Research on deformation characteristics and control technology of surrounding rock for gob‐side entry with small coal pillar in gently inclined coal seam

With the engineering background of the gob‐side entry of panel SA1104 in Heiyanquan Coal Mine, the deformation characteristics and control technology of the surrounding rock for gob‐side entry with small pillar in gently inclined coal seam have been studied by field observation, numerical simulation, and theoretical analysis. The study show that: (1) The surrounding rock has obvious asymmetric deformation characteristics, the roof subsidence on the solid coal side is larger than that on the small coal pillar side; the deformation of small coal pillar side rib is more significant than that of the solid coal side rib, which is concentrated in the middle and upper part, and slip deformation occurs at the upper sharp corner; the middle part of solid coal side rib exhibits prominent deformation; and floor heave is shifted to the small coal pillar side. (2) The stress concentration zone on the small coal pillar side is located 1.9 m from the high rib, while the stress concentration zone on the solid coal side is located 4.1 m from the low rib. The difference in the rib‐to‐rib peak stress is 4.2 MPa, and the stress concentration on the solid coal side is larger than that on the small coal pillar side. (3) Poor physical and mechanical properties of the surrounding rock, high in‐situ stress, and high deterioration degree of the surrounding rock structure are the main internal causes of the entry deformation; strong mining disturbance, arbitrary support parameters, and large section sizes are the main external causes of the entry deformation. (4) The asymmetric combined support program is proposed, featuring “non‐equidistant cable + inclined bolt on the roof, bolt extension + cable supplement on the small coal pillar side rib, and bolt encryption on the solid coal side rib”, and it had a positive on‐site application effect.

Gently inclined coal seam in China has a wide distribution range, extensive mining scale, and the reserves and production account for about 40% of the comparable national value. 1,2Considering the impact of inclination angle, following the entry excavation, the surrounding rock stress redistribution and deformation exhibit distinct asymmetry as a result of the unequal abutment pressure exerted on the roof and floor.4][5][6] To reduce the loss of chain pillars and improve the recovery ratio of coal mine, small coal pillars are currently used to replace the traditional wide coal pillars along the entry, which moderates the matching entry development and panel mining.However, given the gently inclined coal seam's rapid descent into the depths and the dynamic load disturbance of rock layer in the gob, the surrounding rock environment of gob-side entry is more and more complicated. 7Additionally, high ground pressure and strong mining disturbance cause the surrounding rock to turn loose and broken, with intense deformation, low integrity, poor uniformity, and increased support and maintenance difficulties.
Scholars outside China rarely studied the gob-side entry with the small coal pillar, and mostly studied the strength and failure mechanisms of coal pillars.Sakhno et al. 8 studied the stress distribution and displacement variation of the main roof for gob-side entry in the working behind the longwall face.Sakhno et al. 9 studied the stress field evolution around head entries with gob-side entry retained through numerical simulation with two variants of filling wall.Frith et al. 10 explored the limitations of empirically derived coal pillar strength equations and proposed a modified coal pillar design representation and model based on coal pillars acting to reinforce a horizontally stressed overburden.Mortazavi et al. 11 studied the failure mechanism and nonlinear behavior of coal pillars, and carried out numerical analysis of the deformation and failure process of coal pillars under natural loads.Das et al. 12 simulated and analyzed the shear characteristics of the inclined coal seam by using the ubiquitous joint model.They found that the strength of the coal pillar decreased with the increase of inclination angle of the seam, and proposed the procedures to estimate the strength of the inclined coal pillar by using numerical simulation technology.Vardar et al. 13 investigated the effects of discontinuities on the strength and energy release characteristics of pillar-scale coal mass under uniaxial compression.Wattimena et al. 14 constructed a logistic regression model for coal pillar stability prediction and determined the orientation of stablefailure boundary line.Jaiswal et al. 15 developed the statistical expressions for estimation of coal pillar strength and post-failure modulus by numerical simulation.
In China, many scholars have carried out a lot of research on gob-side entry from the deformation mechanism, ground pressure appearance, and control technology.Bai et al. 16 put forward the control mechanism about entry driven along the gob-side in fully mechanized top coal caving face by using the strength reinforcement theory about roof bolting, and examined it by taking 4 m coal pillar entry as a case study.Wang et al. 17 applied the damage theory to analyze the abutment pressure distribution characteristic of solid coal rib along the gobside entry.They noted that when the thickness of the coal seam and immediate roof was large, the abutment pressure was relatively high and the floor heave was prone to occur.Li et al. 18 counted six typical cases of coal mines in east and west China, studied the deformation and failure characteristics of small coal pillars under various influence factors such as coal seam depth, mining thickness, surrounding rock nature, mining disturbance, and support strength.They also proposed control countermeasures to guarantee the uniformity of small coal pillars under various influence factors.Chen et al. 19 studied the deformation characteristics and mechanism of high-stress gob-side entry in 1000 m-deep coal mine, and developed a control technique that combines high-tensioned active support and grouting modified reinforcement.Meng et al. 20 investigated the gateroad protection mechanism of gob-side entry in inclined extra-thick coal seams, and presented an integrated surrounding rock control technique that combines coal pillar reinforcement with precise destress of high-stress zone.Zhang et al. 21studied the mechanism of gob-side entry developed by roof cutting for de-stress and surrounding rock control with a large mining height of 6 m.Zhang et al. 22 examined the form of roof rupture and destabilization about gob-side entry under nonuniform overlying rock strata and suggested a corresponding control method for large deformation of the surrounding rock.Yin et al. 23 explored the reasonable value of coal pillar retention size for inclined trapezoidal gob-side entry in gently inclined coal seam and suggested an asymmetric support scheme.Jiang et al. 24 demonstrated the spatial asymmetry of deformation of deep gateroad, the nonuniformity deformation between shallow and deep surrounding rock, and the jumping progressivity of the rock's time-dependent deformation.By establishing a mechanical model for the uniformity of high and low key blocks, He et al. 25 studied the pillar width of gob-side entry in extra-thick coal seams under hard-thick main roof, and determined that 8 m was the optimal pillar width based on the overburden structure.Zha et al. 26 analyzed the impact on the small coal pillar for entry protection from the fracture position of the main roof, and based on this, studied the selection criteria of reasonable coal pillar width.Xu et al. 27 analyzed the deformation failure patterns of the gobside entry in fully mechanized top coal caving face, and showed that the deformation and failure of the entry was characterized by "two-way" asymmetric failure patterns along vertical and horizontal directions.Sun et al. 28 studied the transmission and evolution law of internal and external stress field in gob-side entry driving, and analyzed the deformation characteristics of the roadway surrounding rock under different widths of chain pillar.Wang et al. 29 studied the large deformation and failure mechanism of surrounding rock for gob-side entry with small coal pillar in the working face with goaf on both sides of the deep soft-broken coal seam in FLAC threedimensional (3D) numerical simulation software.Wei et al. 30 analyzed the strata behavior regularity of gob-side entry with small coal pillar of shallow buried coal seam through on-site monitoring.Based on the stress environment of coal pillar before and after grouting, Zhang et al. 31 proposed that the structure of caving rock mass can be stabilized by grouting backfilling in adjacent goaf, so as to realize effective control of surrounding rock for the gob-side entry.
In summary, the deformation and failure characteristics of gob-side entry under specific conditions are complex and diverse.Nevertheless, there is currently limited research on the gob-side entry of gently inclined coal seam.The high ground pressure, side abutment pressure, and mining-induced stress can easily form the superposition of stress concentration, and the entry surrounding rock is prominently deformed and difficult to control, so it is urgent to carry out a research on the deformation and control of gob-side entry in gently inclined coal seam.Accordingly, this paper takes the gobside entry of gently inclined coal seam in Heiyanquan coal mine as the engineering background, and analyzes the deformation characteristic, stress evolution, and deformation cause of surrounding rock.Moreover, combined with the problems in the entry support, the essential principles of surrounding rock control and support technology are presented.It is successfully applied in the engineering practice, which is aimed at providing theoretical foundation and scientific basis for the support of gob-side entry of the adjacent panel and other similar gently inclined coal seams.

| ENGINEERING OVERVIEW
Heiyanquan Coal Mine is located in Hami City, Xinjiang Province, and its minefield is distinguished by development of folds, scarcity of faults, and absence of magmatic intrusion.The main coal seam A 1 mined, 85-574 m deep, dips 10°-21°, which is a typical gently inclined coal seam.The coal seam is 2.16-6.92m thick with an average of 3.71 m and has 0-2 layer partings with a maximum thickness of 0.48 m.In addition, it is nonuniform and has a simple structure.Panel SA1104 is located in the south of the first district in coal mine, west of the gob in panel SA1103, east of the solid coal area that needs to be mined, north of the blind airshaft station at the second mining level, and south of the pre-excavated setup room in panel SA1104, as shown in Figure 1.The panel is 240 m wide × 2300 m long with 8 m chain pillar between the upper panel.The tailgate is excavated along the roof (leaving 200 mm of top coal) to form a right-angled trapezoidal section, with a width of 5.2 m, a height of 3.8 m for the coal pillar rib, and a height of 2.2 m for the solid coal rib.As shown in Figure 2, the immediate roof is siltstone.The main roof is comprised of gritstone and mudstone, and it is overlain by siltstone and mudstone.
F I G U R E 1 Relation between mining and excavation of panel.
The immediate floor is fine mudstone and the main floor is siltstone.However, in the early stage of excavation, tailgate SA1104 exhibits intense deformation with considerable roof-to-floor convergence, rib-to-rib convergence, and rib corner failure, so the support is particularly difficult.

| Field observation
Considering the tailgate SA1104 as an example, the deformation of gob-side entry with small coal pillar in gently inclined coal seam was analyzed, as shown in Figure 3. Figure 3A illustrates the right-angle trapezoidal section of the tailgate.The deformation of entry surrounding rock was mainly manifested in the roof-tofloor and rib-to-rib contraction toward the section, resulting in the failure of supporting components.The most common ways that entry failure appears were slippage of coal pillar side rib, internal extrusion of solid coal side rib, floor heave, broken bolt, roof subsidence, and roof bulge.The surrounding rock deformation was characterized as follows: The entire area showed the characteristics of spatial asymmetric large deformation.The roof subsidence was serious with a maximum value of 900 mm, and the top corner of small coal pillar side rib appeared the bulge, as shown in Figure 3A; the roof, shoulder nest, and other locations were loose and broken, and the expansion and fragmentation dominated the deformation of the surrounding rock, resulting in the appearance of numerous "net pockets," as shown in Figure 3B; shear slip occurred at the junction between the roof and the upper part of the small coal pillar, leading to the fractured surrounding rock at the upper sharp corner.The mid-upper part of face was squeezed more than 600 mm, accompanied by sloughing, as shown in Figure 3C; the solid coal side rib was less broken, with evident expansion and fragmentation deformation.The compression deformation was not coordinated between the upper and lower part and showed an overall inward extrusion and protrusion, as shown in Figure 3D; floor have occurred in some areas with a maximum value of 500 mm, as shown in Figure 3E; the phenomenon of bolt ejection and wire mesh tearing appeared in the part with severe deformation, as shown in Figure 3F.
According to the field observation and analysis, the asymmetric deformation characteristics of the gob-side entry with small coal pillar in gently inclined coal seam are mainly in the following three aspects: (1) The roof-tofloor convergence is asymmetric along the centerline of the entry.The roof subsidence near the solid coal side is more than that near the small coal pillar side, but the floor heave near the solid coal side is less.(2) The rib-torib convergence is asymmetric.The displacement of the small coal pillar side rib is more than that of the solid coal pillar side rib, as is the damage degree.(3) The upper and lower parts of ribs are asymmetrically deformed.The upper deformation of the small coal pillar side rib is larger, as is the middle deformation of the solid coal side rib, and there is a substantial difference in the deformation and failure of the upper and lower rib corners.

| Numerical simulation
Based on the geological parameters of the panel SA1104, a FLAC 3D numerical simulation model has been constructed to further understand the deformation mechanism of surrounding rock for the gob-side entry with small coal pillar in gently inclined coal seam, as shown in Figure 4. Since this paper focused on the simulation analysis of the overall stress evolution and deformation pattern of the surrounding rock for the gobside entry, it can be seen from the field observation that the surrounding rock still had a large convergence displacement under the original support conditions, so the support elements such as the bolts, which acted only locally in a small range, was not considered in the simulation.The model dimensions were 500 m × 200 m × 217 m, with a entry width of 5.2 m, a high rib of 3.8 m, a low rib of 2.2 m, a small coal pillar width of 8 m, and a coal seam inclination angle of 17°.The Mohr's Coulomb model was adopted to evaluate the rupture of the surrounding rock, and the physical parameters of the model were selected according to the geomechanical parameter assessment report as shown in Table 1.The normal displacement was limited at the side of the model, and the bottom was fixed.According to the results of the field test, the top of the model had a vertical stress of 11.02 MPa given to it to imitate the gravity of the rock layer above it.The model was divided into 359,600 elements and 385,057 gridpoints.Mesh refinement was carried out at the edge of the gob and near the gob-side entry for more detailed observation.Among them, the size of the mesh element near the tailgate was 2 m × 4 m × 2.4/3.2/4m.The calculating procedure of the numerical model was as follows: numerical calculation model establishment→primary rock stress calculation→calculation of upper sublevel panel mining→entry driven along the gob-side→panel mining→output and analysis of the calculation results.
The tailgate SA1104 mining stress distribution is shown in Figure 5. From Figure 5A, it can be seen that the edge coal body of the gob of upper panel has formed a certain range of fissure zone under the action of "given deformation" of the overlying strata.The excavation caused the surrounding rock stress to redistribute, and the small coal pillar's bearing ability was diminished because the fissure zone on both sides of it was connected.The small coal pillar's stress concentration zone was located 1.9 m from the high side rib, with a peak stress of 25.2 MPa.Similarly, the solid coal pillar's stress concentration zone was located 4.1 m from the low side rib, with a peak stress of 29.4 MPa.The difference in peak stress between the ribs was 4.2 MPa.In the case of leaving 8 m small coal pillar, entry driven along the gobside partly contributed to decompression, and the stress concentration was to a certain extent shifted to the deeper part of the solid coal side, with a larger concentration area.Concurrently, there was a rise in the coal pillar's tension.Every entry corner has a different range and value of stress concentration.At the top corner of the high side rib, the stress concentration was focused and extended to the roof above the coal pillar.
During the mining period, the solid coal side rib would be drastically deformed, accompanied by the adjusting process of the mining-induced abutment stress.From Figure 5B, as the face of panel SA1104 advanced to 150 m, the front abutment pressure of entry can be divided into low-value zone (0-3.4 m), high-value zone (3.4-9.6 m), peak-value zone (9.6-21.5 m) and slow descent zone (21.5-37.6 m).Obviously, the surrounding rock stress was higher than during the excavation process.Naturally, since the entry by the influence of the front abutment pressure for a shorter period, generally only 3-5 d, the entry was scrapped as the face advanced.Therefore, the maintenance of gob-side entry in gently inclined coal seam should be focused on the entry excavation-the face premining stage.
Monitoring lines were arranged at the surrounding rock section to monitor the displacement of roof, floor, and ribs of the entry after excavation, as shown in Figure 6.The major area of surrounding rock deformation was mostly centered in the roof and small coal pillar side rib following the entry excavation stress adjustment and stabilization.The surrounding rock deformation was clearly asymmetrical.The solid coal side rib produced deformation protruding to the inside of the entry mainly in the middle of the solid coal side rib, with the maximum transverse displacement value of 353.3 mm, as shown in Figure 6A.The face sloughing and slipping occurred at the upper sharp corner of the high rib on the side of the coal pillar along the direction of BC, showing a positive displacement.The slip amount was 255.1 mm, forming a coal pillar slip triangle area, and the roof overhanging length increased by the length of AC, as shown in Figure 6B.The roof subsidence of the solid coal side was more than that of the small coal pillar side, with the maximum value of 694.9 mm, as shown in Figure 6C.The deformation of floor heave was deflecting to the small coal pillar side, with the maximum value of 329.6 mm, as shown in Figure 6D.The coal pillar deformation was significant, mainly concentrated in the shallow middle and upper part of the pillar, with a maximum lateral displacement of 544.7 mm.The upper part of the coal pillar was primarily characterized by downward sliding deformation, and this displacement gradually transitions into displacement in the lower part of the coal pillar towards the gob direction.As a result, there was a slight dislocation between the coal pillar and the floor, which aligned with the conclusion drawn from observations made during drilling activities at the rear regarding discordance caused by rock surface irregularities and extensive crack development.

| Cause analysis of surrounding rock deformation
The uniformity of the surrounding rock about gob-side entry in gently inclined coal seams is influenced by multiple factors, which can be categorized into geological environmental factors and mining design factors.The geological environmental factors primarily encompass the physical and mechanical properties, primary stress, and structural characteristics of the surrounding rock.
On the other hand, the mining design factors mainly mining-induced influence, supporting materials and parameters, as well as entry section size.

| Geological environmental factor
(1) Physical and mechanical properties of the surrounding rock It is difficult to conduct a large number of in-situ tests in the field, and since the tensile strength of the fractured rock mass is very small, the engineering design generally does not allow the existence of the tensile stress in the rock mass.Accordingly, samples were taken from the roof and floor of tailgate SA1104 and coal mass, and processed into standard samples for coal and rock strength test. 19The measured physical and mechanical parameters of entry surrounding rock were shown in Table 1.| 1765 According to the test results, it is determined that the roof of tailgate SA1104 is a composite roof.The siltstone roof exhibits a compressive strength of 10.75 MPa, which is significantly lower than that of the overlying gritstone layer.Following excavation, due to the weaker strength of the siltstone layer, it experienced greater deflection compared to the overlying gritstone layer.Additionally, there was poor adhesion between these layers, leading to potential separation.The coal mass itself possessed low strength and underwent plastic deformation during excavation, reaching post-peak residual strength with relatively low-stress levels.Cracks developed at the edges of the coal mass resulting in severe deformation and fracture, causing a significant decrease in carrying capacity and making small coal pillars prone to nonuniformity and failure.The surrounding rock exhibited poor mechanical properties characterized by loose structure, uneven hardness, low consolidation degree, and weak cementation; consequently resulting in reduced load resistance for the surrounding rock and leading to substantial entry deformation.
(2) Distribution characteristic of in-situ stress The hollow inclusion (HI) stress relief method was used to conduct the in-situ stress test, as depicted in Figure 7.The test findings indicated that: The in-situ stress of Heiyanquan Coal Mine was characterized by σ H > σ h > σ v .The azimuth of the maximum horizontal primary stress was 197.24°-219.77°,and the angle with horizontal plane was between −3.71°and 9.02°, which was near-horizontal orientation.The maximum horizontal primary stress was 29.35 MPa, the minimum horizontal primary stress was 17.04 MPa, and the vertical primary stress was 11.02 MPa.According to the research results of Kang et al. 32 on in-situ stress, the in-situ stress of Heiyanquan Coal Mine was between 18 and 30 MPa, which was a high-stress area.The ratio of the measured maximum horizontal stress to the theoretically calculated horizontal stress ranged from 3.98 to 10.09, which indicated that it was under the influence of the more obvious horizontal tectonic stress.When the maximum horizontal primary stress was perpendicular to the axis of the roadway, the stress along the inclined seam was enhanced.Meanwhile, the increase of the stress difference between vertical stress and horizontal stress would aggravate the stress concentration degree of horizontal stress in the floor and roof.The tailgate SA1104 depth was 395 m, with an inclination angle ranging from 32°to 55°relative to the maximum horizontal primary stress.This orientation resulted in significant deformation on the right side of the entry (adjacent to small coal pillar).Upon excavation, the surrounding radial stress diminished to zero, causing a sharp decline in surrounding rock strength.Consequently, under the combined influence of high horizontal tectonic stress and side abutment pressure, compression occurred within the coal wall while severe deformation affected the surrounding rock.Therefore, it can be concluded that high ground pressure and fold tectonic setting serve as primary driving forces behind entry deformation and failure.
(3) Deterioration characteristics of the surrounding rock structure To monitor the development of bedding and cracks in the surrounding rock of the entry, three sections were selected for placing observation holes within a distance of 0-30 m from the advancing face of tailgate SA1104.The distribution characteristics of cracks in the roof and rib of the entry were analyzed.The results indicated that there was an obvious nonuniform distribution of cracks in the surrounding rock, with significantly more severe crack occurrence and development on both side rib compared to the roof.Additionally, larger crushing areas were observed, and cracks tended to develop more extensively in small coal pillars than in solid coal.Notably, internal crack development within the roof's surrounding rock exhibited distinct saltatory nature (Figure 8A).The surrounding rock's cracks were broken within the section of 0-0.7 m, and there was a development of large, clearly visible cracks with a large width of separation at 0.4 m, visible longitudinal cracks at 1.3 and 3.7 m, and sparse circumferential cracks at 5.2 m.In contrast, there were no obvious cracks at 1.9-3.6 m, and the distribution of cracks development was not uniform.Cracks also developed extensively along small coal pillar wall (Figure 8B).High levels of fragmentation were seen in the surrounding rock at segment 0-2.4 m.Transverse cracks can be seen at 0.6 m of depth, dense longitudinal cracks at 1.5 m of depth, scarce circumferential cracks at 5.8 m of depth, and fracture void region occurred between 3.2 and 4.4 m of depth.By comparison, solid coal wall had less fracture growth (Figure 8C).Sections 0-1.1 m showed relatively greater degrees of fracturing; tiny longitudinal cracks appearred around depth 1.8 m; intensive cracking occurred between depths 4.0-4.6 m; other sections showed scattered cracking.
Therefore, cracks were highly developed in the 3.2 m range of the entry, while the deterioration of the internal structure in the deep surrounding rock had obvious saltatory nature and nonuniformity.In addition, cracks in the small coal pillar were the most developed.With the continuous disturbance of gob dynamic load and entry excavation, longitudinal cracks gradually evolved into fracture zones, and transverse cracks gradually evolved into separation and radial dislocation, and the two kinds of cracks crossed and extended each other, easily forming abnormal fracture zones. 22The deterioration degree of surrounding rock structure increased and the uniformity decreased.

| Mining design factor (1) Mining disturbance
According to the principle of big and small structures proposed by Hou, 33 the entire surrounding rock structure of the gob-side entry was composed of "big structures" and "small structures."Under specific mining conditions, the stability of the big structure played a crucial role in determining the stability of gob-side entry.As depicted in Figure 9, the dynamic change process of "stability-instability-stability" was constantly repeated by the articulated chain plate structure formed by caving waste in gob and fractured roof rock beams.During excavation, following the upper face mining, main roof fractured first, and the key block B rotated, slipped and contacted with waste while articulating with rock blocks A and C. The overhanging beam created by key block B led to a sharp increase in side abutment pressure along with roof and coal pillar.This concentrated superposition of high in-situ stress and side abutment pressure caused integral squeezing and deformation between rib and rib.Being a supporting point for masonry beam structure of main roof, severe deformation occurred at this stage.Furthermore, during the face mining, new fractures appearred on main roof causing newly produced rock block A to hinge with original key block B resulting in rotating and sinking actions that disrupted equilibrium state within big structure.The intense dynamic pressure from mining formed higher front abutment pressure which further complicated surrounding rock stress environment leading to intensified deformation and failure.Therefore, both during excavation and mining processes, there was formation of highly concentrated side abutment stress which continuously collapsed and destabilized coal bodies on both sides of entry.Rib destabilization eventually led to the big structure becoming unstable as well, and during the process of changing into a new balance, the surrounding rock would experience increased stress and deformation of the dual role.
(2) The choice of supporting materials and parameters The deformation of the surrounding rock can be divided into two main categories: (1) expansion deformation of the rock structural surface such as bed separation, sliding, fissure opening, and fissure expansion, which belongs to discontinuous deformation; (2) elastic deformation of surrounding rock, plastic deformation before peak strength, and overall deformation of the anchoring area, which belongs to continuous deformation.Since the strength of structural surface was generally lower than that of rock mass, after excavation of the entry, discontinuous deformation preceded continuous deformation.So the initial support stiffness and strength of the support system should be greatly improved, and the surrounding rock should be allowed to have larger continuous deformation.The original support design of tailgate SA1104 adopted symmetrical support in the early stage of driving, resulting in obvious asymmetric deformation of surrounding rock.The low tensioned bolts used in tailgate SA1104 led to passive bearing of the bolts, which failed to give full play to the surrounding rock's own bearing capacity.As a result, the discontinuous deformation could not be controlled in a timely manner and the strength and integrity of the surrounding rock decreased.The elongation of the support system was minimal, preventing significant continuous deformation in the surrounding rock of the entry and hindering the release of high stress.This caused the support components to slip and break under high stress.Concurrently, the bolt strength was low, its shear and tensile ability was inadequate to sustain the stress generated by the surrounding rock, and major deformation situations commonly caused the bolts to bend and shear break.Additionally, the support structure failed to reinforce the support for significant deformation areas like shoulder nests, the two bottom corners of the ribs, and the roof beside the gob, which compromised the overall stability of entry.
(3) Entry section size design Research shows that: there is a critical width of the entry section size, generally 4 m; after exceeding the critical value, the entry width has a significant effect on roof subsidence. 34Tailgate SA1104 was a right-angle trapezoidal entry with a width of 5.2 m, and a section area of 15.6 m 2 .Over-excavation occurred during the excavation phase, resulting in section areas that were already 20 m wide in some places.Based on the general F I G U R Structural section of gob-side entry in gently inclined coal seam.description of China's gateroad, the fully mechanized top coal caving or large mining height entry with a net section area of more than 16.5 m 2 was classified as a mega section entry.While the right-angle trapezoidal roadway can partly withstand larger vertical loads, the roof was easy to bend due to large span, making it highly susceptible to roof fall.The height of small coal pillar side rib was 3.8 m, and its weak bearing capacity and high side pressure made it more difficult to support the entry.
In summary, the tailgate SA1104 experienced uneven loads from the surrounding rock due to the inclination of the coal seam.The main power source of this asymmetric deformation were high ground pressure and strong disturbance; the low load resistance and high structural deterioration of the surrounding rock led to the fissure development and large range of fragmentation, which aggravated the expansion deformation of the entry; low tensioned of the bolts and the large span of the entry further deteriorated the stability of the surrounding rock.Therefore, rapid deformation occurred at the early stage of excavation in the tailgate SA1104.Driven by high horizontal stress and side abutment pressure, the surrounding rock strength decayed sharply after plastic zone deterioration, with a high degree of fissure development and prominent expansion and fragmentation deformation, which ultimately led to deformation and instability of the entry.

| The key control principles
Giving full play to the strength of surrounding rock, improving the stress conditions of surrounding rock and reasonable support are the keys to reduce the relaxation range of plastic zone and ensure the stability of surrounding rock.According to the deformation characteristics and mechanism of surrounding rock for gobside entry with small coal pillar in gently inclined coal seam, aiming at the large deformation problem of tailgate SA1104, a combined support method of "bolt + steel wire mesh + rebar ladder beam + cable" was proposed.
(1) High-tensioned strong support was implemented to enhance the compatibility between supporting components.By utilizing high-tensioned strong supporting bolts, the expansion deformation and failure of surrounding rock in the anchoring area, such as separation, sliding, crack opening, and new crack formation can be effectively controlled.This ensured that the surrounding rock remained under pressure while restraining its bending deformation, stretching, and shear failure.Consequently, the surrounding rock became the primary tensioned load-bearing structure with high stiffness in the anchoring area.The aim was to maximize the integrity of the surrounding rock in the anchoring area and improve stress distribution within deep surrounding rock.(2) The roof adopted combined support.Specifically, bolts and cables were utilized to reinforce the shallow surrounding rock and strengthen its mechanical properties.This formed an initial stressbearing ring for surrounding rock.To further control high-stress concentration on solid coal side roof, top bolt arrangement was inclined towards it.Bolt length was determined based on specific relaxation zone range of surrounding rock.Additionally, vertical installation of cables guaranteed that it can be fully and stably anchored in the deep stable rock layer in the entry roof.(3) The stability control of the two ribs was achieved by employing asymmetric supporting parameters.Following entry excavation, the small coal pillar experienced plastic failure with reduced bearing capacity, resulting in more pronounced deformation on the coal pillar side compared to that on the solid coal side, forming a characteristic asymmetric deformation pattern.To provide constraints and reinforce the small coal pillar, thereby enhancing its carrying capacity and restraining deformations and slippage of the coal wall, a combination of lengthening bolts and additional cables was employed.Due to the significant stress concentration on the solid coal side rib, it was necessary to appropriately increase bolt installation density to offer support resistance and mitigate the influence of asymmetric stress on surrounding rock stability.Furthermore, the shoulder corner and bottom corner of the entry were potential to shear deformation and failure, reinforcement measures should be implemented.

| Entry support scheme
The tailgate SA1104 was determined to use the asymmetric combined support scheme of "bolt + steel wire mesh + rebar ladder beam + cable" based on the boltcable support theory and previous analysis.was not less than 250 kN.The side ribs were supported by high-tensioned rebar bolts (ϕ20 × 2500/2200 mm), rebar ladder beam (ϕ12 mm), and steel wire mesh (50 × 50 mm).The small coal pillar side rib adopted the bolts (ϕ20 × 2500 mm), with 5 bolts in each row and an interrow spacing of 800 × 1000 mm.The solid coal side rib adopted the bolts (ϕ20 × 2200 mm), with 4 bolts in each row and an interrow spacing of 600 × 1000 mm.There was a 15°angle between the horizontal line and the top and bottom bolts, and the other bolts were perpendicular to the entry rib.Moreover, each row of small coal pillar side rib was installed with one steel strand anchor cable (ϕ21.6 × 5300 mm) reinforcement support, which was 1800mm away from the roof, the row spacing was 2000 mm, and the angle between it and the horizontal line was 45°.The specific support mode was shown in Figure 10.

| Field application (1) Surface displacement monitoring
To verify the effectiveness of the entry surrounding rock control after the support optimization, a typical section of the tailgate SA1104 was arranged with a measuring station to observe the surrounding rock surface displacement, and Figure 11 showed the monitoring curve of the entry surface displacement.
According to the deformation monitoring results, the deformation velocity of surrounding rock increased continuously during 0-10 days of excavation, and the deformation velocity of ribs and floor reached the maximum on the 10th day, which were 6.4 and 3.3 mm d −1 , respectively.This was because the rapid unloading of the surrounding rock broke the primary rock stress balance during excavation, the stress redistribution of the surrounding rock led to the expansion of the plastic zone and the increase of the displacement of the side ribs, and the fracture or shear displacement of the rock mass occurred since the stress reached the strength limit.The expansion and fragmentation of rock mass caused the displacement of surrounding rock to increase rapidly in the early stage of excavation.After 10 days, the deformation velocity of the side ribs and floor decreased dramatically, whereas the roof deformation velocity continued to increase with a decreased amplification.The roof deformation velocity peaked on the 30th day, which was 6.3 mm d −1 .The maximum rib-to-rib convergence was 99 mm, and gradually stabilized after 40 days.The maximum roof subsidence was 77 mm, which steadily became stable after 30 days.The maximum floor heave was 41 mm and gradually stabilized after 10 days.The deformation of surrounding rock was within a reasonable range, the roof, floor, and side ribs deformation were effectively controlled, and the asymmetric deformation characteristics were weakened.
(2) Roof bed separation monitoring Deep anchors and shallow anchors were set to monitor the roof bed separation.Figure 12 shows the monitoring curve of entry roof bed separation.
The monitoring results for roof bed separation indicated that while the rate of separation was considerable in the early stages of excavation and gradually increased with the length of excavation, it tended to remain steady.Comparing shallow and deep anchors, the former has a greater overall separation amount.The maximum roof bed separation at the shallow anchors and the deep anchors were 28 and 30 mm, respectively.The total roof bed separation was 58 mm, and every part of the roof was stable.
(3) Roof bolt and cable stress monitoring The stress monitoring of bolt and cable supporting components was carried out.Figure 13 shows the stress monitoring curve of bolt and cable.
Bolt, cable force monitoring results showed that: On Days 0-2, the cable force fluctuated slightly, the force of top bolts and top cables grew more quickly, playing a role in active bearing capacity.After a brief period of stabilization, the stress started to increase on Day 10.The force of bolts and cables on the small coal pillar side grew more quickly, and the small coal pillar side rib was affected by the larger load of the overlying strata at this F I G U R E 11 Entry surface displacement monitoring curve.
F I G U R E 12 Entry roof bed separation monitoring curve.
time.The force of the cables and bolts started to vary once more on the 32th day, and it essentially stabilized by the 55th day.The stress of roof and small coal pillar was larger, while that of solid coal was smaller, and the bolt force showed the characteristics of small coal pillar > roof > solid coal.The maximum values of bolts and cables were 68.25 and 71.56 kN respectively, which were both within the yield range and leave enough margin for the mining period.To sum up, the deformation control effect of gob-side entry with small coal pillar in gently inclined coal seam was remarkable.

CONCLUSION
Based on the field measurement and numerical simulation, this paper deeply studied the deformation characteristic of surrounding rock for gob-side entry with 8 m small coal pillar and the deformation cause of surrounding rock of the entry and put forward the corresponding asymmetric combined support design scheme of the entry, which was successfully applied in the field.The following conclusions can be drawn: (1) Gob-side entry with small coal pillar in gently inclined coal seam presented the characteristic of spatial asymmetric deformation, the roof subsidence on the solid coal side was larger than that on the small coal pillar side, and the floor heave was smaller than that on small coal pillar side.The deformation of small coal pillar was larger than that of solid coal, and the damage degree was higher.The deformation of the upper and lower coal wall was uneven, and the mid-upper part of the small coal pillar and the middle part of the solid coal had larger deformation.(2) 8 m small coal pillar played a certain role of pressure relief, and the stress concentration zone of solid coal was larger.The stress concentration zone on the small coal pillar side was located 1.9 m from the high rib, while the stress concentration zone on the solid coal side was located 4.1 m from the low rib.The difference in the ribto-rib peak stress is 4.2 MPa.The front abutment pressure of entry can be divided into low-value zone (0-3.4 m), high-value zone (3.4-9.6 m), peakvalue zone (9.6-21.5 m), and slow descent zone (21.5-37.6 m).The maintenance of gob-side entry should be focused on the entry excavation-the face premining stage.(3) Through the physical and mechanical test of coal rock, in-situ stress test, borehole observation of surrounding rock, and theoretical analysis, it was believed that poor physical and mechanical properties of the surrounding rock, high in-situ stress, and high deterioration degree of the surrounding rock structure were the main internal causes of the entry deformation.The main external causes of the entry deformation were thought to be strong mining disturbance, arbitrary support parameters, and large section sizes.(4) Based on the key control principles of high-tensioned strong support and asymmetric support, a combined support method of "bolt + steel wire mesh + rebar ladder beam + cable" was proposed.Under this support scheme, the field surrounding rock control effect was remarkable, which provided a useful reference for the support of similar gob-side entry in gently inclined coal seam.

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I G U R E 2 Panel SA1104 drilling histogram.F I G U R E 3 The deformation and failure of surrounding rock in right-angle trapezoidal entry with small coal pillar.(A) Roof subsidence, (B) roof bulge, (C) shear slip, (D) extrusion, (E) floor heave, (F) bolt failure.

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Displacement characteristics of surrounding rock in section of tailgate SA1104.(A) Solid coal side rib horizontal displacement, (B) small coal pillar side rib horizontal displacement, (C) roof vertical displacement, (D) floor vertical displacement.WANG ET AL.

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I G U R E 7 In-situ stress test.(A) Monitoring device, (B) underground construction.
Distribution characteristics of entry surrounding rock structure deterioration.(A) roof, (B) small coal pillar side rib, (C) solid coal side rib.
The roof was supported by steel wire mesh (50 × 50 mm), rebar ladder beams (ϕ12 mm), and high-tensioned left-handed nonlongitudinal rebar bolts (ϕ20 × 2900 mm).The bearing plates of bolts (150 × 150 × 8 mm) were high-strength arch-style pallets, including centering ball pads and damping nylon washers.Each row had seven top bolts, with 800 × 1000 mm interrow spacing.The angle of corner bolts with respect to the vertical line of the roof was 15°, and the other bolts were perpendicular to the roof.Each bolt was filled with two MSCK2335 resin cartridges, the bolt pretension was 300 N•m, and the design anchoring force was 120 kN.Stranded cables (ϕ21.6 × 8300 mm) were installed in the roof centerline and side along plumbline to provide reinforcing support.The bearing plates of cables (300 × 300 × 16 mm) were high-strength arch-style pallets.Every row consisted of three top cables, with 1800 × 2000 mm interrow spacing on the small coal pillar side and 1400 × 2000 mm interrow spacing on the solid coal side.Each cable was filled with five MSK2335 resin cartridges, the cable pretension was 250 N•m, and the design anchoring force

F I G U R E 10
Entry support design.(A) Entry supporting section, (B) roof support, (C) solid coal side rib support, (D) small coal pillar side rib support.
Physical and mechanical parameters of coal and rock.
T A B L E 1