Deformation mechanism of gob‐side entry at top coal caving mining face: A numerical study

To investigate the significant deformation of the surrounding rock within the dynamic pressure zone of the gateroad, this study utilized the tailgate of the 1506 longwall face in Anyang Coal Mine as the engineering context. A combined approach of field measurement and numerical analysis was employed to explore the deformation and failure mechanisms of the surrounding rock under dynamic pressure. The findings indicate that before the exploitation of the 1506 longwall face, there was a notable reduction in stress across the floor and immediate roof, with minimal stress concentration observed in the coal pillar and the entity rib side. The plastic zone surrounding the tailgate was limited, suggesting a high degree of stability in the entry's surrounding rock. However, during the extraction of the 1506 longwall face, the tailgate, extending 60 m ahead of the working face, was subjected to dynamic pressure, leading to a marked increase in stress within the floor, coal pillar, and immediate roof. Stress concentration was more pronounced on the entity rib side than on the pillar rib side. This resulted in significant fissuring and severe deformation of the entry's surrounding rock. The primary failure mechanism involved damage to both the roof and floor corners, with the severity of damage escalating alongside the stress in the surrounding rock. This culminated in the failure of roof corner bolts and anchor cables, thereby compromising the stability of the entry within the dynamic pressure zone.


| INTRODUCTION
In China's coal mining sector, underground extraction predominates, with approximately 12,000 km of new roadways being excavated annually.Ensuring the stability and integrity of these roadways is vital for the mine's safe operation. 1Mining along the longwall face disrupts the overburden's originally stable structure, leading to a redistribution of stress in the surrounding rock.This redistribution manifests as advancing and lateral abutment pressures, which significantly affect the gateroad's stability adjacent to the longwall face. 2 The presence of weak and poorly integrated surrounding rock exacerbates deformation and increases the rate of deformation.Consequently, to facilitate the safe extraction of the longwall face, it is imperative to apply suitable reinforcement strategies to mitigate the adverse effects of mining-induced stress on the soft rock entry.Thus, investigating the instability mechanisms of the gateroad's surrounding rock under mininginduced stress is crucial for effective control measures. 3ao et al., 4 Zhang et al., 5 and Zhu and Li 6 identified the primary cause of instability in the surrounding rock of mining dynamic pressure gateroads as the intense dynamic pressure resulting from the instability of a large-sized step rock beam.This beam forms following the fracture of the gob's long cantilever structure.The dynamic pressure impacts the coal pillar and propagates to the entry ahead of the longwall face, leading to the surrounding rock's instability in the gateroad.Du et al., 7 Klemetti et al., 8 and Esterhuizen et al. 9 observed that roadways with weak surrounding rock exhibit rapid early deformation, significant overall deformation, prolonged deformation duration, and comprehensive deformation of the surrounding rock.They attributed the extensive deformation and failure of roadways to high ground stress, the low soft strength of surrounding rock, significant section effect, and inadequate support.Building on this research, a novel technical strategy centered on "high pre-stressed high-strength bolt support, enhanced support for critical areas, integrated full-section support, and a combination of rigid and flexible resistance" has been proposed.
Tang et al. 10 and Tian et al. 11,12 identified that the failure mode of the high-stress soft rock surrounding mine entries is rheological, characterized by rapid deformation rates, significant deformation, prolonged duration, and compromised stability.The underlying failure mechanism involves, under high-stress conditions, the rheological behavior of weak rock leading to fractures in the surrounding rock, causing continuous deformation and ultimately the failure of support structures.Gao et al. 13 conducted theoretical analyses and numerical simulations to explore the stress conditions within the rock mass in the dynamic pressure zone of a longwall face and its failure process due to abutment pressure.Their findings suggest that the peak advance abutment pressure occurs 10 m ahead of the longwall face, with the failure of the coal-rock layer resulting from the synergistic effects of abutment and horizontal stresses, which intensifies with increasing abutment pressure.
Chen et al., 14 He et al., 15 and Wang et al. 16 categorized the influence zone of advance abutment pressure into five distinct areas: the fracture zone, plastic zone, elastic zone, original rock stress zone, and a newly identified plastic zone.This classification is based on the state of coal and rock, as well as a dynamic demarcation point, leading to the division of the advanced support area into three sections: the reinforced support section, the auxiliary support section, and the original support section, delineated by dynamic stress boundaries.The reinforced support section encompasses the fracture zone, plastic zone, and a portion of the elastic zone, necessitating high-strength advance support equipment for enhanced roof support.The auxiliary support section, primarily located in the elastic zone, requires the support of either single hydraulic props or unit hydraulic supports.The original support section, situated entirely within the original rock stress area, does not require additional support reinforcement.
Dong et al., 17 He et al., 18 and Wu et al. 19 utilized numerical software to analyze the distribution characteristics of lateral abutment pressure in hard rocks under geological conditions.Their findings suggest that the superposition of lateral stress during longwall face mining resulting in significant mining pressure within the gateroad, particularly as the coal pillar situated between two longwalls faces experiences this pressure.Ge and Zhang, 20 Zhang et al., 21 and Fan et al. 22 investigated the fracture characteristics of the entry roof, analyzing the formation mechanisms of various fracture structures adjacent to the gob.Furthermore, they examined the mechanical properties of the entry under dynamic mining pressure through numerical simulations, discovering that stress concentration in the roof leads to the instability of the surrounding rock in the gateroad.
According to the research conducted by Xu et al., 23 Quan et al., 24 and Zhang et al., 25 the mining process of a longwall face subjects the laterally fractured rock mass to horizontal forces due to mining-induced pressure.This pressure influences the stress concentration at the joints of the fractured rock mass, altering both the distance between the support pressure and the longwall face and the magnitude of stress concentration.Numerous scholars, both domestically and internationally, have analyzed the structure of roof fracture in the mining-induced stress zone and the stress environment of the coal-rock mass using various theories.][31] In this study, we integrate previous research findings on the deformation mechanism of adjacent roadways to address the significant deformation challenges faced by the tailgate in the dynamic pressure zone of the 1506 top coal caving face at Anyang Coal Mine.Utilizing FLAC 3D , a widely employed continuous medium mechanics analysis software in engineering, we simulate both the excavation of the 1506 tailgate and the mining operations at the 1506 longwall face.
Our analysis encompasses the fracture of overlying strata, deformation and failure characteristics of the surrounding rock, and the stress evolution pattern.We uncover the evolution of advance and lateral abutment stress distribution resulting from mining activities.Understanding the magnitude and peak positions of advance and lateral abutment stresses in roadways is crucial for mitigating local high-stress concentrations in coal and rock masses in subsequent stages.This knowledge serves as a vital foundation for mine disaster prevention and control, as well as for the stability management of surrounding rock in soft rock mining roadways within dynamic pressure zones.

| Mining geological conditions of tested roadway
The Anyang Coal Mine is situated in the southeastern segment of the Loess Plateau, within the northern area of Weinan.This region is predominantly covered by loess, featuring a relatively level landscape.The terrain's highest elevation, found in the northeast, reaches 725 m, whereas its lowest point, located in Jinshuigou, descends to 598 m, yielding a vertical disparity of 127 m.The primary coal seam exploited in this mine is the No. 5 seam, which lies at an average depth of 366 m.The thickness of this seam varies between 2.35 and 6.49 m, averaging 4.42 m, and it possesses an average dip angle of 5°.
Panel 1506, situated in the central region of mining area No. 1 within the No. 5 coal seam, is flanked by Panel 1508 to the west and the 1505 gob to the east.The coal seam floor's elevation ranges from +318 to +350 m.Panel 1506 spans 1903 m in length and 150 m in width, covering an area of 277,697 m², with estimated recoverable reserves of 1.52 Mt.The longwall top coal caving method is utilized here, allowing for the extraction of 2.5 m of the coal seam directly from the longwall panels, and an additional approximate 1.92 m through caving.The layout of the longwall face is depicted in Figure 1.

| The characteristics of rock layers in the roof and floor strata
The immediate roof of panel 1506 predominantly consists of sandy mudstone, encompassing the No. 4 coal seam, and has an approximate thickness of 3.90 m.The main roof is chiefly composed of medium-grained sandstone and siltstone, interspersed with mica flakes, and exhibits considerable hardness, with an average thickness of roughly 9.20 m.The immediate floor comprises sandy mudstone, embedded with plant stem and leaf fossils, and measures approximately 2.00 m in thickness.The main floor is primarily made up of siltstone and fine sandstone, both of which are dense and exhibit significant hardness.Figure 2 illustrates the geological column chart of the coal seam.
Generalized stratigraphy column of the test site.

| Support scheme
The tailgate section 1506 features a straight-sided arch shape, measuring 4400 mm in width and 4000 mm in height, which yields a net sectional area of 14.41 m².It utilizes highstrength bolts, each 20 mm in diameter and 2500 mm in length.These bolts are installed with a spacing of 670 mm × 800 mm for roof bolts and 570 mm × 800 mm for rib bolts.To ensure a minimum anchoring force of 125 kN, each bolt is secured with two Z2360-type anchoring agents.Additionally, high-strength anchor cables, 21.6 mm in diameter and 6800 mm in length, are used with an installation spacing of 1500 mm × 800 mm.Each anchor cable is secured with three Z2360-type anchoring agents.For surface control, steel mesh (6.5 mm in diameter) is installed, featuring a mesh size of 100 mm × 100 mm.The layout of the support structure is depicted in Figure 3.

| Distribution of cracks in the surrounding rock of tailgate
When panel 1506 advanced to a distance of 1000 m, four fracture detection stations, designated H1-H4, were established within tailgate 1506 at intervals of 10, 30, 60, and 90 m ahead of the longwall face of 1506.Each station was equipped with one roof borehole and two rib boreholes.The roof boreholes were drilled to a depth of 8 m, while the rib boreholes reached a depth of 5 m, all with a diameter of 42 mm.For the purpose of fracture observation, CXK-12type borehole endoscope equipment was utilized.Figure 4 illustrates the distribution of cracks in tailgate 1506.
The findings suggest that when the distance between tailgate 1506 and the longwall face exceeds 60 m, the structural integrity of the roof rock layer above tailgate 1506 remains relatively intact, with only dense fractures distributed within a shallow depth of 1.0 m.Additionally, minor transverse and longitudinal fractures are observed within a depth range of 2.75-4.1 m.Conversely, within a 60-m ahead of panel 1506, tailgate 1506 experiences significant structural compromise due to the extraction activities at the 1506 longwall face.This results in diminished integrity of the roof rock layer, evidenced by an increased prevalence of fractures and fragmentation within the shallow sections of the roof.The fragmentation zone predominantly occupies the area up to 1.25 m above the roof, whereas the bedding fracture zone extends from 2.8 to 6.7 m within the roof rock layer.Furthermore, the roof rock layer exhibits a high fracture density, with the majority of fracture apertures ranging between 2 and 3 mm.A narrow fracture zone exists within the 0-0.4-m range of solid coal ribs, accompanied by a limited number of longitudinal cracks developing between 1.85 and 2.0 m.The fracturing of coal pillar ribs intensifies within the 0-0.75-m range, with additional longitudinal cracks appearing between 1.5 and 2.0 m.The integrity of the surrounding rock remains intact in other areas.

| Establish numerical computation model
The numerical model in this study was developed based on the geological and mining conditions of the 1506 longwall face at Anyang Coal Mine, as depicted in Figure 5.It utilizes the FLAC 3D mechanical analysis software, created by Itasca, for simulation purposes.Constructed incrementally, the model employs hexahedral elements as its foundational units.To enhance computational efficiency, the grid density around the tailgate of 1506 was increased, while it was decreased in areas further away.The model comprises 1,076,080 blocks and 1,139,354 nodes, with dimensions of 400 m geological strata.By integrating geological data from the Anyang Coal Mine and outcomes from laboratory rock physics mechanics tests, the mechanical parameters for the coal and rock in the numerical model are determined, as detailed in Table 1.

| Numerical simulation scheme
This simulation encompasses multiple stages of the mining process.Initially, excavation commences on the model's left side with a step size of 10 m, creating a gob of 1505.Subsequently, the tailgate and headgate 1506 undergo excavation, with the implementation of original support parameters to ensure stability.The model's C 0 station is selected as the focal point for research, facilitating separate analyses of the stress and plastic zones in the immediate floor, coal seam, immediate roof, and main roof.Additionally, the failure characteristics and stress distribution within the overlying strata before the mining of panel 1506 are scrutinized.The configuration of the measurement line is depicted in Figure 6A.
In this study, we investigate the stress distribution, displacement, and plastic zone characteristics of the surrounding rock during the excavation of the 1506 longwall face.The panel 1506 is excavated to a depth of 200 m, advancing at a step distance of 5 m.To facilitate our analysis, measurement lines were strategically positioned at distances of 5, 10, 30, 60, and 90 m ahead of the working face, designated as C 1 -C 5 .These lines serve as the basis for our investigation into the specific parameters and characteristics of stress, displacement, and the plastic zone in relation to the mining activities.For a detailed layout of the measurement line arrangement, Figure 6B is provided for reference.

| Vertical stress distribution
Following the analysis of the 1506 headgate and tailgate excavation, it is evident that the stress distribution within the model is marginally impacted.The observed stress fluctuations in the vicinity of the excavation site suggest a localized redistribution of stress, as illustrated in Figure 7.This figure distinctly shows a marked decrease in internal stress levels for both the floor and the immediate roof of the gateroad.Notably, the vertical stress on the floor is reduced from 10.3 to 0.59 MPa, and the vertical stress on the immediate roof drops from 14.3 to 2.36 MPa.Conversely, the reduction in internal stress within the main roof is comparatively minor, indicating its significant role in controlling the deformation of the surrounding rock mass during tunnel excavation.Additionally, an area of stress concentration is identified along the rib sides of the 1506 gateroad, with higher stress levels observed in the tailgate than in the headgate.The stress concentration on the pillar rib side, peaking at 19.75 MPa approximately 1.25 m from the rib, exceeds that | 2461 on the entity rib side, which reaches 18.7 MPa about 1.50 m from the rib.The stress distribution pattern on both the pillar and entity rib sides is asymmetric and resembles a saddle shape.

| Plastic zone distribution
Figure 8 illustrates the distribution of plastic zones in both the tailgate and headgate at station C 0 .The analysis indicates that the tailgate has incurred significant damage, primarily due to the impact of the 1505 gob and the inherent weak roof strength.In the tailgate, the number of failed bricks within the plastic zone surpasses that in the headgate, with a broader distribution range.The plastic zone in the roof peaks at a height of 1.5 m, whereas the plastic zone along the coal pillar rib extends up to 2.5 m.Furthermore, the plastic zone of the entity rib spans 1.5 m, and the floor exhibits failure to a depth of 0.6 m.In contrast, the headgate, flanked by solid coal on both sides, is less influenced by gob 1505, resulting in a comparatively smaller plastic zone than that observed in the tailgate.Upon excavating the 1506 longwall face to a depth of 200 m, the vertical stress distribution along the measurement line C 1 -C 5 in front of the longwall face is illustrated in Figure 10.Areas of stress concentration are identified in the coal and rock mass surrounding the entry, with the stress distribution on the pillar and the adjacent rib side forming an asymmetrical saddle shape.Notably, within 30 m (C 3 ) ahead of the 1506 longwall face, the stress concentration on the entity rib side surpasses that on the pillar rib side, attributed to mining activities.Within a 60-m range (C 4 ) ahead, the surrounding rock exhibits a significant increase in stress rate.The peak stress concentration on the entity rib side of the tailgate, at a depth of 1.5 m, reaches 33.5 MPa (C 1 ), marking an increase of 14.8 MPa above the premining stress level at panel 1506.Similarly, the peak stress on the pillar rib side of the tailgate, located at a depth of 2.5 m, attains a maximum of 29.6 MPa (C 1 ), which is 9.85 MPa higher than the premining stress level at panel 1506.
Figure 10 illustrates that the peak stress concentration on the tailgate's entity rib side occurs at a depth of 1.5 m.Similarly, the maximum stress concentration on the pillar rib side is found at a depth of 2.5 m.To comprehensively analyze the stress distribution on the floor, seam, and roof of both the entity rib side and the pillar rib side, including their specific locations, four stress measurement lines were established at depths of 1.5 m along the entity rib side and 2.5 m along the coal pillar ribs.Additionally, three measurement lines were positioned on the floor and roof along the gateroad's centerline to assess stress levels.The findings from this monitoring are presented in Figure 11.
Figure 11A demonstrates that the peak vertical stress on the rib side of the entity occurs 3 m ahead of the longwall face, reaching 36.29 MPa.This represents an increase of 17.59 MPa over the stress levels in panel 1506 before mining.As depicted in Figure 11B, a pronounced stress concentration is observed on the pillar rib side up to 60 m ahead of the longwall face, with the maximum vertical stress recorded at 31.68 MPa, an elevation of 11.93 MPa from the premining conditions of panel 1506.Furthermore, Figure 11C indicates that the main roof of the gateroad, within a 60-m range ahead of the longwall face, undergoes the most significant stress escalation, peaking at 16.8 MPa directly at the working face.This marks an increase of 4.47 MPa compared with the stress levels in panel 1506 before mining commenced.

| Vertical displacement distribution
When the 1506 longwall face is mined to a length of 200 m, the 3D mapping of the vertical displacement of the coal and rock mass is depicted in Figure 12.This figure demonstrates that the overburden in the goaf undergoes a basin-like subsidence pattern during the mining operations at the 1506 longwall face.The entry, situated less than 60 m ahead of panel 1506, is subjected to the most significant impact from the mining activities, exhibiting a markedly increased rate of vertical displacement in comparison to the period before mining.Within the area affected by mining, the gateroad is subject to both floor heave and roof subsidence, with the floor exhibiting a maximum heave of 350 mm and the roof a maximum subsidence of 431 mm.Additionally, the coal pillar ribs and the entire coal ribs undergo maximum subsidences of 144 and 225 mm, respectively.
Upon mining the 1506 longwall face to a depth of 200 m, the vertical displacement distribution along the C 1 -C 5 measurement line ahead of the longwall face is depicted in Figure 13.Initially, Figure 13A clearly shows that the floor heave near the tailgate is significantly greater than that near the headgate, with the highest floor heave rate observed within 30 m (C 3 ) of the longwall face, reaching a maximum of 350 mm.
In Figure 13B, it is noted that the tailgate ribs within 60 m of the longwall face undergo noticeable subsidence.This is attributed to the front abutment pressure, which causes the vertical displacement of the entity ribs within this 60 m range to exceed that of the coal pillar ribs, thereby resulting in higher subsidence rates for these ribs.The maximum subsidence recorded on the coal rib side of the tailgate is 225 mm, compared with 144 mm on the coal pillar rib side.
Figure 13C illustrates that the immediate roof subsidence near the tailgate consistently surpasses that near the headgate.The subsidence rate of the immediate roof is highest within 30 m of the longwall face, with the subsidence of the entity rib increasing closer to the working face.Beyond this 30 m range, the immediate roof subsidence rate near the tailgate decreases and stabilizes.The peak subsidence of the tailgate roof is 431 mm, occurring in the entry's midpoint, with the entity rib side experiencing a maximum subsidence of 417 mm and the pillar rib side 347 mm.
Finally, Figure 13D indicates that the main roof subsidence near the tailgate generally exceeds that near the headgate, with the subsidence on the coal pillar rib side of the tailgate being more pronounced than on the entity rib side.The subsidence rate of the main roof is highest within the 0-10-m range in front of the longwall face and diminishes from 10 to 90 m.The maximum subsidence of the main roof at the tailgate is recorded at 263 mm.

| Horizontal displacement distribution
Two measurement lines were positioned at the midpoint between the ribs of the 1506 tailgate to monitor their displacement before and after mining activities.Upon mining the 1506 longwall face to a depth of 200 m, Figure 14 illustrates the horizontal displacement distribution of the tailgate's ribs.This figure indicates that significant deformation occurs within 0-60 m ahead of the 1506 longwall face, whereas deformation beyond 60 m is relatively minor.
Utilizing premining and postmining rib displacement data from panel 1506, Table 2 illustrates the relative displacement of the two ribs throughout the mining process.The data reveal that the pillar rib side undergoes more significant horizontal movement than the entity rib side, with the extent of movement intensifying as proximity to the longwall face decreases.Specifically, at a distance of 5 m from the longwall face of panel 1506, the entity rib undergoes a lateral displacement of 61.8 mm, whereas the pillar rib experiences a lateral displacement of 48.7 mm, culminating in a combined displacement of 110.5 mm for both ribs.   of the space.Shear failure predominates as the primary failure mechanism within the rock mass.Notably, the plastic zone attains its maximum elevation at the center of gob 1506, aligning with the peak height observed in gob 1505, which is 97 m.In contrast, the plastic zone situated above the coal pillar exhibits a considerably reduced height.When the 1506 longwall face is mined to a depth of 200 m, the 3D distribution diagram in Figure 16 depicts the plastic zone at measurement lines C 1 , C 3 , C 4 , and C 5 , situated ahead of the longwall face.The analysis indicates that the area within 5 m in front of the 1506 longwall face, specifically the tailgate, is most affected by mining activities.This area exhibits the highest number of failure units and the broadest distribution of the plastic zone.The plastic zone's maximum height on the roof reaches 4.1 m, the plastic zone along the two ribs extends beyond 2.5 m in depth, and the floor's failure depth is 1.8 m.Severe failures of the rib bolt, roof bolt, and roof corner anchor cable are evident, as shown in Figure 16A.
The tailgate, situated within a 30-60-m range ahead of the 1506 longwall face, experiences significant impacts from mining activities.The immediate roof and floor's low mechanical strength contribute to considerable deformation and failure of the surrounding rock in the roadway.Notably, The primary damage occurs in the corner of the roof and floor, where an increase in damage severity is evident.The plastic zone at the roof corner expands upwards, leading to the failure of four rows of anchor cables in that area, which diminishes the anchoring efficacy.This deformation, predominantly caused by shear and tensile failure, accelerates in the roof and floor.Conversely, the plastic zones along the two ribs remain relatively unchanged, as depicted in Figure 16B,C.
Beyond a distance of 60 m in front of the longwall face, the tailgate exhibits minimal disturbance from the mining activities at the 1506 longwall face.This results in a limited extent of the plastic zone, which remains relatively stable when compared with the conditions before mining commenced.The plastic zone within the roof is predominantly localized at the corner of the roof adjacent to the gateroad, where the maximum failure height reaches 1.3 m.The depth of the plastic zone along the pillar rib side is measured at 2.5 m, in contrast to 1.5 m on the entity rib side.The depth of floor failure is negligible, as illustrated in Figure 16D.(1) The findings from borehole assessments suggest that the structural integrity of the entry roof improves significantly when the separation between the roadway and the longwall face exceeds 60 m.Conversely, within a 60-m ahead of the longwall face, the exerted mining dynamic pressure detrimentally impacts the roof's integrity, leading to extensive roof cracking.These fractures predominantly occur at depths ranging from 2.8 to 6.7 m and are densely packed, with the majority of the cracks being 2-3 mm wide.(2) When the 1506 longwall face remains unmined, both the floor stress and the immediate roof stress of tailgate 1506 significantly decrease.However, the stress reduction in the main roof is minimal.The stress concentration on the pillar side exceeds that on the entity rib side.Specifically, the maximum stress on the pillar rib side of the tailgate reaches 19.75 MPa, located 1.25 m from the pillar rib, whereas the maximum stress on the entity rib side is 18.7 MPa, situated 1.50 m from the entity rib.This results in an asymmetrical stress distribution between the pillar and entity rib sides, resembling a saddle shape.The maximum height of the plastic zone in the tailgate's roof is 1.5 m, compared with 2.5 m on the pillar side and 1.5 m on the entity rib side.The floor's failure depth is 0.6 m.Overall, the gateroad's surrounding rock demonstrates good stability.(3) During the extraction of the 1506 longwall face, the tailgate area within 60 m ahead of the working face experiences significant impacts from mining activities.The stress levels on the floor, pillar, and immediate roof exhibit noticeable increases.Stress concentration on the entity rib side surpasses that on the pillar rib side.Specifically, the maximum vertical stress on the entity rib side, positioned 3 m ahead of the longwall face, escalates from 18.7 MPa before mining to 36.29 MPa.In a similar vein, the maximum vertical stress on the pillar rib side rises from 19.75 MPa before mining to 31.68 MPa.The tailgate's maximum floor heave reaches 350 mm, and the roof's maximum subsidence stands at 431 mm.Moreover, the maximum lateral displacement of the walls is 110.5 mm.The roof's plastic zone extends to a height of 4.1 m, while the plastic zones along the ribs measure 2.5 m in depth.The floor's failure depth is recorded at 1.8 m, with the surrounding rock undergoing significant deformation and failure.(4) During the extraction process at the working face, the stress environment in the surrounding rock deteriorates.A section of the entry ahead of the working face is subjected to high-stress conditions, resulting in failures predominantly at the corners of the roof and floor.As the stress on the surrounding rock intensifies, the extent of failure expands.Should the damage surpass the maximum capacity of the support structures, including bolts and anchor cables, the deformation of the gateroad in the impacted area will escalate rapidly.

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I G U R E 3 Support scheme of the tailgate 1506 (units: mm).(A) Roof anchor bolt and cable layout plan and (B) 1-1 cross section.ZHANG ET AL. | 2459 in length, 379 m in width, and 246 m in height.Adjacent to the tailgate's left side is a 30-m coal pillar, beyond which lies the gob 1505, measuring 360 m × 150 m.The 1506 longwall face, situated to the right, spans 150 m in width.The inclination of the rock layers is assumed to be 0°for simplification.Reflecting the actual conditions of the rock strata, cable elements were employed to represent bolts and anchor cables, with their installation in the gateroad detailed in Section 2.3.The model's lower boundary is subject to displacement constraints, whereas its upper and surrounding boundaries are governed by stress boundary conditions.Owing to the significant depth of mining, the initial horizontal stress is set to match the vertical stress, featuring a stress gradient of 2.5 × 10 −2 MPa/m.The entire model is subjected to a gravitational acceleration of 9.81 m/s 2 .This study adopts the Mohr-Coulomb model as the constitutive framework to simulate the F I G U R E 4 Distribution law of surrounding rock crack in the tailgate 1506.(A) H1 station, (B) H2 station, (C) H3 station, and (D) H4 station.F I G U R E 5 Numerical computation model.

6 1 |
Layout of measuring lines.(A) Unmined panel 1506 and (B) mining panel 1506.F I G U R E 7 Stress distribution diagram when the panel 1506 unmined.(A) Immediate floor, (B) coal seam, (C) immediate roof, and (D) main roof.STRESS AND DEFORMATION WHEN THE PANEL 1506 IS MINED5.Vertical stress distributionUpon mining the 1506 longwall face over a distance of 200 m, the three-dimensional (3D) mapping of the vertical stress on the coal and rock mass is depicted in Figure9.This figure illustrates the significant stress redistribution within the model due to mining activities in panel 1506.The retreat of this panel markedly affects the stress distribution around the model's entry, leading to extensive stress variations in the coal and rock masses adjacent to the gateroad.Notably, the stress levels in the floor, pillar, and immediate roof of the 1506 tailgate, located ahead of the working face, have increased significantly.In contrast, the internal stress within the main roof has only slightly increased, suggesting that the mining operations at the 1506 face exert a minimal impact on the main roof.

F I G U R E 8
The plastic zone diagram of the roadway when the panel 1506 unmined.(A) Tailgate and (B) headgate.F I G U R E 9 Three-dimensional mapping surface of vertical stress when the panel 1506 is mined.(A) Floor, (B) coal seam, (C) immediate roof, and (D) main roof.F I G U R E 10 Vertical stress when the panel 1506 is mined.(A) Floor, (B) coal seam, (C) immediate roof, and (D) main roof.

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I G U R E 11 Vertical stress when the panel 1506 is mined.(A) 1.5 m depth on the entity rib side, (B) 2.5 m depth on the pillar rib side, and (C) tailgate 1506 center line.

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I G U R E 12 Three-dimensional mapping surface of vertical displacement when the panel 1506 is mined.(A) Floor, (B) coal seam, (C) immediate roof, and (D) main roof.

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I G U R E 13 Vertical displacement when the panel 1506 is mined.(A) Floor, (B) coal seam, (C) immediate roof, and (D) main roof.ZHANG ET AL. | 2467 5.4 | Plastic zone distribution When the 1506 longwall face is extended to a depth of 200 m, Figure 15 depicts the 3D distribution of the plastic zone within the model.This figure demonstrates that the distribution of the plastic zone in gob 1506 is distinct from that in gob 1505, attributable to the adoption of the semi-infinite mining method in panel 1506.Specifically, in gob 1506, the plastic zone distribution takes an ellipsoidal shape, occupying approximately one-quarter F I G U R E 14 Horizontal displacement distribution.T A B L E 2 Tailgate 1506 rib convergence.

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I G U R E 15 Three-dimensional distribution map of the plastic zone during mining of 1506 longwall face.

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I G U R E 16 Plastic zone layout of tailgate 1506.(A) Measurement lines C 1 , (B) measurement lines C 3 , (C) measurement lines C 4 , and (D) measurement lines C 5 .
Mechanical parameters of coal and rock.
T A B L E 1