studyonthegroundmovementinanopen-pitmineinthe...

Research ArticleStudy on the Ground Movement in an Open-Pit Mine in theCase of Combined Surface and Underground Mining

Zhenwei Wang, Gaofeng Song , and Kuo Ding

School of Civil Engineering, North China University of Technology, Beijing 100144, China

Correspondence should be addressed to Gaofeng Song; [email protected]

Received 14 December 2019; Revised 23 April 2020; Accepted 14 May 2020; Published 9 June 2020

Academic Editor: Stefano Bellucci

Copyright © 2020 Zhenwei Wang et al. ,is is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

,e combined surface and underground mining method is typically used in an open-pit mine for better production and profits.However, the improved scale of mining operations at the combined mining conditions results in even more intensive stratamovement and massive ground damages. ,is paper assesses the progressive development of the characteristics of roofmovement with the longwall face advance and its influence on the ground movement at the slope area using physical models.,e identification of strata zones at the combined mining conditions is also included. ,e results show the following: (1) thefailure of the competent strong roof creates an inverse arch-shaped rock block structure, which compacts the loose rockfragments in the caved zone; (2) a bed separation occurs above the inverse structure at the top of the disturbed strataconfiguration and extends upward with the face advance until it approaches the continuous bending zone; (3) more intensivestrata movement and ground damages are produced by the large-scale multiseam mining operations, while regular and moredistinct strata zones in the disturbed configuration are identified for less intensive single-seam mining; and (4) the intensiveand massive underground mining activities increase the slope strata movement at the surface mining side. ,is researchsuggests that a less intensive mining activity is preferred in the combined surface and underground mining conditions from thepoint of view of ground control.

1. Introduction

,e surface miningmethod is mainly used for recovering thereserves at a shallow depth for economic reasons, while theundergroundmining method is adopted for deep seams.,ecombined surface and underground mining method is re-ferred to as extracting the reserves using the undergroundmining method at an open-pit mine [1]. ,e surface andunderground mining operations can be performed either atthe same time or separately (simultaneous and nonsimul-taneous combined mining operations). ,e study on thecombined surface and underground mining in the case ofiron mine has been extensively performed by Bakhtavar. Hedefined the two major problems in the field of combinedmining, which is to find a practicable underground miningmethod for an open-pit mine for both simultaneous andnonsimultaneous combined mining and to optimize the

schedule for the transition from surface mining to under-ground block cave, particularly in the case of nonsimulta-neous mode [2]. For the nonsimultaneous combined miningcase, the optimal time for the transition from the surfacemining to underground mining was determined from astochastic mathematical model, and the favourable depth forthe transition in an iron mine was determined by consid-ering both geological and economic models [3, 4]. ,egeotechnical challenges were identified as the induced stress,subsidence, and instability of mine structures which can bemitigated or eliminated by optimization of the crown pillardimensions [5]. ,e optimum size of the pillar was calcu-lated from a dimensional analysis to balance the geotechnicalproblems and economic benefits [6]. In the field of coalmining, the combined mining system can be classified intotwo categories according to the seam inclination [7]. Forsteeply inclined seams, the surface mining is operated at the

HindawiAdvances in Materials Science and EngineeringVolume 2020, Article ID 8728653, 13 pageshttps://doi.org/10.1155/2020/8728653

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https://doi.org/10.1155/2020/8728653

shallow part, while the underground faces (typically thelongwall top coal caving method) are used to extract the coalat a deeper depth. On the other hand, the near-flat seams areworked by both the surface and underground mining op-erations at approximately the same level. ,e combinedmethod has been practiced for decades in a number of coalmines in China with further improved production andeconomic profits. However, one would intuitively expect amore intensive mining-induced ground movement andtherefore more damages to the overlying strata and theground structures especially when the underground long-wall faces approach the slopes of the open pit or the dump.,is is not only because the mining scale and intensity aresignificantly increased, but also because it is less likely for theshallow overburden strata in an open-pit mine to form a self-supporting structure. As a result, the combined miningmethod differs from the other mining technologies in theexcavated strata configuration (i.e., the open-pit slope) andthe improved ground movement caused by the extensivescale of the surface and underground mining activities.Figure 1 shows the ground movement of the surface and theslope steps captured at Anjialing mine, Shanxi Province.,eground is influenced multiple times by the surface andunderground mining activities, with the massive crack ob-served on the ground and the shear movement at the slopestrata.

Numerical and physical modelling of the roof cavingbehaviours has been frequently documented in previousstudies for understanding the progressive development ofmining-induced fractures and mechanism of strata move-ment around underground openings. ,e models haveconsidered the face advance rate on groundmovement [8], theextension of caved and fractured zone heights [9], longwallface failure and shield-strata interactions [10–12], and theimpact loadings on shields [13]. ,ese models have notconsidered the combined surface and underground miningmethod. Sun believes that the damages to the ground areenhanced due to the overlap of the ground movement areascaused by the surface and underground mining activities[14]. Zhu et al. optimized the dimensions of the solid coalpillar (located from one end of the underground longwallface to the slope edge) to improve the stability of the slope[15, 16]. Liu et al. analyzed the influence of the criticaldistance (the distance from the face position at the slope sideperpendicular to the upper slope) on the failure of the slope,based on which they suggested the proper undergroundmining sequences [17]. However, these models mainlyconsidered the stability and movement of the pit slope; thedistribution of the three zones in the overburden for acombined surface and underground is not included.

,e three-zone theories of the strata movement in thevertical and horizontal directions are widely accepted bymost researchers [18, 19]. ,e disturbed overlying stratacaused by the longwall mining can be identified as threezones in the vertical direction, i.e., the caved zone (Zone I),the fractured zone (Zone II), and the continuous bendingzone (Zone III) as shown in Figure 2 [18].,e fractured zoneis further divided into the fragmented rock blocks zone, thevertical fracture propagating-through zone, and the

horizontally separated zone [18, 19]. ,e three zones may bereduced to two zones with an absence of the continuousdeformation zone at the top in the shallow coal mines inInner Mongolia of China where the top cover is mainly theunsolidified loose formation [20, 21]. ,e fractured zonetherefore extends to the ground surface as the face advances,leading to an extensive ground movement. ,e height of thethree zones (especially the fractured zone) is closely relatedto the connectivity of the waste gas and water [22] and can bedetermined from the empirical equations [23–25], numer-ical and physical models [26–28], vertical borehole obser-vations [29, 30], and the microseismic monitoring method[31]. Meanwhile, the movement of the overlying strata canalso be divided into three zones in the horizontal directionaccording to the surface subsidence curve. ,e three hori-zontal zones are identified as the solid coal support zone(Zone A, where the curve shows the least deformation andthe subsidence rate), the separation zone (Zone B, where thebed separation mostly occurs), and the recompacted zone(Zone C, where the loose gobmaterials in the caved zone andthe horizontal bed separation in the fractured zone arerecompacted and the surface settlement reaches themaximum).

,is paper attempts to study the influence of under-ground mining on both the overburden migration and theslope stability from physical models. ,e model configu-rations are selected from different geological sections of anopen-pit mine. ,e 3 models in the work include differentface dimensions: the adjacent-seam mining and multiple-seam mining. ,e paper has three goals: (1) obtain theprogressive development of roof failure andmining-inducedstrata movement for the combined surface and undergroundmining conditions; (2) identify the vertical and horizontalzones of the disturbed overburden; and (3) assess the slopebehaviours influenced by the underground mining activity.

2. Model Development

2.1. Mine Description. ,e representative physical model inthis study is based on the geological and mining conditionsat the Anjialing open-pit mine, Pingshuo, Shanxi Province,where the combined surface and underground miningmethod is adopted for recovering the 4th and 9th seams atthe mine site. ,e total thickness of No. 4 seam is 8.6–16.8mand averages at 13.64m. ,e immediate roof above No. 4seam is sandstone with an average thickness of 7.33m. ,eweak mudstone and sandy mudstone occasionally occurabove the coal seam but cave in upon the face advance. ,eimmediate roof belowNo. 4 seam is mainly mudstone, belowwhich is the 6.9–16.7m thick No. 9 seam averaging at 13.8m.,e rock strata between the two near-flat seams are17.0–57.2m (average 31.1m). No. 4 and No. 9 seams wereworked at a depth of 170–250m using the surface miningmethod. Currently, the coal mine is also recovering the coalseams using the underground longwall top coal cavingmethod. ,e width of the underground longwall face is240m, while the length and the position of the longwall facerelative to the slope should be determined based on itsinfluence on the ground movement and the slope stability.

2 Advances in Materials Science and Engineering

2.2. Similarity Principles and Construction Materials. ,ephysical modelling approach is utilized in this work to re-produce the progressive development of the groundmovement and the slope stability of an open-pit mine usingthe combined surface and underground mining method.One of the most important aspects of the physical modellingis that the physical model must follow a number of similarityprinciples, according to which the physical features of themodel and the prototype case (full-scale case) should besimilar in terms of geometry, time, density, and strength.,emeasurements in the physical model can, therefore, becompared with the real case. ,e above coefficients aredefined as the ratios of corresponding real case parametersover the small-scale physical model and must be kept asconstants, which are determined based on the mechanicalproperties of the modelling materials and the size of thephysical modelling rig [8, 12]. In this work, the geometry,time, density, and strength similarity coefficients are de-termined as 250, 10, 1.47, and 147, respectively.

Proper geomechanical modelling materials are carefullyselected to construct the physical model for realisticallyrepresenting the strata movement. In this work, the con-struction materials are a mixture of sand, gypsum, and limewith a proper proportion of water. ,e solid materials are

fully mixed before the addition of water to ensure the generalhomogeneity. ,e proportions of the physical materials arecarefully determined through a trial-and-error process, sothat the strata could behave and cave in in a similar way tothe real case. ,e proportions and mechanical properties ofthe physical materials are given in Tables 1 and 2,respectively.

2.3. Physical Modelling Rig and Model Preparation. ,e 2Dstrain plain physical modelling rig used in this research is500 cm in length, 40 cm in width, and 150 cm in height (seeFigure 3).,e frame is larger than the traditional ones in orderto incorporate the large-scale surface and undergroundmining activities. Physical materials are first placed in themodelling steel frame and are then compacted to the designedheight. ,e physical model is constructed layer by layer toensure the overall strength and height of the model. No. 4 andNo. 9 seams and the strata shown in Figure 4(a) at theAnjialing open-pit mine are modelled in this study. ,e rightside of the physical model is a slope created by the previoussurface mining, while the underground longwall faces arecurrently operated in the physical models starting from the leftside. ,e physical model has roller boundaries along the leftand right side and at the bottom. Figure 3 shows the overallgeometry of the physical model. Note that the geometricsimilarity is 250 :1 in this work; therefore, the 500 cm longphysical model simulates a total of 1250m in the real case.

,ree models are developed in this research (Models I,II, and III) for representing three geological sections in thestudied mine site. In Model I, Face A1 and Face A2 areadvanced a total of 200m in the upper No. 4 seam, followedby a 500m development of Face B1 in the lower No. 9 seam.A 50m wide solid coal pillar is maintained between Faces A1and A2. In Model II, Face B2 and Face B3 are located in thelower No. 9 seam and are worked a total distance of 170mand 370m, respectively. A 50m wide solid coal pillar is leftbetween the two faces. In Model III, Face B4 is mined a totalof 500m in No. 9 seam. ,e models are developed to studythe progressive development of the ground movement, thevertical displacement of the roof strata, and the displacementfield of the slope. A 3D digital close-range industrial pho-togrammetry system is used in this research for providing a

(a) (b) (c)

Figure 1: Ground movement caused by the surface and underground mining activities at the Anjialing open-pit mine, Shanxi Province.(a) Massive ground crack on step 1405. (b) Shear movement of rock strata on step 1300. (c) Shear movement on step 1270.

I: caved zoneII: fractured zoneIII: continuous bending zone

A: solid coal support zoneB: separation zoneC: recompacted zone

III

I

IIA B

C

Figure 2: ,e three-zone distribution of the overlying stratamovement in the vertical and horizontal directions caused bylongwall mining.

Advances in Materials Science and Engineering 3

precise, rapid, automatic, and continuous measurement ofthe strata vertical displacement above the longwall workings.,e ground movement in the slope area is also provided byrecording the slope deformation field using the digitalspeckle correlation method. ,e camera measuring systemyields more accurate strata movement results than thetraditional electronic theodolite.

3. Results and Analysis

3.1. Model I. Figure 5 plots the progressive development ofthe ground movement in Model I for recovering both No. 4

and No. 9 seams. Face A1 in the upper seam was first mined.,e face starts at approximately 180m away from the leftboundary of the rig. ,e roof caves in regularly in the gobarea with the face advance, and the caved zone heightreaches 10m above the seam after 120m of the face advance(Figure 5(a)). With a further 50m of face development, themore component sandstone roof bends and forms an in-verse arch-shaped structure (see the dashed line inFigure 5(b)), beneath which the previously caved loosematerials are compacted. A bed separation is also observedabove the arch structure at the top of the disturbedoverburden, which extends 42.5m above the seam. Face A1advances another 30m and ceases after a total of 200mdevelopment. During this period, the immediate roof cavesin upon the face advance (Figure 5(c)). ,e main roof beamabove the shield is fixed at one end at the solid coal face sideand is supported by the inverse arch-shaped structure at theother. As a result, the convergence of the roof should besmall and the loading condition of the longwall shieldshould be better off at this position.

Table 1: Proportions of the physical materials in terms of weight to construct different lithologies in the physical model.

Lithologies in the modelPercentage of the solid materials by weight Percentage of the water over solid

materials by weight (%)Sand (%) Gypsum (%) Lime (%)Artificial fill 87.5 8.75 3.75 10Loose soil 87.5 8.75 3.75 10Weathered sandstone 80 10 10 10Sandstone 75 12.5 12.5 10Clay 85.7 4.3 10 10No. 4 coal seam 83.3 8.35 8.35 10Sandy mudstone 85.7 4.3 10 10Fine sandstone 80 6 14 10Siltstone 80 6 14 10Malmstone 80 10 10 10No. 9 coal seam 83.3 8.35 8.35 10Mudstone 80 10 10 10No. 11 coal seam 83.3 8.35 8.35 10Medium-fine sandstone 75 7.5 17.5 10

Table 2: Mechanical properties for different lithologies.

Lithology Compression strength(MPa)

Internal frictionangle (°)

Cohesion(MPa)

Unit weight(kg·m−3)

Elastic modulus(MPa)

Poisson’sratio

Artificial fill 1.8 16 0.112 1650 12 0.45Loose soil 1.8 18 0.125 1960 15 0.42Weatheredsandstone 56.3 38 2.5 2300 2000 0.36

Sandstone 95.1 39 3 2380 4200 0.33Clay 58.1 40 0.5 2355 2015 0.23No. 4 coal seam 40.7 36 1.62 1440 1000 0.38Sandy mudstone 63.8 39 0.5 2360 2050 0.24Fine sandstone 99.4 40 3.1 2380 4300 0.33Siltstone 107.4 38 5 2600 4800 0.32Malmstone 54.4 38 0.4 2350 2000 0.25No. 9 coal seam 38.4 39 1.62 1330 1200 0.36Mudstone 54.5 38 0.4 2300 2000 0.25No. 11 coal seam 36.6 36 1.62 1400 1300 0.35Medium-finesandstone 99.8 40 3.1 2390 4300 0.33

Figure 3: ,e physical modelling rig and the finished 2D model.


1400 Loose formation

No. 4 coal seam

Borehole

1300

1200

Sandy mudstone and sandstone

Sandstone and mudstone

Mudstone and sandstone No. 5 coal seam No. 9 coal seam

No. 11coal seam

(a)

Scale1 : 2000

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3~1612

13

10~25

42

65~95

41~89

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sandstone, coarse-grainedsandstone, and siltstone

Mudstone and sandymudstone

Mudstone, sandymudstone, and coal seam

Mudstone, sandymudstone, and siltstone

Limestone

(b)

Figure 4: (a) Geological section profile. (b) General stratigraphy for the Anjialing mine.


After completion of Face A1, a 50m wide coal pillar wasleft between the previous Face A1 and the adjacent activeFace A2. ,e strata movement for mining Face A2 is shownin Figure 6, which is similar to that of the previous face. ,eextension of the broken strata increases with the face de-velopment. It also reaches 42.5m after the completion ofFace A2 at a total of 200m face advance (Figure 6(c)).However, the overall dimension of the disturbed configu-ration for Face A2 is slightly larger than the previous one,and the gob materials are less compacted by the above in-verse arch-shaped strata. ,e overburden strata above thecoal pillar (outlined by the blue dot lines in Figure 6(c))remain stable and intact. ,e angle of break (defined as theangle between the vertical line and the edge of stable strataabove the pillar) is found at approximately 26° for bothpanels. ,e solid coal pillar may yield due to the com-pression of the stable configuration and the abutmentpressures. Mine hazards such as violent coal bursts andextraordinary distortion of the pillar or mine entries mayoccur.

,e influence of multiple-seam mining operations isincluded in this work by mining Face B1 in the lower seamafter the completion of the two faces in the upper seam. Inthe beginning, the immediate roof collapses in the gob areawith the face advance, but the main roof remains intact andthe caved height has not reached the previously mined-outgob in the upper seam (Figures 7(a) and 7(b)). In otherwords, the disturbed strata for Face B1 in the lower seamhave not reached the upper gob area; the upper and lowergobs are separated.,e loading conditions for the shield andlongwall face should be better off at this position since theoverburden pressure is partially released through the pre-vious upper gob. After the face has developed 200m, themain roof caves in as the limit of overhang length is reached(Figure 7(c)). ,e roof channels between the two coal seamsfail completely and the upper gob collapses. ,e caved zonefor the active face extends upward and connects with theupper gob. ,is may increase the intensity of the roofweighting as well as the shield loading.,e channels form aninverse arch-shaped structure compacting the below gobmaterials and a load-bearing structure supporting theoverburden. A huge separation is created at the top of thegob above the structure. ,e angles of break for both upperand lower faces are similar.

Face B1 advances to the above coal pillar position(Figure 7(d)). ,e above stable configuration (sitting on thecoal pillar between the two upper faces, see Figure 6(c))transfers the overburden pressures to the active longwallpanel and shield. Hence, poor face stability and the tre-mendous difficulty in advancing the shields might be ex-pected. ,e load environment in the open face area couldreturn to the previous level only after the face passes the coalpillar area. ,e face then advances under the upper gob areaof Face A2 (Figure 7(e)). ,e lower gob connects the upperone upon the failure of the roof channel (Figure 7(f )).

Face B1 ceases after 500m of face development. ,e finalview of the ground movement and the three zones in thehorizontal and vertical directions are shown in Figure 8. ,ezones are different with the typical zone identification shownin Figure 2 because of the extraction of multiple seams. ,eintensity and extension of ground movement is increased bythe multiple-seam mining. A massive ground fracture isobserved on the surface above the start position of thelongwall face, as well as the shear movement of strata on theslope step. ,is is consistent with the field observationshown in Figure 1.

Figure 9 shows the vertical displacement of the strataalong the face length at different layers above No. 9 seam.Generally, the displacement maximizes at the middle po-sitions of Faces A1 and A2 and reaches a smaller plateauabove the solid coal pillar between the two faces. ,e two-peak shaped curves are formed because of the existence ofthe stable configuration above the solid coal pillar. It is alsonoted that the strata at a deeper depth (closer to No. 9 seam)show a larger vertical displacement, with an exception thatthe roof at 15m above the seam presents the least verticaldisplacement and stabilizes at this level (see the blue line).,is differs from the other two-peak curves because the roofat 15m above No. 9 seam stays in the roof channel betweenthe two seams; therefore, the displacement simply representsthe strata movement for mining Face B1 rather than forextracting the multiple seams.

,e vertical displacement at the slope area is recordedusing the digital speckle correlation method. It may notproduce the most refined details of the slope displacementfield, but the general influence of the underground miningactivity to the slope behaviour can be obtained. Figure 10shows the displacement field after completion of Faces A1,

(a) (b) (c)

Figure 5: Progressive development of strata movement with advance of Face A1. Face advance distance: (a) 120m, (b) 170m, and (c) 200m.


A2, and B1, respectively. ,e extraction of Face A1 showslimited impact on the subsidence of the slope(Figure 10(a)), since the finish position of Face A1 is lo-cated more than 250m away from the slope area. ,emagnitude of the displacement increases with the com-pletion of Faces A2 and B1 (Figures 10(b) and 10(c)).Hence, it is inferred that the slope movement is gradually

increased with the increase of the scale and intensity ofmining activities.

3.2.Model II. In Model II, Faces B2 and B3 in the lower seamare mined with a 50m coal pillar left between the two panels.,e above No. 4 seam remains unmined. ,e strata

(a) (b) (c)

Figure 6: Progressive development of strata movement with advance of Face A2. Face advance distance: (a) 120m, (b) 170m, and (c) 200m.

(a) (b) (c)

(d) (e) (f )

Figure 7: Progressive development of strata movement with advance of Face B1. Face advance distance: (a) 120m, (b) 150m, (c) 200m,(d) 250m, (e) 325m, and (f) 400m.

III

II

III

AC C B ABA B

Ground crack Shear movement

I

Figure 8: Final view of the ground movement at completion of Face B1. I: caved zone; II: fractured zone; and III: continuous bending zone.(A) stable zone; (B) separation zone; and (C) recompacted zone.


movement with the advance of Face B2 is shown in Figure 11.,e caved height extends upward with the face advance(Figures 11(a)–11(c)). ,e inverse arch-shaped structurecompacts the caved loose materials in the gob area(Figure 11(d)). Face B2 ceases after 170m of facedevelopment.

,e adjacent Face B3 is mined after the completion of theprevious Face B2. ,e immediate roof caves in upon the faceadvance. Loose gob materials are regularly placed in the gobarea and form the caved zone (Figures 12(a) and 12(b)).With the further face development, the disturbed over-burden develops to the fractured zone, and the gob wastesare compacted by the inverse arch-shaped structure(Figures 12(c)–12(e)). ,e huge bed separation at the top ofthe disturbed configuration is closed and recreated with theface advance at each face position (see Figures 12(a)–12(e)).However, the recreation of the bed separation terminatesafter 370m of face advance when the disturbed strata

configuration extends to the continuous bending zone(Figure 12(f )).

Figure 13 provides a final view of the strata movementafter completion of Faces B2 and B3. ,e caved zone,fractured zone, and continuous zone in the vertical directionand the stable zone, separation zone, and the recompactedzone in the horizontal direction are also identified. SinceFace B2 only develops a limited distance, no bending zone isobserved. By comparison, the bending zone for Face B3develops to the ground surface and causes the groundsubsidence and surface cracks that are less significant thanModel I.

,e roof vertical displacement for Model II is given inFigure 14.,emaximum displacement is approximately 6mmeasuring at the strata at 15m above No. 9 seam. It de-creases with the increase of the distance above the seam andreduces to approximately 3.2m for the strata at 165m abovethe seam. ,e roof vertical displacement above Face B2

300400500600700800900

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(c)

Figure 10: Slope vertical displacement contour after completion of Faces (a) A1, (b) A2, and (c) B1.

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t (m

)

15m above coal seam65m above coal seam90m above coal seam115m above coal seam

No. 4 seamNo. 9 seam

Face A2

Face B1

140m above coal seam165m above coal seam190m above coal seam

Face A1Mining direction–12

–10

–8

–6

–4

–2

00 200 400 600 800 1000

Face advance distance (m)

Figure 9: Strata vertical displacement after completion of Faces A1, A2, and B1.


becomes unnoticeable starting at the strata 40m above No.9 seam; therefore, the two-peak curves turn one-peak. It isalso observed that, for the vertical displacement above FaceB3, the slope rates of the curves at the left side are largerthan the right side, corresponding to the more fracturedseparation zone observed on the left side of Face B3 (seeFigure 13).

,e contours of the slope vertical displacement aftercompletion of Faces B2 and B3 are shown in Figure 15. ,eoverall vertical displacement is around −0.5 to −1m at thecease of Face B2 (Figure 15(a)) and −1 to −1.5m for Face B3(Figure 15(b)). ,e movement at the slope area is increasedas the face approaches the slope [20].

3.3.Model III. InModel III, the only longwall face, Face B4, inthe lower seam is mined. ,e development of strata move-ment with the face advance is shown in Figure 16. ,e overallintensity and extension of ground movement is mitigated ascompared to the multiple-seam mining in Model I.Figure 16(a) plots the cave of the lower immediate roofextending 5m above the seam; the cave of the immediate roofoccurring at 140m of face advance is shown in Figure 16(b),extending 12m above the seam.When the face advances 75m

from the start position, the inverse arch-shaped roof structureand compaction of caved loose gob materials are observed(Figure 16(c)). Extension of disturbed overburden reaches75m above the seam, with a notable bed separation at the topof the disturbed configuration. ,e disturbed strata reach105m above the seam after 310m of the face development(Figure 16(d)).,e strata then extend to the ground surface ata further 60m of face advance, where the bed separation isclosed (Figure 16(e)). ,e dimension of the broken stratagrows gradually with face development, but the generalconfigurations look similar (Figures 16(f) and 16(g)). Face B4ceases after 500m of face advance.

Figure 17 identifies the caved zone, the fractured zone,and the bending zone in the vertical direction as well as thestable zone supported by the solid coal, the bed separationzone, and recompacted zone in the horizontal direction afterthe completion of Face B4. Since no pillar is maintainedbetween panels, no superposition of the zones occurs.,erefore, the identified zones in the vertical and horizontaldirections are more distinct and are similar to the typicalzones shown in Figure 2. It also shows that the groundsubsidence and cracks seem less distinct than Models I andII. ,e angle of break is about 30°, which is similar toprevious models.

(a) (b) (c)

(d) (e) (f )

Figure 12: Progressive development of strata movement with advance of Face B3. Face advance distance: (a) 120m, (b) 170m, (c) 230m,(d) 270m, (e) 320m, and (f) 370m.

(a) (b) (c) (d)

Figure 11: Progressive development of strata movement with advance of Face B2. Face advance distance: (a) 70m, (b) 90m, (c) 120m, and(d) 170m.


,e vertical displacement of the strata at different dis-tance above No. 9 seam is shown in Figure 18. ,e stratadisplacement starts to increase at the junction between thestable zone and separation zone. It reaches the maximum inthe recompacted zone. Generally, the overburden stratacloser to No. 9 seam show a larger displacement. At 15mabove the seam, the largest vertical displacement is about3.8m, compared with only 2m of vertical displacement at165m above the seam.

,e comparison of roof vertical displacement for dif-ferent models is given in Table 3. Due to the bed separation

in the caved and fractured zones, it is more proper tocompare the displacement at 165m above the seam, whichrepresents the ground movement in the continuous bendingzone. Model I shows the largest strata displacement mainlydue to the intensive multiseam mining activity, followed byModels II and III in the descending order. ,e shallowerdepth of cover for Model II might be responsible for therelative larger displacement than Model III.

,e slope vertical displacement field after completion ofFace B4 is shown in Figure 19. ,e overall vertical dis-placement is around 0 to −2m. As compared toModels I and

I III

II

III

C B AA B

Ground crack and subsidence

Figure 13: Final view of the ground movement at completion of Faces B2 and B3. I: caved zone; II: fractured zone; and III: continuousbending zone. (A) stable zone; (B) separation zone; and (C) recompacted zone.

No. 9 seamFace B2 Face B3


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t (m

)


No. 4 seamMining direction

–6

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Face advance distance (m)

Figure 14: Strata vertical displacement after completion of Faces B2 and B3.

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(b)

Figure 15: Slope vertical displacement contour after completion of (a) Faces B2 and (b) B3.


II, the slope movement is slightly mitigated. ,e groundmovement at the slope area may depend on the intensity ofmining activities and the mining positions relative to the

(a) (b) (c) (d)

(e) (f ) (g)

Figure 16: Progressive development of strata movement with advance of Face B4. Face advance distance: (a) 70m, (b) 140m, (c) 220m,(d) 310m, (e) 370m, (f ) 470m, and (g) 500m.

I

II

III

C BB AA

30°

Figure 17: Final view of the groundmovement at completion of Face B4. I: caved zone; II: fractured zone; and III: continuous bending zone.(A) stable zone; (B) separation zone; and (C) recompacted zone.


No. 9 seamFace B4

Ver

tical

disp

lace

men

t (m

)

115m above coal seam140m above coal seam165m above coal seam

No. 4 seamMining direction–4

–3

–2

–1

0 0 200 400 600 800 1000Face advance distance (m)

Figure 18: Strata vertical displacement after completion of Face B4.

Table 3: Comparison of roof vertical displacement at 15m and165m above No. 9 seam.

Strata verticaldisplacement

Model I(m)

Model II(m)

Model III(m)

15m above No. 9 seam 10 6 3.8165m above No. 9 seam 7 3.2 2

300

400

500

600

700

800

900200 400 600 800 1000 1200 1400

86420–2–4

Figure 19: Slope vertical displacement contour after completion ofFace B4.


mine slope. ,e slope movement is increased with the in-crease of mining intensity and the approach of longwall faceto the slope.

4. Discussion and Conclusions

With the development of the dimensions of the open pit, thecoal mine starts to adopt the underground longwall miningmethod for recovering the coal seam to improve the pro-duction and profits. However, the ground movement andsurface subsidencemay also be increased due to the intensiveand large-scale combined surface and underground miningactivities. In this paper, the ground movement of thecombined surface and underground mining of an open-pitmine is reproduced on a large-size physical modelling rig.,ree physical models are developed to assess the rooffailure characteristics, the strata movement process, stratavertical displacement, and the zone identification along thehorizontal and vertical directions. ,e influence of theunderground mining on the ground movement at the slopearea is also included. Important findings of this study arelisted below.

(1) Faces A1 and A2 in the upper seam and Face B1 in thelower seam are included in Model I. Faces A1 and A2are first mined, followed by the extraction of Face B1.An inverse arch-shaped roof block structure iscreated upon the failure of the competent strongroof. ,e structure compacts the loose gob materialsin the caved zone but leaves a bed separation at thetop of the disturbed configuration. During the ex-traction of Face B1, the caved and fractured zones forthe lower seam extend and connect with the upperones, leading to the massive collapse of the over-burden strata. Massive ground cracks and shearmovement on the slope step are created by the large-scale multiseam mining activities. ,e maximumroof vertical displacement in the continuous bendingzone reaches 6m.

(2) Faces B1 and B2 in the lower seam are mined inModel II. A 50m coal pillar is maintained betweenthe two faces. ,e ground movement is muchmitigated as compared to Model I. ,e bed sepa-ration above the inverse arch-shaped structure ex-tends upward with the face advance and is closedwhen it reaches the continuous bending zone. Onlythe caved and fractured zones are observed aboveFace B1 due to the limited distance of face advance,while the continuous bending zone is found aboveFace B2. ,e ground crack is also observed on thesurface but is significantly smaller than the massivecrack caused by the intensive multiseammining.,evertical displacement of the strata in the continuouszone (at 165m above No. 9 seam) is about 3.2m.

(3) Face B4 in the lower seam advances a total of 500min Model III. ,is face is the only longwall face in themodel; therefore, the three zones in the vertical andhorizontal directions are clearly identified and nosuperposition of the zones is observed. ,e angle of

break after completion of the face is approximately30°, similar to the rest of the faces. ,e strata in thecontinuous bending zone displace 2m in the verticaldirection.

(4) ,e ground movement in the slope area is mostlyaffected by the underground mining in Model I,followed by Models II and III in the descendingorder. ,e extent of slope movement increases withnot only the intensity and scale of the mining ac-tivity, but also as the longwall face approaches theslope.

Data Availability

,e data used to support the findings of this study are in-cluded within the article.

Conflicts of Interest

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

Acknowledgments

,e financial support from the National Natural ScienceFoundation of China (51974320, 51904082, and 51774184),Beijing Natural Science Foundation (2204080), Science andTechnology Plan of Beijing Municipal Education Com-mission (KM202010009001), and North China University ofTechnology (110051360002, 107051360019XN134/017, and107051360019XN134/020) is greatly appreciated.

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