Research on gob‐side entry‐retaining technology with coal rib and corner strengthened support in soft rock strata

The gob‐side entry‐retaining by roof cutting and pressure relief (GERRC) method is an advanced no‐pillar mining method that can significantly improve coal recovery and reduce roadway accidents. However, according to case studies, there is relatively little research on the application of the GERRC method in geological conditions of soft rock formations (especially when the roof and floor are both mudstone), and such research is needed. Therefore, based on the typical soft strata of the Xinyi Coal Mine, this paper analyzes the failure mechanism of the roadways and proposes the GERRC method with coal rib and corner strengthened support (CRCSS). The evaluation indexes of the roof convergence rate and cable safety margin are established, and the effects of roof cutting and CRCSS on roadway deformation control are explored by a numerical comparison test. The results show that roof cutting has a limited effect on roadway deformation. The deformation can be reduced by 75.77% by implementing CRCSS on the basis of GERRC. Then, the evolution characteristics of roadway stress and deformation in the process of mining are further discussed through field contrast tests. The field application results show that compared with that of GERRC with conventional support, the surrounding rock deformation of GERRC with CRCSS is reduced by more than 55%, the safety margin of the cable is improved. The overpressure of the hydraulic prop is diminished, and the stability distance of the roadway is reduced by approximately 30%. Finally, this paper discusses the mechanism to control roadway deformation by CRCSS. This research expounds the rationality of CRCSS, and the results can provide a reference for other situations with similar geological conditions.

Coal plays an important role in China's energy structure. 1he problems of low resource recovery and severe waste caused by coal pillar retention mining are becoming increasingly prominent.3][4] Therefore, gob-side entryretaining (GER) stress transfer technology has been promoted and developed. 5,6][9] With the advancement of technology and the deepening of research, GER technology has undergone great innovation.He et al. 10 and Wang et al. 11 proposed the GER formed by roof cutting and pressure relief (GERRC).Their major innovation was the elimination of filling materials.GERRC uses presplit blasting to change the stress distribution and uses the features of broken and caving gangue expansion to achieve the GER. 12,13s a new mining technology developed in recent years, 14 GERRC has become a requirement for building green mines. 15Many scholars and technicians are exploring the parameters of presplitting and roadway support in different geological conditions from the aspects of theoretical analysis, numerical simulation, similar experiments, and engineering tests.It is expected that the system of presplitting parameters and supporting parameters can be selected according to geological conditions.For example, Sun et al. 16 studied the presplitting blasting and supporting parameters of thin coal seams and realized GERRC in thin coal seams.Ma et al. 17 explored the key parameters of GERRC under composite roof conditions, and the study results were successfully applied in the testing ground.Wang et al. 18 proposed the method of high-strength bolt grouting with GERRC and extended this technology to the geological conditions of deeply buried broken roofs.At present, GERRC has been successfully applied at shallow to deep depth, 19,20 for thin to medium coal seams 16,21,22 and thick coal seams, 23 in hard roofs to composite roofs, 17,24 and for highly dipping coal seams and broken roofs. 18,257][28][29] Additionally, the new presplitting blasting method has achieved fruitful results. 30In terms of support, studies on roof deformation mechanisms, 31 roof structure evolution characteristics, 32 roof control mechanisms, technology, and methods 33 have achieved fruitful results.However, a literature review found that the application of GERRC in the geological conditions of soft strata is rarely studied, with few successful engineering cases for reference.China has complex coal resources and abundant reserves in soft strata. 34In these cases, the soft strata are the soft roof and floor of the coal seam, and the compressive strengths of coal and rock are low. 35,368][39][40][41] However, it is rare for a roadway to have both a roof and floor made of mudstone.The adaptability of the original technical scheme can be limited due to changes in geological conditions. 42herefore, the application and popularization of GERRC are particularly important under soft strata. 43n addition, an insufficient bearing capacity and easily crushed sidewall are geological characteristics of soft strata, and examples of roadway deformation caused by sidewall crushing are not uncommon. 44In this regard, Zhu et al. 45,46 believed that the main reason for the collapse of the sidewall is that the bolt is in the broken surrounding rock and cannot fully achieve its support potential, and they proposed a support scheme.Wang et al. 47 deduced the theoretical formulas for calculating the range of the limit equilibrium zone (LQZ) and the stress and displacement of a sidewall and elaborated the factors influencing the distribution stress in the surrounding rock and LQZ to provide a theoretical basis for sidewall support.However, to summarize the support system of GERRC, roof control is key for roadway support.GERRC regards the sidewall as the bearing foundation, despite a lack of research on sidewall deformation control, and does not form a complete sidewall supporting system.This is a major challenge for the application and popularization of GERRC in soft strata.
On the basis of the above analysis, this paper research the mechanism of the deformation of roadways in typical soft rock with the geological conditions of the 1706 panel of the Xinyi Coal Mine.Combined with the research results of GERRC and cable support technology, the method of GERRC with coal rib and corner strengthened support (CRCSS) is proposed.Numerical and field comparison tests are carried out on GERRC with CRCSS and GERRC with conventional support.The influence of the cutting roof and CRCSS parameters on roadway surrounding rock deformation control is clarified, and the optimal CRCSS parameters are obtained.The stress and deformation evolutions of the roadways with the two GERRC methods are clarified, and the rationality of GERRC with CRCSS is expounded.The results have important reference significance for the extension of GERRC in similar geological conditions.

| Engineering background
The Xinyi Coal Mine is situated in northern Jining city, Shandong Province, China, as illustrated in Figure 1A.The mine has coal reserves of approximately 324 million tons, with a set annual production capacity of 0.9 million tons.The burial depth of the 1706 panel is 242.6-300.4m.The average thickness of the coal is 2.4 m.By testing the mechanics of the rock, it was determined that the mudstone of the roof and floor had a low compressive strength, corresponding to a typical soft rock. 48A drilling machine was used to core the coal seam roof and floor rock layers.Three standard mechanical samples were made for each stratum.After testing, it was found that the mechanical properties of the same type of rock in the roof and floor were similar, with an error of less than 7%.Here, average values are used to represent the rock properties.
Roadway roof is supported by a "bolt-cablemesh + W-shaped steel strip" system, and the rib is supported with "bolt-mesh," as shown in Figure 1B.The length of the roof cable is 8.3 m, and the spacing is 1.1 m × 1.1 m.The specifications of the roof and rib bolts are the same, with a length of 2.1 m and a spacing of 1.0 m × 1.1 m.The roof bolts and cables are fitted with W-shaped steel strips.The mesh diameter is 4 mm, and the mesh size is 60 mm.In the early stage of implementation, when the above supporting methods are used with GERRC, the roadway deformation is large, and the roof bolt and cable are seriously broken.

| On-site deformation characteristics
To clarify the mechanism of the large deformation of a roadway, reasonable control methods are proposed.
During mining, the roof and floor deformation, roadway deformation, roof separation layer, and roof cable stress are monitored.Figure 2A shows the monitoring results of the cable force.In the coal mine production, the cable stress first increases, then decreases, and then stabilizes.The stress rapidly increases at 0-37 m behind the longwall face, which is caused by roof collapse and overburden deformation.A brief stabilization phase occurs at a distance of 40 m behind the longwall face, where the stress peaks.With caving gangue supporting the goaf roof and being gradually compacted, the cable axial force decreases, and the fluctuation during this period is caused by the nonuniform support of the caving gangue for the roof.Approximately 190 m behind the longwall face is basically stable, the roadway is stable, and no deformation occurs.The above analysis shows that the cable has a long stability time, that is, the deformation duration of the roadway is long.Meanwhile, the increments of cable stress are as follows: gob side (246 kN) > coal rib side (214 kN) > roof middle (154 kN), which indicates that the coal rib side fails to support the roof effectively.In addition, the breaking force of this type of cable is 408 kN, the utilization rate of the gob-side cable is 93.87%, and the safety margin is small.
During the mining process, the roadway surrounding rock approximately 20 m ahead of the longwall face begins to deform, as shown in Figure 2B.As the mining progresses, the deformation increases rapidly, and the rate of increase slows farther than approximately 35 m behind the longwall face.At 150 and 190 m behind the longwall face, the shallow and deep separation layers of the roof are stable, and at about 200 m behind the longwall face, the roadway is stable.Finally, the vertical deformation is 963 mm, the sidewall deformation is 726 mm, and the vertical and horizontal convergence rates of the roadway are 40.13%and 18.15%, respectively.The displacement characteristics of the roof separation layer are similar to the surface displacement of the roadway, and the final roof depth base point (11.5 m) corresponds to a separation layer of 531 mm.
In addition, by observing the supporting members, it is found that under such a large deformation of the sidewall, the sidewall bolt does not appear to be broken or the faceplate is flattened, but the bolt is extruded along with the crushed coal of the sidewall.This occurs because the anchoring end of the sidewall bolt is in the broken surrounding rock or part of the anchoring end is in the broken surrounding rock, which does not fully achieve the anchoring potential, allowing the large range of loosening and failure inside the sidewall.The applicability of support structures to limiting the deformation of surrounding rock needs to be improved.stress of 6 MPa is applied to the top to simulate the influence of the dead weight of rock strata. 52,53he roadway size is 4.0 m × 2.4 m (width × height), and the roof support is the same as the original scheme.During the numerical modeling process, separate groups are set up on the roof of the panel.Depending on the coal seam mining situation of the panel, Null elements are assigned to the roof-cutting group to achieve cutting. 54his section investigates the stress and deformation properties of the roadway under the original support.The deformation of roadway rapidly increases in the range of 0-40 m behind the longwall face.Two typical cross sections, 20 and 40 m, are chosen for the analysis, as shown in Figure 4. Rock near the roadway is always in a high-stress zone.The lateral stress concentration zone is adjacent to the coal rib edge, which has a much larger stress concentration range than the roof, making it an extremely unstable area.The deformation of the rib gradually increases as the panel is mined.The deformation of the rib affects the failure of the roof.
During mining, compressive-shear failure and slip failure occur primarily in the coal rib. 55The vertical load on the coal rib gradually increases during GER.Compressive-shear failure occurs when the final shear strength of the coal rib is reached, resulting in a shear plane.The mechanical model of coal rib compressive-shear failure is shown in Figure 5. Assume that the coal rib compressive-shear failure conforms to the Mohr-Coulomb criterion. 55The stress of the surrounding rock before the roadway excavation is illustrated by Mohr Circle A. After roadway excavation, σ 3 decreases, and the stress of the surrounding rock becomes Mohr Circle B. Due to coal mining of the panel, σ 1 increases, and the stress environment of the surrounding rock becomes Mohr Circle C. In addition, compressive-shear failure further reduces the strength and carrying capacity of the coal rib, and shear failure envelope 1 develops into shear failure envelope 2. The compressive-shear failure of the coal rib will be further exacerbated by changes in the shear failure envelope.
The original plan focused on the roof as the support point, with the combination of bolts and cables.However, the stability of the coal rib is the basis for the safety of the roof.The deformation of the coal rib leads to a reduction in the load carrying capacity, which in turn induces roof deformation.The vertical load borne by the coal rib gradually increases as it moves away from the longwall face during GER.When the final shear strength is reached, the coal rib undergoes compressive-shear failure, and an internal shear plane is formed.Monitoring at the site confirms that the length of the bolts provided to support the coal rib is insufficient.The anchoring zone is located within the shear plane, with the bolts jutting out together with the shattered coal rib.The gradual failure of the coal rib leads to the instability of the bearing foundation of the roof.The instability of the bearing foundation will induce a continued downward deformation of the roof, further exacerbating the inward failure of the coal rib.Therefore, the stability of the coal rib is an important aspect of GER, which is more of a concern in soft strata.

| Strengthened sidewall improves the stability of the roadway
In the process of mining, the LQZ appears in the sidewall under the influence of mine pressure redistribution.| 3601 Assuming that the limit equilibrium of the sidewall satisfies the Mohr-Coulomb yield criterion, 56 the principal stress is as follows: where σ 1f is the vertical compressive strength corresponding to σ 3 , MPa; σ c is the uniaxial compressive strength in the LQZ, MPa; and φ is the internal friction angle of the LQZ, °.
The coordinate of the center of the circle is ( ) , and the radius is away from the shear failure envelope.Finally, the LQZ enters the elastic state between circle D to circle E, as shown in Figure 6.It can be seen that the stability of the sidewall can be improved by reinforcing the sidewall.Theoretically, the greater the supporting strength of the sidewall is, the farther the Mohr circle from the shear failure envelope, and the more stable the sidewall.In fact, the sidewall strength cannot increase infinitely, and there is a limit value.

| The roof span can be reduced by strengthening the sidewall
Both roadway excavation and mining will cause an LQZ in the sidewall.Compressive-shear failure and slip failure mainly occur in the sidewall, and the two failure modes intensify each other, resulting in a larger LQZ of the sidewall, which leads to an increase in the roof span and roof deformation.Meanwhile, the vertical load transferred to the roadway sidewall increases with the roof span, which further aggravates the sidewall deformation, as shown in Figure 7.
The width formula of the LQZ of the sidewall is 55 where X 0 is the LQZ width, m; M is the mining height, m; A is the lateral pressure ratio; K is the lateral pressure ratio; γ is the roof bulk density, kN/m 3 ; H s is the depth of the roadway, m; P s is the supporting force of the sidewall, MPa; c s is the residual cohesion of the joint, MPa; and φ s is the residual internal friction angle of the upper part, °.
According to Equation ( 2), the deformation of the rib can be diminished by strengthening the supporting of the sidewall.The supported bib is a stable foundation that reduces the span of the roof and controls the roof deformation.

| Strengthening the sidewall shoulder corner realizes a benign bearing mechanism of the roadway
The supporting effect of the cable on the sidewall is shown in Figure 6.In the shear plane, the support force F can be decomposed into forward force F n and tangential force F τ .F n increases the normal stress in the shear plane, thus increasing the ultimate shear strength.F τ counteracts part of the shear stress in the shear plane.A certain elevation angle is set for the cable at the shoulder corner of the sidewall so that the anchorage end is in the roof rock.On the one hand, the cable can counteract shear stress to a greater extent and prevent shear failure.F I G U R E 7 Sketch of the limit equilibrium zone.
On the other hand, enhancing the cohesion, internal friction angle, and strength of the interface can effectively prevent slip failure.In addition, the roof and sidewall form a unified whole, enhancing the overall bearing capacity of the roadway and forming a good mechanism of joint bearing of the roof and sidewall.Therefore, the CRCSS method is applicable to GER in soft rock.
In summary, on the basis of ensuring the cable support force, the cable support enables the coal rib to be in a 3D stress state.It improves the strength of the coal rib and can effectively reduce compressive-shear failure.At the same time, the cable supports the rock between the shallow LQZ and the deep stability zone of the coal rib, and its cohesion and friction angle increase with the support strength, further increasing the strength of the coal rib.In addition, the cable supports the rock between the coal rib and the roof, improving the mechanical parameters at the interface.This not only effectively reduces slip failure but also improves the overall strength of the roadway.
On the basis of the above theoretical analysis, we design different support parameters by considering the original support and validate them through numerical simulations.

| Necessity of roof cutting
The mining of the panel results in the roof of the gob being hollowed out.Under the dual effects of overlying strata weight and stress, the roof of the gob will gradually deform downward, as shown in Figure 8A.The direct roof is relatively weak and can result in direct collapse.However, the base roof may not collapse immediately, which would place a load on the coal pillars. 57As deformation accumulates, once the base roof is broken, it forms a masonry beam structure, such as Rock Block B. The location of the fracture is inside the gob, forming a cantilever beam with a larger span (Rock Block A). 58 Therefore, the coal pillar bears most of the stress of the roof and overlying strata, leading to an increase in the coal pillar vertical stress.Compressive-shear failure occurs when it reaches the final shear strength of the coal rib, resulting in serious deformation of the sidewalls of the gob-side entry. 59,60In addition, the lithology of the direct roof is relatively weak.A large deformation of the coal slope will cause deformation and failure of the roof, causing in the overall failure of the retained roadway.
The roof cantilever length can be controlled manually using a roof-cutting technique.The presplit blasting disconnects the physical link between the roof.The movement of the gob roof and the retained roadway roof are independent of each other.After the coal is mined, the gob roof can smoothly slide toward the gob floor. 41nce "Rock Block D" slides down to the gob floor along the roof-cutting surface, the stress transmission in the rock strata can be effectively stopped, as described in Figure 8B.Cutting the roof can greatly reduce the stress along the roadway roof. 57The stability of the roadway can be maintained under the support of the roof "bolt-mesh-cable" and temporary support of the retained roadway.Therefore, cutting the roof is a critical link in preserving the roadway.
Assuming that the surrounding rock meets the Mohr-Coulomb yield criterion, the Mohr circle of the surrounding rock is illustrated in Figure 8. Circle Ⅰ is the balance stage before mining, Circle Ⅱ is the mining stage without roof cutting, Circle Ⅲ is after the mining stage without roof cutting, Circle Ⅳ is the mining stage after roof cutting, and Circle Ⅴ is after the mining stage with roof cutting.
In the uncut roof state, the surrounding rock close to the longwall face is greatly affected by dynamic pressure, and the principal stress increases obviously.In the process of mining, the principal stress of the surrounding rock in front gradually increases, the abscissa of the Mohr circle increases, the radius increases, and Circle Ⅰ develops into Circle Ⅱ. Mohr Circle Ⅱ is likely to intersect with the failure envelope, causing roadway failure.The roof stress behind the longwall face is released through deformation.The principal stress of the roadway increases again, and Circle Ⅱ develops into Circle Ⅲ, which further intersects with the failure envelope, resulting in continuous roadway failure.
In the roof-cutting state, the stress transmission of the panel roof and the roadway roof is blocked and greatly reduces the effect of mining on the roadway.After the coal is mined, the Mohr circle of the advanced surrounding rock develops from Circle Ⅰ to Circle Ⅳ, changing little and remining far from the failure envelope, reflecting a safe state.The pressure will be relieved in a large area behind the longwall face.At this time, the principal stress will be reduced, the radius of the Mohr circle will be reduced, Circle Ⅳ will be developed into Circle Ⅴ, and the surrounding rock will remain safe.

| The control method
To solve the above difficulties, this paper proposes a method of "CRCSS," as shown in Figure 9.The method of CRCSS is based on the method of GERRC.On the basis of the actual condition of the soft strata, additional supports were provided for the coal rib and corners of the roadway in addition to the original supports.On the one hand, presplit blasting is used to reduce the roadway stress.On the other hand, the roadway bearing capacity is improved by using the CRCSS method, and the joint bearing mechanism of the roof and sidewall is formed.On the basis of roof support, the roadway deformation process consists of three typical phases: At present, there is no basis for selecting the CRCSS parameters.The main issues are as follows: ① An insufficient support strength causes large deformation, and an excessive support causes material waste.② An insufficient cable length leads to the anchoring ending in broken surrounding rock, which cannot fully achieve its supporting potential, and an excessive cable length that leads to construction difficulty and higher costs.③ If the cable angle is too large, the anchorage depth is shallow and can anchor only the LQZ of the shallow sidewall; if it is too small, it cannot anchor into the roof to stabilize the rock layer.

| NUMERICAL TEST OF GERRC WITH CRCSS
In this paper, a total of five schemes are designed to reach the roof deformation and cable force characteristics for the cutting roof and CRCSS technologies.

| Conceptual design
In this section, taking the burial depth, size, and roof support scheme of the roadway as invariants, the roofcutting parameters are explored.Then, on the basis of roof cutting, four comparison schemes are designed for the cable length, the preload force, the interrow distance, and the cable angle at the shoulder corner.

| Roof-cutting parameters (1) The height of roof cutting
This parameter needs to consider factors such as the mining height and the expansion coefficient of rock fragmentation to ensure that the gangue can fill the gob fully and quickly.The cut height should meet the following equation under ideal conditions 61 : where H g is the height of roof cutting, m; M is the mining height, m; ΔH 1 is roof deformation, m; ΔH 2 is floor heave, m; and K is the average crushing and heaving coefficient of roof strata.Without considering the vertical deformation of the gob, according to the lithological parameters of the 1706 panel, mining height, and overlying rock structure of the Xinyi Coal Mine, the calculation is H g = 8.2 m.To test the impact of the roof-cutting height of the roadway, a 3D roof-cutting model is constructed.The stress distribution and deformation properties are studied at three heights of 6, 8.2, and 9.7 m, as shown in Figure 10.The height of 9.7 m completely cuts off the main roof sandstone.When the roof-cutting height is 6, 8.2, and 9.7 m, the peak stress concentration is 20.2, 18.8, and 18.5 MPa, respectively.The extreme values of the roadway roof deformation are 189, 124, and 133 mm, respectively.As the cutting height increases, the peak stress concentration on the coal rib of the retained roadway decreases.Continuing to increase the cutting height beyond 8.2 m has a smaller effect on the pressure relief benefit of the retained roadway roof.On the basis of a comprehensive analysis, the optimal cutting height is 8.2 m.
(2) The angle of roof cutting Many scholars have shown that when the angle is small (<10°), the roof friction is large, and the gob roof caving has a great influence on the deformation of the roadway, which cannot reflect the effect of roof cutting.When the angle of roof cutting is too large (>25°), the cantilever length of the roof will increase, and the roof of the roadway will rotate and sink. 22,62The roof-cutting height (8.2 m) remains unchanged, and the stress and deformation characteristics of the roadway are simulated when the roof-cutting angles are 10°, 15°, and 20°.The results are shown in Figure 11.
When the angle increases from 0°to 15°, the peak stress concentration and vertical deformation of the roof on the left side of the roadway gradually decrease.The stress concentration region gradually shifts to deeper layers, and the stress near the retained roadway decreases.When the roof-cutting angle increases to 15°, continuing to increase the angle will lead to an increase in the stress and vertical deformation of the retained roadway roof.On the basis of this comprehensive analysis, it is reasonable to choose a roof-cutting angle of 15°.
(3) The spacing of roof-cutting holes A diameter of 50 mm is chosen for the roof-cutting hole based on engineering experience and drilling equipment.To ensure that the rock roof is cut by shaped charge blasting, three operating conditions are designed: The blast holes are spaced 400, 500, and 600 mm apart.Using the numerical software ANSYS/LS-DYNA, we simulate and analyze the propagation of rock cracks with different hole spacings in the bidirectional charge blasting mode.Sandstone is chosen as the rock.Figure 12 reflects the crack propagation 180 μs after blasting.
When the spacing between the blast holes is 400 mm, the cracks generated by the explosion are superimposed at the center of the two holes, indicating excess explosion energy.When the spacing between the blast holes is 500 mm, the cracks have propagated between the holes to achieve a good blast effect.However, when the distance between the two holes is 600 mm, the crack has basically stopped expanding at the center position, resulting in a poor blasting effect at this spacing.Therefore, the spacing between the roof-cutting holes of the rock roof is chosen to be 500 mm.

| CRCSS parameters
The control single variable method is used to study the control mechanism of CRCSS.Numerical tests are carried out with cable length, preload, interrow distance, and cable angle at the shoulder corner as variables, as illustrated in Table 2.

| Evaluation indicators of the numerical comparison test
To quantify the effect of roof deformation control and pressure release under each scheme, two evaluation indicators of roof convergence rate χ ij and cable safety margin ξ ij are established.
The roof convergence rate is the percentage of roof subsidence and the height of the original roadway, and the calculation formula is where χ ij is the roof convergence rate of the ij scheme, where i represents the four designed test schemes and j represents the subschemes of the test scheme.l ij is the roof deformation of the ij scheme, mm; l s is the design height of the roadway, mm.The cable safety margin is the percentage of cable residual strength and cable ultimate breaking value, and the calculation formula is where ξ ij is the cable safety margin of the ij scheme, where i represents the four test schemes designed, and j represents the subscheme of the test scheme.F s is the ultimate breaking force of the cable, kN; F ij is the peak cable axial force of the ij scheme, kN.

| Test results of GERRC with conventional support
The track grooving of the panel is carried out with GERRC, and the belt grooving is not precracked and has no retained roadway.The results of the two roadways are compared, as illustrated in Figure 13.The cross section of the surrounding rock at the stable stage 50 m behind the longwall face is selected for analysis.
The peak vertical stress of the roadway roof without roof cutting is 7.19 MPa.The lateral stress concentration area is adjacent to the sidewall, and the maximum lateral stress is about 13.93 MPa.After mining, the roadway roof deformation is large, approximately 710 mm, and the maximum value appears on the gob side.However, the roof stress decreases significantly after roof cutting, with the maximum roof stress being 1.8 MPa, which decreases by 74.97%.The maximum vertical stress is 8.4 MPa, which decreases by 25.07%.This improves the stability of roadways.After mining, the extreme value of the roof deformation is 388 mm, reduced by 45.35%, and the peak deformation is still on the side of the gob.

| Analysis of the comparison results of different cable lengths
In the comparative analysis, the cable length has an important influence on roadway deformation control, as illustrated in Figure 14A.As the cable length increases, the roof deformation and cable axial force first decrease and then remain basically unchanged.The roof deformation and cable axial force of schemes A1-A3 are significantly reduced.The deformation control effect of schemes A1 and A2 is not good, and the roof deformation and cable axial force are large.The cables of schemes A1 and A2 are not anchored into the stable rock mass, which cannot fully achieve the support potential.The sidewall cannot effectively support the roof, resulting in large roof deformation and large cable stress.With increasing cable length (A3-A5), the deformation characteristics of the roadway basically remain unchanged, which indicates that the cable length is suitable in the A3 scheme.| 3609

| Analysis of comparative results of different cable preloading forces
In the comparative analysis, the effect of the cable preloading force on roadway deformation control varies, as illustrated in Figure 14B.As the cable preloading force increases, the roof deformation and cable axial force gradually decrease and then become basically stable.In schemes B1-B3, the roof convergence rate is significantly reduced, and the cable safety margin is greatly increased.In schemes B3 and B4, the convergence rate of the roof decreases, and the safety margin of the cable does not obviously increase.The preloading force causes the sidewall to be in three stress states.With increasing preloading force, the sidewall strength increases, the shear capacity increases, and the deformation of the roadway and roof decreases.The cable is mainly used to suspend the roof; when the deformation is reduced, the peak axial force of the cable is reduced.However, the strength of the sidewall will not increase endlessly, and the effect of increasing the preloading force is not obvious.

| Analysis of comparative results of spacing between different cables
In the comparative analysis, different row spacings of cables have different control effects on roadway deformation, as illustrated in Figure 14C.As the row spacing between cables decreases, the roof convergence first decreases and then stays basically unchanged, and the cable safety margin first increases and then stays basically unchanged.The roof convergence rate of schemes C1-C3 is significantly reduced, and the cable safety margin is improved.In scheme C3, the roadway roof deformation is 94 mm, and the cable safety margin is 45.5%.Compared with C3, the convergence rate of the roadway roof in scheme C4 is reduced by 0.29%, and the safety margin of the cable is only increased by 0.49%.Through the above analysis, it can be seen that if the row spacing is too small, the supporting surface area of the cable will coincide, and the deformation control will not increase significantly.If the spacing between rows is too high, the supporting force on some parts of the roadway will be small, that is, the supporting strength is insufficient, and the roadway will be deformed.In the comparative analysis, different cable angles at the shoulder corner have different control effects on roadway deformation, as illustrated in Figure 14D.As the elevation angle of the cable at the shoulder angle increases, the roof convergence first decreases and then increases, with the opposite for the cable safety margin.The elevation angle from 0°to 45°has an obvious effect on reducing roof deformation and cable peak axial force because the elevation angle of the cable at the shoulder corner improves the parameters of the interface of the roof and the sidewall and makes the roof and the sidewall form a unified whole, which effectively couples the bearing capacities of the roof and the sidewall.In addition, cable support can counteract the shear stress of the sidewall to a greater extent.From 45°to 60°, the roof deformation increases, and the cable safety margin decreases.When the cable elevation angle is too large, the cable can anchor only the LQZ of the shallow sidewall; then, the normal stress acting on the roadway is small, and the anchoring effect is not good.
The following conclusions are drawn: (1) Roof cutting reduces the stress and moves the lateral stress concentration away from the sidewall, which improves the stability of the roadway.The roof deformation is reduced by 45.35% compared with that of the original retained roadway.However, the deformation is still large and needs to be further controlled.(2) The joint action of roof cutting and CRCSS can effectively control soft rock roadway deformation.Compared with that of the GERRC with conventional support, the deformation of GERRC with CRCSS is reduced by 75.77%.(3) The CRCSS parameters are sensitive to the control of roof deformation and the improvement in the cable safety margin.The roadway deformation can be effectively controlled by changing the cable parameters within a certain range, while the influence on the control of roof deformation beyond this range is small.In this test, the best supporting parameters of CRCSS are shown in Table 3.

| Analysis of monitoring results
The consistency of the change rules of each monitoring point under the two working conditions is high.In this paper, only data from representative monitoring sites are selected for analysis.

| Surface displacement monitoring results
To avoid the influence of support mode transformation, the roof and floor monitoring data of D2, D3 and D5, D6 are selected for GERRC with conventional support and GERRC with CRCSS, respectively, and plotted in Figure 17A.The sidewall data of the J2, J3, J5, and J6 monitoring points are selected and drawn in Figure 17B.
The roadway vertical deformation of the two types of GER has experienced three stages: rapid growth, slow growth, and stability, which is caused by the collapse and gradual compaction of the panel roof.However, the slow growth stage of GERRC with CRCSS is short, which decreases by 36.59% compared with GERRC with conventional support.Meanwhile, the vertical deformation of GERRC with CRCSS is 56.80% less than that of GERRC with conventional support.The deformation characteristics of the two ways of GERRC are similar to the vertical deformation, and their causes are basically the same.However, the control effect of the deformation of the two sidewalls is more remarkable.Compared with GERRC with conventional support, the deformation of the two sidewalls of GERRC with CRCSS was reduced by approximately 75.76%, and the stability distance decreased by approximately 33.45%.In addition, it is found that the leading influence distance is reduced by 35%.The reasons are analyzed, and the bearing capacity of the sidewall is improved by the CRCSS, so the deformation time is late under the same pressure.

| Roof separation monitoring results
L3-L6 monitoring points are selected for GERRC with conventional support, and L9-L12 monitoring points are selected for GERRC with CRCSS.The separation amount  of GERRC with conventional support is large, and the development time of the separation is long, as illustrated in Figure 18.
The separation of the deep and shallow base points of the roof is approximately 306 and 142.25 mm, respectively, and the stability distance of the separation is approximately 157.75 and 140.25 m, respectively.By analyzing the reasons, the sidewall directly supports the direct roof, so the crushing of the sidewall has the greatest impact on the direct roof, leading to the separation of layers.GERRC with CRCSS effectively solves the problems of large roof separation amount and long development time of separation.Compared with GERRC with conventional support, the deep and shallow basis-point separation is reduced by 66.09% and 73.81%, respectively, and the development time of separation is reduced by 48.49% and 49.20%, respectively.

| Cable force monitoring results
M3-M6 monitoring points are selected for the GERRC with conventional support, and M9-M12 monitoring points are selected for the GERRC with CRCSS.The stress monitoring curves of the cables are shown in Figure 19.The peak axial force of the cable of GERRC with conventional support is relatively high, and the failure phenomenon occurs.In contrast, the cable of GERRC is healthy and has a sufficient safety margin.The main function of the cable is to suspend the direct roof strata.The large deformation of the direct roof strata of GERRC with conventional support leads to the high stress of the cable.In addition, the load stability time of the cable of GERRC with CRCSS is reduced by 30.72%.This is caused by the decrease in the stabilization time of the roof behind the longwall face.

| Hydraulic prop stress monitoring results
Monitoring points Y2 and Y3 are selected for the GERRC with conventional support, while Y5 and Y6 are selected for the GERRC with CRCSS.As illustrated in Figure 20, in the vertical roadway direction, the hydraulic prop force of the two ways of GERRC is as follows: gob side > roof middle > coal rib side, which is caused by the cantilever structure formed in the roadway roof by roof cutting.However, the gob side and roof middle hydraulic support of GERRC with conventional support leaks or becomes overpressured, and the coal rib side hydraulic support pressure is also high.The deformation of the roof is small in GERRC with CRCSS, so the hydraulic prop is reasonable.Along the roadway direction, the hydraulic prop of the two ways of GERRC has a large force within 25 m behind the longwall face, which is caused by the caving roof of the gob.With the caving gangue being compacted, the force slows and fluctuates and then becomes stable.The stability distance of the hydraulic prop of GERRC with CRCSS decreases by 32.78%.
In summary, GERRC with conventional support results in large surrounding rock deformation and a large supporting construction bearing capacity, which does not meet the requirements of GER.GERRC with CRCSS can effectively reduce the roadway deformation, shorten the stability time of the surrounding rock, maintain safe supporting construction conditions, and realize the GER of soft strata.

| CONCLUSIONS
On the basis of the 1706 panel of the Xinyi Coal Mine, this paper studies the technology of CRCSS in soft rock roadways.The influence of roof cutting and the mechanism of the CRCSS method are analyzed.The main findings are as follows: (1) The monitoring of the original GER technology shows that the support form of "strong roof and weak sidewall" releases the mining pressure from the roadway sidewall, increases the damage depth of the sidewall, and causes the anchor bolt anchorage section to be in the broken surrounding rock, which cannot fully achieve its support potential.The above scheme can only strengthen the roof support and cannot control the deformation of the roadway.The order of roadway deformation and failure is sidewall collapse, overall roof subsidence, and support structure failure.rock under GERRC with conventional support starts early, takes a long time, and increase greatly.The stability time of the roadway under GERRC with CRCSS decreases by 30%, and the roadway surface convergence and roof separation decrease by 55%.
The new support scheme also improves the safety margin of the cable and reduces the leakage of the hydraulic prop.(4) Theoretical analysis shows that roof cutting and CRCSS can keep the stress circle of the roadway far from the failure envelope during the mining process by improving the roadway stress, enhancing the roadway bearing capacity, and improving the surrounding rock bearing mechanism.The roadway is always in a safe state.The CRCSS method is applicable to GER in soft rock.
The results can provide guidance for follow-up test research, support scheme optimization, GER for similar projects, and so forth.

2. 2 . 2 |
The stress and deformation distributions of the retained roadway According to the geological conditions of the 1706 panel, a three-dimensional (3D) model is established with FLAC 3D , as shown in Figure 3.The elastic modulus and Poisson's ratio are converted into the bulk modulus and shear modulus recognized by FLAC 3D .The parameters of strata are shown in Table 1.A 3D model size is 130 m × 120 m × 71 m.The bottom of the 3D model is fixed vertically, and the front and back, left and right surfaces are fixed horizontally. 50,51A compensation F I G U R E 2 Monitoring results of surrounding rock deformation and cable axial force.Monitoring results of (A) cable force and (B) surrounding rock deformation.

F I G U R E 3
Numerical model.GERRC, gob-side entryretaining by roof cutting and pressure relief.T A B L E 1 Physical parameters of the trata.

σ σ − 2 1 3 .
D is the Mohr circle of the sidewall in limit equilibrium, and E is the Mohr circle after the sidewall support.With increasing support strength, the LQZ σ 3 of the sidewall gradually increases.According to Equation (1), the vertical compressive strength σ 1 increases more than σ 3 , which greatly increases the bearing capacity of the surrounding rock.Due to the increase in σ 3 , the abscissa of the Mohr circle increases, the radius decreases, and it gradually movesF I G U R E 4 Stress and deformation characteristics of the roadway behind the longwall face.F I G U R E 5 Sketch of coal rib shear failure.

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I G U R E 6 Sketch of the coal rib mechanical model and Mohr circles.
(a) CRCSS phase.The sidewall 40-50 m in front of the longwall face is strengthened to improve the bearing capacity, as shown in section A-A.(b) Pressure relief phase.Approximately 25 m in front of the longwall, directional blasting is used to reduce the stress in the roadway roof, as shown in section B-B.(c) Forming phase.At this stage, temporary support and gangue support are used to support the roof and caved gangue, as shown in section C-C.F I G U R E 9 Schematic diagram of GERRC with CRCSS.CRCSS, coal rib and corner strengthened support; GER, gob-side entryretaining; GERRC, gob-side entry-retaining by roof cutting and pressure relief.

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I G U R E 10 Simulation results of roof-cutting height.F I G U R E 11 Numerical simulation of roof-cutting angle parameters.F I G U R E 12 Crack propagation between blast holes under different spacings.

F I G U R E 13
The vertical stress and deformation of the roadway.The state of (A) roof uncutting and (B) roof cutting.F I G U R E 14 Simulation test results of different schemes.Simulation test results of different (A) cable lengths, (B) cable preloading forces, (C) row spacing between cables, and (D) cable angles at shoulder corners.different cable angles

T A B L E 3
Optimal parameters of coal rib and corner strengthened support.

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I G U R E 15 Supporting parameters of the roadway.(A) GERRC with conventional support and (B) GERRC with CRCSS.CRCSS, coal rib and corner strengthened support; GERRC, gob-side entry-retaining by roof cutting and pressure relief.

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I G U R E 16 Mine pressure monitoring points of the roadway.CRCSS, coal rib and corner strengthened support; GER, gob-side entryretaining; GERRC, gob-side entry-retaining by roof cutting and pressure relief.

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I G U R E 17 Monitoring curve of roadway deformation.Monitoring curve of (A) roof and floor deformation and (B) sidewalls deformation.F I G U R E 18 Monitoring curve of roof separation of roadway.Monitoring data of (A) L3-L6 and (B) L9-L12.

( 2 )
The results of the numerical comparison test show that roof cutting can significantly improve the stress environment of the roadway and that CRCSS can form a cooperative support system between the roof and roadway sidewall, which can effectively diminish the deformation of the roadway.The CRCSS parameters are sensitive to the deformation control of the roadway.The evaluation indexes of the roof convergence rate and cable safety margin are established, and the best parameters are studied and determined.(3) The field comparison test results show that the stress and deformation development of the surrounding F I G U R E 19 Monitoring curve of roadway cable force.CRCSS, coal rib and corner strengthened support; GERRC, gob-side entryretaining by roof cutting and pressure relief.F I G U R E 20 Stress of the hydraulic prop.Monitoring data of (A) Y2 and Y3 and (B) Y5 and Y6. )