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Research ArticleThe Influences of Key Strata Compound Breakage on theOverlying Strata Movement and Strata Pressure Behavior inFully Mechanized Caving Mining of Shallow and ExtremelyThick Seams: A Case Study

Junmeng Li ,1,2 Yanli Huang ,1 Jixiong Zhang ,1 Meng Li,1 Ming Qiao,1

and Fengwan Wang1

1State Key Laboratory of Coal Resources and Safe Mining, School of Mines, China University of Mining & Technology,Xuzhou 221116, China2School of Mechanical and Mining Engineering, $e University of Queensland, Brisbane 4072, Queensland, Australia

Correspondence should be addressed to Yanli Huang; [email protected]

Received 30 August 2018; Accepted 26 December 2018; Published 3 February 2019

Guest Editor: Dengke Wang

Copyright © 2019 Junmeng Li et al. .is is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In order to analyze the impact of compound breakage of key strata on overlying strata movement and strata pressure behaviorduring the fully mechanized caving mining in shallow and extremely thick seams, this paper took the 1322 fully mechanizedcaving face in Jindi Coal Mine in Xing County as the engineering background. Under the special mining and geological conditionmentioned above, UDEC numerical simulation software was applied to research the engineering problems, and results ofnumerical simulation were verified through the in-site measurement. .e research results showed that during the fullymechanized caving mining in shallow and extremely thick seams, the inferior key strata affected by mining movement behaved inthe mode of sliding instability and could not form the stable structure of the voussoir beam after breaking and caving. In addition,the main key strata behaved in the mode of rotary instability, and the caving rocks behind the goaf were gradually compactedbecause of the periodic instability of the main key strata. With the continuous advance of the working face, the abutment pressureof the working face was affected by the compound breakage and periodic instability of both the inferior key strata and the main keystrata, and the peaks of the abutment pressure presented small-big-small-big periodical change characteristics. Meanwhile, therisk of rib spalling ahead of the working face presented different levels of acute or slowing trends. .e actual measurement resultsof ground pressure in the working face showed that, in the working process, the first weighting interval of the inferior key stratawas about 51m and its average periodic weighting interval was about 12.6m, both of which were basically consistent with theresults of numerical simulation. .e research has great significance in providing theoretical guidance and practical experience forpredicting and controlling the ground pressure under the similar mining and geological conditions.

1. Introduction

Many areas in the west of China, mainly including Xinjiang,Inner Mongolia, and Shaanxi and Shanxi provinces, are richin coal resources, accounting for 81.3% of total nationalresources. Among these coal resources, the reserve of boththick and extrathick coal seams take up more than 44% oftotal reserves, which has laid a solid foundation for safe andefficient production of coal resources and rapid economicdevelopment [1–3]. With the implementation of the

national strategy for developing the western areas, theexploitation scale of coal resources in these areas hasgradually expanded, and shallow-buried working face arebecoming increasingly common [4–6]. At present, the XingCounty mining area in Shanxi province which is underdevelopment has a total reserve of approximately 39.639billion tons, and its average seam thickness is 12m, which isregarded as extrathick coal seam. .e distinguishing fea-ture of occurrence of coal seam in the Xing County miningarea is shallow-buried depth and thicker loess layers above

HindawiAdvances in Civil EngineeringVolume 2019, Article ID 5929635, 11 pageshttps://doi.org/10.1155/2019/5929635

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https://doi.org/10.1155/2019/5929635

the bedrock, which is similar to that in the adjacent Shenfu-Yuheng mining area, but also has significant differences. Asfor the Xing County mining area, the buried depth of coalseam is deeper than that in the Shenfu mining area, and thethickness of both coal seam and bedrock is larger than thatin the Shenfu mining area, which belongs to the sandy-soil-rock-type bedrock coal seam with shallow-buried depthand thin thickness [7, 8]. Under the special mining andgeological condition in the Xing County mining area, boththe movement and breakage law of overlying strata and thepressure behavior law of stope in the mining process ofshallow and extremely thick seams are significantly dif-ferent from those in conventional mining.

At present, many scholars have conducted extensiveresearch on the breaking mechanism and pressure be-havior law of key strata in overlying strata under theconventional mining condition of shallow coal seams. Laiet al. studied the coupling mechanical properties of the“support-surrounding rock” in the large-height workingface with shallow-buried and extremely thick coal seamand obtained the features, mainly including period, in-tensity, and range, of the pressure behavior of the roof inthe large-height working face with shallow-buried depthand extremely thick seam [9]. Aimed at the fact that thekey strata of shallow-buried seams in the Shendongmining area are easy to cave and cause many problems,such as roof failure due to weighting over great extent anddynamic hazards. Jiang et al. studied the initial breakingcharacteristics and caving mechanism of the key strata inshallow coal seams [10]. Xie and Zhao took the Qianshutacoal mine as the research background, and an integratedmethod, mainly including numerical simulation, theo-retical analysis, and on-site measurement, was used tostudy caving feature and structure feature of the top coalduring fully mechanized top-coal caving mining in theshallow-buried, hard, and extremely thick coal seam [11].Wang et al. took the geological condition of the fullymechanized working face with a high mining height in theSandaokou coal mine as the engineering background, andsimilar material simulation was applied to research thebreaking laws of the key strata in the working face withlarge mining height and approximate shallow depth [12].Yang [13] carried out research on the mechanisms oflarge-scale roof cutting and support crushing and pressurebehavior law of the working face in the process of high-intensity exploitation of shallow-buried coal seams. Liu etal. built the height control model of the water-flowingfractured zone based on the background of a shallow thickcoal seam in North Shaanxi. .rough the experimentalsimulation and analysis of rock bearing capacity andfracture evolution rule, the control model of structuralstability of the key strata layer was built and the effectiveprotection of the coal seam in northern Shaanxi was re-alized [14]. Ju and Xu studied surface stepped subsidencerelated to top-coal caving longwall mining of extremelythick coal seam under shallow cover. .e formationmechanism of the surface stepped subsidence case wasexplained. .e surface subsidence profile after the sequentmining activity was predicted. .e possible controls of

the stepped subsidence were lastly proposed [15]. Wanget al. took the Shendong coal mine as the researchbackground and investigated the form and stability of theroof-bearing structure of a shallow coal seam underlongwall mining, in order to determine the fundamentalmechanism of roof step-form subsidence [16].

.e research mentioned above is mostly based on theengineering condition of coal seams in the Shenfu-Yongheng mining area in China, the breaking mecha-nism of the key strata and pressure behavior law in themining process of shallow-buried coal seams have beenstudied. However, there is little research on the influence ofcompound breakage of the key strata on the movement ofoverlying strata and pressure behavior law under thespecial mining and geological condition in Xing Countrymining area. .is paper takes the 1322 fully mechanizedcaving face of Jindi Coal Mine in Xing County mining areaas a case study, and some research is conducted on theproblems mentioned above. .e research results are ofgreat significance to enrich mine pressure theory andprovide guidance for mine pressure control under similarmining and geological conditions.

2. Engineering Background

.e Xing County mining area is located in the Lvliangmountain system and presents the typical landscape of LoessPlateau. .e geographical location is shown in Figure 1. .ecoal seams of Xing County mining area mainly characterizesand-base bedrock with a shallow-buried depth and thinthickness, and the main structure of overlying strata ischaracterized by loess-thin bedrock-seam structure. In ad-dition, the main feature of this type of overburden stratastructure is that most of the loose layers of the quaternarysystem are made up of loess layers with large thickness, andthe components of bedrock are mainly sandstone andmudstone. .e 1322 working face of the Jindi Coal Mine inthe Xing County mining area is located in the north-centralregion of the minefield, with 1324 working face in the west,the boundary pillar in the north, the industrial square pillar inthe south, and the goaf of 1313 working face, south trans-portation roadway, and other major roadways in the east. .eprofile of 1322 working face is shown in Figure 2. .e mainexploiting seam at 1322 working face is 13# coal seam with aburied depth of 258m∼329.3m, which is a shallow-buriedcoal seam; the thickness of coal seam is 8.8∼13.65m with anaverage thickness of 11.9m, which belongs to extremely thickcoal seam. .e fully mechanized top-coal caving miningmethod was used to exploit the 13# coal seam with a miningheight of 3.2m and a roof caving height of 8.7m..e inclinedlength of the working face is 180m, and the strike length is1230m. .e dip angle of the coal seam is 18°∼25° with anaverage angle of 22°, which is a gently inclined coal seam..ethickness of immediate roof is 0.8∼2m. .e immediate roofmainly consists of sandy mudstone, mudstone, and siltstonewith sandstones distributing locally; the basic top is made upof medium-coarse sandstone; the floor is mainly made up offine-grained sandstone, mudstone, and sandy mudstone.

2 Advances in Civil Engineering

3. The Establishment of Numerical Model andSimulation Scheme

�is paper took the 1322 fully mechanized caving mining facein Jindi Coal Mine as the engineering background, and theUDEC discrete element numerical model was established tostudy the movement law and pressure behavior law of over-lying strata in the stope under the special mining and geologicalcondition of sallow-buried and extremely thick coal seam.

3.1.�e Establishment of Numerical Models. Taking the 1322fully mechanized top-coal caving face in Jindi Coal Mine as

the engineering background, the UDEC numerical simulationsoftware was used to establish a discrete element numericalmodel to research the overlying strata movement and pres-sure behavior of the working face under the condition of thefully mechanized top-coal caving mining face in the shallow-buried thick coal seam. �e length and height of the modelwere 500m and 120.6m, and the average buried depth of thecoal seam was 266.6m. As for the numerical model, a total of17 layers of strata which contained both coal and roof and�oor were established, and the topsoil layer was simpli�ed andreplaced by an equivalent stress of 2.63MPa. Considering thein�uence of the mining boundary, boundary protectionpillars with a thickness of 100m were remained on both sidesof coal seams. �e horizontal constraint was imposed on theleft and right boundaries of the model, and the verticalconstraint was imposed on the bottom boundary of themodel. �e model was divided into 7871 blocks, 35874 cells,and 44648 nodes. According to the key strata theory [17–20],it could be obtained that there were two layers of the key stratawithin the coal-bearing strata. �e main key strata of theoverlying strata is the 9th layer of �ne sandstone above the 13#coal seam, and the inferior key strata is the 2nd layer ofgritstone above the 13# coal. �e subsidence monitoring linesof overburden strata were, respectively, placed in the mainand inferior key strata, and vertical monitoring lines of stresswere arranged in the immediate roof. �e numerical modelestablished and boundary conditions are shown in Figure 3.

3.2. Selection of Mechanical Parameters. �e physical andmechanical parameters of each stratum applied in the nu-merical model were measured by experiments in the labo-ratory, as are shown in Table 1. �e mechanical parametersof jointed rock mass are shown in Table 1.

Beijing

Xing Country

Jindi Coal Mine

Xing Country

Figure 1: �e geographic location of Jindi Coal Mine.

Setup en

try

1313 working face1322 working face1324 working face

1322 tail entry

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ing lin

e

Stop

ping

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Figure 2: Overview of the 1322 working face at Jindi Coal Mine.

Advances in Civil Engineering 3

3.3. Numerical Simulation Schemes. �is paper mainlyresearched the movement law and pressure behavior law ofoverburden strata at fully mechanized caving face withshallow-buried and extremely thick coal seam. As the fullymechanized top-coal caving face with shallow-buried andextremely thick coal seam continuously advanced, theemphasis of the research was to simulate the breaking law ofthe key strata at the fully mechanized top-coal caving facewith shallow-buried and extremely thick coal seam and thepressure behavior law caused by the breakage of the keystrata.

In the process of numerical simulation, the length ofexcavation was 10m every time and the total length ofexcavation was 300m.�e speci�c process of simulation is asfollows:

(1) �e initial stress calculation function of UDEC wasused to generate the gravitational �eld to simulate theoriginal stress state of the strata before excavation.

(2) Under the e�ect of the initial stress �eld, themovement and deformation characteristics and thepressure behavior law of overlying strata in themining process of shallow-buried and extremelythick coal seams were simulated. A method ofcontinuous excavation was applied in the simulation,and the length of excavation was 10m every time,with a total excavation length of 300m.

(3) At di�erent advance distances of the working face,the movement, caving characteristics, and stressdistribution of the overlying strata were comparedand analyzed.

(4) �e monitoring data of the survey lines #1 and #2arranged in the main and inferior key strata wereextracted, and based on the data obtained, thesubsidence curves of the main and inferior key strataunder the condition of di�erent advance distances ofthe working face were respectively drawn; the

100

q = 2.63 MPa

Coal seam

Primary key stratum

Sub-key stratum

Measurement line #1

Measurement line #2Measurement line #3

Mining direction

Figure 3: Schematic diagram of the numerical model and boundary conditions.

Table 1: Coal and rock interface mechanical parameters used in the model.

Strata number Lithology Normalsti�ness (GPa)

Shearsti�ness (GPa) Cohesion (MPa) Friction angle (°) Tensile

strength (MPa)1 Sandy mudstone 1.0 2.5 0 10 02 Siltstone 3.5 7.2 0 23 03 Sandy mudstone 1.0 2.5 0 10 04 Medium sandstone 3.5 7.0 0 20 05 Siltstone 3.5 7.2 0 23 06 Fine sandstone 4 8.2 0 22 07 Gritstone 3.6 8.0 0 14 08 Mudstone 0.6 2.3 0 20 09 Medium sandstone 3.5 7.0 0 20 010 Sandy mudstone 1.0 2.5 0 10 011 Siltstone 3.5 7.2 0 23 012 Mudstone 0.6 2.3 0 20 013 Gritstone 3.6 8.0 0 14 014 Sandy mudstone 1.0 2.5 0 10 015 Coal 0.5 2.0 0 16 016 Fine sandstone 4 8.2 0 22 017 Sandy mudstone 1.0 2.5 0 10 0


monitoring data of the survey line #3 placed in theimmediate were extracted, and based on the datagained, the vertical stress curves of the survey line #3under the condition of di�erent advance distanceswere drawn. By comparing and analyzing the curvesdrawn, some conclusions could be obtained.

4. Study on Strata Pressure Behavior

4.1. Analysis of Overlying StrataMovement. Monitoring dataof survey lines #1 and #2 placed in the inferior key strata andthe main key strata were extracted, and subsidence curves ofthe inferior key strata and the main key strata under thecondition of di�erent advancing distances of the workingface could be obtained, as are shown in Figures 4 and 5,respectively.

It is shown in both Figures 4 and 5 that when thecontinuous advance distance of the working face was 20m,there was no obvious subsidence for the inferior key strataand there was almost no subsidence for the main key strata.When the working face advanced 50m, the subsidence of theinferior key strata increased to 11m, meaning that the in-ferior key strata caved for the �rst time. At the moment, thesubsidence of the inferior key strata beside the working facerose dramatically to 8.5m. It could be obtained that theinferior key strata showed the characteristics of sliding in-stability. �ere was no obvious subsidence for the main keystrata, with only 0.3m. When the continuous advancedistance of the working face reached 70m, the maximumsubsidence of the inferior key strata was 11.5m, whichmeant that the inferior key strata occurred the periodicalcaving for the �rst time. It could be obtained from thesubsidence curves of the inferior key strata that the inferiorkey strata showed the characteristics of sliding instability atthis time. In addition, the subsidence of the main key strataincreased to 0.8m. When the working face advanced 90m,the inferior key strata occurred the periodical caving for thesecond time, and its subsidence was 12m. Compared to thesubsidence of the main key strata with the advance distanceof 70m, the subsidence of the main key strata with theadvance distance of 90m increased to 3m, rising by 2.2mand with an increment of 275%. At this time, the main keystrata broke for the �rst time and they showed the char-acteristics of rotary instability. When the continuous ad-vance distance reached 110m, the maximum subsidence ofthe inferior key strata and themain key strata was 11.5m and4.2m, respectively. Compared to the subsidence of the mainkey strata with the advance distance of 110m, the subsidenceof the main key strata with the advance distance of 150mincreased by 1.8m, with an increment of 43%, which meantthat the main key strata occurred the periodical breakage forthe �rst time and they showed the characteristics of rotaryinstability. When the continuous advance distances of theworking face were 200m and 300m, the maximum sub-sidence of the main key strata was 8.8m and the subsidenceof the inferior key strata also rose, meaning that the cavingrocks located at the rear of the goaf were gradually com-pacted with the periodical instability of the main key strata.

4.2. �e Impact of Key Strata Compound Breakage on theStrata Pressure Behavior. �e plastic zone distribution ofsurrounding rocks in the working face under the conditionof di�erent advance distances of the working face is shown inFigure 6. �e advanced support pressure and the subsidenceof the main key strata under the condition of di�erentadvance distances are shown in Figure 7.

It is showed in Figures 6 and 7 that with the continuousadvance of the working face, the maximum subsidence of themain key strata showed a gradually upward trend, and theabutment pressure ahead of the working face showed a wavyincreasing trend. To be more speci�c, before the continuousadvance distance of the working face reached 90m, becausethe inferior key strata had occurred the initial caving and

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Figure 5: Subsidence curves of the main key strata.


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(f )

Figure 6: �e plastic zone distribution of surrounding rocks in the working face under di�erent advance distances of the working face.(a) 50m. (b) 70m. (c) 90m. (d) 110m. (e) 150m. (f ) 200m.


periodical caving, the fracture in the main key strata de-veloped, but the main key strata did not break and losestability, and they could still be regarded as the supportingbody of the overlying strata. At the moment, both themaximum abutment pressure ahead of the working face andthe maximum subsidence of the main key strata increasedwith a relatively slow speed..e rib ahead of the working facehad the risk of spalling. When the working face advanced90m, the main key strata located at the anterior and upperside of the coal rib broke and lost the stability because ofstretching, and they broke for the first time. At this time, boththe maximum abutment pressure ahead of the working faceand themaximum subsidence of themain key strata increaseddramatically with a fast speed, which increased risk of ribspalling ahead of the working face. After the working face hadadvanced 90m, the main key strata affected by different levelsof stretching occurred the periodical breakage. When theworking face advanced 110m and 170m, because the abutmentpressure ahead of the working face was affected by the com-pound breakage and periodical instability of both the inferiorandmain key strata, themaximumabutment pressure showed adownward trend compared with that when the advance dis-tances of the working face were 90m, 150m, and 200m. .emaximum of the abutment pressure ahead of the working facepresented small-big-small-big periodical change characteristics.Meanwhile, the risk of rib spalling ahead of the working facepresented different levels of acute or slowing trends. .emaximumof the abutment pressure occurred 10m∼40m aheadof the working face, with an affected area of 110m∼160m.

5. Field Test

In the mining process of 1322 fully mechanized top-coalcaving face, the working resistance of the hydraulic supportwas measured in situ. By analyzing the monitoring results ofworking resistance of 4 supports placed at 1322 fullymechanized top-coal caving face, the working resistancedistribution of 4 supports could be obtained along with the

continuous advance of working face at the initial miningstage, as are shown in Figure 8.

It could be concluded from Figure 8 that with thecontinuous advance of the working face, the working re-sistance of the hydraulic support increased steadily. Whenthe advance distance of the working face reached 50m, theworking resistance values of the 25#, 50#, and 60# supportssoared, which were close to the rated working resistance ofthe hydraulic support. .is meant that the inferior key strataof the working face collapsed for the first time, resulting inhydraulic supports placed at the working face crushing witha large area. .e first weighting interval of the inferior keystrata was about 51m, which was basically consistent withthe results obtained from numerical simulation.

.e working resistance distribution of the 4 hydraulicsupports along with the advance of the working face at thenormal mining stage is shown in Figure 9.

It could be obtained from Figure 9 that, at the normalmining stage, when the working face advanced from 567.9mto 698.3m with advance distance of about 130m, theworking resistance of 4 hydraulic supports occurred the peakpressure for 8∼10 times, meaning that the roof regularlybroke and caved for many times. .e average periodicalweighting interval of the roof at the 25# and 50# support wasabout 15.1m and 12.1m, respectively..e average periodicalweighting interval of the 4 supports was about 12.6m. .erewas no significant difference in the average periodicalweighting interval of each support. .e pressure duration ofthe roof in the middle of the working face was relativelylonger, while the pressure duration of the head and tail wasrelatively shorter.

6. Conclusion

.is paper took the 1322 fully mechanized caving face inJindi Coal Mine in Xing County as the engineering back-ground. Under the condition of different advance distances,UDEC numerical simulation software was applied to analyze

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eme v

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Figure 7: .e advanced support pressure and the subsidence of the main key strata.


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Figure 8:�e working resistance distribution of hydraulic supports placed at di�erent parts of working face at the initial mining stage.(a) 25# support. (b) 50# support. (c) 60# support. (d) 110# support.


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3:09

-09.

08

17:1

3:16

-10.

08

04:2

7:43

-10.

0805

:11:

01-1

0.08

10:0

8:01

-10.

0809

:13:

00-1

0.08

10:2

1:15

-10.

08

18:1

5:01

-10.

0819

:13:

16-1

0.08

23:4

1:01

-10.

08

15:3

2:23

-12.

08

13:2

4:09

-11.

0821

:36:

01-1

2.08

12:3

6:01

-11.

0807

:41:

21-1

1.08

05:5

1:01

-11.

0801

:27:

43-1

1.08

23:0

3:09

-13.

08

18:1

6:16

-16.

08

01:2

7:43

-14.

0805

:11:

01-1

4.08

15:2

1:01

-15.

0810

:35:

01-1

5.08

05:2

5:11

-16.

08

15:0

1:09

-17.

08

Setting loadRating working resistanceEnd resistance

(a)


Time

570 580 590 600 610 620 630 640 650 660 670 680 690 7003000

4000

5000

6000

7000

8000

9000

10000

11000

12000

Advance rate (m)

22:5

4:12

-14.

08

06:3

6:11

-15.

08

06:2

1:46

-16.

08

kN

23:4

6:25

-17.

08

00:1

7:05

-04.

0819

:45:

00-0

3.08

02:5

2:20

-04.

08

17:3

9:12

-04.

08

23:3

4:03

-05.

08

17:2

9:31

-04.

08

22:5

5:10

-04.

0803

:54:

56-0

5.08

18:5

3:29

-05.

0819

:53:

46-0

5.08

06:2

5:10

-06.

08

15:2

2:02

-07.

08

02:0

7:18

-07.

0804

:24:

13-0

7.08

00:2

1:41

-07.

0818

:41:

25-0

6.08

14:5

0:33

-06.

08

02:4

7:57

-06.

08

21:0

0:05

-07.

08

00:1

4:17

-09.

08

03:4

7:20

-08-

0805

:11:

01-0

8.08

09:5

3:01

-08.

0816

:43:

40-0

8.08

20:5

8:06

-08.

08

04:0

0:49

-09.

08

23:5

9:00

-10.

08

01:5

3:02

-10.

0813

:53:

11-1

0.08

00:2

0:14

-10.

0820

:59:

12-0

9.08

17:4

1:23

-09.

0811

:44:

48-0

9.08

18:4

9:29

-10.

08

19:3

6:50

-13.

08

04:2

2:54

-10.

0820

:57:

57-1

1.08

15:1

7:40

-12.

0801

:42:

17-1

2.08

01:3

4:12

-13.

08

23:1

5:41

-13.

0805

:47:

17-1

4.08

10:5

6:06

-14.

08

11:1

2:52

-16.

08

16:4

4:57

-15.

0821

:19:

35-1

5.08

13:5

6:49

-15.

08

00:4

9:10

-15.

08

18:1

2:39

-14.

0813

:44:

34-1

4.08

14:5

1:55

-16.

08

22:0

6:03

-17.

08

18:2

0:36

-16.

0800

:28:

20-1

7.08

12:2

5:39

-17.

0804

:48:

17-1

7.08

18:2

5:16

-17.

08

01:1

0:03

-03.

0805

:36:

03-0

3.08

13:4

3:59

-03.

0817

:12:

27-0

3.08

(b)

Setting loadEnd resistanceRating working resistance

Time

570 580 590 600 610 620 630 640 650 660 670 680 690 7003000

4000

5000

6000

7000

8000

9000

10000

11000

12000

19:3

6:45

-10.

08

21:5

8:08

-17.

08

02:1

6:20

-17.

0812

:18:

05-1

7.08

22:1

2:52

-17.

08

03:3

7:41

-04.

0800

:10:

38-0

4.08

10:1

3:36

-04.

08

19:4

5:56

-03.

0813

:37:

54-0

3.08

10:3

8:12

-03.

0804

:28:

12-0

3.08

16:3

2:24

-04.

08

16:3

1:09

-05.

08

22:4

8:53

-04.

0803

:56:

54-0

5.08

16:3

1:09

-05.

0822

:48:

53-0

5.08

03:5

6:54

-05.

08

22:0

3:53

-05.

08

11:5

9:10

-07.

08

21:0

7:20

-06.

0800

:37:

38-0

7.08

15:4

5:25

-06.

0812

:25:

03-0

6.08

02:3

2:30

-06.

08

17:4

7:44

-05.

08

16:3

2:27

-07.

08

02:0

3:07

-09.

08

22:1

5:21

-07.

0801

:58:

01-0

8.08

09:5

4:32

-08.

0815

:25:

32-0

8.08

22:4

3:34

-08.

08

05:5

3:01

-09.

08

04:1

2:50

-11.

08

18:4

5:08

-10.

08

21:3

5:45

-10.

08

13:2

1:41

-10.

0801

:23:

27-1

0.08

21:1

2:10

-09.

0814

:35:

59-0

9.08

23:5

9:50

-11.

08

16:1

6:59

-12.

08

15:2

8:22

-11.

0819

:43:

10-1

1.08

05:1

4:47

-12.

0800

:03:

36-1

2.08

12:0

7:33

-12.

08

01:0

1:29

-13.

0805

:30:

00-1

3.08

21:4

5:52

-13.

08

20:0

7:02

-15.

08

12:0

8:33

-15.

0816

:35:

59-1

5.08

17:3

6:16

-14.

0812

:49:

51-1

4.08

07:0

7:03

-14.

0801

:47:

43-1

4.08

00:1

0:27

-15.

08

20:0

9:17

-16.

08

14:4

6:43

-16.

0816

:09:

26-1

6.08

19:2

1:01

-16.

0817

:09:

17-1

6.08

20:0

1:11

-16.

08

18:2

0:47

-17.

08

kN

Advance rate (m)

(c)


Time

kN

570 580 590 600 610 620 630 640 650 660 670 680 690 7003000

4000

5000

6000

7000

8000

9000

10000

11000

12000

23:3

5:36

-16.

0810

:49:

47-1

7.08

19:1

5:16

-16.

08

Advance rate (m)

11:2

7:41

-04.

0823

:22:

34-0

4.08

08:1

7:34

-04.

08

18:5

0:38

-03.

0815

:11:

03-0

3.08

05:2

6:39

-03.

0800

:43:

08-0

3.08

17:0

2:18

-04.

08

23:1

3:16

-05.

08

21:4

8:35

-04.

0801

:23:

32-0

5.08

11:3

6:04

-05.

0815

:11:

01-0

5.08

18:1

1:01

-05.

08

04:5

5:01

-05.

08

20:1

8:01

-07.

08

11:2

2:32

-07.

0815

:31:

00-0

7.08

02:5

6:24

-07.

0823

:02:

04-0

6.08

14:0

9:14

-06.

08

06:1

3:25

-05.

08

23:0

3:09

-07.

08

17:1

3:16

-08.

08

02:5

4:50

-08.

0805

:11:

01-0

8.08

08:5

4:01

-08.

0812

:11:

01-0

8.08

15:1

7:01

-08.

08

21:1

3:01

-08.

08

23:0

1:01

-09.

08

17:4

5:08

-09.

0821

:16:

21-0

9.08

15:2

1:41

-09.

0808

:30:

01-0

9.08

05:1

1:01

-09.

0801

:27:

43-0

9.08

03:0

3:09

-10.

08

17:1

3:16

-10.

08

04:2

7:43

-10.

0805

:11:

01-1

0.08

10:0

8:01

-10.

0809

:13:

00-1

0.08

10:2

1:15

-10.

08

18:1

5:01

-10.

0819

:13:

16-1

0.08

23:4

1:01

-10.

08

15:3

2:23

-12.

08

13:2

4:09

-11.

0821

:36:

01-1

2.08

12:3

6:01

-11.

0807

:41:

21-1

1.08

05:5

1:01

-11.

0801

:27:

43-1

1.08

23:0

3:09

-13.

08

17:1

3:16

-15.

08

01:2

7:43

-14.

0805

:11:

01-1

4.08

15:2

1:01

-15.

0810

:35:

01-1

5.08

05:2

5:11

-16.

08

17:1

5:02

-17.

08

(d)

Figure 9: Working resistance distribution of hydraulic supports at di�erent positions of working face at the normal mining stage. (a) 25#support. (b) 50# support. (c) 60# support. (d) 110# support.


overlying strata movement and strata pressure behaviorduring the fully mechanized caving mining in shallow andextremely thick seams..e impact of compound breakage ofboth the inferior and main key strata on strata pressurebehavior was analyzed, and the results of numerical simu-lation were verified by in situ measurement. .e conclusionsare as follows:

(1) After the inferior key strata above fully mechanizedcaving face in shallow and extremely thick seamsbroke and caved, they could not form stable blockstructure of the voussoir beam. .e initial cavingdistance of the inferior key strata was 50m, and theinferior key strata presented the characteristics ofsliding instability. In addition, their periodical cavingdistance was about 20m and showed the character-istics of sliding instability of the step voussoir beam.

(2) .e initial and periodical caving distances of the mainkey strata above fully mechanized caving face in shallowand extremely thick seams were about 90m and 50mrespectively, presenting the characteristics of rotaryinstability. .e caving rocks behind the goaf weregradually compacted because of the periodic instabilityof the main key strata. With the continuous advance ofthe working face, the maximum subsidence of the mainkey strata showed a gradually increasing trend.

(3) With the continuous advance of the working face,the abutment pressure ahead of the working facepresented a wavy increasing trend. After the workingface had advanced 90m, because the abutmentpressure ahead of the working face was affected bythe compound breakage and periodical instability ofboth the inferior and main key strata, its maximumpresented small-big-small-big periodical changecharacteristics. What is more, the risk of rib spallingahead of the working face presented different levelsof acute or slowing trends. When the main key strataoccurred the initial breakage and periodical break-age, the maximum value of the advanced supportpressure was relatively large, and the location of themaximum was away from the working face with arelatively large affected area.

(4) It could be obtained from the in situ measurement ofstrata pressure behavior in the mining process of theworking face that the initial weighting interval of theinferior key strata and the average periodicalweighting interval were about 51m and 12.6m, re-spectively, which were consistent with the resultsobtained by numerical simulation.

Data Availability

.e data used to support the findings of this study areavailable from the corresponding author upon request.

Conflicts of Interest

.e authors declare that there are no conflicts of interestregarding the publication of this article.

Acknowledgments

.is work was supported by Independent Research Projectsof State Key Laboratory of Coal Resources and Safe Mining,CUMT (SKLCRSM18X004), the National Natural ScienceFoundation of China (no. 51774269), the National Key R&DProgram of China (2018YFC0604705), and China Schol-arship Council (CSC).

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