ndash90000E + 06ndash10000E + 07ndash11000E + 07ndash11428E + 07

(b)

Figure 5+e vertical stress distribution before and after the room pillar was damaged (a) Before the room pillar was damaged (b) After theroom pillar was damaged

Shock and Vibration 7

5 Similar Material Simulation Study

51Establishmentof theModel For themodel the geometricsimilarity ratio was 1100 For the volume weight similarityratio it was 075 (the practical volume weight was 24 gcm3

and the model volume weight was 18 gcm3) +e filling sizeof the model was 5000mmtimes 400mmtimes 2000mm(lengthtimeswidthtimes height) +e top boundary of the modelwas up to the ground surface+erefore it was not necessaryto apply pressure

To regenerate the coal pillar in the room gob the paraffinwas used Room retreat is simulated by heating to melt theparaffin +e coal rooms were mined and the coal pillarswere remained +en the coal pillars were destroyedmanually to simulate the instability of coal pillars +e coalpillars of 20m and the large coal pillars of 50m wereremained as shown in Figure 9

52 Overlying Strata Movement When the Working FaceCrossed Pillars +e overlying rock strata movement wasstudied to explain the violent dynamic pressure whencrossing the coal pillar of 50m Below the coal pillar of 50mthe roof was caved along the coal wall +e direction was

basically consistent with the left boundary of the reversedtrapezoid rock column +e top and bottom gobs wereconnected +e working face bears the weight of the in-terlayer strata between two coal seams and the weight of thereversed rock column above the coal pillar of 50m+eminepressure appearance was violent as shown in Figure 10(a)With the working face advancing there was a plastic zone atthe boundary of the large coal pillar When the working faceentered the right plastic zone range the coal pillar stabilitywas influenced Coal pillars cannot effectively support theweight of the overlying rock strata +e coal pillar of 50 mand the above rock column caved +is leaded to the col-lapsing of the rock strata above the rock column +efractures along two sides of the reversed trapezoid werecompressed again +e acting force among rock strata wasrelatively large +e roof was cut along the coal wall verti-cally +e coal masses in the working face that was notexploited were squeezed out Compared with the rightboundary of the reversed trapezoid rock column the di-rection shows the reversed symmetry form +e dynamicpressure was violent as shown in Figure 10(b)

When the coal seam 3minus1 crossed the large coal pillar thecollapsed height of overlying strata was relatively large +e

Contour of ZZ-stress32592E + 0500000E + 00ndash20000E + 06ndash40000E + 06ndash60000E + 06ndash80000E + 06ndash10000E + 07ndash12000E + 07ndash14000E + 07ndash16000E + 07ndash18000E + 07ndash20000E + 07ndash21340E + 07

(a)Contour of ZZ-stress

79775E + 0500000E + 00ndash20000E + 06ndash40000E + 06ndash60000E + 06ndash80000E + 06ndash10000E + 07ndash12000E + 07ndash14000E + 07ndash16000E + 07ndash18000E + 07ndash20000E + 07ndash20031E + 07

(b)

Contour of ZZ-stress78440E + 0500000E + 00ndash25000E + 06ndash50000E + 06ndash75000E + 06ndash10000E + 07ndash12500E + 07ndash15000E + 07ndash17500E + 07ndash20000E + 07ndash22500E + 07

(c)

Figure 8 Stress distribution contour when the working face advancing (a) Working face advanced to 100m (b) Working face advanced to200m (c) Working face advanced to 240m

8 Shock and Vibration

stress concentration of the upper coal seam and the mininginduced stress of the lower coal seamwere overlaid+e rockstrata between two coal seams caved thoroughly +e dy-namic pressure was violent When it was not able to supportthe reversed trapezoid rock column above the coal pillar of50m the high elastic-plastic energy that was accumulatedreleased +is induced the instability of the overlying rockstructure If this impact load was applied on the workingface the accident of the hydraulic supports crush wouldoccur

6 Conclusions

(1) In the Huoluowan Coal Mine for the room gob inthe shallow buried coal seam 2minus2 the stress distri-bution under coal pillar group was determinedWhen the room pillars of 8m lost stability they haddisturbing effect on the stress field of the rock strata5ndash8m lower than the coal seam As for the coalpillars of 20m and 50m the maximum influencingrange on the lower rock strata was 42m and 58mFor the lower coal seam of 3minus1 the vertical stressconcentration coefficients were 131 and 124respectively

(2) +e characteristic of the abutment pressure when theworking face crossed large coal pillars was revealed+e concentration coefficient reached 444ndash500Moreover it was maximal at the position in front ofthe working face 4ndash6m

(3) Above the large coal pillar there was no acting forceon the key strata along two sides Moreover therewas large rotating space It was determined that forthe working face in the coal seam 3minus1 when the largecoal pillars were mined the overlying rock strata loststability

(4) +e joint acting dynamic pressure mechanism of theworking face 3minus1 was revealed Specifically it was thehigh stress environment (the mining inducedstresses of the lower coal seam and upper coal seamwere overlaid) and the overall instability of the re-versed trapezoid rock column above the coal pillar+is would induce the accident of roof cutting andthe support compressing

Mining layouts and geological conditions vary from siteto site Although our study is based on the specific miningcondition it provides guidance and reference to thosestudies with similar conditions

44 m

20 m coal pillar Main key strata 50 m coal pillar

5 m remaining room pillar

3ndash1 coal seam

2ndash2 coal seam

Figure 9 Similarity simulation of the coal pillar in the room gob

50 m coal pillar

3ndash1 coal seam

(a)

Extrude coal 15 m

(b)

Figure 10 +e overlying rock strata movement when crossing the coal pillar (a) When crossing the coal pillar of 50m (b) When the coalpillar of 50m was mined

Shock and Vibration 9

Data Availability

Some or all data models or codes generated or used duringthe study are included within the article and are alsoavailable from the corresponding author upon request

Conflicts of Interest

+e authors declare that they have no conflicts of interestregarding the publication of this paper

Acknowledgments

+e authors gratefully acknowledge the National NaturalScience Foundation of China Project (no 51504127) forproviding partial financial support for the present study

References

[1] Q X Huang ldquoGround pressure behavior and definition ofshallow seamsrdquo Chinese Journal of Rock Mechanics and En-gineering vol 21 no 8 pp 1174ndash1177 2002

[2] Y F Ren and Q X Qi ldquoStudy on characteristic of stress fieldin surrounding rocks of shallow coal face under long wallminingrdquo Journal of China Coal Society vol 10 no 36pp 1612ndash1618 2011

[3] Y F Ren ldquoStudy on mine pressure and overlying stratamovement law of contiguous seams with shallow depthrdquo CoalScience and Technology vol 43 no 7 pp 11ndash14 2015

[4] T Zhu B S Zhang G R Feng and X Zhang ldquoRoof structureand control in the lower seam mining field in the ultra-closemultiple seamsrdquo Journal of China Coal Society vol 35 no 2pp 190ndash193 2010

[5] Z Yang B Tong C C Huang et al ldquoStudy on the movementlaw of overlying strata in mining face under room and pillargoafrdquo Journal of Mining amp Safety Engineering vol 29 no 2pp 157ndash161 2012

[6] Q S Bai S H Tu F T Wang et al ldquo+e evolution of miningstress and the mechanism of disaster caused by shallow buriedcoal pillarrdquo Chinese Journal of Rock Mechanics and Engi-neering vol 31 no S2 pp 3772ndash3778 2012

[7] P Małkowski Z Niedbalski and T Balarabe ldquoA statisticalanalysis of geomechanical data and its effect on rock massnumerical modeling a case studyrdquo International Journal ofCoal Science amp Technology vol 8 no 4 pp 1ndash12 2020

[8] D Cao A Wang S Ning et al ldquoCoalfield structure andstructural controls on coal in Chinardquo International Journal ofCoal Science amp Technology vol 7 no 5 pp 220ndash239 2020

[9] S Mao ldquoDevelopment of coal geological information tech-nologies in Chinardquo International Journal of Coal Science ampTechnology vol 7 no 4 pp 320ndash328 2020

[10] J Han H Liang C Cao and Z Bi ldquoA mechanical model forsheared joints based on Mohr-Coulomb material propertiesrdquoGeotechnique Letters vol 8 no 2 pp 1ndash14 2018

[11] D J Xue L L Lu J Zhou and L Lu ldquoCluster modeling of theshort-range correlation of acoustically emitted scatteringsignalsrdquo International Journal of Coal Science amp Technologypp 1ndash15 2020

[12] D J Xue J Zhou Y T Liu and L Gau ldquoOn the excavation-induced stress drop in damaged coal considering a coupledyield and failure criterionrdquo International Journal of CoalScience amp Technology vol 7 no 5 pp 58ndash67 2020

[13] D J Xue Y T Liu H W Zhou J Q Wang J F Liu andJ Zhou ldquoFractal characterization on anisotropy and fractalreconstruction of rough surface of granite under orthogonalshearrdquo Rock Mechanics and Rock Engineering vol 53 no 3pp 1225ndash1242 2020

[14] L Zhang S Chen C Zhang X Fang and S Li ldquo+echaracterization of bituminous coal microstructure andpermeability by liquid nitrogen fracturing based on μCTtechnologyrdquo Fuel vol 262 Article ID 116635 2020

[15] L Zhang J H Li J H Xue C Zhang and X Fang ldquoEx-perimental studies on the changing characteristics of the gasflow capacity on bituminous coal in CO2-ECBM and N2-ECBMrdquo Fuel vol 291 Article ID 120115 2021

[16] J Lin T Ren Y P Cheng J Nimcik and G Wang ldquoCyclicN2 injection for enhanced coal seam gas recovery a labo-ratory studyrdquo Energy vol 188 Article ID 116115 2019

[17] J Lin T Ren G Wang P Booth and J Nemcik ldquoExperi-mental investigation of N2 injection to enhance gas drainagein CO2-rich low permeable seamrdquo Fuel vol 215 pp 665ndash6742018

[18] B Chen ldquoStress-induced trend the clustering feature of coalmine disasters and earthquakes in Chinardquo InternationalJournal of Coal Science amp Technology vol 7 no 4 pp 676ndash692 2020

[19] X Wu Y Peng J Xu Q Yan W Nie and T Zhang ldquoEx-perimental study on evolution law for particle breakageduring coal and gas outburstrdquo International Journal of CoalScience amp Technology vol 7 no 1 pp 97ndash106 2020

[20] 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

[21] X Z Jie ldquoStudy on the characteristics of strata behavior inshallow seam longwall mining under the room-and-pillarmining goafrdquo Journal of China Coal Society vol 37 no 6pp 898ndash902 2012

[22] S H Tu F J Dou Z J Wan F Wang and Y Yuan ldquoStratacontrol technology of the fully mechanized face in shallowcoal seam close to the above room and pillar gobrdquo Journal ofChina Coal Society vol 36 no 3 pp 366ndash370 2011

[23] X Lian H Hu T Li and D Hu ldquoMain geological and miningfactors affecting ground cracks induced by underground coalmining in Shanxi Province Chinardquo International Journal ofCoal Science amp Technology vol 7 no 2 pp 362ndash370 2020

[24] D Xue H Zhou Y Zhao L Zhang L Deng and X WangldquoReal-time SEM observation of mesoscale failures underthermal-mechanical coupling sequences in graniterdquo Inter-national Journal of Rock Mechanics and Mining Sciencesvol 112 pp 35ndash46 2018

[25] Z L Yang ldquoStudy of controlling catastrophe for roof strata inshallow seam longwall miningrdquo Rock and Soil Mechanicsvol 29 no s1 pp 459ndash463 2011

[26] H D Li H H Yang B Zhang and G Chen ldquoControl studyof strong strata behaviors during the fully mechanizedworking face out of concentrated coal pillar in a shallow depthseam in proximity beneath a room mining goafrdquo Journal ofChina Coal Society vol 40 no s1 pp 6ndash11 2015

[27] J Y Zhao S L Wu J P Ma et al ldquoStudy on strata behaviorsregulation of coal working face under room and pillar mininggoafrdquo China Energy and Environmental Protection vol 6pp 60ndash63 2015

[28] J X Yang C Y Liu and B Yu ldquo+e effects of load distri-bution exerting on coal pillars on the stress and energy

10 Shock and Vibration

distribution of the floor stratardquo Acta Montanistica Slovacavol 21 pp 102ndash112 2016

[29] S Li L F Zhou M K Luo et al ldquoStrata behaviors analysis ofstage coal pillar in Tong Xinmine caused by repeatedminingrdquoJournal of Liaoning Technical University Natural Sciencevol 34 no 6 pp 661ndash667 2015

[30] S K Zhang L G Wang X D Zhang et al ldquoLoad transfereffects of the longwall sustaining coal pillars on the safety ofthe mining goaf systemrdquo Journal of Safety and Environmentvol 16 no 3 pp 116ndash119 2016

[31] L Li F Li Y Zhang D Yang and X Liu ldquoFormationmechanism and height calculation of the caved zone andwater-conducting fracture zone in solid backfill miningrdquoInternational Journal of Coal Science amp Technology vol 7no 1 pp 208ndash215 2020

[32] L Xu H X Wei Z Y Xiao and B Li ldquoEngineering cases andcharacteristics of deviatoric stressunder coal pillar in regionalfloorrdquo Coal Science and Technology vol 40 no 4 pp 23ndash252012

[33] Z J Zhu H W Zhang T W Lan B Huo and Y ChenldquoStudy on surrounding rock stress evolution law of fully-mechanized top coal caving mining face under role ofoverlapping coal pillarsrdquo Coal Science and Technology vol 45no 3 pp 26ndash31 2017

[34] C Zhu M He M Karakus X Zhang and Z Tao ldquoNumericalsimulations of the failure process of anaclinal slope physicalmodel and control mechanism of negative Poissonrsquos ratiocablerdquo Bulletin of Engineering Geology and the Environmentvol 80 no 4 pp 3365ndash3380 2021

[35] J G Men S Wang R P Yuan et al ldquoStudy on overburdenstrata structure features and stress distribution law of coalpillar in inclined seamrdquo Coal Science and Technology vol 42no 5 pp 21ndash24 2014

[36] M X Ren ldquoRational decision of pillarrsquos size of Wongawillimining methodrdquo Journal of Mining amp Safety Engineeringvol 21 no 1 pp 42-43+118 2004

[37] B Yu C Y Liu J X Yang and J Liu ldquoMechanism of strongpressure reveal under the influence of miningdual system ofcoal pillar in Datong mining areardquo Journal of China CoalSociety vol 39 no 1 pp 40ndash46 2014

[38] J F Ju ldquoMechanism and prevention of support crushingdisaster while mining out of the upper coal pillar in closedistance shallow seamsrdquo PhD thesis China University ofMining and Technology Xuzhou China 2013

Shock and Vibration 11

bulking effect the gob was fully filled by coal mass finallyand then strata become stable +e load bearing capacity offractured coal mass in the gob was weak +erefore theremaining coal pillar would bear the weight of the rock stratain the collapsing area Coal pillars of 20m and 50m bearmajority load of the overlying rock strata

According to the above analysis it was assumed that theoriginal height of the room pillar is h After the room pillarfractured the height is h1 +en the following equation canbe acquired

h minus h1( 1113857a times η times m h1 times w times n (3)

where h was the original height of the room pillar m h1 wasthe height after the room pillar fractured m η was the bulkexpansion coefficient of the coal mass n was the number ofcoal rooms m and m was the number of coal pillars +ebulk expansion coefficient of coal usually ranged from 13 to15 In this calculation 14 was used+en the parameters ofthe Huoluowan Coal Mine were substituted into equation(3) It can be acquired that h1 235m that is the shorteningof the coal pillar is around 3m +e load that the coal pillarsof 20m and 50m beared was

σi nw + ma + wi( 1113857 times H minus 025(nw + ma)

2 cot δwic

(4)

where wi was the width of the coal pillar in the miningsection and the isolated coal pillar m and δ was the col-lapsing angle of the rock strata above the coal room It can becalculated that the load of the 20m and 50m coal pillars is153MPa and 97MPa respectively Substituting them intoequation (1) it was easy to acquire that the coal pillars of20m and 50m can maintain long-term stability

32 Stress Distribution under Upper Coal Pillars Due to thelong-term stable equilibrium the load above the coal pillarswas almost uniformly distributed To conveniently calculatethe influence of the coal pillar width a uniform load for eachsegment was assumed +e loading model of the underlyingstrata of the coal pillars with different sizes in coal seam 2minus2

in the Huoluowan Coal Mine was established as shown inFigure 2

+e origin of the coordinate was set at the rightboundary of the coal pillarM1 andM8 indicate the isolatingcoal pillars of 20m and 50m in coal seam 2minus2 respectively+e loads of coal pillars in M1 and M8 were λ1q0 and λ2q0where λ1 and λ2 were stress concentration coefficient and q0was the intact stress of the coal seam +e widths were L1and L4 +e load of room pillars afterM2ndashM7 failure is λ3q0And the width was L3 +e coal room width was L2According to the loading condition and geometry of the

Mining section I

Coal pillar of 20 m

Mining boundary

Rooms 6 times 8Coal seam 3ndash1

Coal seam 2ndash2Mining section II

Coal pillar of 50 m

Mining section IIIMining boundary

Figure 1 Characters of coal pillars in the room gob in coal seam 2minus2

Table 1 Mechanical parameters of the coal seam 2minus2-3minus1 in the Huoluowan Coal Mine

Rock strata Property +ickness(m)

Volume force (kNm3)

Tensile strength(MPa)

Shear modulus(GPa)

Bulk modulus(GPa)

Fine sandstone Key rock strata 2026 2353 195 397 602Sandymudstone mdash 151 233 218 421 612

Coal seam 2minus2 mdash 536 147 082 067 160Mudstone mdash 421 238 158 273 435Sandymudstone mdash 209 233 218 421 612

Fine sandstone mdash 254 2353 195 397 602Sandymudstone mdash 47 233 218 421 612

Fine sandstone Second key rockstrata 1171 2353 195 397 602

Siltstone mdash 786 2209 424 346 526Sandymudstone mdash 188 233 218 421 612

Coal seam 3minus1 mdash 416 147 082 067 160Mudstone mdash 194 238 218 273 435

Shock and Vibration 3

rock strata in different zones in the coal seam the load ofdifferent zones in coal seam 2minus2 the distance between twosides of each zone and the coordinate origin (a and b) areshown in Table 2

+e coal pillar floor stress transfer equations under theuniform loading condition are [37]

σx minusq0

2π2 θ2 minus θ1( 1113857 + sin 2θ2 minus sin 2θ1( 11138571113858 1113859 (5)

σy minusq0

2π2 θ2 minus θ1( 1113857 minus sin 2θ2 minus sin 2θ1( 11138571113858 1113859 (6)

τxy minusq0

2πcos 2θ2 minus cos 2θ1( 1113857 (7)

θ1 arctanyA minus a

xA

θ2 arctanyA minus b

xA

(8)

where σx was the horizontal stress of an arbitrary point A inthe rock strata below the coal pillar σy was the vertical stressof point A τxy was the shear stress of point A θ1 and θ2 werethe vertical intersection angles between point A and the twoboundary positions of the coal pillar XA and XB were thevertical and horizontal coordinates of the point A and a andb were the distances between the coordinate origin and theright and left boundaries of the coal pillar

According to equations (5)ndash(8) the vertical stress ap-plied to point A by each coal pillar in the lower coal seam canbe calculated +rough superposition the vertical stress thatall coal pillars applied on it can be calculated+e dimensionof the instable room pillar in the room gob in the coal seam2minus2 was 8m +e dimension of the isolated coal pillars was20m and 50m +en L1 20m L2 6m L3 8m andL4 50m According to the above theoretical calculation thesoftware of MATLAB was applied +en the stress distri-bution contour was acquired as shown in Figure 3

According to Figure 3(a) the vertical stress in the middleof the gob in the mining section II was relatively small +estress disturbing under room pillars of 8m was relativelysmall +e influencing depth is from 5m to 8m Around the

coal pillars of 20m and 50m the vertical stress was high Itinfluences the lower rock strata up to 42m and 58m re-spectively According to Figure 3(b) the influence of theroom pillar of 8m was relatively small up to 4m Aroundthe coal pillars of 20m and 50m the horizontal stress hasinfluence on the lower rock strata in the range of 10ndash18mand 16ndash23m According to Figure 3(c) the coal pillar of 8mhas already been damaged It cannot transfer shear stress+e shear stress mainly concentrated at the boundary of twosides of the coal pillars of 20m and 50m +eir influencingdepths on the lower rock strata were 25ndash30m and 30ndash50mrespectively +e shear stress below the coal pillar shows thereversed symmetric distribution pattern It suggests thatshear failure was likely occurred when mining the coal seam3minus1 under coal pillars of 20m and 50m

+e vertical stress and horizontal stress under coal pillarsof 20m and 50m were relatively high It accumulates largeamount of elastic-plastic energy Furthermore the influencingrange of the vertical stress was from 42m to 58m howeverthe distance between two coal seams was only 30m Miningactivity was influenced by the stress concentration of coalpillars of 20m and 50m+e influencing range of shear stressof coal pillars 20m and 50m was relatively larger Howeverthe magnitude was relatively small It suggests that when theworking face coal seam of 3minus1 crossed the above room gobhigh ground stress could appear

33 Roof Structure andDynamicMechanism Much researchhas been conducted on the dynamic pressure Most of themregard the instability of the three-hinged structure of the keyblock However according to the stability equation of thevoussoir beam and the geological condition in the Huo-luowan Coal Mine the key block of the main roof can easilyform two structures around the edge of the large coal pillarOne of them is the voussoir beam whose rupture line isinside of the coal pillar It was believed that when the ruptureline is inside of the coal pillar due to the supporting effect ofthe lower rock strata the load would not completely transferto the main roof above the lower coal seam when thestructural block lost stability In this case the stress at theworking face would be weak [38]

y A

0

λ1q0

M1 M2 M3 M4 M5 M6 M7 M8

L4L2L2L2L2

L2L2L2L120 m L3

L3L3L3L3L3

x

λ3q0 λ3q0 λ3q0 λ3q0 λ3q0 λ3q0

λ2q0

Figure 2 Loading model of the floor rock strata of the coal seam 2minus2

4 Shock and Vibration

+e dynamic pressure phenomenon was different in theHuoluowan Coal Mine It means that the key block along thelarge coal pillar side has already slipped and lost stability inadvance +ere was no acting force on the key block abovethe coal pillar and the key block along two sides So the mainkey block had relatively large rotation space as shown inFigure 4

+e working face stress in the coal seam 3minus1 below thelarge coal pillar was concentrated +e stress of two coalseam layers was overlaid +e second key strata below the

coal pillar was easy caving behind working face +eoverlying strata between coal seams caved +e pressure atworking face was relatively violent and frequent

When the coal under the pillar was gradually exploitedthe upper seam coal pillar was not able to steadily supportthe weight of the overlying rock strata Furthermore it waseasy to be cut off along the edge of the coal pillar influencingrange +en dynamic impact was applied on the second keyrock strata At this time the dynamic mine pressure wasdifferent from the previous Not only did it have an impact

Table 2 Loading and geometric parameters of different zones in the gob of the coal seam 2minus2

Different coal pillars M1 M2 M3 M4 M5 M6 M7 M8

q λ1q0 λ2q0 λ2q0 λ2q0 λ2q0 λ2q0 λ2q0 λ1q0a 20 20 + L1 + L2 20 + L1 + 7L2 + 6L3b 20 + L1 20 + L1 + L2 + L3 20 + L1 + 7L2 + 6L3 + L4

020 40 60 80 100 120 140 160 180 200

y (m)

10

20

30

40

50

60

x (m

)

14121086420

MPa

(a)

20 40 60 80 100 120 140 160 180 200y (m)

0

10

20

30

40

50

60

x (m

)

14121086420

MPa

(b)

20 40 60 80 100 120 140 160 180 200y (m)

10

20

30

40

50

60

0

x (m

)

3210ndash1ndash2ndash3ndash4

MPa

(c)

Figure 3 Stress contour under the upper seam floor (a) Vertical stress (b) Horizontal stress (c) Shear stress

Shock and Vibration 5

on the working face but also it caused damage to the retreatroadway to a certain distance

4 Numerical Simulation

41 Stress Field of the Coal Seam 3minus1 +e working faces incoal seams 2minus2 and 3minus1 in the Huoluowan Coal Mine werestudied According to the practical status and consideringthe boundary effect isolating pillars of 50m were set as twoboundaries +e model dimension was 500mtimes 400mtimes 200m For the numerical model the boundary conditionand loading state were explained as follows for the boundaryof two sides along the direction of X the displacement alongthe direction of X was fixed It means that for the boundarythe displacement along the direction ofXwas zero Similarlythe displacement of the two sides along the direction of Ywas fixed +e bottom boundary of the model was fixed Atthe top of the model it was free Along the direction of Z theself-weight capacity was applied According to the measuredstress data in the mine site a stress of 462MPa was appliedalong the direction of X Along the direction of Y a stress of22MPa was applied

After the coal seam of 2minus2 was exploited stress con-centration occurred in the coal pillar +e vertical stressdistributions before and after the room pillars were damagedare shown in Figure 5 Before the coal pillars was damagedthe maximum vertical stress was 146MPa and the stressconcentration coefficient was 43 All stresses were in thecentral area of the coal pillar indicating that two sides of thecoal pillar have already entered the plastic zone +e rela-tively high stress is transferred to the internal area of the coalpillar after they lost the bearing capacity +ey did not in-tersect in the zone where the stress concentration wasmaximal in the room pillar In fact they were located alongtwo sides of the centre +e vertical stress in the room pillarcentre were lower than that at two sides +is indicates thatin the room pillar there was still the elastic core which hascertain loading capacity After the coal pillar was damaged

the stress in the coal pillar decreased to 82MPa higher thanthe intact stress +is indicates that after the room pillar wasdamaged it still had residual load bearing capacity

After the coal seam 2minus2 was mined the stress field in therock strata is shown in Figure 6 In the figure the blackbreak line shows the stress distribution along the per-pendicular direction of the coal seam 3minus1 According to therelative position relationship it can be known that the coalseam 3minus1 below the gob was in pressure-relieving area afterthe coal seam 2minus2 was mined +e vertical stress is from36MPa to 43MPa decreasing by 44ndash20 +e accu-mulated elastic energy in the coal seam 3minus1 was effectivelyreleased +e possibility of dynamic disaster decreasedwhich was beneficial for the exploiting of the coal seam 3minus1However below the coal pillars of 20m and 50m thepressure increases Among them in the coal seam 3minus1 thevertical stresses were 59MPa and 56MPa respectively+e stress concentration coefficients were 131 and 124respectively

42 FailureCharacteristic of the SurroundingRock afterUpperSeam Mining For the horizontally bedded coal seam afterthe roadway was excavated the stress distribution of thesurrounding rock was relatively uniform +ey often dis-tribute symmetrically Under the influence of the high stressthe surrounding rock masses in the working face generatedplastic deformation +e mining section II was regarded asan example It was analysed that the failure characteristic ofthe surrounding rock masses is shown in Figure 7 after thecoal seam 2minus2 was mined It can be known that the failurezones of the surrounding rock in the working face showed asymmetric distribution For the surrounding rock aroundthe coal rooms the main failure mode was tensile andshearing For the upper strata it was mainly of shearingfailure +e room pillar of 8m lost stability For the floorbelow the coal pillar there was no failure And this wasconsistent with the theoretical analysis

Main key rock strata

Second key rock strata

Figure 4 +e roof structure when coal pillars were mined

6 Shock and Vibration

43 Mining-Induced Stress under Upper Coal PillarsWhen the working face advanced to 100m the stressdistribution in the surrounding rock is shown inFigure 8(a)+e influencing range of the abutment pressurewas around 52m in front of the working face+e abutmentpressure was around 45MPa to 213MPa +e maximumabutment pressure occurred in front of the working face6m +e stress concentration coefficient was around 473+e working face was below the coal pillar of 20m

When the working face advanced to 200m the stressdistribution in the surrounding rock was shown inFigure 8(b) +e influencing range of the abutment pressurewas around 58m in front of the working face +e abutment

pressure was around 45MPa to 200MPa Among them themaximum stress occurred in front of the working face 50m+e stress concentration coefficient was around 444 +eworking face started entering the coal pillar of 50m

When the working face arrived at 240m the stressdistribution in the surrounding rock is shown in Figure 8(c)+e influencing range of the abutment pressure was in frontof the working face around 39m+e abutment pressure wasaround 45MPa to 225MPa Among them the maximumstress occurred in front of the working face 40m +e stressconcentration coefficient was around 500 At this point theright boundary of the coal pillar of 50m was located in frontof the working face 10m

3ndash1 coal seam

Distance (times100)

ZZ-s

tres

s (times1

0^6)

500

450

400

00 05 10 15 20 25 30 35 40

Figure 6 Vertical stress distribution of the coal seam 3minus1 under the mining influence of coal seam 2minus2

NoneShear-n shear-pShear-n shear-p tension-pShear-pShear-p tension-pTension-p

ZonePlane active onColor by state-average

Figure 7 +e plastic zone distribution in the surrounding rock

Contour of ZZ-stress34509E + 0400000E + 00ndash10000E + 06ndash20000E + 06ndash30000E + 06ndash40000E + 06ndash50000E + 06ndash60000E + 06ndash70000E + 06ndash80000E + 06ndash90000E + 06ndash10000E + 07ndash12000E + 07ndash13000E + 07ndash14000E + 07ndash14656E + 07

(a)

Contour of ZZ-stress

ndash14443E + 04ndash10000E + 06ndash20000E + 06ndash30000E + 06ndash40000E + 06ndash50000E + 06ndash60000E + 06ndash70000E + 06ndash80000E + 06ndash90000E + 06ndash10000E + 07ndash11000E + 07ndash11428E + 07

(b)

Figure 5+e vertical stress distribution before and after the room pillar was damaged (a) Before the room pillar was damaged (b) After theroom pillar was damaged

Shock and Vibration 7

5 Similar Material Simulation Study

51Establishmentof theModel For themodel the geometricsimilarity ratio was 1100 For the volume weight similarityratio it was 075 (the practical volume weight was 24 gcm3

and the model volume weight was 18 gcm3) +e filling sizeof the model was 5000mmtimes 400mmtimes 2000mm(lengthtimeswidthtimes height) +e top boundary of the modelwas up to the ground surface+erefore it was not necessaryto apply pressure

To regenerate the coal pillar in the room gob the paraffinwas used Room retreat is simulated by heating to melt theparaffin +e coal rooms were mined and the coal pillarswere remained +en the coal pillars were destroyedmanually to simulate the instability of coal pillars +e coalpillars of 20m and the large coal pillars of 50m wereremained as shown in Figure 9

52 Overlying Strata Movement When the Working FaceCrossed Pillars +e overlying rock strata movement wasstudied to explain the violent dynamic pressure whencrossing the coal pillar of 50m Below the coal pillar of 50mthe roof was caved along the coal wall +e direction was

basically consistent with the left boundary of the reversedtrapezoid rock column +e top and bottom gobs wereconnected +e working face bears the weight of the in-terlayer strata between two coal seams and the weight of thereversed rock column above the coal pillar of 50m+eminepressure appearance was violent as shown in Figure 10(a)With the working face advancing there was a plastic zone atthe boundary of the large coal pillar When the working faceentered the right plastic zone range the coal pillar stabilitywas influenced Coal pillars cannot effectively support theweight of the overlying rock strata +e coal pillar of 50 mand the above rock column caved +is leaded to the col-lapsing of the rock strata above the rock column +efractures along two sides of the reversed trapezoid werecompressed again +e acting force among rock strata wasrelatively large +e roof was cut along the coal wall verti-cally +e coal masses in the working face that was notexploited were squeezed out Compared with the rightboundary of the reversed trapezoid rock column the di-rection shows the reversed symmetry form +e dynamicpressure was violent as shown in Figure 10(b)

When the coal seam 3minus1 crossed the large coal pillar thecollapsed height of overlying strata was relatively large +e

Contour of ZZ-stress32592E + 0500000E + 00ndash20000E + 06ndash40000E + 06ndash60000E + 06ndash80000E + 06ndash10000E + 07ndash12000E + 07ndash14000E + 07ndash16000E + 07ndash18000E + 07ndash20000E + 07ndash21340E + 07

(a)Contour of ZZ-stress

79775E + 0500000E + 00ndash20000E + 06ndash40000E + 06ndash60000E + 06ndash80000E + 06ndash10000E + 07ndash12000E + 07ndash14000E + 07ndash16000E + 07ndash18000E + 07ndash20000E + 07ndash20031E + 07

(b)

Contour of ZZ-stress78440E + 0500000E + 00ndash25000E + 06ndash50000E + 06ndash75000E + 06ndash10000E + 07ndash12500E + 07ndash15000E + 07ndash17500E + 07ndash20000E + 07ndash22500E + 07

(c)

Figure 8 Stress distribution contour when the working face advancing (a) Working face advanced to 100m (b) Working face advanced to200m (c) Working face advanced to 240m

8 Shock and Vibration

stress concentration of the upper coal seam and the mininginduced stress of the lower coal seamwere overlaid+e rockstrata between two coal seams caved thoroughly +e dy-namic pressure was violent When it was not able to supportthe reversed trapezoid rock column above the coal pillar of50m the high elastic-plastic energy that was accumulatedreleased +is induced the instability of the overlying rockstructure If this impact load was applied on the workingface the accident of the hydraulic supports crush wouldoccur

6 Conclusions

(1) In the Huoluowan Coal Mine for the room gob inthe shallow buried coal seam 2minus2 the stress distri-bution under coal pillar group was determinedWhen the room pillars of 8m lost stability they haddisturbing effect on the stress field of the rock strata5ndash8m lower than the coal seam As for the coalpillars of 20m and 50m the maximum influencingrange on the lower rock strata was 42m and 58mFor the lower coal seam of 3minus1 the vertical stressconcentration coefficients were 131 and 124respectively

(2) +e characteristic of the abutment pressure when theworking face crossed large coal pillars was revealed+e concentration coefficient reached 444ndash500Moreover it was maximal at the position in front ofthe working face 4ndash6m

(3) Above the large coal pillar there was no acting forceon the key strata along two sides Moreover therewas large rotating space It was determined that forthe working face in the coal seam 3minus1 when the largecoal pillars were mined the overlying rock strata loststability

(4) +e joint acting dynamic pressure mechanism of theworking face 3minus1 was revealed Specifically it was thehigh stress environment (the mining inducedstresses of the lower coal seam and upper coal seamwere overlaid) and the overall instability of the re-versed trapezoid rock column above the coal pillar+is would induce the accident of roof cutting andthe support compressing

Mining layouts and geological conditions vary from siteto site Although our study is based on the specific miningcondition it provides guidance and reference to thosestudies with similar conditions

44 m

20 m coal pillar Main key strata 50 m coal pillar

5 m remaining room pillar

3ndash1 coal seam

2ndash2 coal seam

Figure 9 Similarity simulation of the coal pillar in the room gob

50 m coal pillar

3ndash1 coal seam

(a)

Extrude coal 15 m

(b)

Figure 10 +e overlying rock strata movement when crossing the coal pillar (a) When crossing the coal pillar of 50m (b) When the coalpillar of 50m was mined

Shock and Vibration 9

Data Availability

Some or all data models or codes generated or used duringthe study are included within the article and are alsoavailable from the corresponding author upon request

Conflicts of Interest

+e authors declare that they have no conflicts of interestregarding the publication of this paper

Acknowledgments

+e authors gratefully acknowledge the National NaturalScience Foundation of China Project (no 51504127) forproviding partial financial support for the present study

References

[1] Q X Huang ldquoGround pressure behavior and definition ofshallow seamsrdquo Chinese Journal of Rock Mechanics and En-gineering vol 21 no 8 pp 1174ndash1177 2002

[2] Y F Ren and Q X Qi ldquoStudy on characteristic of stress fieldin surrounding rocks of shallow coal face under long wallminingrdquo Journal of China Coal Society vol 10 no 36pp 1612ndash1618 2011

[3] Y F Ren ldquoStudy on mine pressure and overlying stratamovement law of contiguous seams with shallow depthrdquo CoalScience and Technology vol 43 no 7 pp 11ndash14 2015

[4] T Zhu B S Zhang G R Feng and X Zhang ldquoRoof structureand control in the lower seam mining field in the ultra-closemultiple seamsrdquo Journal of China Coal Society vol 35 no 2pp 190ndash193 2010

[5] Z Yang B Tong C C Huang et al ldquoStudy on the movementlaw of overlying strata in mining face under room and pillargoafrdquo Journal of Mining amp Safety Engineering vol 29 no 2pp 157ndash161 2012

[6] Q S Bai S H Tu F T Wang et al ldquo+e evolution of miningstress and the mechanism of disaster caused by shallow buriedcoal pillarrdquo Chinese Journal of Rock Mechanics and Engi-neering vol 31 no S2 pp 3772ndash3778 2012

[7] P Małkowski Z Niedbalski and T Balarabe ldquoA statisticalanalysis of geomechanical data and its effect on rock massnumerical modeling a case studyrdquo International Journal ofCoal Science amp Technology vol 8 no 4 pp 1ndash12 2020

[8] D Cao A Wang S Ning et al ldquoCoalfield structure andstructural controls on coal in Chinardquo International Journal ofCoal Science amp Technology vol 7 no 5 pp 220ndash239 2020

[9] S Mao ldquoDevelopment of coal geological information tech-nologies in Chinardquo International Journal of Coal Science ampTechnology vol 7 no 4 pp 320ndash328 2020

[10] J Han H Liang C Cao and Z Bi ldquoA mechanical model forsheared joints based on Mohr-Coulomb material propertiesrdquoGeotechnique Letters vol 8 no 2 pp 1ndash14 2018

[11] D J Xue L L Lu J Zhou and L Lu ldquoCluster modeling of theshort-range correlation of acoustically emitted scatteringsignalsrdquo International Journal of Coal Science amp Technologypp 1ndash15 2020

[12] D J Xue J Zhou Y T Liu and L Gau ldquoOn the excavation-induced stress drop in damaged coal considering a coupledyield and failure criterionrdquo International Journal of CoalScience amp Technology vol 7 no 5 pp 58ndash67 2020

[13] D J Xue Y T Liu H W Zhou J Q Wang J F Liu andJ Zhou ldquoFractal characterization on anisotropy and fractalreconstruction of rough surface of granite under orthogonalshearrdquo Rock Mechanics and Rock Engineering vol 53 no 3pp 1225ndash1242 2020

[14] L Zhang S Chen C Zhang X Fang and S Li ldquo+echaracterization of bituminous coal microstructure andpermeability by liquid nitrogen fracturing based on μCTtechnologyrdquo Fuel vol 262 Article ID 116635 2020

[15] L Zhang J H Li J H Xue C Zhang and X Fang ldquoEx-perimental studies on the changing characteristics of the gasflow capacity on bituminous coal in CO2-ECBM and N2-ECBMrdquo Fuel vol 291 Article ID 120115 2021

[16] J Lin T Ren Y P Cheng J Nimcik and G Wang ldquoCyclicN2 injection for enhanced coal seam gas recovery a labo-ratory studyrdquo Energy vol 188 Article ID 116115 2019

[17] J Lin T Ren G Wang P Booth and J Nemcik ldquoExperi-mental investigation of N2 injection to enhance gas drainagein CO2-rich low permeable seamrdquo Fuel vol 215 pp 665ndash6742018

[18] B Chen ldquoStress-induced trend the clustering feature of coalmine disasters and earthquakes in Chinardquo InternationalJournal of Coal Science amp Technology vol 7 no 4 pp 676ndash692 2020

[19] X Wu Y Peng J Xu Q Yan W Nie and T Zhang ldquoEx-perimental study on evolution law for particle breakageduring coal and gas outburstrdquo International Journal of CoalScience amp Technology vol 7 no 1 pp 97ndash106 2020

[20] 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

[21] X Z Jie ldquoStudy on the characteristics of strata behavior inshallow seam longwall mining under the room-and-pillarmining goafrdquo Journal of China Coal Society vol 37 no 6pp 898ndash902 2012

[22] S H Tu F J Dou Z J Wan F Wang and Y Yuan ldquoStratacontrol technology of the fully mechanized face in shallowcoal seam close to the above room and pillar gobrdquo Journal ofChina Coal Society vol 36 no 3 pp 366ndash370 2011

[23] X Lian H Hu T Li and D Hu ldquoMain geological and miningfactors affecting ground cracks induced by underground coalmining in Shanxi Province Chinardquo International Journal ofCoal Science amp Technology vol 7 no 2 pp 362ndash370 2020

[24] D Xue H Zhou Y Zhao L Zhang L Deng and X WangldquoReal-time SEM observation of mesoscale failures underthermal-mechanical coupling sequences in graniterdquo Inter-national Journal of Rock Mechanics and Mining Sciencesvol 112 pp 35ndash46 2018

[25] Z L Yang ldquoStudy of controlling catastrophe for roof strata inshallow seam longwall miningrdquo Rock and Soil Mechanicsvol 29 no s1 pp 459ndash463 2011

[26] H D Li H H Yang B Zhang and G Chen ldquoControl studyof strong strata behaviors during the fully mechanizedworking face out of concentrated coal pillar in a shallow depthseam in proximity beneath a room mining goafrdquo Journal ofChina Coal Society vol 40 no s1 pp 6ndash11 2015

[27] J Y Zhao S L Wu J P Ma et al ldquoStudy on strata behaviorsregulation of coal working face under room and pillar mininggoafrdquo China Energy and Environmental Protection vol 6pp 60ndash63 2015

[28] J X Yang C Y Liu and B Yu ldquo+e effects of load distri-bution exerting on coal pillars on the stress and energy

10 Shock and Vibration

distribution of the floor stratardquo Acta Montanistica Slovacavol 21 pp 102ndash112 2016

[29] S Li L F Zhou M K Luo et al ldquoStrata behaviors analysis ofstage coal pillar in Tong Xinmine caused by repeatedminingrdquoJournal of Liaoning Technical University Natural Sciencevol 34 no 6 pp 661ndash667 2015

[30] S K Zhang L G Wang X D Zhang et al ldquoLoad transfereffects of the longwall sustaining coal pillars on the safety ofthe mining goaf systemrdquo Journal of Safety and Environmentvol 16 no 3 pp 116ndash119 2016

[31] L Li F Li Y Zhang D Yang and X Liu ldquoFormationmechanism and height calculation of the caved zone andwater-conducting fracture zone in solid backfill miningrdquoInternational Journal of Coal Science amp Technology vol 7no 1 pp 208ndash215 2020

[32] L Xu H X Wei Z Y Xiao and B Li ldquoEngineering cases andcharacteristics of deviatoric stressunder coal pillar in regionalfloorrdquo Coal Science and Technology vol 40 no 4 pp 23ndash252012

[33] Z J Zhu H W Zhang T W Lan B Huo and Y ChenldquoStudy on surrounding rock stress evolution law of fully-mechanized top coal caving mining face under role ofoverlapping coal pillarsrdquo Coal Science and Technology vol 45no 3 pp 26ndash31 2017

[34] C Zhu M He M Karakus X Zhang and Z Tao ldquoNumericalsimulations of the failure process of anaclinal slope physicalmodel and control mechanism of negative Poissonrsquos ratiocablerdquo Bulletin of Engineering Geology and the Environmentvol 80 no 4 pp 3365ndash3380 2021

[35] J G Men S Wang R P Yuan et al ldquoStudy on overburdenstrata structure features and stress distribution law of coalpillar in inclined seamrdquo Coal Science and Technology vol 42no 5 pp 21ndash24 2014

[36] M X Ren ldquoRational decision of pillarrsquos size of Wongawillimining methodrdquo Journal of Mining amp Safety Engineeringvol 21 no 1 pp 42-43+118 2004

[37] B Yu C Y Liu J X Yang and J Liu ldquoMechanism of strongpressure reveal under the influence of miningdual system ofcoal pillar in Datong mining areardquo Journal of China CoalSociety vol 39 no 1 pp 40ndash46 2014

[38] J F Ju ldquoMechanism and prevention of support crushingdisaster while mining out of the upper coal pillar in closedistance shallow seamsrdquo PhD thesis China University ofMining and Technology Xuzhou China 2013

Shock and Vibration 11

rock strata in different zones in the coal seam the load ofdifferent zones in coal seam 2minus2 the distance between twosides of each zone and the coordinate origin (a and b) areshown in Table 2

+e coal pillar floor stress transfer equations under theuniform loading condition are [37]

σx minusq0

2π2 θ2 minus θ1( 1113857 + sin 2θ2 minus sin 2θ1( 11138571113858 1113859 (5)

σy minusq0

2π2 θ2 minus θ1( 1113857 minus sin 2θ2 minus sin 2θ1( 11138571113858 1113859 (6)

τxy minusq0

2πcos 2θ2 minus cos 2θ1( 1113857 (7)

θ1 arctanyA minus a

xA

θ2 arctanyA minus b

xA

(8)

where σx was the horizontal stress of an arbitrary point A inthe rock strata below the coal pillar σy was the vertical stressof point A τxy was the shear stress of point A θ1 and θ2 werethe vertical intersection angles between point A and the twoboundary positions of the coal pillar XA and XB were thevertical and horizontal coordinates of the point A and a andb were the distances between the coordinate origin and theright and left boundaries of the coal pillar

According to equations (5)ndash(8) the vertical stress ap-plied to point A by each coal pillar in the lower coal seam canbe calculated +rough superposition the vertical stress thatall coal pillars applied on it can be calculated+e dimensionof the instable room pillar in the room gob in the coal seam2minus2 was 8m +e dimension of the isolated coal pillars was20m and 50m +en L1 20m L2 6m L3 8m andL4 50m According to the above theoretical calculation thesoftware of MATLAB was applied +en the stress distri-bution contour was acquired as shown in Figure 3

According to Figure 3(a) the vertical stress in the middleof the gob in the mining section II was relatively small +estress disturbing under room pillars of 8m was relativelysmall +e influencing depth is from 5m to 8m Around the

coal pillars of 20m and 50m the vertical stress was high Itinfluences the lower rock strata up to 42m and 58m re-spectively According to Figure 3(b) the influence of theroom pillar of 8m was relatively small up to 4m Aroundthe coal pillars of 20m and 50m the horizontal stress hasinfluence on the lower rock strata in the range of 10ndash18mand 16ndash23m According to Figure 3(c) the coal pillar of 8mhas already been damaged It cannot transfer shear stress+e shear stress mainly concentrated at the boundary of twosides of the coal pillars of 20m and 50m +eir influencingdepths on the lower rock strata were 25ndash30m and 30ndash50mrespectively +e shear stress below the coal pillar shows thereversed symmetric distribution pattern It suggests thatshear failure was likely occurred when mining the coal seam3minus1 under coal pillars of 20m and 50m

+e vertical stress and horizontal stress under coal pillarsof 20m and 50m were relatively high It accumulates largeamount of elastic-plastic energy Furthermore the influencingrange of the vertical stress was from 42m to 58m howeverthe distance between two coal seams was only 30m Miningactivity was influenced by the stress concentration of coalpillars of 20m and 50m+e influencing range of shear stressof coal pillars 20m and 50m was relatively larger Howeverthe magnitude was relatively small It suggests that when theworking face coal seam of 3minus1 crossed the above room gobhigh ground stress could appear

33 Roof Structure andDynamicMechanism Much researchhas been conducted on the dynamic pressure Most of themregard the instability of the three-hinged structure of the keyblock However according to the stability equation of thevoussoir beam and the geological condition in the Huo-luowan Coal Mine the key block of the main roof can easilyform two structures around the edge of the large coal pillarOne of them is the voussoir beam whose rupture line isinside of the coal pillar It was believed that when the ruptureline is inside of the coal pillar due to the supporting effect ofthe lower rock strata the load would not completely transferto the main roof above the lower coal seam when thestructural block lost stability In this case the stress at theworking face would be weak [38]

y A

0

λ1q0

M1 M2 M3 M4 M5 M6 M7 M8

L4L2L2L2L2

L2L2L2L120 m L3

L3L3L3L3L3

x

λ3q0 λ3q0 λ3q0 λ3q0 λ3q0 λ3q0

λ2q0

Figure 2 Loading model of the floor rock strata of the coal seam 2minus2

4 Shock and Vibration

+e dynamic pressure phenomenon was different in theHuoluowan Coal Mine It means that the key block along thelarge coal pillar side has already slipped and lost stability inadvance +ere was no acting force on the key block abovethe coal pillar and the key block along two sides So the mainkey block had relatively large rotation space as shown inFigure 4

+e working face stress in the coal seam 3minus1 below thelarge coal pillar was concentrated +e stress of two coalseam layers was overlaid +e second key strata below the

coal pillar was easy caving behind working face +eoverlying strata between coal seams caved +e pressure atworking face was relatively violent and frequent

When the coal under the pillar was gradually exploitedthe upper seam coal pillar was not able to steadily supportthe weight of the overlying rock strata Furthermore it waseasy to be cut off along the edge of the coal pillar influencingrange +en dynamic impact was applied on the second keyrock strata At this time the dynamic mine pressure wasdifferent from the previous Not only did it have an impact

Table 2 Loading and geometric parameters of different zones in the gob of the coal seam 2minus2

Different coal pillars M1 M2 M3 M4 M5 M6 M7 M8

q λ1q0 λ2q0 λ2q0 λ2q0 λ2q0 λ2q0 λ2q0 λ1q0a 20 20 + L1 + L2 20 + L1 + 7L2 + 6L3b 20 + L1 20 + L1 + L2 + L3 20 + L1 + 7L2 + 6L3 + L4

020 40 60 80 100 120 140 160 180 200

y (m)

10

20

30

40

50

60

x (m

)

14121086420

MPa

(a)

20 40 60 80 100 120 140 160 180 200y (m)

0

10

20

30

40

50

60

x (m

)

14121086420

MPa

(b)

20 40 60 80 100 120 140 160 180 200y (m)

10

20

30

40

50

60

0

x (m

)

3210ndash1ndash2ndash3ndash4

MPa

(c)

Figure 3 Stress contour under the upper seam floor (a) Vertical stress (b) Horizontal stress (c) Shear stress

Shock and Vibration 5

on the working face but also it caused damage to the retreatroadway to a certain distance

4 Numerical Simulation

41 Stress Field of the Coal Seam 3minus1 +e working faces incoal seams 2minus2 and 3minus1 in the Huoluowan Coal Mine werestudied According to the practical status and consideringthe boundary effect isolating pillars of 50m were set as twoboundaries +e model dimension was 500mtimes 400mtimes 200m For the numerical model the boundary conditionand loading state were explained as follows for the boundaryof two sides along the direction of X the displacement alongthe direction of X was fixed It means that for the boundarythe displacement along the direction ofXwas zero Similarlythe displacement of the two sides along the direction of Ywas fixed +e bottom boundary of the model was fixed Atthe top of the model it was free Along the direction of Z theself-weight capacity was applied According to the measuredstress data in the mine site a stress of 462MPa was appliedalong the direction of X Along the direction of Y a stress of22MPa was applied

After the coal seam of 2minus2 was exploited stress con-centration occurred in the coal pillar +e vertical stressdistributions before and after the room pillars were damagedare shown in Figure 5 Before the coal pillars was damagedthe maximum vertical stress was 146MPa and the stressconcentration coefficient was 43 All stresses were in thecentral area of the coal pillar indicating that two sides of thecoal pillar have already entered the plastic zone +e rela-tively high stress is transferred to the internal area of the coalpillar after they lost the bearing capacity +ey did not in-tersect in the zone where the stress concentration wasmaximal in the room pillar In fact they were located alongtwo sides of the centre +e vertical stress in the room pillarcentre were lower than that at two sides +is indicates thatin the room pillar there was still the elastic core which hascertain loading capacity After the coal pillar was damaged

the stress in the coal pillar decreased to 82MPa higher thanthe intact stress +is indicates that after the room pillar wasdamaged it still had residual load bearing capacity

After the coal seam 2minus2 was mined the stress field in therock strata is shown in Figure 6 In the figure the blackbreak line shows the stress distribution along the per-pendicular direction of the coal seam 3minus1 According to therelative position relationship it can be known that the coalseam 3minus1 below the gob was in pressure-relieving area afterthe coal seam 2minus2 was mined +e vertical stress is from36MPa to 43MPa decreasing by 44ndash20 +e accu-mulated elastic energy in the coal seam 3minus1 was effectivelyreleased +e possibility of dynamic disaster decreasedwhich was beneficial for the exploiting of the coal seam 3minus1However below the coal pillars of 20m and 50m thepressure increases Among them in the coal seam 3minus1 thevertical stresses were 59MPa and 56MPa respectively+e stress concentration coefficients were 131 and 124respectively

42 FailureCharacteristic of the SurroundingRock afterUpperSeam Mining For the horizontally bedded coal seam afterthe roadway was excavated the stress distribution of thesurrounding rock was relatively uniform +ey often dis-tribute symmetrically Under the influence of the high stressthe surrounding rock masses in the working face generatedplastic deformation +e mining section II was regarded asan example It was analysed that the failure characteristic ofthe surrounding rock masses is shown in Figure 7 after thecoal seam 2minus2 was mined It can be known that the failurezones of the surrounding rock in the working face showed asymmetric distribution For the surrounding rock aroundthe coal rooms the main failure mode was tensile andshearing For the upper strata it was mainly of shearingfailure +e room pillar of 8m lost stability For the floorbelow the coal pillar there was no failure And this wasconsistent with the theoretical analysis

Main key rock strata

Second key rock strata

Figure 4 +e roof structure when coal pillars were mined

6 Shock and Vibration

43 Mining-Induced Stress under Upper Coal PillarsWhen the working face advanced to 100m the stressdistribution in the surrounding rock is shown inFigure 8(a)+e influencing range of the abutment pressurewas around 52m in front of the working face+e abutmentpressure was around 45MPa to 213MPa +e maximumabutment pressure occurred in front of the working face6m +e stress concentration coefficient was around 473+e working face was below the coal pillar of 20m

When the working face advanced to 200m the stressdistribution in the surrounding rock was shown inFigure 8(b) +e influencing range of the abutment pressurewas around 58m in front of the working face +e abutment

pressure was around 45MPa to 200MPa Among them themaximum stress occurred in front of the working face 50m+e stress concentration coefficient was around 444 +eworking face started entering the coal pillar of 50m

When the working face arrived at 240m the stressdistribution in the surrounding rock is shown in Figure 8(c)+e influencing range of the abutment pressure was in frontof the working face around 39m+e abutment pressure wasaround 45MPa to 225MPa Among them the maximumstress occurred in front of the working face 40m +e stressconcentration coefficient was around 500 At this point theright boundary of the coal pillar of 50m was located in frontof the working face 10m

3ndash1 coal seam

Distance (times100)

ZZ-s

tres

s (times1

0^6)

500

450

400

00 05 10 15 20 25 30 35 40

Figure 6 Vertical stress distribution of the coal seam 3minus1 under the mining influence of coal seam 2minus2

NoneShear-n shear-pShear-n shear-p tension-pShear-pShear-p tension-pTension-p

ZonePlane active onColor by state-average

Figure 7 +e plastic zone distribution in the surrounding rock

Contour of ZZ-stress34509E + 0400000E + 00ndash10000E + 06ndash20000E + 06ndash30000E + 06ndash40000E + 06ndash50000E + 06ndash60000E + 06ndash70000E + 06ndash80000E + 06ndash90000E + 06ndash10000E + 07ndash12000E + 07ndash13000E + 07ndash14000E + 07ndash14656E + 07

(a)

Contour of ZZ-stress

ndash14443E + 04ndash10000E + 06ndash20000E + 06ndash30000E + 06ndash40000E + 06ndash50000E + 06ndash60000E + 06ndash70000E + 06ndash80000E + 06ndash90000E + 06ndash10000E + 07ndash11000E + 07ndash11428E + 07

(b)

Figure 5+e vertical stress distribution before and after the room pillar was damaged (a) Before the room pillar was damaged (b) After theroom pillar was damaged

Shock and Vibration 7

5 Similar Material Simulation Study

51Establishmentof theModel For themodel the geometricsimilarity ratio was 1100 For the volume weight similarityratio it was 075 (the practical volume weight was 24 gcm3

and the model volume weight was 18 gcm3) +e filling sizeof the model was 5000mmtimes 400mmtimes 2000mm(lengthtimeswidthtimes height) +e top boundary of the modelwas up to the ground surface+erefore it was not necessaryto apply pressure

To regenerate the coal pillar in the room gob the paraffinwas used Room retreat is simulated by heating to melt theparaffin +e coal rooms were mined and the coal pillarswere remained +en the coal pillars were destroyedmanually to simulate the instability of coal pillars +e coalpillars of 20m and the large coal pillars of 50m wereremained as shown in Figure 9

52 Overlying Strata Movement When the Working FaceCrossed Pillars +e overlying rock strata movement wasstudied to explain the violent dynamic pressure whencrossing the coal pillar of 50m Below the coal pillar of 50mthe roof was caved along the coal wall +e direction was

basically consistent with the left boundary of the reversedtrapezoid rock column +e top and bottom gobs wereconnected +e working face bears the weight of the in-terlayer strata between two coal seams and the weight of thereversed rock column above the coal pillar of 50m+eminepressure appearance was violent as shown in Figure 10(a)With the working face advancing there was a plastic zone atthe boundary of the large coal pillar When the working faceentered the right plastic zone range the coal pillar stabilitywas influenced Coal pillars cannot effectively support theweight of the overlying rock strata +e coal pillar of 50 mand the above rock column caved +is leaded to the col-lapsing of the rock strata above the rock column +efractures along two sides of the reversed trapezoid werecompressed again +e acting force among rock strata wasrelatively large +e roof was cut along the coal wall verti-cally +e coal masses in the working face that was notexploited were squeezed out Compared with the rightboundary of the reversed trapezoid rock column the di-rection shows the reversed symmetry form +e dynamicpressure was violent as shown in Figure 10(b)

When the coal seam 3minus1 crossed the large coal pillar thecollapsed height of overlying strata was relatively large +e

Contour of ZZ-stress32592E + 0500000E + 00ndash20000E + 06ndash40000E + 06ndash60000E + 06ndash80000E + 06ndash10000E + 07ndash12000E + 07ndash14000E + 07ndash16000E + 07ndash18000E + 07ndash20000E + 07ndash21340E + 07

(a)Contour of ZZ-stress

79775E + 0500000E + 00ndash20000E + 06ndash40000E + 06ndash60000E + 06ndash80000E + 06ndash10000E + 07ndash12000E + 07ndash14000E + 07ndash16000E + 07ndash18000E + 07ndash20000E + 07ndash20031E + 07

(b)

Contour of ZZ-stress78440E + 0500000E + 00ndash25000E + 06ndash50000E + 06ndash75000E + 06ndash10000E + 07ndash12500E + 07ndash15000E + 07ndash17500E + 07ndash20000E + 07ndash22500E + 07

(c)

Figure 8 Stress distribution contour when the working face advancing (a) Working face advanced to 100m (b) Working face advanced to200m (c) Working face advanced to 240m

8 Shock and Vibration

stress concentration of the upper coal seam and the mininginduced stress of the lower coal seamwere overlaid+e rockstrata between two coal seams caved thoroughly +e dy-namic pressure was violent When it was not able to supportthe reversed trapezoid rock column above the coal pillar of50m the high elastic-plastic energy that was accumulatedreleased +is induced the instability of the overlying rockstructure If this impact load was applied on the workingface the accident of the hydraulic supports crush wouldoccur

6 Conclusions

(1) In the Huoluowan Coal Mine for the room gob inthe shallow buried coal seam 2minus2 the stress distri-bution under coal pillar group was determinedWhen the room pillars of 8m lost stability they haddisturbing effect on the stress field of the rock strata5ndash8m lower than the coal seam As for the coalpillars of 20m and 50m the maximum influencingrange on the lower rock strata was 42m and 58mFor the lower coal seam of 3minus1 the vertical stressconcentration coefficients were 131 and 124respectively

(2) +e characteristic of the abutment pressure when theworking face crossed large coal pillars was revealed+e concentration coefficient reached 444ndash500Moreover it was maximal at the position in front ofthe working face 4ndash6m

(3) Above the large coal pillar there was no acting forceon the key strata along two sides Moreover therewas large rotating space It was determined that forthe working face in the coal seam 3minus1 when the largecoal pillars were mined the overlying rock strata loststability

(4) +e joint acting dynamic pressure mechanism of theworking face 3minus1 was revealed Specifically it was thehigh stress environment (the mining inducedstresses of the lower coal seam and upper coal seamwere overlaid) and the overall instability of the re-versed trapezoid rock column above the coal pillar+is would induce the accident of roof cutting andthe support compressing

Mining layouts and geological conditions vary from siteto site Although our study is based on the specific miningcondition it provides guidance and reference to thosestudies with similar conditions

44 m

20 m coal pillar Main key strata 50 m coal pillar

5 m remaining room pillar

3ndash1 coal seam

2ndash2 coal seam

Figure 9 Similarity simulation of the coal pillar in the room gob

50 m coal pillar

3ndash1 coal seam

(a)

Extrude coal 15 m

(b)

Figure 10 +e overlying rock strata movement when crossing the coal pillar (a) When crossing the coal pillar of 50m (b) When the coalpillar of 50m was mined

Shock and Vibration 9

Data Availability

Some or all data models or codes generated or used duringthe study are included within the article and are alsoavailable from the corresponding author upon request

Conflicts of Interest

+e authors declare that they have no conflicts of interestregarding the publication of this paper

Acknowledgments

+e authors gratefully acknowledge the National NaturalScience Foundation of China Project (no 51504127) forproviding partial financial support for the present study

References

[1] Q X Huang ldquoGround pressure behavior and definition ofshallow seamsrdquo Chinese Journal of Rock Mechanics and En-gineering vol 21 no 8 pp 1174ndash1177 2002

[2] Y F Ren and Q X Qi ldquoStudy on characteristic of stress fieldin surrounding rocks of shallow coal face under long wallminingrdquo Journal of China Coal Society vol 10 no 36pp 1612ndash1618 2011

[3] Y F Ren ldquoStudy on mine pressure and overlying stratamovement law of contiguous seams with shallow depthrdquo CoalScience and Technology vol 43 no 7 pp 11ndash14 2015

[4] T Zhu B S Zhang G R Feng and X Zhang ldquoRoof structureand control in the lower seam mining field in the ultra-closemultiple seamsrdquo Journal of China Coal Society vol 35 no 2pp 190ndash193 2010

[5] Z Yang B Tong C C Huang et al ldquoStudy on the movementlaw of overlying strata in mining face under room and pillargoafrdquo Journal of Mining amp Safety Engineering vol 29 no 2pp 157ndash161 2012

[6] Q S Bai S H Tu F T Wang et al ldquo+e evolution of miningstress and the mechanism of disaster caused by shallow buriedcoal pillarrdquo Chinese Journal of Rock Mechanics and Engi-neering vol 31 no S2 pp 3772ndash3778 2012

[7] P Małkowski Z Niedbalski and T Balarabe ldquoA statisticalanalysis of geomechanical data and its effect on rock massnumerical modeling a case studyrdquo International Journal ofCoal Science amp Technology vol 8 no 4 pp 1ndash12 2020

[8] D Cao A Wang S Ning et al ldquoCoalfield structure andstructural controls on coal in Chinardquo International Journal ofCoal Science amp Technology vol 7 no 5 pp 220ndash239 2020

[9] S Mao ldquoDevelopment of coal geological information tech-nologies in Chinardquo International Journal of Coal Science ampTechnology vol 7 no 4 pp 320ndash328 2020

[10] J Han H Liang C Cao and Z Bi ldquoA mechanical model forsheared joints based on Mohr-Coulomb material propertiesrdquoGeotechnique Letters vol 8 no 2 pp 1ndash14 2018

[11] D J Xue L L Lu J Zhou and L Lu ldquoCluster modeling of theshort-range correlation of acoustically emitted scatteringsignalsrdquo International Journal of Coal Science amp Technologypp 1ndash15 2020

[12] D J Xue J Zhou Y T Liu and L Gau ldquoOn the excavation-induced stress drop in damaged coal considering a coupledyield and failure criterionrdquo International Journal of CoalScience amp Technology vol 7 no 5 pp 58ndash67 2020

[13] D J Xue Y T Liu H W Zhou J Q Wang J F Liu andJ Zhou ldquoFractal characterization on anisotropy and fractalreconstruction of rough surface of granite under orthogonalshearrdquo Rock Mechanics and Rock Engineering vol 53 no 3pp 1225ndash1242 2020

[14] L Zhang S Chen C Zhang X Fang and S Li ldquo+echaracterization of bituminous coal microstructure andpermeability by liquid nitrogen fracturing based on μCTtechnologyrdquo Fuel vol 262 Article ID 116635 2020

[15] L Zhang J H Li J H Xue C Zhang and X Fang ldquoEx-perimental studies on the changing characteristics of the gasflow capacity on bituminous coal in CO2-ECBM and N2-ECBMrdquo Fuel vol 291 Article ID 120115 2021

[16] J Lin T Ren Y P Cheng J Nimcik and G Wang ldquoCyclicN2 injection for enhanced coal seam gas recovery a labo-ratory studyrdquo Energy vol 188 Article ID 116115 2019

[17] J Lin T Ren G Wang P Booth and J Nemcik ldquoExperi-mental investigation of N2 injection to enhance gas drainagein CO2-rich low permeable seamrdquo Fuel vol 215 pp 665ndash6742018

[18] B Chen ldquoStress-induced trend the clustering feature of coalmine disasters and earthquakes in Chinardquo InternationalJournal of Coal Science amp Technology vol 7 no 4 pp 676ndash692 2020

[19] X Wu Y Peng J Xu Q Yan W Nie and T Zhang ldquoEx-perimental study on evolution law for particle breakageduring coal and gas outburstrdquo International Journal of CoalScience amp Technology vol 7 no 1 pp 97ndash106 2020

[20] 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

[21] X Z Jie ldquoStudy on the characteristics of strata behavior inshallow seam longwall mining under the room-and-pillarmining goafrdquo Journal of China Coal Society vol 37 no 6pp 898ndash902 2012

[22] S H Tu F J Dou Z J Wan F Wang and Y Yuan ldquoStratacontrol technology of the fully mechanized face in shallowcoal seam close to the above room and pillar gobrdquo Journal ofChina Coal Society vol 36 no 3 pp 366ndash370 2011

[23] X Lian H Hu T Li and D Hu ldquoMain geological and miningfactors affecting ground cracks induced by underground coalmining in Shanxi Province Chinardquo International Journal ofCoal Science amp Technology vol 7 no 2 pp 362ndash370 2020

[24] D Xue H Zhou Y Zhao L Zhang L Deng and X WangldquoReal-time SEM observation of mesoscale failures underthermal-mechanical coupling sequences in graniterdquo Inter-national Journal of Rock Mechanics and Mining Sciencesvol 112 pp 35ndash46 2018

[25] Z L Yang ldquoStudy of controlling catastrophe for roof strata inshallow seam longwall miningrdquo Rock and Soil Mechanicsvol 29 no s1 pp 459ndash463 2011

[26] H D Li H H Yang B Zhang and G Chen ldquoControl studyof strong strata behaviors during the fully mechanizedworking face out of concentrated coal pillar in a shallow depthseam in proximity beneath a room mining goafrdquo Journal ofChina Coal Society vol 40 no s1 pp 6ndash11 2015

[27] J Y Zhao S L Wu J P Ma et al ldquoStudy on strata behaviorsregulation of coal working face under room and pillar mininggoafrdquo China Energy and Environmental Protection vol 6pp 60ndash63 2015

[28] J X Yang C Y Liu and B Yu ldquo+e effects of load distri-bution exerting on coal pillars on the stress and energy

10 Shock and Vibration

distribution of the floor stratardquo Acta Montanistica Slovacavol 21 pp 102ndash112 2016

[29] S Li L F Zhou M K Luo et al ldquoStrata behaviors analysis ofstage coal pillar in Tong Xinmine caused by repeatedminingrdquoJournal of Liaoning Technical University Natural Sciencevol 34 no 6 pp 661ndash667 2015

[30] S K Zhang L G Wang X D Zhang et al ldquoLoad transfereffects of the longwall sustaining coal pillars on the safety ofthe mining goaf systemrdquo Journal of Safety and Environmentvol 16 no 3 pp 116ndash119 2016

[31] L Li F Li Y Zhang D Yang and X Liu ldquoFormationmechanism and height calculation of the caved zone andwater-conducting fracture zone in solid backfill miningrdquoInternational Journal of Coal Science amp Technology vol 7no 1 pp 208ndash215 2020

[32] L Xu H X Wei Z Y Xiao and B Li ldquoEngineering cases andcharacteristics of deviatoric stressunder coal pillar in regionalfloorrdquo Coal Science and Technology vol 40 no 4 pp 23ndash252012

[33] Z J Zhu H W Zhang T W Lan B Huo and Y ChenldquoStudy on surrounding rock stress evolution law of fully-mechanized top coal caving mining face under role ofoverlapping coal pillarsrdquo Coal Science and Technology vol 45no 3 pp 26ndash31 2017

[34] C Zhu M He M Karakus X Zhang and Z Tao ldquoNumericalsimulations of the failure process of anaclinal slope physicalmodel and control mechanism of negative Poissonrsquos ratiocablerdquo Bulletin of Engineering Geology and the Environmentvol 80 no 4 pp 3365ndash3380 2021

[35] J G Men S Wang R P Yuan et al ldquoStudy on overburdenstrata structure features and stress distribution law of coalpillar in inclined seamrdquo Coal Science and Technology vol 42no 5 pp 21ndash24 2014

[36] M X Ren ldquoRational decision of pillarrsquos size of Wongawillimining methodrdquo Journal of Mining amp Safety Engineeringvol 21 no 1 pp 42-43+118 2004

[37] B Yu C Y Liu J X Yang and J Liu ldquoMechanism of strongpressure reveal under the influence of miningdual system ofcoal pillar in Datong mining areardquo Journal of China CoalSociety vol 39 no 1 pp 40ndash46 2014

[38] J F Ju ldquoMechanism and prevention of support crushingdisaster while mining out of the upper coal pillar in closedistance shallow seamsrdquo PhD thesis China University ofMining and Technology Xuzhou China 2013

Shock and Vibration 11

+e dynamic pressure phenomenon was different in theHuoluowan Coal Mine It means that the key block along thelarge coal pillar side has already slipped and lost stability inadvance +ere was no acting force on the key block abovethe coal pillar and the key block along two sides So the mainkey block had relatively large rotation space as shown inFigure 4

+e working face stress in the coal seam 3minus1 below thelarge coal pillar was concentrated +e stress of two coalseam layers was overlaid +e second key strata below the

coal pillar was easy caving behind working face +eoverlying strata between coal seams caved +e pressure atworking face was relatively violent and frequent

When the coal under the pillar was gradually exploitedthe upper seam coal pillar was not able to steadily supportthe weight of the overlying rock strata Furthermore it waseasy to be cut off along the edge of the coal pillar influencingrange +en dynamic impact was applied on the second keyrock strata At this time the dynamic mine pressure wasdifferent from the previous Not only did it have an impact

Table 2 Loading and geometric parameters of different zones in the gob of the coal seam 2minus2

Different coal pillars M1 M2 M3 M4 M5 M6 M7 M8

q λ1q0 λ2q0 λ2q0 λ2q0 λ2q0 λ2q0 λ2q0 λ1q0a 20 20 + L1 + L2 20 + L1 + 7L2 + 6L3b 20 + L1 20 + L1 + L2 + L3 20 + L1 + 7L2 + 6L3 + L4

020 40 60 80 100 120 140 160 180 200

y (m)

10

20

30

40

50

60

x (m

)

14121086420

MPa

(a)

20 40 60 80 100 120 140 160 180 200y (m)

0

10

20

30

40

50

60

x (m

)

14121086420

MPa

(b)

20 40 60 80 100 120 140 160 180 200y (m)

10

20

30

40

50

60

0

x (m

)

3210ndash1ndash2ndash3ndash4

MPa

(c)

Figure 3 Stress contour under the upper seam floor (a) Vertical stress (b) Horizontal stress (c) Shear stress

Shock and Vibration 5

on the working face but also it caused damage to the retreatroadway to a certain distance

4 Numerical Simulation

41 Stress Field of the Coal Seam 3minus1 +e working faces incoal seams 2minus2 and 3minus1 in the Huoluowan Coal Mine werestudied According to the practical status and consideringthe boundary effect isolating pillars of 50m were set as twoboundaries +e model dimension was 500mtimes 400mtimes 200m For the numerical model the boundary conditionand loading state were explained as follows for the boundaryof two sides along the direction of X the displacement alongthe direction of X was fixed It means that for the boundarythe displacement along the direction ofXwas zero Similarlythe displacement of the two sides along the direction of Ywas fixed +e bottom boundary of the model was fixed Atthe top of the model it was free Along the direction of Z theself-weight capacity was applied According to the measuredstress data in the mine site a stress of 462MPa was appliedalong the direction of X Along the direction of Y a stress of22MPa was applied

After the coal seam of 2minus2 was exploited stress con-centration occurred in the coal pillar +e vertical stressdistributions before and after the room pillars were damagedare shown in Figure 5 Before the coal pillars was damagedthe maximum vertical stress was 146MPa and the stressconcentration coefficient was 43 All stresses were in thecentral area of the coal pillar indicating that two sides of thecoal pillar have already entered the plastic zone +e rela-tively high stress is transferred to the internal area of the coalpillar after they lost the bearing capacity +ey did not in-tersect in the zone where the stress concentration wasmaximal in the room pillar In fact they were located alongtwo sides of the centre +e vertical stress in the room pillarcentre were lower than that at two sides +is indicates thatin the room pillar there was still the elastic core which hascertain loading capacity After the coal pillar was damaged

the stress in the coal pillar decreased to 82MPa higher thanthe intact stress +is indicates that after the room pillar wasdamaged it still had residual load bearing capacity

After the coal seam 2minus2 was mined the stress field in therock strata is shown in Figure 6 In the figure the blackbreak line shows the stress distribution along the per-pendicular direction of the coal seam 3minus1 According to therelative position relationship it can be known that the coalseam 3minus1 below the gob was in pressure-relieving area afterthe coal seam 2minus2 was mined +e vertical stress is from36MPa to 43MPa decreasing by 44ndash20 +e accu-mulated elastic energy in the coal seam 3minus1 was effectivelyreleased +e possibility of dynamic disaster decreasedwhich was beneficial for the exploiting of the coal seam 3minus1However below the coal pillars of 20m and 50m thepressure increases Among them in the coal seam 3minus1 thevertical stresses were 59MPa and 56MPa respectively+e stress concentration coefficients were 131 and 124respectively

42 FailureCharacteristic of the SurroundingRock afterUpperSeam Mining For the horizontally bedded coal seam afterthe roadway was excavated the stress distribution of thesurrounding rock was relatively uniform +ey often dis-tribute symmetrically Under the influence of the high stressthe surrounding rock masses in the working face generatedplastic deformation +e mining section II was regarded asan example It was analysed that the failure characteristic ofthe surrounding rock masses is shown in Figure 7 after thecoal seam 2minus2 was mined It can be known that the failurezones of the surrounding rock in the working face showed asymmetric distribution For the surrounding rock aroundthe coal rooms the main failure mode was tensile andshearing For the upper strata it was mainly of shearingfailure +e room pillar of 8m lost stability For the floorbelow the coal pillar there was no failure And this wasconsistent with the theoretical analysis

Main key rock strata

Second key rock strata

Figure 4 +e roof structure when coal pillars were mined

6 Shock and Vibration

43 Mining-Induced Stress under Upper Coal PillarsWhen the working face advanced to 100m the stressdistribution in the surrounding rock is shown inFigure 8(a)+e influencing range of the abutment pressurewas around 52m in front of the working face+e abutmentpressure was around 45MPa to 213MPa +e maximumabutment pressure occurred in front of the working face6m +e stress concentration coefficient was around 473+e working face was below the coal pillar of 20m

When the working face advanced to 200m the stressdistribution in the surrounding rock was shown inFigure 8(b) +e influencing range of the abutment pressurewas around 58m in front of the working face +e abutment

pressure was around 45MPa to 200MPa Among them themaximum stress occurred in front of the working face 50m+e stress concentration coefficient was around 444 +eworking face started entering the coal pillar of 50m

When the working face arrived at 240m the stressdistribution in the surrounding rock is shown in Figure 8(c)+e influencing range of the abutment pressure was in frontof the working face around 39m+e abutment pressure wasaround 45MPa to 225MPa Among them the maximumstress occurred in front of the working face 40m +e stressconcentration coefficient was around 500 At this point theright boundary of the coal pillar of 50m was located in frontof the working face 10m

3ndash1 coal seam

Distance (times100)

ZZ-s

tres

s (times1

0^6)

500

450

400

00 05 10 15 20 25 30 35 40

Figure 6 Vertical stress distribution of the coal seam 3minus1 under the mining influence of coal seam 2minus2

NoneShear-n shear-pShear-n shear-p tension-pShear-pShear-p tension-pTension-p

ZonePlane active onColor by state-average

Figure 7 +e plastic zone distribution in the surrounding rock

Contour of ZZ-stress34509E + 0400000E + 00ndash10000E + 06ndash20000E + 06ndash30000E + 06ndash40000E + 06ndash50000E + 06ndash60000E + 06ndash70000E + 06ndash80000E + 06ndash90000E + 06ndash10000E + 07ndash12000E + 07ndash13000E + 07ndash14000E + 07ndash14656E + 07

(a)

Contour of ZZ-stress

ndash14443E + 04ndash10000E + 06ndash20000E + 06ndash30000E + 06ndash40000E + 06ndash50000E + 06ndash60000E + 06ndash70000E + 06ndash80000E + 06ndash90000E + 06ndash10000E + 07ndash11000E + 07ndash11428E + 07

(b)

Figure 5+e vertical stress distribution before and after the room pillar was damaged (a) Before the room pillar was damaged (b) After theroom pillar was damaged

Shock and Vibration 7

5 Similar Material Simulation Study

51Establishmentof theModel For themodel the geometricsimilarity ratio was 1100 For the volume weight similarityratio it was 075 (the practical volume weight was 24 gcm3

and the model volume weight was 18 gcm3) +e filling sizeof the model was 5000mmtimes 400mmtimes 2000mm(lengthtimeswidthtimes height) +e top boundary of the modelwas up to the ground surface+erefore it was not necessaryto apply pressure

To regenerate the coal pillar in the room gob the paraffinwas used Room retreat is simulated by heating to melt theparaffin +e coal rooms were mined and the coal pillarswere remained +en the coal pillars were destroyedmanually to simulate the instability of coal pillars +e coalpillars of 20m and the large coal pillars of 50m wereremained as shown in Figure 9

52 Overlying Strata Movement When the Working FaceCrossed Pillars +e overlying rock strata movement wasstudied to explain the violent dynamic pressure whencrossing the coal pillar of 50m Below the coal pillar of 50mthe roof was caved along the coal wall +e direction was

basically consistent with the left boundary of the reversedtrapezoid rock column +e top and bottom gobs wereconnected +e working face bears the weight of the in-terlayer strata between two coal seams and the weight of thereversed rock column above the coal pillar of 50m+eminepressure appearance was violent as shown in Figure 10(a)With the working face advancing there was a plastic zone atthe boundary of the large coal pillar When the working faceentered the right plastic zone range the coal pillar stabilitywas influenced Coal pillars cannot effectively support theweight of the overlying rock strata +e coal pillar of 50 mand the above rock column caved +is leaded to the col-lapsing of the rock strata above the rock column +efractures along two sides of the reversed trapezoid werecompressed again +e acting force among rock strata wasrelatively large +e roof was cut along the coal wall verti-cally +e coal masses in the working face that was notexploited were squeezed out Compared with the rightboundary of the reversed trapezoid rock column the di-rection shows the reversed symmetry form +e dynamicpressure was violent as shown in Figure 10(b)

When the coal seam 3minus1 crossed the large coal pillar thecollapsed height of overlying strata was relatively large +e

Contour of ZZ-stress32592E + 0500000E + 00ndash20000E + 06ndash40000E + 06ndash60000E + 06ndash80000E + 06ndash10000E + 07ndash12000E + 07ndash14000E + 07ndash16000E + 07ndash18000E + 07ndash20000E + 07ndash21340E + 07

(a)Contour of ZZ-stress

79775E + 0500000E + 00ndash20000E + 06ndash40000E + 06ndash60000E + 06ndash80000E + 06ndash10000E + 07ndash12000E + 07ndash14000E + 07ndash16000E + 07ndash18000E + 07ndash20000E + 07ndash20031E + 07

(b)

Contour of ZZ-stress78440E + 0500000E + 00ndash25000E + 06ndash50000E + 06ndash75000E + 06ndash10000E + 07ndash12500E + 07ndash15000E + 07ndash17500E + 07ndash20000E + 07ndash22500E + 07

(c)

Figure 8 Stress distribution contour when the working face advancing (a) Working face advanced to 100m (b) Working face advanced to200m (c) Working face advanced to 240m

8 Shock and Vibration

stress concentration of the upper coal seam and the mininginduced stress of the lower coal seamwere overlaid+e rockstrata between two coal seams caved thoroughly +e dy-namic pressure was violent When it was not able to supportthe reversed trapezoid rock column above the coal pillar of50m the high elastic-plastic energy that was accumulatedreleased +is induced the instability of the overlying rockstructure If this impact load was applied on the workingface the accident of the hydraulic supports crush wouldoccur

6 Conclusions

(1) In the Huoluowan Coal Mine for the room gob inthe shallow buried coal seam 2minus2 the stress distri-bution under coal pillar group was determinedWhen the room pillars of 8m lost stability they haddisturbing effect on the stress field of the rock strata5ndash8m lower than the coal seam As for the coalpillars of 20m and 50m the maximum influencingrange on the lower rock strata was 42m and 58mFor the lower coal seam of 3minus1 the vertical stressconcentration coefficients were 131 and 124respectively

(2) +e characteristic of the abutment pressure when theworking face crossed large coal pillars was revealed+e concentration coefficient reached 444ndash500Moreover it was maximal at the position in front ofthe working face 4ndash6m

(3) Above the large coal pillar there was no acting forceon the key strata along two sides Moreover therewas large rotating space It was determined that forthe working face in the coal seam 3minus1 when the largecoal pillars were mined the overlying rock strata loststability

(4) +e joint acting dynamic pressure mechanism of theworking face 3minus1 was revealed Specifically it was thehigh stress environment (the mining inducedstresses of the lower coal seam and upper coal seamwere overlaid) and the overall instability of the re-versed trapezoid rock column above the coal pillar+is would induce the accident of roof cutting andthe support compressing

Mining layouts and geological conditions vary from siteto site Although our study is based on the specific miningcondition it provides guidance and reference to thosestudies with similar conditions

44 m

20 m coal pillar Main key strata 50 m coal pillar

5 m remaining room pillar

3ndash1 coal seam

2ndash2 coal seam

Figure 9 Similarity simulation of the coal pillar in the room gob

50 m coal pillar

3ndash1 coal seam

(a)

Extrude coal 15 m

(b)

Figure 10 +e overlying rock strata movement when crossing the coal pillar (a) When crossing the coal pillar of 50m (b) When the coalpillar of 50m was mined

Shock and Vibration 9

Data Availability

Some or all data models or codes generated or used duringthe study are included within the article and are alsoavailable from the corresponding author upon request

Conflicts of Interest

+e authors declare that they have no conflicts of interestregarding the publication of this paper

Acknowledgments

+e authors gratefully acknowledge the National NaturalScience Foundation of China Project (no 51504127) forproviding partial financial support for the present study

References

[1] Q X Huang ldquoGround pressure behavior and definition ofshallow seamsrdquo Chinese Journal of Rock Mechanics and En-gineering vol 21 no 8 pp 1174ndash1177 2002

[2] Y F Ren and Q X Qi ldquoStudy on characteristic of stress fieldin surrounding rocks of shallow coal face under long wallminingrdquo Journal of China Coal Society vol 10 no 36pp 1612ndash1618 2011

[3] Y F Ren ldquoStudy on mine pressure and overlying stratamovement law of contiguous seams with shallow depthrdquo CoalScience and Technology vol 43 no 7 pp 11ndash14 2015

[4] T Zhu B S Zhang G R Feng and X Zhang ldquoRoof structureand control in the lower seam mining field in the ultra-closemultiple seamsrdquo Journal of China Coal Society vol 35 no 2pp 190ndash193 2010

[5] Z Yang B Tong C C Huang et al ldquoStudy on the movementlaw of overlying strata in mining face under room and pillargoafrdquo Journal of Mining amp Safety Engineering vol 29 no 2pp 157ndash161 2012

[6] Q S Bai S H Tu F T Wang et al ldquo+e evolution of miningstress and the mechanism of disaster caused by shallow buriedcoal pillarrdquo Chinese Journal of Rock Mechanics and Engi-neering vol 31 no S2 pp 3772ndash3778 2012

[7] P Małkowski Z Niedbalski and T Balarabe ldquoA statisticalanalysis of geomechanical data and its effect on rock massnumerical modeling a case studyrdquo International Journal ofCoal Science amp Technology vol 8 no 4 pp 1ndash12 2020

[8] D Cao A Wang S Ning et al ldquoCoalfield structure andstructural controls on coal in Chinardquo International Journal ofCoal Science amp Technology vol 7 no 5 pp 220ndash239 2020

[9] S Mao ldquoDevelopment of coal geological information tech-nologies in Chinardquo International Journal of Coal Science ampTechnology vol 7 no 4 pp 320ndash328 2020

[10] J Han H Liang C Cao and Z Bi ldquoA mechanical model forsheared joints based on Mohr-Coulomb material propertiesrdquoGeotechnique Letters vol 8 no 2 pp 1ndash14 2018

[11] D J Xue L L Lu J Zhou and L Lu ldquoCluster modeling of theshort-range correlation of acoustically emitted scatteringsignalsrdquo International Journal of Coal Science amp Technologypp 1ndash15 2020

[12] D J Xue J Zhou Y T Liu and L Gau ldquoOn the excavation-induced stress drop in damaged coal considering a coupledyield and failure criterionrdquo International Journal of CoalScience amp Technology vol 7 no 5 pp 58ndash67 2020

[13] D J Xue Y T Liu H W Zhou J Q Wang J F Liu andJ Zhou ldquoFractal characterization on anisotropy and fractalreconstruction of rough surface of granite under orthogonalshearrdquo Rock Mechanics and Rock Engineering vol 53 no 3pp 1225ndash1242 2020

[14] L Zhang S Chen C Zhang X Fang and S Li ldquo+echaracterization of bituminous coal microstructure andpermeability by liquid nitrogen fracturing based on μCTtechnologyrdquo Fuel vol 262 Article ID 116635 2020

[15] L Zhang J H Li J H Xue C Zhang and X Fang ldquoEx-perimental studies on the changing characteristics of the gasflow capacity on bituminous coal in CO2-ECBM and N2-ECBMrdquo Fuel vol 291 Article ID 120115 2021

[16] J Lin T Ren Y P Cheng J Nimcik and G Wang ldquoCyclicN2 injection for enhanced coal seam gas recovery a labo-ratory studyrdquo Energy vol 188 Article ID 116115 2019

[17] J Lin T Ren G Wang P Booth and J Nemcik ldquoExperi-mental investigation of N2 injection to enhance gas drainagein CO2-rich low permeable seamrdquo Fuel vol 215 pp 665ndash6742018

[18] B Chen ldquoStress-induced trend the clustering feature of coalmine disasters and earthquakes in Chinardquo InternationalJournal of Coal Science amp Technology vol 7 no 4 pp 676ndash692 2020

[19] X Wu Y Peng J Xu Q Yan W Nie and T Zhang ldquoEx-perimental study on evolution law for particle breakageduring coal and gas outburstrdquo International Journal of CoalScience amp Technology vol 7 no 1 pp 97ndash106 2020

[20] 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

[21] X Z Jie ldquoStudy on the characteristics of strata behavior inshallow seam longwall mining under the room-and-pillarmining goafrdquo Journal of China Coal Society vol 37 no 6pp 898ndash902 2012

[22] S H Tu F J Dou Z J Wan F Wang and Y Yuan ldquoStratacontrol technology of the fully mechanized face in shallowcoal seam close to the above room and pillar gobrdquo Journal ofChina Coal Society vol 36 no 3 pp 366ndash370 2011

[23] X Lian H Hu T Li and D Hu ldquoMain geological and miningfactors affecting ground cracks induced by underground coalmining in Shanxi Province Chinardquo International Journal ofCoal Science amp Technology vol 7 no 2 pp 362ndash370 2020

[24] D Xue H Zhou Y Zhao L Zhang L Deng and X WangldquoReal-time SEM observation of mesoscale failures underthermal-mechanical coupling sequences in graniterdquo Inter-national Journal of Rock Mechanics and Mining Sciencesvol 112 pp 35ndash46 2018

[25] Z L Yang ldquoStudy of controlling catastrophe for roof strata inshallow seam longwall miningrdquo Rock and Soil Mechanicsvol 29 no s1 pp 459ndash463 2011

[26] H D Li H H Yang B Zhang and G Chen ldquoControl studyof strong strata behaviors during the fully mechanizedworking face out of concentrated coal pillar in a shallow depthseam in proximity beneath a room mining goafrdquo Journal ofChina Coal Society vol 40 no s1 pp 6ndash11 2015

[27] J Y Zhao S L Wu J P Ma et al ldquoStudy on strata behaviorsregulation of coal working face under room and pillar mininggoafrdquo China Energy and Environmental Protection vol 6pp 60ndash63 2015

[28] J X Yang C Y Liu and B Yu ldquo+e effects of load distri-bution exerting on coal pillars on the stress and energy

10 Shock and Vibration

distribution of the floor stratardquo Acta Montanistica Slovacavol 21 pp 102ndash112 2016

[29] S Li L F Zhou M K Luo et al ldquoStrata behaviors analysis ofstage coal pillar in Tong Xinmine caused by repeatedminingrdquoJournal of Liaoning Technical University Natural Sciencevol 34 no 6 pp 661ndash667 2015

[30] S K Zhang L G Wang X D Zhang et al ldquoLoad transfereffects of the longwall sustaining coal pillars on the safety ofthe mining goaf systemrdquo Journal of Safety and Environmentvol 16 no 3 pp 116ndash119 2016

[31] L Li F Li Y Zhang D Yang and X Liu ldquoFormationmechanism and height calculation of the caved zone andwater-conducting fracture zone in solid backfill miningrdquoInternational Journal of Coal Science amp Technology vol 7no 1 pp 208ndash215 2020

[32] L Xu H X Wei Z Y Xiao and B Li ldquoEngineering cases andcharacteristics of deviatoric stressunder coal pillar in regionalfloorrdquo Coal Science and Technology vol 40 no 4 pp 23ndash252012

[33] Z J Zhu H W Zhang T W Lan B Huo and Y ChenldquoStudy on surrounding rock stress evolution law of fully-mechanized top coal caving mining face under role ofoverlapping coal pillarsrdquo Coal Science and Technology vol 45no 3 pp 26ndash31 2017

[34] C Zhu M He M Karakus X Zhang and Z Tao ldquoNumericalsimulations of the failure process of anaclinal slope physicalmodel and control mechanism of negative Poissonrsquos ratiocablerdquo Bulletin of Engineering Geology and the Environmentvol 80 no 4 pp 3365ndash3380 2021

[35] J G Men S Wang R P Yuan et al ldquoStudy on overburdenstrata structure features and stress distribution law of coalpillar in inclined seamrdquo Coal Science and Technology vol 42no 5 pp 21ndash24 2014

[36] M X Ren ldquoRational decision of pillarrsquos size of Wongawillimining methodrdquo Journal of Mining amp Safety Engineeringvol 21 no 1 pp 42-43+118 2004

[37] B Yu C Y Liu J X Yang and J Liu ldquoMechanism of strongpressure reveal under the influence of miningdual system ofcoal pillar in Datong mining areardquo Journal of China CoalSociety vol 39 no 1 pp 40ndash46 2014

[38] J F Ju ldquoMechanism and prevention of support crushingdisaster while mining out of the upper coal pillar in closedistance shallow seamsrdquo PhD thesis China University ofMining and Technology Xuzhou China 2013

Shock and Vibration 11

on the working face but also it caused damage to the retreatroadway to a certain distance

4 Numerical Simulation

41 Stress Field of the Coal Seam 3minus1 +e working faces incoal seams 2minus2 and 3minus1 in the Huoluowan Coal Mine werestudied According to the practical status and consideringthe boundary effect isolating pillars of 50m were set as twoboundaries +e model dimension was 500mtimes 400mtimes 200m For the numerical model the boundary conditionand loading state were explained as follows for the boundaryof two sides along the direction of X the displacement alongthe direction of X was fixed It means that for the boundarythe displacement along the direction ofXwas zero Similarlythe displacement of the two sides along the direction of Ywas fixed +e bottom boundary of the model was fixed Atthe top of the model it was free Along the direction of Z theself-weight capacity was applied According to the measuredstress data in the mine site a stress of 462MPa was appliedalong the direction of X Along the direction of Y a stress of22MPa was applied

After the coal seam of 2minus2 was exploited stress con-centration occurred in the coal pillar +e vertical stressdistributions before and after the room pillars were damagedare shown in Figure 5 Before the coal pillars was damagedthe maximum vertical stress was 146MPa and the stressconcentration coefficient was 43 All stresses were in thecentral area of the coal pillar indicating that two sides of thecoal pillar have already entered the plastic zone +e rela-tively high stress is transferred to the internal area of the coalpillar after they lost the bearing capacity +ey did not in-tersect in the zone where the stress concentration wasmaximal in the room pillar In fact they were located alongtwo sides of the centre +e vertical stress in the room pillarcentre were lower than that at two sides +is indicates thatin the room pillar there was still the elastic core which hascertain loading capacity After the coal pillar was damaged

the stress in the coal pillar decreased to 82MPa higher thanthe intact stress +is indicates that after the room pillar wasdamaged it still had residual load bearing capacity

After the coal seam 2minus2 was mined the stress field in therock strata is shown in Figure 6 In the figure the blackbreak line shows the stress distribution along the per-pendicular direction of the coal seam 3minus1 According to therelative position relationship it can be known that the coalseam 3minus1 below the gob was in pressure-relieving area afterthe coal seam 2minus2 was mined +e vertical stress is from36MPa to 43MPa decreasing by 44ndash20 +e accu-mulated elastic energy in the coal seam 3minus1 was effectivelyreleased +e possibility of dynamic disaster decreasedwhich was beneficial for the exploiting of the coal seam 3minus1However below the coal pillars of 20m and 50m thepressure increases Among them in the coal seam 3minus1 thevertical stresses were 59MPa and 56MPa respectively+e stress concentration coefficients were 131 and 124respectively

42 FailureCharacteristic of the SurroundingRock afterUpperSeam Mining For the horizontally bedded coal seam afterthe roadway was excavated the stress distribution of thesurrounding rock was relatively uniform +ey often dis-tribute symmetrically Under the influence of the high stressthe surrounding rock masses in the working face generatedplastic deformation +e mining section II was regarded asan example It was analysed that the failure characteristic ofthe surrounding rock masses is shown in Figure 7 after thecoal seam 2minus2 was mined It can be known that the failurezones of the surrounding rock in the working face showed asymmetric distribution For the surrounding rock aroundthe coal rooms the main failure mode was tensile andshearing For the upper strata it was mainly of shearingfailure +e room pillar of 8m lost stability For the floorbelow the coal pillar there was no failure And this wasconsistent with the theoretical analysis

Main key rock strata

Second key rock strata

Figure 4 +e roof structure when coal pillars were mined

6 Shock and Vibration

43 Mining-Induced Stress under Upper Coal PillarsWhen the working face advanced to 100m the stressdistribution in the surrounding rock is shown inFigure 8(a)+e influencing range of the abutment pressurewas around 52m in front of the working face+e abutmentpressure was around 45MPa to 213MPa +e maximumabutment pressure occurred in front of the working face6m +e stress concentration coefficient was around 473+e working face was below the coal pillar of 20m

When the working face advanced to 200m the stressdistribution in the surrounding rock was shown inFigure 8(b) +e influencing range of the abutment pressurewas around 58m in front of the working face +e abutment

pressure was around 45MPa to 200MPa Among them themaximum stress occurred in front of the working face 50m+e stress concentration coefficient was around 444 +eworking face started entering the coal pillar of 50m

When the working face arrived at 240m the stressdistribution in the surrounding rock is shown in Figure 8(c)+e influencing range of the abutment pressure was in frontof the working face around 39m+e abutment pressure wasaround 45MPa to 225MPa Among them the maximumstress occurred in front of the working face 40m +e stressconcentration coefficient was around 500 At this point theright boundary of the coal pillar of 50m was located in frontof the working face 10m

3ndash1 coal seam

Distance (times100)

ZZ-s

tres

s (times1

0^6)

500

450

400

00 05 10 15 20 25 30 35 40

Figure 6 Vertical stress distribution of the coal seam 3minus1 under the mining influence of coal seam 2minus2

NoneShear-n shear-pShear-n shear-p tension-pShear-pShear-p tension-pTension-p

ZonePlane active onColor by state-average

Figure 7 +e plastic zone distribution in the surrounding rock

Contour of ZZ-stress34509E + 0400000E + 00ndash10000E + 06ndash20000E + 06ndash30000E + 06ndash40000E + 06ndash50000E + 06ndash60000E + 06ndash70000E + 06ndash80000E + 06ndash90000E + 06ndash10000E + 07ndash12000E + 07ndash13000E + 07ndash14000E + 07ndash14656E + 07

(a)

Contour of ZZ-stress

ndash14443E + 04ndash10000E + 06ndash20000E + 06ndash30000E + 06ndash40000E + 06ndash50000E + 06ndash60000E + 06ndash70000E + 06ndash80000E + 06ndash90000E + 06ndash10000E + 07ndash11000E + 07ndash11428E + 07

(b)

Figure 5+e vertical stress distribution before and after the room pillar was damaged (a) Before the room pillar was damaged (b) After theroom pillar was damaged

Shock and Vibration 7

5 Similar Material Simulation Study

51Establishmentof theModel For themodel the geometricsimilarity ratio was 1100 For the volume weight similarityratio it was 075 (the practical volume weight was 24 gcm3

and the model volume weight was 18 gcm3) +e filling sizeof the model was 5000mmtimes 400mmtimes 2000mm(lengthtimeswidthtimes height) +e top boundary of the modelwas up to the ground surface+erefore it was not necessaryto apply pressure

To regenerate the coal pillar in the room gob the paraffinwas used Room retreat is simulated by heating to melt theparaffin +e coal rooms were mined and the coal pillarswere remained +en the coal pillars were destroyedmanually to simulate the instability of coal pillars +e coalpillars of 20m and the large coal pillars of 50m wereremained as shown in Figure 9

52 Overlying Strata Movement When the Working FaceCrossed Pillars +e overlying rock strata movement wasstudied to explain the violent dynamic pressure whencrossing the coal pillar of 50m Below the coal pillar of 50mthe roof was caved along the coal wall +e direction was

basically consistent with the left boundary of the reversedtrapezoid rock column +e top and bottom gobs wereconnected +e working face bears the weight of the in-terlayer strata between two coal seams and the weight of thereversed rock column above the coal pillar of 50m+eminepressure appearance was violent as shown in Figure 10(a)With the working face advancing there was a plastic zone atthe boundary of the large coal pillar When the working faceentered the right plastic zone range the coal pillar stabilitywas influenced Coal pillars cannot effectively support theweight of the overlying rock strata +e coal pillar of 50 mand the above rock column caved +is leaded to the col-lapsing of the rock strata above the rock column +efractures along two sides of the reversed trapezoid werecompressed again +e acting force among rock strata wasrelatively large +e roof was cut along the coal wall verti-cally +e coal masses in the working face that was notexploited were squeezed out Compared with the rightboundary of the reversed trapezoid rock column the di-rection shows the reversed symmetry form +e dynamicpressure was violent as shown in Figure 10(b)

When the coal seam 3minus1 crossed the large coal pillar thecollapsed height of overlying strata was relatively large +e

Contour of ZZ-stress32592E + 0500000E + 00ndash20000E + 06ndash40000E + 06ndash60000E + 06ndash80000E + 06ndash10000E + 07ndash12000E + 07ndash14000E + 07ndash16000E + 07ndash18000E + 07ndash20000E + 07ndash21340E + 07

(a)Contour of ZZ-stress

79775E + 0500000E + 00ndash20000E + 06ndash40000E + 06ndash60000E + 06ndash80000E + 06ndash10000E + 07ndash12000E + 07ndash14000E + 07ndash16000E + 07ndash18000E + 07ndash20000E + 07ndash20031E + 07

(b)

Contour of ZZ-stress78440E + 0500000E + 00ndash25000E + 06ndash50000E + 06ndash75000E + 06ndash10000E + 07ndash12500E + 07ndash15000E + 07ndash17500E + 07ndash20000E + 07ndash22500E + 07

(c)

Figure 8 Stress distribution contour when the working face advancing (a) Working face advanced to 100m (b) Working face advanced to200m (c) Working face advanced to 240m

8 Shock and Vibration

stress concentration of the upper coal seam and the mininginduced stress of the lower coal seamwere overlaid+e rockstrata between two coal seams caved thoroughly +e dy-namic pressure was violent When it was not able to supportthe reversed trapezoid rock column above the coal pillar of50m the high elastic-plastic energy that was accumulatedreleased +is induced the instability of the overlying rockstructure If this impact load was applied on the workingface the accident of the hydraulic supports crush wouldoccur

6 Conclusions

(1) In the Huoluowan Coal Mine for the room gob inthe shallow buried coal seam 2minus2 the stress distri-bution under coal pillar group was determinedWhen the room pillars of 8m lost stability they haddisturbing effect on the stress field of the rock strata5ndash8m lower than the coal seam As for the coalpillars of 20m and 50m the maximum influencingrange on the lower rock strata was 42m and 58mFor the lower coal seam of 3minus1 the vertical stressconcentration coefficients were 131 and 124respectively

(2) +e characteristic of the abutment pressure when theworking face crossed large coal pillars was revealed+e concentration coefficient reached 444ndash500Moreover it was maximal at the position in front ofthe working face 4ndash6m

(3) Above the large coal pillar there was no acting forceon the key strata along two sides Moreover therewas large rotating space It was determined that forthe working face in the coal seam 3minus1 when the largecoal pillars were mined the overlying rock strata loststability

(4) +e joint acting dynamic pressure mechanism of theworking face 3minus1 was revealed Specifically it was thehigh stress environment (the mining inducedstresses of the lower coal seam and upper coal seamwere overlaid) and the overall instability of the re-versed trapezoid rock column above the coal pillar+is would induce the accident of roof cutting andthe support compressing

Mining layouts and geological conditions vary from siteto site Although our study is based on the specific miningcondition it provides guidance and reference to thosestudies with similar conditions

44 m

20 m coal pillar Main key strata 50 m coal pillar

5 m remaining room pillar

3ndash1 coal seam

2ndash2 coal seam

Figure 9 Similarity simulation of the coal pillar in the room gob

50 m coal pillar

3ndash1 coal seam

(a)

Extrude coal 15 m

(b)

Figure 10 +e overlying rock strata movement when crossing the coal pillar (a) When crossing the coal pillar of 50m (b) When the coalpillar of 50m was mined

Shock and Vibration 9

Data Availability

Some or all data models or codes generated or used duringthe study are included within the article and are alsoavailable from the corresponding author upon request

Conflicts of Interest

+e authors declare that they have no conflicts of interestregarding the publication of this paper

Acknowledgments

+e authors gratefully acknowledge the National NaturalScience Foundation of China Project (no 51504127) forproviding partial financial support for the present study

References

[1] Q X Huang ldquoGround pressure behavior and definition ofshallow seamsrdquo Chinese Journal of Rock Mechanics and En-gineering vol 21 no 8 pp 1174ndash1177 2002

[2] Y F Ren and Q X Qi ldquoStudy on characteristic of stress fieldin surrounding rocks of shallow coal face under long wallminingrdquo Journal of China Coal Society vol 10 no 36pp 1612ndash1618 2011

[3] Y F Ren ldquoStudy on mine pressure and overlying stratamovement law of contiguous seams with shallow depthrdquo CoalScience and Technology vol 43 no 7 pp 11ndash14 2015

[4] T Zhu B S Zhang G R Feng and X Zhang ldquoRoof structureand control in the lower seam mining field in the ultra-closemultiple seamsrdquo Journal of China Coal Society vol 35 no 2pp 190ndash193 2010

[5] Z Yang B Tong C C Huang et al ldquoStudy on the movementlaw of overlying strata in mining face under room and pillargoafrdquo Journal of Mining amp Safety Engineering vol 29 no 2pp 157ndash161 2012

[6] Q S Bai S H Tu F T Wang et al ldquo+e evolution of miningstress and the mechanism of disaster caused by shallow buriedcoal pillarrdquo Chinese Journal of Rock Mechanics and Engi-neering vol 31 no S2 pp 3772ndash3778 2012

[7] P Małkowski Z Niedbalski and T Balarabe ldquoA statisticalanalysis of geomechanical data and its effect on rock massnumerical modeling a case studyrdquo International Journal ofCoal Science amp Technology vol 8 no 4 pp 1ndash12 2020

[8] D Cao A Wang S Ning et al ldquoCoalfield structure andstructural controls on coal in Chinardquo International Journal ofCoal Science amp Technology vol 7 no 5 pp 220ndash239 2020

[9] S Mao ldquoDevelopment of coal geological information tech-nologies in Chinardquo International Journal of Coal Science ampTechnology vol 7 no 4 pp 320ndash328 2020

[10] J Han H Liang C Cao and Z Bi ldquoA mechanical model forsheared joints based on Mohr-Coulomb material propertiesrdquoGeotechnique Letters vol 8 no 2 pp 1ndash14 2018

[11] D J Xue L L Lu J Zhou and L Lu ldquoCluster modeling of theshort-range correlation of acoustically emitted scatteringsignalsrdquo International Journal of Coal Science amp Technologypp 1ndash15 2020

[12] D J Xue J Zhou Y T Liu and L Gau ldquoOn the excavation-induced stress drop in damaged coal considering a coupledyield and failure criterionrdquo International Journal of CoalScience amp Technology vol 7 no 5 pp 58ndash67 2020

[13] D J Xue Y T Liu H W Zhou J Q Wang J F Liu andJ Zhou ldquoFractal characterization on anisotropy and fractalreconstruction of rough surface of granite under orthogonalshearrdquo Rock Mechanics and Rock Engineering vol 53 no 3pp 1225ndash1242 2020

[14] L Zhang S Chen C Zhang X Fang and S Li ldquo+echaracterization of bituminous coal microstructure andpermeability by liquid nitrogen fracturing based on μCTtechnologyrdquo Fuel vol 262 Article ID 116635 2020

[15] L Zhang J H Li J H Xue C Zhang and X Fang ldquoEx-perimental studies on the changing characteristics of the gasflow capacity on bituminous coal in CO2-ECBM and N2-ECBMrdquo Fuel vol 291 Article ID 120115 2021

[16] J Lin T Ren Y P Cheng J Nimcik and G Wang ldquoCyclicN2 injection for enhanced coal seam gas recovery a labo-ratory studyrdquo Energy vol 188 Article ID 116115 2019

[17] J Lin T Ren G Wang P Booth and J Nemcik ldquoExperi-mental investigation of N2 injection to enhance gas drainagein CO2-rich low permeable seamrdquo Fuel vol 215 pp 665ndash6742018

[18] B Chen ldquoStress-induced trend the clustering feature of coalmine disasters and earthquakes in Chinardquo InternationalJournal of Coal Science amp Technology vol 7 no 4 pp 676ndash692 2020

[19] X Wu Y Peng J Xu Q Yan W Nie and T Zhang ldquoEx-perimental study on evolution law for particle breakageduring coal and gas outburstrdquo International Journal of CoalScience amp Technology vol 7 no 1 pp 97ndash106 2020

[20] 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

[21] X Z Jie ldquoStudy on the characteristics of strata behavior inshallow seam longwall mining under the room-and-pillarmining goafrdquo Journal of China Coal Society vol 37 no 6pp 898ndash902 2012

[22] S H Tu F J Dou Z J Wan F Wang and Y Yuan ldquoStratacontrol technology of the fully mechanized face in shallowcoal seam close to the above room and pillar gobrdquo Journal ofChina Coal Society vol 36 no 3 pp 366ndash370 2011

[23] X Lian H Hu T Li and D Hu ldquoMain geological and miningfactors affecting ground cracks induced by underground coalmining in Shanxi Province Chinardquo International Journal ofCoal Science amp Technology vol 7 no 2 pp 362ndash370 2020

[24] D Xue H Zhou Y Zhao L Zhang L Deng and X WangldquoReal-time SEM observation of mesoscale failures underthermal-mechanical coupling sequences in graniterdquo Inter-national Journal of Rock Mechanics and Mining Sciencesvol 112 pp 35ndash46 2018

[25] Z L Yang ldquoStudy of controlling catastrophe for roof strata inshallow seam longwall miningrdquo Rock and Soil Mechanicsvol 29 no s1 pp 459ndash463 2011

[26] H D Li H H Yang B Zhang and G Chen ldquoControl studyof strong strata behaviors during the fully mechanizedworking face out of concentrated coal pillar in a shallow depthseam in proximity beneath a room mining goafrdquo Journal ofChina Coal Society vol 40 no s1 pp 6ndash11 2015

[27] J Y Zhao S L Wu J P Ma et al ldquoStudy on strata behaviorsregulation of coal working face under room and pillar mininggoafrdquo China Energy and Environmental Protection vol 6pp 60ndash63 2015

[28] J X Yang C Y Liu and B Yu ldquo+e effects of load distri-bution exerting on coal pillars on the stress and energy

10 Shock and Vibration

distribution of the floor stratardquo Acta Montanistica Slovacavol 21 pp 102ndash112 2016

[29] S Li L F Zhou M K Luo et al ldquoStrata behaviors analysis ofstage coal pillar in Tong Xinmine caused by repeatedminingrdquoJournal of Liaoning Technical University Natural Sciencevol 34 no 6 pp 661ndash667 2015

[30] S K Zhang L G Wang X D Zhang et al ldquoLoad transfereffects of the longwall sustaining coal pillars on the safety ofthe mining goaf systemrdquo Journal of Safety and Environmentvol 16 no 3 pp 116ndash119 2016

[31] L Li F Li Y Zhang D Yang and X Liu ldquoFormationmechanism and height calculation of the caved zone andwater-conducting fracture zone in solid backfill miningrdquoInternational Journal of Coal Science amp Technology vol 7no 1 pp 208ndash215 2020

[32] L Xu H X Wei Z Y Xiao and B Li ldquoEngineering cases andcharacteristics of deviatoric stressunder coal pillar in regionalfloorrdquo Coal Science and Technology vol 40 no 4 pp 23ndash252012

[33] Z J Zhu H W Zhang T W Lan B Huo and Y ChenldquoStudy on surrounding rock stress evolution law of fully-mechanized top coal caving mining face under role ofoverlapping coal pillarsrdquo Coal Science and Technology vol 45no 3 pp 26ndash31 2017

[34] C Zhu M He M Karakus X Zhang and Z Tao ldquoNumericalsimulations of the failure process of anaclinal slope physicalmodel and control mechanism of negative Poissonrsquos ratiocablerdquo Bulletin of Engineering Geology and the Environmentvol 80 no 4 pp 3365ndash3380 2021

[35] J G Men S Wang R P Yuan et al ldquoStudy on overburdenstrata structure features and stress distribution law of coalpillar in inclined seamrdquo Coal Science and Technology vol 42no 5 pp 21ndash24 2014

[36] M X Ren ldquoRational decision of pillarrsquos size of Wongawillimining methodrdquo Journal of Mining amp Safety Engineeringvol 21 no 1 pp 42-43+118 2004

[37] B Yu C Y Liu J X Yang and J Liu ldquoMechanism of strongpressure reveal under the influence of miningdual system ofcoal pillar in Datong mining areardquo Journal of China CoalSociety vol 39 no 1 pp 40ndash46 2014

[38] J F Ju ldquoMechanism and prevention of support crushingdisaster while mining out of the upper coal pillar in closedistance shallow seamsrdquo PhD thesis China University ofMining and Technology Xuzhou China 2013

Shock and Vibration 11

43 Mining-Induced Stress under Upper Coal PillarsWhen the working face advanced to 100m the stressdistribution in the surrounding rock is shown inFigure 8(a)+e influencing range of the abutment pressurewas around 52m in front of the working face+e abutmentpressure was around 45MPa to 213MPa +e maximumabutment pressure occurred in front of the working face6m +e stress concentration coefficient was around 473+e working face was below the coal pillar of 20m

When the working face advanced to 200m the stressdistribution in the surrounding rock was shown inFigure 8(b) +e influencing range of the abutment pressurewas around 58m in front of the working face +e abutment

pressure was around 45MPa to 200MPa Among them themaximum stress occurred in front of the working face 50m+e stress concentration coefficient was around 444 +eworking face started entering the coal pillar of 50m

When the working face arrived at 240m the stressdistribution in the surrounding rock is shown in Figure 8(c)+e influencing range of the abutment pressure was in frontof the working face around 39m+e abutment pressure wasaround 45MPa to 225MPa Among them the maximumstress occurred in front of the working face 40m +e stressconcentration coefficient was around 500 At this point theright boundary of the coal pillar of 50m was located in frontof the working face 10m

3ndash1 coal seam

Distance (times100)

ZZ-s

tres

s (times1

0^6)

500

450

400

00 05 10 15 20 25 30 35 40

Figure 6 Vertical stress distribution of the coal seam 3minus1 under the mining influence of coal seam 2minus2

NoneShear-n shear-pShear-n shear-p tension-pShear-pShear-p tension-pTension-p

ZonePlane active onColor by state-average

Figure 7 +e plastic zone distribution in the surrounding rock

Contour of ZZ-stress34509E + 0400000E + 00ndash10000E + 06ndash20000E + 06ndash30000E + 06ndash40000E + 06ndash50000E + 06ndash60000E + 06ndash70000E + 06ndash80000E + 06ndash90000E + 06ndash10000E + 07ndash12000E + 07ndash13000E + 07ndash14000E + 07ndash14656E + 07

(a)

Contour of ZZ-stress

ndash14443E + 04ndash10000E + 06ndash20000E + 06ndash30000E + 06ndash40000E + 06ndash50000E + 06ndash60000E + 06ndash70000E + 06ndash80000E + 06ndash90000E + 06ndash10000E + 07ndash11000E + 07ndash11428E + 07

(b)

Figure 5+e vertical stress distribution before and after the room pillar was damaged (a) Before the room pillar was damaged (b) After theroom pillar was damaged

Shock and Vibration 7

5 Similar Material Simulation Study

51Establishmentof theModel For themodel the geometricsimilarity ratio was 1100 For the volume weight similarityratio it was 075 (the practical volume weight was 24 gcm3

and the model volume weight was 18 gcm3) +e filling sizeof the model was 5000mmtimes 400mmtimes 2000mm(lengthtimeswidthtimes height) +e top boundary of the modelwas up to the ground surface+erefore it was not necessaryto apply pressure

To regenerate the coal pillar in the room gob the paraffinwas used Room retreat is simulated by heating to melt theparaffin +e coal rooms were mined and the coal pillarswere remained +en the coal pillars were destroyedmanually to simulate the instability of coal pillars +e coalpillars of 20m and the large coal pillars of 50m wereremained as shown in Figure 9

52 Overlying Strata Movement When the Working FaceCrossed Pillars +e overlying rock strata movement wasstudied to explain the violent dynamic pressure whencrossing the coal pillar of 50m Below the coal pillar of 50mthe roof was caved along the coal wall +e direction was

basically consistent with the left boundary of the reversedtrapezoid rock column +e top and bottom gobs wereconnected +e working face bears the weight of the in-terlayer strata between two coal seams and the weight of thereversed rock column above the coal pillar of 50m+eminepressure appearance was violent as shown in Figure 10(a)With the working face advancing there was a plastic zone atthe boundary of the large coal pillar When the working faceentered the right plastic zone range the coal pillar stabilitywas influenced Coal pillars cannot effectively support theweight of the overlying rock strata +e coal pillar of 50 mand the above rock column caved +is leaded to the col-lapsing of the rock strata above the rock column +efractures along two sides of the reversed trapezoid werecompressed again +e acting force among rock strata wasrelatively large +e roof was cut along the coal wall verti-cally +e coal masses in the working face that was notexploited were squeezed out Compared with the rightboundary of the reversed trapezoid rock column the di-rection shows the reversed symmetry form +e dynamicpressure was violent as shown in Figure 10(b)

When the coal seam 3minus1 crossed the large coal pillar thecollapsed height of overlying strata was relatively large +e

Contour of ZZ-stress32592E + 0500000E + 00ndash20000E + 06ndash40000E + 06ndash60000E + 06ndash80000E + 06ndash10000E + 07ndash12000E + 07ndash14000E + 07ndash16000E + 07ndash18000E + 07ndash20000E + 07ndash21340E + 07

(a)Contour of ZZ-stress

79775E + 0500000E + 00ndash20000E + 06ndash40000E + 06ndash60000E + 06ndash80000E + 06ndash10000E + 07ndash12000E + 07ndash14000E + 07ndash16000E + 07ndash18000E + 07ndash20000E + 07ndash20031E + 07

(b)

Contour of ZZ-stress78440E + 0500000E + 00ndash25000E + 06ndash50000E + 06ndash75000E + 06ndash10000E + 07ndash12500E + 07ndash15000E + 07ndash17500E + 07ndash20000E + 07ndash22500E + 07

(c)

Figure 8 Stress distribution contour when the working face advancing (a) Working face advanced to 100m (b) Working face advanced to200m (c) Working face advanced to 240m

8 Shock and Vibration

stress concentration of the upper coal seam and the mininginduced stress of the lower coal seamwere overlaid+e rockstrata between two coal seams caved thoroughly +e dy-namic pressure was violent When it was not able to supportthe reversed trapezoid rock column above the coal pillar of50m the high elastic-plastic energy that was accumulatedreleased +is induced the instability of the overlying rockstructure If this impact load was applied on the workingface the accident of the hydraulic supports crush wouldoccur

6 Conclusions

(1) In the Huoluowan Coal Mine for the room gob inthe shallow buried coal seam 2minus2 the stress distri-bution under coal pillar group was determinedWhen the room pillars of 8m lost stability they haddisturbing effect on the stress field of the rock strata5ndash8m lower than the coal seam As for the coalpillars of 20m and 50m the maximum influencingrange on the lower rock strata was 42m and 58mFor the lower coal seam of 3minus1 the vertical stressconcentration coefficients were 131 and 124respectively

(2) +e characteristic of the abutment pressure when theworking face crossed large coal pillars was revealed+e concentration coefficient reached 444ndash500Moreover it was maximal at the position in front ofthe working face 4ndash6m

(3) Above the large coal pillar there was no acting forceon the key strata along two sides Moreover therewas large rotating space It was determined that forthe working face in the coal seam 3minus1 when the largecoal pillars were mined the overlying rock strata loststability

(4) +e joint acting dynamic pressure mechanism of theworking face 3minus1 was revealed Specifically it was thehigh stress environment (the mining inducedstresses of the lower coal seam and upper coal seamwere overlaid) and the overall instability of the re-versed trapezoid rock column above the coal pillar+is would induce the accident of roof cutting andthe support compressing

Mining layouts and geological conditions vary from siteto site Although our study is based on the specific miningcondition it provides guidance and reference to thosestudies with similar conditions

44 m

20 m coal pillar Main key strata 50 m coal pillar

5 m remaining room pillar

3ndash1 coal seam

2ndash2 coal seam

Figure 9 Similarity simulation of the coal pillar in the room gob

50 m coal pillar

3ndash1 coal seam

(a)

Extrude coal 15 m

(b)

Figure 10 +e overlying rock strata movement when crossing the coal pillar (a) When crossing the coal pillar of 50m (b) When the coalpillar of 50m was mined

Shock and Vibration 9

Data Availability

Some or all data models or codes generated or used duringthe study are included within the article and are alsoavailable from the corresponding author upon request

Conflicts of Interest

+e authors declare that they have no conflicts of interestregarding the publication of this paper

Acknowledgments

+e authors gratefully acknowledge the National NaturalScience Foundation of China Project (no 51504127) forproviding partial financial support for the present study

References

[1] Q X Huang ldquoGround pressure behavior and definition ofshallow seamsrdquo Chinese Journal of Rock Mechanics and En-gineering vol 21 no 8 pp 1174ndash1177 2002

[2] Y F Ren and Q X Qi ldquoStudy on characteristic of stress fieldin surrounding rocks of shallow coal face under long wallminingrdquo Journal of China Coal Society vol 10 no 36pp 1612ndash1618 2011

[3] Y F Ren ldquoStudy on mine pressure and overlying stratamovement law of contiguous seams with shallow depthrdquo CoalScience and Technology vol 43 no 7 pp 11ndash14 2015

[4] T Zhu B S Zhang G R Feng and X Zhang ldquoRoof structureand control in the lower seam mining field in the ultra-closemultiple seamsrdquo Journal of China Coal Society vol 35 no 2pp 190ndash193 2010

[5] Z Yang B Tong C C Huang et al ldquoStudy on the movementlaw of overlying strata in mining face under room and pillargoafrdquo Journal of Mining amp Safety Engineering vol 29 no 2pp 157ndash161 2012

[6] Q S Bai S H Tu F T Wang et al ldquo+e evolution of miningstress and the mechanism of disaster caused by shallow buriedcoal pillarrdquo Chinese Journal of Rock Mechanics and Engi-neering vol 31 no S2 pp 3772ndash3778 2012

[7] P Małkowski Z Niedbalski and T Balarabe ldquoA statisticalanalysis of geomechanical data and its effect on rock massnumerical modeling a case studyrdquo International Journal ofCoal Science amp Technology vol 8 no 4 pp 1ndash12 2020

[8] D Cao A Wang S Ning et al ldquoCoalfield structure andstructural controls on coal in Chinardquo International Journal ofCoal Science amp Technology vol 7 no 5 pp 220ndash239 2020

[9] S Mao ldquoDevelopment of coal geological information tech-nologies in Chinardquo International Journal of Coal Science ampTechnology vol 7 no 4 pp 320ndash328 2020

[10] J Han H Liang C Cao and Z Bi ldquoA mechanical model forsheared joints based on Mohr-Coulomb material propertiesrdquoGeotechnique Letters vol 8 no 2 pp 1ndash14 2018

[11] D J Xue L L Lu J Zhou and L Lu ldquoCluster modeling of theshort-range correlation of acoustically emitted scatteringsignalsrdquo International Journal of Coal Science amp Technologypp 1ndash15 2020

[12] D J Xue J Zhou Y T Liu and L Gau ldquoOn the excavation-induced stress drop in damaged coal considering a coupledyield and failure criterionrdquo International Journal of CoalScience amp Technology vol 7 no 5 pp 58ndash67 2020

[13] D J Xue Y T Liu H W Zhou J Q Wang J F Liu andJ Zhou ldquoFractal characterization on anisotropy and fractalreconstruction of rough surface of granite under orthogonalshearrdquo Rock Mechanics and Rock Engineering vol 53 no 3pp 1225ndash1242 2020

[14] L Zhang S Chen C Zhang X Fang and S Li ldquo+echaracterization of bituminous coal microstructure andpermeability by liquid nitrogen fracturing based on μCTtechnologyrdquo Fuel vol 262 Article ID 116635 2020

[15] L Zhang J H Li J H Xue C Zhang and X Fang ldquoEx-perimental studies on the changing characteristics of the gasflow capacity on bituminous coal in CO2-ECBM and N2-ECBMrdquo Fuel vol 291 Article ID 120115 2021

[16] J Lin T Ren Y P Cheng J Nimcik and G Wang ldquoCyclicN2 injection for enhanced coal seam gas recovery a labo-ratory studyrdquo Energy vol 188 Article ID 116115 2019

[17] J Lin T Ren G Wang P Booth and J Nemcik ldquoExperi-mental investigation of N2 injection to enhance gas drainagein CO2-rich low permeable seamrdquo Fuel vol 215 pp 665ndash6742018

[18] B Chen ldquoStress-induced trend the clustering feature of coalmine disasters and earthquakes in Chinardquo InternationalJournal of Coal Science amp Technology vol 7 no 4 pp 676ndash692 2020

[19] X Wu Y Peng J Xu Q Yan W Nie and T Zhang ldquoEx-perimental study on evolution law for particle breakageduring coal and gas outburstrdquo International Journal of CoalScience amp Technology vol 7 no 1 pp 97ndash106 2020

[20] 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

[21] X Z Jie ldquoStudy on the characteristics of strata behavior inshallow seam longwall mining under the room-and-pillarmining goafrdquo Journal of China Coal Society vol 37 no 6pp 898ndash902 2012

[22] S H Tu F J Dou Z J Wan F Wang and Y Yuan ldquoStratacontrol technology of the fully mechanized face in shallowcoal seam close to the above room and pillar gobrdquo Journal ofChina Coal Society vol 36 no 3 pp 366ndash370 2011

[23] X Lian H Hu T Li and D Hu ldquoMain geological and miningfactors affecting ground cracks induced by underground coalmining in Shanxi Province Chinardquo International Journal ofCoal Science amp Technology vol 7 no 2 pp 362ndash370 2020

[24] D Xue H Zhou Y Zhao L Zhang L Deng and X WangldquoReal-time SEM observation of mesoscale failures underthermal-mechanical coupling sequences in graniterdquo Inter-national Journal of Rock Mechanics and Mining Sciencesvol 112 pp 35ndash46 2018

[25] Z L Yang ldquoStudy of controlling catastrophe for roof strata inshallow seam longwall miningrdquo Rock and Soil Mechanicsvol 29 no s1 pp 459ndash463 2011

[26] H D Li H H Yang B Zhang and G Chen ldquoControl studyof strong strata behaviors during the fully mechanizedworking face out of concentrated coal pillar in a shallow depthseam in proximity beneath a room mining goafrdquo Journal ofChina Coal Society vol 40 no s1 pp 6ndash11 2015

[27] J Y Zhao S L Wu J P Ma et al ldquoStudy on strata behaviorsregulation of coal working face under room and pillar mininggoafrdquo China Energy and Environmental Protection vol 6pp 60ndash63 2015

[28] J X Yang C Y Liu and B Yu ldquo+e effects of load distri-bution exerting on coal pillars on the stress and energy

10 Shock and Vibration

distribution of the floor stratardquo Acta Montanistica Slovacavol 21 pp 102ndash112 2016

[29] S Li L F Zhou M K Luo et al ldquoStrata behaviors analysis ofstage coal pillar in Tong Xinmine caused by repeatedminingrdquoJournal of Liaoning Technical University Natural Sciencevol 34 no 6 pp 661ndash667 2015

[30] S K Zhang L G Wang X D Zhang et al ldquoLoad transfereffects of the longwall sustaining coal pillars on the safety ofthe mining goaf systemrdquo Journal of Safety and Environmentvol 16 no 3 pp 116ndash119 2016

[31] L Li F Li Y Zhang D Yang and X Liu ldquoFormationmechanism and height calculation of the caved zone andwater-conducting fracture zone in solid backfill miningrdquoInternational Journal of Coal Science amp Technology vol 7no 1 pp 208ndash215 2020

[32] L Xu H X Wei Z Y Xiao and B Li ldquoEngineering cases andcharacteristics of deviatoric stressunder coal pillar in regionalfloorrdquo Coal Science and Technology vol 40 no 4 pp 23ndash252012

[33] Z J Zhu H W Zhang T W Lan B Huo and Y ChenldquoStudy on surrounding rock stress evolution law of fully-mechanized top coal caving mining face under role ofoverlapping coal pillarsrdquo Coal Science and Technology vol 45no 3 pp 26ndash31 2017

[34] C Zhu M He M Karakus X Zhang and Z Tao ldquoNumericalsimulations of the failure process of anaclinal slope physicalmodel and control mechanism of negative Poissonrsquos ratiocablerdquo Bulletin of Engineering Geology and the Environmentvol 80 no 4 pp 3365ndash3380 2021

[35] J G Men S Wang R P Yuan et al ldquoStudy on overburdenstrata structure features and stress distribution law of coalpillar in inclined seamrdquo Coal Science and Technology vol 42no 5 pp 21ndash24 2014

[36] M X Ren ldquoRational decision of pillarrsquos size of Wongawillimining methodrdquo Journal of Mining amp Safety Engineeringvol 21 no 1 pp 42-43+118 2004

[37] B Yu C Y Liu J X Yang and J Liu ldquoMechanism of strongpressure reveal under the influence of miningdual system ofcoal pillar in Datong mining areardquo Journal of China CoalSociety vol 39 no 1 pp 40ndash46 2014

[38] J F Ju ldquoMechanism and prevention of support crushingdisaster while mining out of the upper coal pillar in closedistance shallow seamsrdquo PhD thesis China University ofMining and Technology Xuzhou China 2013

Shock and Vibration 11

5 Similar Material Simulation Study

51Establishmentof theModel For themodel the geometricsimilarity ratio was 1100 For the volume weight similarityratio it was 075 (the practical volume weight was 24 gcm3

and the model volume weight was 18 gcm3) +e filling sizeof the model was 5000mmtimes 400mmtimes 2000mm(lengthtimeswidthtimes height) +e top boundary of the modelwas up to the ground surface+erefore it was not necessaryto apply pressure

To regenerate the coal pillar in the room gob the paraffinwas used Room retreat is simulated by heating to melt theparaffin +e coal rooms were mined and the coal pillarswere remained +en the coal pillars were destroyedmanually to simulate the instability of coal pillars +e coalpillars of 20m and the large coal pillars of 50m wereremained as shown in Figure 9

52 Overlying Strata Movement When the Working FaceCrossed Pillars +e overlying rock strata movement wasstudied to explain the violent dynamic pressure whencrossing the coal pillar of 50m Below the coal pillar of 50mthe roof was caved along the coal wall +e direction was

basically consistent with the left boundary of the reversedtrapezoid rock column +e top and bottom gobs wereconnected +e working face bears the weight of the in-terlayer strata between two coal seams and the weight of thereversed rock column above the coal pillar of 50m+eminepressure appearance was violent as shown in Figure 10(a)With the working face advancing there was a plastic zone atthe boundary of the large coal pillar When the working faceentered the right plastic zone range the coal pillar stabilitywas influenced Coal pillars cannot effectively support theweight of the overlying rock strata +e coal pillar of 50 mand the above rock column caved +is leaded to the col-lapsing of the rock strata above the rock column +efractures along two sides of the reversed trapezoid werecompressed again +e acting force among rock strata wasrelatively large +e roof was cut along the coal wall verti-cally +e coal masses in the working face that was notexploited were squeezed out Compared with the rightboundary of the reversed trapezoid rock column the di-rection shows the reversed symmetry form +e dynamicpressure was violent as shown in Figure 10(b)

When the coal seam 3minus1 crossed the large coal pillar thecollapsed height of overlying strata was relatively large +e

Contour of ZZ-stress32592E + 0500000E + 00ndash20000E + 06ndash40000E + 06ndash60000E + 06ndash80000E + 06ndash10000E + 07ndash12000E + 07ndash14000E + 07ndash16000E + 07ndash18000E + 07ndash20000E + 07ndash21340E + 07

(a)Contour of ZZ-stress

79775E + 0500000E + 00ndash20000E + 06ndash40000E + 06ndash60000E + 06ndash80000E + 06ndash10000E + 07ndash12000E + 07ndash14000E + 07ndash16000E + 07ndash18000E + 07ndash20000E + 07ndash20031E + 07

(b)

Contour of ZZ-stress78440E + 0500000E + 00ndash25000E + 06ndash50000E + 06ndash75000E + 06ndash10000E + 07ndash12500E + 07ndash15000E + 07ndash17500E + 07ndash20000E + 07ndash22500E + 07

(c)

Figure 8 Stress distribution contour when the working face advancing (a) Working face advanced to 100m (b) Working face advanced to200m (c) Working face advanced to 240m

8 Shock and Vibration

stress concentration of the upper coal seam and the mininginduced stress of the lower coal seamwere overlaid+e rockstrata between two coal seams caved thoroughly +e dy-namic pressure was violent When it was not able to supportthe reversed trapezoid rock column above the coal pillar of50m the high elastic-plastic energy that was accumulatedreleased +is induced the instability of the overlying rockstructure If this impact load was applied on the workingface the accident of the hydraulic supports crush wouldoccur

6 Conclusions

(1) In the Huoluowan Coal Mine for the room gob inthe shallow buried coal seam 2minus2 the stress distri-bution under coal pillar group was determinedWhen the room pillars of 8m lost stability they haddisturbing effect on the stress field of the rock strata5ndash8m lower than the coal seam As for the coalpillars of 20m and 50m the maximum influencingrange on the lower rock strata was 42m and 58mFor the lower coal seam of 3minus1 the vertical stressconcentration coefficients were 131 and 124respectively

(2) +e characteristic of the abutment pressure when theworking face crossed large coal pillars was revealed+e concentration coefficient reached 444ndash500Moreover it was maximal at the position in front ofthe working face 4ndash6m

(3) Above the large coal pillar there was no acting forceon the key strata along two sides Moreover therewas large rotating space It was determined that forthe working face in the coal seam 3minus1 when the largecoal pillars were mined the overlying rock strata loststability

(4) +e joint acting dynamic pressure mechanism of theworking face 3minus1 was revealed Specifically it was thehigh stress environment (the mining inducedstresses of the lower coal seam and upper coal seamwere overlaid) and the overall instability of the re-versed trapezoid rock column above the coal pillar+is would induce the accident of roof cutting andthe support compressing

Mining layouts and geological conditions vary from siteto site Although our study is based on the specific miningcondition it provides guidance and reference to thosestudies with similar conditions

44 m

20 m coal pillar Main key strata 50 m coal pillar

5 m remaining room pillar

3ndash1 coal seam

2ndash2 coal seam

Figure 9 Similarity simulation of the coal pillar in the room gob

50 m coal pillar

3ndash1 coal seam

(a)

Extrude coal 15 m

(b)

Figure 10 +e overlying rock strata movement when crossing the coal pillar (a) When crossing the coal pillar of 50m (b) When the coalpillar of 50m was mined

Shock and Vibration 9

Data Availability

Some or all data models or codes generated or used duringthe study are included within the article and are alsoavailable from the corresponding author upon request

Conflicts of Interest

+e authors declare that they have no conflicts of interestregarding the publication of this paper

Acknowledgments

+e authors gratefully acknowledge the National NaturalScience Foundation of China Project (no 51504127) forproviding partial financial support for the present study

References

[1] Q X Huang ldquoGround pressure behavior and definition ofshallow seamsrdquo Chinese Journal of Rock Mechanics and En-gineering vol 21 no 8 pp 1174ndash1177 2002

[2] Y F Ren and Q X Qi ldquoStudy on characteristic of stress fieldin surrounding rocks of shallow coal face under long wallminingrdquo Journal of China Coal Society vol 10 no 36pp 1612ndash1618 2011

[3] Y F Ren ldquoStudy on mine pressure and overlying stratamovement law of contiguous seams with shallow depthrdquo CoalScience and Technology vol 43 no 7 pp 11ndash14 2015

[4] T Zhu B S Zhang G R Feng and X Zhang ldquoRoof structureand control in the lower seam mining field in the ultra-closemultiple seamsrdquo Journal of China Coal Society vol 35 no 2pp 190ndash193 2010

[5] Z Yang B Tong C C Huang et al ldquoStudy on the movementlaw of overlying strata in mining face under room and pillargoafrdquo Journal of Mining amp Safety Engineering vol 29 no 2pp 157ndash161 2012

[6] Q S Bai S H Tu F T Wang et al ldquo+e evolution of miningstress and the mechanism of disaster caused by shallow buriedcoal pillarrdquo Chinese Journal of Rock Mechanics and Engi-neering vol 31 no S2 pp 3772ndash3778 2012

[7] P Małkowski Z Niedbalski and T Balarabe ldquoA statisticalanalysis of geomechanical data and its effect on rock massnumerical modeling a case studyrdquo International Journal ofCoal Science amp Technology vol 8 no 4 pp 1ndash12 2020

[8] D Cao A Wang S Ning et al ldquoCoalfield structure andstructural controls on coal in Chinardquo International Journal ofCoal Science amp Technology vol 7 no 5 pp 220ndash239 2020

[9] S Mao ldquoDevelopment of coal geological information tech-nologies in Chinardquo International Journal of Coal Science ampTechnology vol 7 no 4 pp 320ndash328 2020

[10] J Han H Liang C Cao and Z Bi ldquoA mechanical model forsheared joints based on Mohr-Coulomb material propertiesrdquoGeotechnique Letters vol 8 no 2 pp 1ndash14 2018

[11] D J Xue L L Lu J Zhou and L Lu ldquoCluster modeling of theshort-range correlation of acoustically emitted scatteringsignalsrdquo International Journal of Coal Science amp Technologypp 1ndash15 2020

[12] D J Xue J Zhou Y T Liu and L Gau ldquoOn the excavation-induced stress drop in damaged coal considering a coupledyield and failure criterionrdquo International Journal of CoalScience amp Technology vol 7 no 5 pp 58ndash67 2020

[13] D J Xue Y T Liu H W Zhou J Q Wang J F Liu andJ Zhou ldquoFractal characterization on anisotropy and fractalreconstruction of rough surface of granite under orthogonalshearrdquo Rock Mechanics and Rock Engineering vol 53 no 3pp 1225ndash1242 2020

[14] L Zhang S Chen C Zhang X Fang and S Li ldquo+echaracterization of bituminous coal microstructure andpermeability by liquid nitrogen fracturing based on μCTtechnologyrdquo Fuel vol 262 Article ID 116635 2020

[15] L Zhang J H Li J H Xue C Zhang and X Fang ldquoEx-perimental studies on the changing characteristics of the gasflow capacity on bituminous coal in CO2-ECBM and N2-ECBMrdquo Fuel vol 291 Article ID 120115 2021

[16] J Lin T Ren Y P Cheng J Nimcik and G Wang ldquoCyclicN2 injection for enhanced coal seam gas recovery a labo-ratory studyrdquo Energy vol 188 Article ID 116115 2019

[17] J Lin T Ren G Wang P Booth and J Nemcik ldquoExperi-mental investigation of N2 injection to enhance gas drainagein CO2-rich low permeable seamrdquo Fuel vol 215 pp 665ndash6742018

[18] B Chen ldquoStress-induced trend the clustering feature of coalmine disasters and earthquakes in Chinardquo InternationalJournal of Coal Science amp Technology vol 7 no 4 pp 676ndash692 2020

[19] X Wu Y Peng J Xu Q Yan W Nie and T Zhang ldquoEx-perimental study on evolution law for particle breakageduring coal and gas outburstrdquo International Journal of CoalScience amp Technology vol 7 no 1 pp 97ndash106 2020

[20] 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

[21] X Z Jie ldquoStudy on the characteristics of strata behavior inshallow seam longwall mining under the room-and-pillarmining goafrdquo Journal of China Coal Society vol 37 no 6pp 898ndash902 2012

[22] S H Tu F J Dou Z J Wan F Wang and Y Yuan ldquoStratacontrol technology of the fully mechanized face in shallowcoal seam close to the above room and pillar gobrdquo Journal ofChina Coal Society vol 36 no 3 pp 366ndash370 2011

[23] X Lian H Hu T Li and D Hu ldquoMain geological and miningfactors affecting ground cracks induced by underground coalmining in Shanxi Province Chinardquo International Journal ofCoal Science amp Technology vol 7 no 2 pp 362ndash370 2020

[24] D Xue H Zhou Y Zhao L Zhang L Deng and X WangldquoReal-time SEM observation of mesoscale failures underthermal-mechanical coupling sequences in graniterdquo Inter-national Journal of Rock Mechanics and Mining Sciencesvol 112 pp 35ndash46 2018

[25] Z L Yang ldquoStudy of controlling catastrophe for roof strata inshallow seam longwall miningrdquo Rock and Soil Mechanicsvol 29 no s1 pp 459ndash463 2011

[26] H D Li H H Yang B Zhang and G Chen ldquoControl studyof strong strata behaviors during the fully mechanizedworking face out of concentrated coal pillar in a shallow depthseam in proximity beneath a room mining goafrdquo Journal ofChina Coal Society vol 40 no s1 pp 6ndash11 2015

[27] J Y Zhao S L Wu J P Ma et al ldquoStudy on strata behaviorsregulation of coal working face under room and pillar mininggoafrdquo China Energy and Environmental Protection vol 6pp 60ndash63 2015

[28] J X Yang C Y Liu and B Yu ldquo+e effects of load distri-bution exerting on coal pillars on the stress and energy

10 Shock and Vibration

distribution of the floor stratardquo Acta Montanistica Slovacavol 21 pp 102ndash112 2016

[29] S Li L F Zhou M K Luo et al ldquoStrata behaviors analysis ofstage coal pillar in Tong Xinmine caused by repeatedminingrdquoJournal of Liaoning Technical University Natural Sciencevol 34 no 6 pp 661ndash667 2015

[30] S K Zhang L G Wang X D Zhang et al ldquoLoad transfereffects of the longwall sustaining coal pillars on the safety ofthe mining goaf systemrdquo Journal of Safety and Environmentvol 16 no 3 pp 116ndash119 2016

[31] L Li F Li Y Zhang D Yang and X Liu ldquoFormationmechanism and height calculation of the caved zone andwater-conducting fracture zone in solid backfill miningrdquoInternational Journal of Coal Science amp Technology vol 7no 1 pp 208ndash215 2020

[32] L Xu H X Wei Z Y Xiao and B Li ldquoEngineering cases andcharacteristics of deviatoric stressunder coal pillar in regionalfloorrdquo Coal Science and Technology vol 40 no 4 pp 23ndash252012

[33] Z J Zhu H W Zhang T W Lan B Huo and Y ChenldquoStudy on surrounding rock stress evolution law of fully-mechanized top coal caving mining face under role ofoverlapping coal pillarsrdquo Coal Science and Technology vol 45no 3 pp 26ndash31 2017

[34] C Zhu M He M Karakus X Zhang and Z Tao ldquoNumericalsimulations of the failure process of anaclinal slope physicalmodel and control mechanism of negative Poissonrsquos ratiocablerdquo Bulletin of Engineering Geology and the Environmentvol 80 no 4 pp 3365ndash3380 2021

[35] J G Men S Wang R P Yuan et al ldquoStudy on overburdenstrata structure features and stress distribution law of coalpillar in inclined seamrdquo Coal Science and Technology vol 42no 5 pp 21ndash24 2014

[36] M X Ren ldquoRational decision of pillarrsquos size of Wongawillimining methodrdquo Journal of Mining amp Safety Engineeringvol 21 no 1 pp 42-43+118 2004

[37] B Yu C Y Liu J X Yang and J Liu ldquoMechanism of strongpressure reveal under the influence of miningdual system ofcoal pillar in Datong mining areardquo Journal of China CoalSociety vol 39 no 1 pp 40ndash46 2014

[38] J F Ju ldquoMechanism and prevention of support crushingdisaster while mining out of the upper coal pillar in closedistance shallow seamsrdquo PhD thesis China University ofMining and Technology Xuzhou China 2013

Shock and Vibration 11

stress concentration of the upper coal seam and the mininginduced stress of the lower coal seamwere overlaid+e rockstrata between two coal seams caved thoroughly +e dy-namic pressure was violent When it was not able to supportthe reversed trapezoid rock column above the coal pillar of50m the high elastic-plastic energy that was accumulatedreleased +is induced the instability of the overlying rockstructure If this impact load was applied on the workingface the accident of the hydraulic supports crush wouldoccur

6 Conclusions

(1) In the Huoluowan Coal Mine for the room gob inthe shallow buried coal seam 2minus2 the stress distri-bution under coal pillar group was determinedWhen the room pillars of 8m lost stability they haddisturbing effect on the stress field of the rock strata5ndash8m lower than the coal seam As for the coalpillars of 20m and 50m the maximum influencingrange on the lower rock strata was 42m and 58mFor the lower coal seam of 3minus1 the vertical stressconcentration coefficients were 131 and 124respectively

(2) +e characteristic of the abutment pressure when theworking face crossed large coal pillars was revealed+e concentration coefficient reached 444ndash500Moreover it was maximal at the position in front ofthe working face 4ndash6m

(3) Above the large coal pillar there was no acting forceon the key strata along two sides Moreover therewas large rotating space It was determined that forthe working face in the coal seam 3minus1 when the largecoal pillars were mined the overlying rock strata loststability

(4) +e joint acting dynamic pressure mechanism of theworking face 3minus1 was revealed Specifically it was thehigh stress environment (the mining inducedstresses of the lower coal seam and upper coal seamwere overlaid) and the overall instability of the re-versed trapezoid rock column above the coal pillar+is would induce the accident of roof cutting andthe support compressing

Mining layouts and geological conditions vary from siteto site Although our study is based on the specific miningcondition it provides guidance and reference to thosestudies with similar conditions

44 m

20 m coal pillar Main key strata 50 m coal pillar

5 m remaining room pillar

3ndash1 coal seam

2ndash2 coal seam

Figure 9 Similarity simulation of the coal pillar in the room gob

50 m coal pillar

3ndash1 coal seam

(a)

Extrude coal 15 m

(b)

Figure 10 +e overlying rock strata movement when crossing the coal pillar (a) When crossing the coal pillar of 50m (b) When the coalpillar of 50m was mined

Shock and Vibration 9

Data Availability

Some or all data models or codes generated or used duringthe study are included within the article and are alsoavailable from the corresponding author upon request

Conflicts of Interest

+e authors declare that they have no conflicts of interestregarding the publication of this paper

Acknowledgments

+e authors gratefully acknowledge the National NaturalScience Foundation of China Project (no 51504127) forproviding partial financial support for the present study

References

[1] Q X Huang ldquoGround pressure behavior and definition ofshallow seamsrdquo Chinese Journal of Rock Mechanics and En-gineering vol 21 no 8 pp 1174ndash1177 2002

[2] Y F Ren and Q X Qi ldquoStudy on characteristic of stress fieldin surrounding rocks of shallow coal face under long wallminingrdquo Journal of China Coal Society vol 10 no 36pp 1612ndash1618 2011

[3] Y F Ren ldquoStudy on mine pressure and overlying stratamovement law of contiguous seams with shallow depthrdquo CoalScience and Technology vol 43 no 7 pp 11ndash14 2015

[4] T Zhu B S Zhang G R Feng and X Zhang ldquoRoof structureand control in the lower seam mining field in the ultra-closemultiple seamsrdquo Journal of China Coal Society vol 35 no 2pp 190ndash193 2010

[5] Z Yang B Tong C C Huang et al ldquoStudy on the movementlaw of overlying strata in mining face under room and pillargoafrdquo Journal of Mining amp Safety Engineering vol 29 no 2pp 157ndash161 2012

[6] Q S Bai S H Tu F T Wang et al ldquo+e evolution of miningstress and the mechanism of disaster caused by shallow buriedcoal pillarrdquo Chinese Journal of Rock Mechanics and Engi-neering vol 31 no S2 pp 3772ndash3778 2012

[7] P Małkowski Z Niedbalski and T Balarabe ldquoA statisticalanalysis of geomechanical data and its effect on rock massnumerical modeling a case studyrdquo International Journal ofCoal Science amp Technology vol 8 no 4 pp 1ndash12 2020

[8] D Cao A Wang S Ning et al ldquoCoalfield structure andstructural controls on coal in Chinardquo International Journal ofCoal Science amp Technology vol 7 no 5 pp 220ndash239 2020

[9] S Mao ldquoDevelopment of coal geological information tech-nologies in Chinardquo International Journal of Coal Science ampTechnology vol 7 no 4 pp 320ndash328 2020

[10] J Han H Liang C Cao and Z Bi ldquoA mechanical model forsheared joints based on Mohr-Coulomb material propertiesrdquoGeotechnique Letters vol 8 no 2 pp 1ndash14 2018

[11] D J Xue L L Lu J Zhou and L Lu ldquoCluster modeling of theshort-range correlation of acoustically emitted scatteringsignalsrdquo International Journal of Coal Science amp Technologypp 1ndash15 2020

[12] D J Xue J Zhou Y T Liu and L Gau ldquoOn the excavation-induced stress drop in damaged coal considering a coupledyield and failure criterionrdquo International Journal of CoalScience amp Technology vol 7 no 5 pp 58ndash67 2020

[13] D J Xue Y T Liu H W Zhou J Q Wang J F Liu andJ Zhou ldquoFractal characterization on anisotropy and fractalreconstruction of rough surface of granite under orthogonalshearrdquo Rock Mechanics and Rock Engineering vol 53 no 3pp 1225ndash1242 2020

[14] L Zhang S Chen C Zhang X Fang and S Li ldquo+echaracterization of bituminous coal microstructure andpermeability by liquid nitrogen fracturing based on μCTtechnologyrdquo Fuel vol 262 Article ID 116635 2020

[15] L Zhang J H Li J H Xue C Zhang and X Fang ldquoEx-perimental studies on the changing characteristics of the gasflow capacity on bituminous coal in CO2-ECBM and N2-ECBMrdquo Fuel vol 291 Article ID 120115 2021

[16] J Lin T Ren Y P Cheng J Nimcik and G Wang ldquoCyclicN2 injection for enhanced coal seam gas recovery a labo-ratory studyrdquo Energy vol 188 Article ID 116115 2019

[17] J Lin T Ren G Wang P Booth and J Nemcik ldquoExperi-mental investigation of N2 injection to enhance gas drainagein CO2-rich low permeable seamrdquo Fuel vol 215 pp 665ndash6742018

[18] B Chen ldquoStress-induced trend the clustering feature of coalmine disasters and earthquakes in Chinardquo InternationalJournal of Coal Science amp Technology vol 7 no 4 pp 676ndash692 2020

[19] X Wu Y Peng J Xu Q Yan W Nie and T Zhang ldquoEx-perimental study on evolution law for particle breakageduring coal and gas outburstrdquo International Journal of CoalScience amp Technology vol 7 no 1 pp 97ndash106 2020

[20] 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

[21] X Z Jie ldquoStudy on the characteristics of strata behavior inshallow seam longwall mining under the room-and-pillarmining goafrdquo Journal of China Coal Society vol 37 no 6pp 898ndash902 2012

[22] S H Tu F J Dou Z J Wan F Wang and Y Yuan ldquoStratacontrol technology of the fully mechanized face in shallowcoal seam close to the above room and pillar gobrdquo Journal ofChina Coal Society vol 36 no 3 pp 366ndash370 2011

[23] X Lian H Hu T Li and D Hu ldquoMain geological and miningfactors affecting ground cracks induced by underground coalmining in Shanxi Province Chinardquo International Journal ofCoal Science amp Technology vol 7 no 2 pp 362ndash370 2020

[24] D Xue H Zhou Y Zhao L Zhang L Deng and X WangldquoReal-time SEM observation of mesoscale failures underthermal-mechanical coupling sequences in graniterdquo Inter-national Journal of Rock Mechanics and Mining Sciencesvol 112 pp 35ndash46 2018

[25] Z L Yang ldquoStudy of controlling catastrophe for roof strata inshallow seam longwall miningrdquo Rock and Soil Mechanicsvol 29 no s1 pp 459ndash463 2011

[26] H D Li H H Yang B Zhang and G Chen ldquoControl studyof strong strata behaviors during the fully mechanizedworking face out of concentrated coal pillar in a shallow depthseam in proximity beneath a room mining goafrdquo Journal ofChina Coal Society vol 40 no s1 pp 6ndash11 2015

[27] J Y Zhao S L Wu J P Ma et al ldquoStudy on strata behaviorsregulation of coal working face under room and pillar mininggoafrdquo China Energy and Environmental Protection vol 6pp 60ndash63 2015

[28] J X Yang C Y Liu and B Yu ldquo+e effects of load distri-bution exerting on coal pillars on the stress and energy

10 Shock and Vibration

distribution of the floor stratardquo Acta Montanistica Slovacavol 21 pp 102ndash112 2016

[29] S Li L F Zhou M K Luo et al ldquoStrata behaviors analysis ofstage coal pillar in Tong Xinmine caused by repeatedminingrdquoJournal of Liaoning Technical University Natural Sciencevol 34 no 6 pp 661ndash667 2015

[30] S K Zhang L G Wang X D Zhang et al ldquoLoad transfereffects of the longwall sustaining coal pillars on the safety ofthe mining goaf systemrdquo Journal of Safety and Environmentvol 16 no 3 pp 116ndash119 2016

[31] L Li F Li Y Zhang D Yang and X Liu ldquoFormationmechanism and height calculation of the caved zone andwater-conducting fracture zone in solid backfill miningrdquoInternational Journal of Coal Science amp Technology vol 7no 1 pp 208ndash215 2020

[32] L Xu H X Wei Z Y Xiao and B Li ldquoEngineering cases andcharacteristics of deviatoric stressunder coal pillar in regionalfloorrdquo Coal Science and Technology vol 40 no 4 pp 23ndash252012

[33] Z J Zhu H W Zhang T W Lan B Huo and Y ChenldquoStudy on surrounding rock stress evolution law of fully-mechanized top coal caving mining face under role ofoverlapping coal pillarsrdquo Coal Science and Technology vol 45no 3 pp 26ndash31 2017

[34] C Zhu M He M Karakus X Zhang and Z Tao ldquoNumericalsimulations of the failure process of anaclinal slope physicalmodel and control mechanism of negative Poissonrsquos ratiocablerdquo Bulletin of Engineering Geology and the Environmentvol 80 no 4 pp 3365ndash3380 2021

[35] J G Men S Wang R P Yuan et al ldquoStudy on overburdenstrata structure features and stress distribution law of coalpillar in inclined seamrdquo Coal Science and Technology vol 42no 5 pp 21ndash24 2014

[36] M X Ren ldquoRational decision of pillarrsquos size of Wongawillimining methodrdquo Journal of Mining amp Safety Engineeringvol 21 no 1 pp 42-43+118 2004

[37] B Yu C Y Liu J X Yang and J Liu ldquoMechanism of strongpressure reveal under the influence of miningdual system ofcoal pillar in Datong mining areardquo Journal of China CoalSociety vol 39 no 1 pp 40ndash46 2014

[38] J F Ju ldquoMechanism and prevention of support crushingdisaster while mining out of the upper coal pillar in closedistance shallow seamsrdquo PhD thesis China University ofMining and Technology Xuzhou China 2013

Shock and Vibration 11

Data Availability

Some or all data models or codes generated or used duringthe study are included within the article and are alsoavailable from the corresponding author upon request

Conflicts of Interest

+e authors declare that they have no conflicts of interestregarding the publication of this paper

Acknowledgments

+e authors gratefully acknowledge the National NaturalScience Foundation of China Project (no 51504127) forproviding partial financial support for the present study

References

[1] Q X Huang ldquoGround pressure behavior and definition ofshallow seamsrdquo Chinese Journal of Rock Mechanics and En-gineering vol 21 no 8 pp 1174ndash1177 2002

[2] Y F Ren and Q X Qi ldquoStudy on characteristic of stress fieldin surrounding rocks of shallow coal face under long wallminingrdquo Journal of China Coal Society vol 10 no 36pp 1612ndash1618 2011

[3] Y F Ren ldquoStudy on mine pressure and overlying stratamovement law of contiguous seams with shallow depthrdquo CoalScience and Technology vol 43 no 7 pp 11ndash14 2015

[4] T Zhu B S Zhang G R Feng and X Zhang ldquoRoof structureand control in the lower seam mining field in the ultra-closemultiple seamsrdquo Journal of China Coal Society vol 35 no 2pp 190ndash193 2010

[5] Z Yang B Tong C C Huang et al ldquoStudy on the movementlaw of overlying strata in mining face under room and pillargoafrdquo Journal of Mining amp Safety Engineering vol 29 no 2pp 157ndash161 2012

[6] Q S Bai S H Tu F T Wang et al ldquo+e evolution of miningstress and the mechanism of disaster caused by shallow buriedcoal pillarrdquo Chinese Journal of Rock Mechanics and Engi-neering vol 31 no S2 pp 3772ndash3778 2012

[7] P Małkowski Z Niedbalski and T Balarabe ldquoA statisticalanalysis of geomechanical data and its effect on rock massnumerical modeling a case studyrdquo International Journal ofCoal Science amp Technology vol 8 no 4 pp 1ndash12 2020

[8] D Cao A Wang S Ning et al ldquoCoalfield structure andstructural controls on coal in Chinardquo International Journal ofCoal Science amp Technology vol 7 no 5 pp 220ndash239 2020

[9] S Mao ldquoDevelopment of coal geological information tech-nologies in Chinardquo International Journal of Coal Science ampTechnology vol 7 no 4 pp 320ndash328 2020

[10] J Han H Liang C Cao and Z Bi ldquoA mechanical model forsheared joints based on Mohr-Coulomb material propertiesrdquoGeotechnique Letters vol 8 no 2 pp 1ndash14 2018

[11] D J Xue L L Lu J Zhou and L Lu ldquoCluster modeling of theshort-range correlation of acoustically emitted scatteringsignalsrdquo International Journal of Coal Science amp Technologypp 1ndash15 2020

[12] D J Xue J Zhou Y T Liu and L Gau ldquoOn the excavation-induced stress drop in damaged coal considering a coupledyield and failure criterionrdquo International Journal of CoalScience amp Technology vol 7 no 5 pp 58ndash67 2020

[13] D J Xue Y T Liu H W Zhou J Q Wang J F Liu andJ Zhou ldquoFractal characterization on anisotropy and fractalreconstruction of rough surface of granite under orthogonalshearrdquo Rock Mechanics and Rock Engineering vol 53 no 3pp 1225ndash1242 2020

[14] L Zhang S Chen C Zhang X Fang and S Li ldquo+echaracterization of bituminous coal microstructure andpermeability by liquid nitrogen fracturing based on μCTtechnologyrdquo Fuel vol 262 Article ID 116635 2020

[15] L Zhang J H Li J H Xue C Zhang and X Fang ldquoEx-perimental studies on the changing characteristics of the gasflow capacity on bituminous coal in CO2-ECBM and N2-ECBMrdquo Fuel vol 291 Article ID 120115 2021

[16] J Lin T Ren Y P Cheng J Nimcik and G Wang ldquoCyclicN2 injection for enhanced coal seam gas recovery a labo-ratory studyrdquo Energy vol 188 Article ID 116115 2019

[17] J Lin T Ren G Wang P Booth and J Nemcik ldquoExperi-mental investigation of N2 injection to enhance gas drainagein CO2-rich low permeable seamrdquo Fuel vol 215 pp 665ndash6742018

[18] B Chen ldquoStress-induced trend the clustering feature of coalmine disasters and earthquakes in Chinardquo InternationalJournal of Coal Science amp Technology vol 7 no 4 pp 676ndash692 2020

[19] X Wu Y Peng J Xu Q Yan W Nie and T Zhang ldquoEx-perimental study on evolution law for particle breakageduring coal and gas outburstrdquo International Journal of CoalScience amp Technology vol 7 no 1 pp 97ndash106 2020

[20] 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

[21] X Z Jie ldquoStudy on the characteristics of strata behavior inshallow seam longwall mining under the room-and-pillarmining goafrdquo Journal of China Coal Society vol 37 no 6pp 898ndash902 2012

[22] S H Tu F J Dou Z J Wan F Wang and Y Yuan ldquoStratacontrol technology of the fully mechanized face in shallowcoal seam close to the above room and pillar gobrdquo Journal ofChina Coal Society vol 36 no 3 pp 366ndash370 2011

[23] X Lian H Hu T Li and D Hu ldquoMain geological and miningfactors affecting ground cracks induced by underground coalmining in Shanxi Province Chinardquo International Journal ofCoal Science amp Technology vol 7 no 2 pp 362ndash370 2020

[24] D Xue H Zhou Y Zhao L Zhang L Deng and X WangldquoReal-time SEM observation of mesoscale failures underthermal-mechanical coupling sequences in graniterdquo Inter-national Journal of Rock Mechanics and Mining Sciencesvol 112 pp 35ndash46 2018

[25] Z L Yang ldquoStudy of controlling catastrophe for roof strata inshallow seam longwall miningrdquo Rock and Soil Mechanicsvol 29 no s1 pp 459ndash463 2011

[26] H D Li H H Yang B Zhang and G Chen ldquoControl studyof strong strata behaviors during the fully mechanizedworking face out of concentrated coal pillar in a shallow depthseam in proximity beneath a room mining goafrdquo Journal ofChina Coal Society vol 40 no s1 pp 6ndash11 2015

[27] J Y Zhao S L Wu J P Ma et al ldquoStudy on strata behaviorsregulation of coal working face under room and pillar mininggoafrdquo China Energy and Environmental Protection vol 6pp 60ndash63 2015

[28] J X Yang C Y Liu and B Yu ldquo+e effects of load distri-bution exerting on coal pillars on the stress and energy

10 Shock and Vibration

distribution of the floor stratardquo Acta Montanistica Slovacavol 21 pp 102ndash112 2016

[29] S Li L F Zhou M K Luo et al ldquoStrata behaviors analysis ofstage coal pillar in Tong Xinmine caused by repeatedminingrdquoJournal of Liaoning Technical University Natural Sciencevol 34 no 6 pp 661ndash667 2015

[30] S K Zhang L G Wang X D Zhang et al ldquoLoad transfereffects of the longwall sustaining coal pillars on the safety ofthe mining goaf systemrdquo Journal of Safety and Environmentvol 16 no 3 pp 116ndash119 2016

[31] L Li F Li Y Zhang D Yang and X Liu ldquoFormationmechanism and height calculation of the caved zone andwater-conducting fracture zone in solid backfill miningrdquoInternational Journal of Coal Science amp Technology vol 7no 1 pp 208ndash215 2020

[32] L Xu H X Wei Z Y Xiao and B Li ldquoEngineering cases andcharacteristics of deviatoric stressunder coal pillar in regionalfloorrdquo Coal Science and Technology vol 40 no 4 pp 23ndash252012

[33] Z J Zhu H W Zhang T W Lan B Huo and Y ChenldquoStudy on surrounding rock stress evolution law of fully-mechanized top coal caving mining face under role ofoverlapping coal pillarsrdquo Coal Science and Technology vol 45no 3 pp 26ndash31 2017

[34] C Zhu M He M Karakus X Zhang and Z Tao ldquoNumericalsimulations of the failure process of anaclinal slope physicalmodel and control mechanism of negative Poissonrsquos ratiocablerdquo Bulletin of Engineering Geology and the Environmentvol 80 no 4 pp 3365ndash3380 2021

[35] J G Men S Wang R P Yuan et al ldquoStudy on overburdenstrata structure features and stress distribution law of coalpillar in inclined seamrdquo Coal Science and Technology vol 42no 5 pp 21ndash24 2014

[36] M X Ren ldquoRational decision of pillarrsquos size of Wongawillimining methodrdquo Journal of Mining amp Safety Engineeringvol 21 no 1 pp 42-43+118 2004

[37] B Yu C Y Liu J X Yang and J Liu ldquoMechanism of strongpressure reveal under the influence of miningdual system ofcoal pillar in Datong mining areardquo Journal of China CoalSociety vol 39 no 1 pp 40ndash46 2014

[38] J F Ju ldquoMechanism and prevention of support crushingdisaster while mining out of the upper coal pillar in closedistance shallow seamsrdquo PhD thesis China University ofMining and Technology Xuzhou China 2013

Shock and Vibration 11

distribution of the floor stratardquo Acta Montanistica Slovacavol 21 pp 102ndash112 2016

[29] S Li L F Zhou M K Luo et al ldquoStrata behaviors analysis ofstage coal pillar in Tong Xinmine caused by repeatedminingrdquoJournal of Liaoning Technical University Natural Sciencevol 34 no 6 pp 661ndash667 2015

[30] S K Zhang L G Wang X D Zhang et al ldquoLoad transfereffects of the longwall sustaining coal pillars on the safety ofthe mining goaf systemrdquo Journal of Safety and Environmentvol 16 no 3 pp 116ndash119 2016

[31] L Li F Li Y Zhang D Yang and X Liu ldquoFormationmechanism and height calculation of the caved zone andwater-conducting fracture zone in solid backfill miningrdquoInternational Journal of Coal Science amp Technology vol 7no 1 pp 208ndash215 2020

[32] L Xu H X Wei Z Y Xiao and B Li ldquoEngineering cases andcharacteristics of deviatoric stressunder coal pillar in regionalfloorrdquo Coal Science and Technology vol 40 no 4 pp 23ndash252012

[33] Z J Zhu H W Zhang T W Lan B Huo and Y ChenldquoStudy on surrounding rock stress evolution law of fully-mechanized top coal caving mining face under role ofoverlapping coal pillarsrdquo Coal Science and Technology vol 45no 3 pp 26ndash31 2017

[34] C Zhu M He M Karakus X Zhang and Z Tao ldquoNumericalsimulations of the failure process of anaclinal slope physicalmodel and control mechanism of negative Poissonrsquos ratiocablerdquo Bulletin of Engineering Geology and the Environmentvol 80 no 4 pp 3365ndash3380 2021

[35] J G Men S Wang R P Yuan et al ldquoStudy on overburdenstrata structure features and stress distribution law of coalpillar in inclined seamrdquo Coal Science and Technology vol 42no 5 pp 21ndash24 2014

[36] M X Ren ldquoRational decision of pillarrsquos size of Wongawillimining methodrdquo Journal of Mining amp Safety Engineeringvol 21 no 1 pp 42-43+118 2004

[37] B Yu C Y Liu J X Yang and J Liu ldquoMechanism of strongpressure reveal under the influence of miningdual system ofcoal pillar in Datong mining areardquo Journal of China CoalSociety vol 39 no 1 pp 40ndash46 2014

[38] J F Ju ldquoMechanism and prevention of support crushingdisaster while mining out of the upper coal pillar in closedistance shallow seamsrdquo PhD thesis China University ofMining and Technology Xuzhou China 2013

Shock and Vibration 11