Study on the new layout pattern about the gob‐side entry under dynamic pressure and its surrounding rock stability control

Though the recovery rate of coal resources can be effectively improved by adopting the gob‐side entry longwall face, which is also easy to cause tension between mining and excavation. Therefore, a new type of roadway layout, narrow coal pillar entry retained (NCPER), was invented in the study. The NCPER is excavated together with the tailgate of next longwall face before the mining of prior longwall face, which is retained and reused during the mining of the next longwall face. The layout is based on the advantages and successful experience of the double roadways excavation, the gob‐side entry retained, and the roadway driven along gob‐side and heading for adjacent advancing coal face; moreover, the NCPER significantly shortens the preparation time of the longwall face while improving the recovery rate. Taking the production geological conditions of Yushuling Mine as the engineering background, the influence of factors such as roof‐cutting pressure relief and coal pillar width on the stability of the narrow coal pillar from the perspective of improving the stress environment of the roadway and strengthening the weak structure support of the surrounding rock was studied, and reasonable suggestions for selecting parameters of roof cutting and coal pillar width were given. Furthermore, the bidirectional grouting cable and its joist were used to strengthen the narrow coal pillar and the roadway roof, respectively, and a good effect of roadway control has been obtained. The invention and application of NCPER provided a new approach for the longwall face roadway layout and effectively eased the strained relationship between mining and excavating, which has a strong popularization and application value.


| INTRODUCTION
China's energy resources are characterized by "more coal, less oil, and less gas."Coal, as the major energy source, has greatly contributed to promoting economic development and ensuring the livelihood of people. 1 According to statistics, coal still accounts for more than 55% of the energy consumption structure of China currently, and its principal status is unlikely to change in a long time. 2 Thus, ensuring sustainable development of the coal industry is crucial for maintaining national energy security until alternative new energy sources are found.To this end, the country has given strong support to the coal industry from the policy and financial aspects.
Improving the coal recovery ratio is one of the critical means to ensure the sustainable development of mines.As for the layout of the mining roadways, wide coal pillar roadway protection (WCPRP) is often replaced by nonpillar roadway protection (NPRP) to reduce the loss of coal pillar in China. 3NPRP includes entry driven along the gob-side (EDGS) and gob-side entry retained (GSER).Due to factors such as construction safety and cost constraints, the application frequency of EDGS is remarkably higher than that of GSER. 4,5However, EDGS can easily cause a strained relationship between mining and excavating, especially when using single-wing district or strip district layout. 6Thus, to ensure continuous production, some mines are forced to abandon EDGS, but to adopt WCPRP, to shorten longwall face preparation time by increasing the coal pillar width.
In the field application, improving the coal recovery rate is in contradiction to alleviating the tense relationship between mining and excavating.If double roadways excavation (DRE) and EDGS can be integrated by using their advantages and avoid their disadvantages, the problems of low coal recovery rate and tense relationship of mining and excavating can be effectively solved.Ma et al., 7 Yu et al., 8 and Zhang et al. 9 have successfully tested the roadway driven along gob-side and heading for adjacent advancing coal face (RAACF) in the field, and at the same time of mining at the previous longwall face, the roadway of the next longwall face is excavated by setting narrow coal pillar (5-8 m).RAACF application is innovative to the conventional gob-side entry (GSE) layout, which can reduce the longwall face preparation time by 2-3 months while improving the coal resources recovery rate. 10However, the disadvantage is that the excavated section of the advanced longwall face of RAACF needs to experience the combined disturbance from the front abutment pressure of the longwall face and the rear residual abutment pressure.Compared with the conventional EDGS, the roadway deformation and maintenance difficulty of RAACF are increased, resulting in extremely strict requirements for the applicable conditions of the mine. 8,11 the basis of the previous study, this study improved RAACF and put forward narrow coal pillar entry retained (NCPER) layout mode.EDGS and GSE are combined in this layout mode, the roadway of the next longwall face is excavated by setting narrow coal pillar before mining the previous longwall face.During the previous longwall face mining, the roadway is retained to continue serving the next longwall face.Successful RAACF application can provide a reference for the attempt at NCPER layout mode because the stress disturbance and deformation stages of them are similar.The difference is that the whole NCPER section will experience the mining influence of the adjacent longwall face, and the maintenance time and difficulty are even more than that of RAACF.Therefore, the key to the success of NCPER technology is to effectively control the stability of GSE surrounding rock during the adjacent longwall face mining.Taking Yushuling Coal Mine in Xinjiang as an example, the theoretical analysis methods, numerical simulation, and field industrial test are adopted in the study.On the premise of researching and developing a new layout mode of narrow coal pillar, the focus is on evaluating the influence of roof-cutting depth, cantilever beam length, and the reasonable determination of the size and control technology of narrow coal pillar provides a theoretical and application basis for the further promotion of NCPER technology.

| Common layout mode of longwall mining roadway
Unlike major coal-producing countries such as the United States and Australia, 95% of coal production in China is through underground coal mining, almost all of which is contributed by longwall mining, and the output from room and pillar extraction is negligible. 12,13There are two kinds of layout modes in longwall face: double roadways and single roadway layout, among which the double roadways layout mode usually adopts WCPRP mode, while the single roadway layout mode usually refers to the GSE, including GSER and EDGS. 4 The common roadway layout mode in China will be introduced in detail in Section 2.1.1-2.1.3.

| DRE layout mode
DRE layout mode has become the major longwall mining layout in China.In the 1990s, the country proposed the concept of sustainable development to improve the recovery rate of coal resources and the DRE layout mode was gradually replaced by the NPRP layout mode. 14Currently, in the central and eastern mining areas where resources are scarce or on the verge of exhaustion, DRE has been eliminated.However, in the large mines with superior mining conditions in Inner Mongolia, Northern Shaanxi, and Shanxi, due to the fast-advancing speed of longwall face and high production efficiency, the DRE layout mode is still used to avoid the strained relationship between mining and excavating.
DRE layout is also known as WCPRP (see Figure 1).When arranging panel 1, roadways 2 and 4 are excavated simultaneously, and the size of the coal pillar is generally set at 20-30 m.During panel 1 mining, roadways 2 and 4 are used as headgate and auxiliary headgate, respectively.After panel 1 mining, roadway 2 is abandoned and roadway 4 is retained as panel 2's tailgate. 15The DRE layout mode is relatively mature from the technical level.Since the wide coal pillar has a higher bearing capacity and can isolate the gob, it is even easier to be maintained than NPRP in some cases. 15,16The reason why this technology is replaced by NPRP is completely due to the low recovery rate, which is not conducive to the sustainable development of the mine and coal industry. 17,181.2| GSER layout mode NPRP includes GSER and EDGS.Among which, only GSER could be strictly called NPRP strictly, because there is no coal pillar between the adjacent working face.19 There are two kinds of commonly used GSER: building filling body entry retained (BFBER) and roof cutting and pressure relief entry retained (RCPRER), 20 as depicted in Figures 2 and 3, respectively.
As shown in Figure 2, BFBER is referred to the construction of the backfilling body along the gob behind the end support in the headgate (roadway 3) during panel 1 mining.Simultaneously, necessary reinforcement measures are taken for the roof and ribs of the roadway, and the roadway 3 is completely or partially retained as the panel 2's tailgate. 21The reinforced support predominantly includes bolt (anchor cable), grouting bolt (anchor cable), and individual hydraulic prop.The construction mode of the filling body is basically mechanized, and the construction technology remains roughly the same as before, the only difference is in the filling body materials.Currently, the commonly used filling materials include paste, high-water, and concrete materials. 22,23The coal pillar is eliminated by BFBER, and the coal recovery rate is significantly increased.However, considering the influence of longwall face mining, the requirement of filling body strength is high, which leads to the high cost of constructing a filling body. 24s shown in Figure 3, in RCPRER the roof first needs to be precracked to cut off key block B in advance by hydraulic fracturing, hydraulic slotting, blasting, or other methods outside the influence range of front abutment pressure at the longwall face 1, to reduce the cantilever beam length of the longwall face and the stress of the roadway surrounding rock in the roadway. 25,26Simultaneously, roadway 2 is retained for use as a panel 2 tailgate by the means of a hanging net, building gangue retaining device, and strengthening support (as depicted in Figure 3B 6, 4, and 3). 27The gangue retaining device is generally made of U-shaped steel, and the steel body is fixed in the position of roof and floor.The purpose of hanging net is to reduce the impact of gangue on U-shaped steel and prevent the steel frame movement towards the roadway.Strengthening support generally adopts anchor cable, grouting anchor cable, or individual hydraulic prop to prevent roof fall during dynamic pressure disturbance. 28he strengthening support technology of RCPRER is roughly the same as that of BFBER, but RCPRER has eliminated the filling body, which has its advantage and disadvantage.The advantage is that filling materials are saved, especially for medium and thick coal seam GSER; the cancellation of filling body means that a lot of capital investment can be saved. 29I G U R E 1 (A) DRE layout mode.(B) A-A section map.DRE, double roadways excavation.
The disadvantage is that after the filling body is canceled, there is a risk of toxic and harmful gases (such as CH 4 and CO) and gob water overflowing in the gob under the condition of negative pressure ventilation, which is not conducive to the safety prevention and the control of longwall face and the roadway.At the early stage of the RCPRER application, some mines had made an attempt to use a wind curtain to temporarily close the gob; however, the ideal effect could not be achieved.Later, guniting, grouting, and other methods are used to plug the leakage, but the effect is still not significant. 30,31herefore, the use of RCPRER should be combined with the geological conditions of the mine, and can be used after passing a safety risk assessment.

| EDGS layout mode
As the most commonly used NPRP mode in China, EDGS has been popularized and applied in several mining areas. 32As shown in Figure 4, the conventional EDGS is to excavate the panel 2 tailgate (roadway 2) by setting a narrow coal pillar (generally 3-8 m) along the edge of the gob after panel 1 mining is completed and the overburden of gob is stable (generally 3-6 months). 33,346][37] First, the stable articulated structure (voussoir beam) formed by the key block of overburden can actively bear the overhead load, which has a protective effect on the GSE.Second, narrow coal pillar roadway protection is used to arrange the roadway in the plastic zone of coal rib (pressure relief range), where the overall stress level of the surrounding rock in the roadway is lower, and the major attention should be paid to reducing the surrounding rock deformation of roadway.Lastly, the narrow coal pillar can effectively isolate the gob, which has great benefits for gas prevention, mine water, coal spontaneous combustion, and other associated disasters.
However, there are also drawbacks in EDGS, such as the need to wait for the overburden in the gob to stabilize (3-6 months) before excavation.For several mines, in particular, the layout of mines in the single-wing district (strip) can easily lead to the tense relationship between mining and excavating, and even worse, lead to months of inability to produce. 38Thus, some mines even drive roadway along the gob without waiting for the stability of overburden in the gob, or directly abandon EDGS and adopt the DRE mode (see Figure 1) to shorten the preparation time of the longwall face and to ensure the normal operation of the mine. 39o solve the problem of the tense relationship of mining and excavating, some scholars invented the RAACF through field tests. 38,40As depicted in Figure 5, while using RAACF, it is not necessary to wait for panel 1 mining to be completed, and according to the design of coal pillar size, panel 2 tailgate is directly excavated (roadway 4).Compared with the conventional EDGS (see Figure 4), using RAACF can generally save 2-3 months of the longwall face preparation time.However, the disadvantage is that some roadways have to experience the influence of longwall face mining and under the combination of the front abutment pressure and gob-side abutment pressure, large deformation in the roadway is easy to occur.Field tests conducted by Ma et al., 7 Yu et al., 8 and Wang et al. 11 found that RAACF tended to occur the problems, such as asymmetric roof subsidence, rib spalling, floor heave, and so forth.If no strengthening support is adopted, the roadway will be difficult to maintain.Therefore, the requirements for the applicable conditions of the mine are more stringent for RAACF.Moreover, there is a problem in adopting RAACF, wherein the construction direction in which the roadway excavated and longwall face is opposite, which is not conducive to the prevention and control of rock bursts.It is clearly stipulated in Coal Mine Safety Regulations that when excavating face and longwall face are opposite and the distance between them is less than 350 m, either the longwall face or the excavated face must be stopped (generally excavating face), which means that using RAACF cannot completely resolve the strained relationship between mining and excavating even though at the cost of large deformation in the roadway.

| New GSE layout mode: NCPER
The usage of DRE layout mode (wide coal pillar) causes a low recovery rate, and using EDGS is easy to cause a tense relationship between mining and excavating, which is contradictory between them.In the field, the problems of recovery rate and tense relationship between mining and excavating can be effectively solved if DRE and EDGS layout could be combined.Thus, we invented a new GSE layout mode: NCPER.As depicted in Figure 6, the characteristic of this layout is as follows: when arranging panel 1, roadways 2 and 4 are excavated simultaneously, and roadway 4 is retained as the panel 2 tailgate after panel 1 mining.The NCPER layout mode is similar to the DRE layout mode, the difference is that roadway 4 does not serve panel 1 after excavating but only to solve the problem of the tense relationship of mining and excavating.
The research and development of NCPER is predominantly draws on the successful experience of EDGS and RAACF.From the perspective of stress environment and deformation stage of roadway, NCPER is similar to GSER and RAACF, and all have to experience the influence of panel 1 mining.However, the field application of GSER and RAACF demonstrates that large deformation easily occurs at the roadway after undergoing the mining influence of longwall face, both have strict applied conditions for mine.Thus, the next major work will focus on the control of NCPER under the influence of longwall face mining, provide theoretical support for the promotion and application of NCPER technology, and solve the problems of low recovery rate or the tense relationship between mining and excavating.

| STABILITY ANALYSIS OF NCPER
On the basis of GSER, RAACF, and other layout modes, the NCPER layout mode is more developed.To control NCPER stability, it is essential to reveal the deformation and failure characteristics of the entry based on the real stress evolution path of the entry, and then to determine the width of coal pillar and support technology.Thus, based on the analysis of the overburden structure and stress environment of NCPER, the influence of the cantilever beam length of key block B on NCPER stability is analyzed in this section, designs the width of chain pillar based on roof cutting and pressure relief, and identifies the weak link of roadway deformation and strengthens the support to form a technical system for controlling the stability of the surrounding rocks of NCPER.

| Characteristics of overburden structure of NCPER
According to the "key stratum theory," 41 after the longwall face mining, caving occurs at the immediate roof, and when the cantilever beam length of the key stratum exceeds its limit span, "O-X" fracture occurs along the advancing direction of longwall face (see Figure 7A), 42,43 and a triangular block structure (key block B) is formed at the end of the longwall face.Key block B is hinged with adjacent rock beams A and C to form a voussoir beam structure.This structure has a great influence on GSE stability.Thus, when setting up the mechanical and numerical analysis model, we need to combine with the site condition, and determine the key block size parameters, namely, block B fracture position in the coal rib x 1 and cantilever beam length x 2 , among them, x 1 can be computed by the following formula 44 : The cantilever beam length of key block B can be computed by the following formula 44 : Other variables can be computed by the following formula 44 :

∆
In the above formulas, R T is the tensile strength of the main roof, MPa; b is the sectional width of the main roof, taking the unit length 1; h is the main roof thickness, m; γ is the bulk density of the main roof, MN/m 3 ; q b is load on the main roof, MPa; r b is main roof overhanging section load (including dead weight), MPa; k is the Winkler foundation coefficient; E′ is the elastic modulus of main roof, MPa; I is the inertia moment, m 4 ; m is the mining height, m; h 1 is the immediate thickness, m; and K p is the initial bulking coefficient.
The influences of different rock strata coefficients, cantilever beam length of the main roof, and other factors that impact the lateral fracture position of the main roof are analyzed in the literature, 45,46 and the fracture position of key block B is generally located in the coal rib within the range of 3-14 m.By evaluating the document database, the coal pillar setting the width in GSE was done statistically, and it demonstrated that the ratio of 3-5 and 5-8 m accounted for 74% and 26%, which means that most GSE is arranged within the fracture line of the main roof, as depicted in Figure 7B, and the stability of the entry will be affected by the structural characteristics and movement of overburden.

| Characteristics of NCPER stress distribution
After NCPER is affected by the advance mining of panel 2, according to the distribution characteristics of the front abutment pressure of the longwall face (Figure 8 σ D ), the stress stages of the GSE can be divided into three stress adjustment stages in the NCPER: roadway excavated stage, panel 1 advance influence stage, and panel 1 rear influence stage. 47The roadway excavated stage (see A stage in Figure 8) is similar to solid coal excavation, which is unaffected by longwall face mining.The roadway deformation is small, the coal pillar damage degree is low, and an elastic zone may exist.The stress distribution in the coal rib and coal pillar are as depicted in Figure 8 σ A and Figure 8 σ′ A .After heading into the advance affection stage of panel 1 (see B stage in Figure 8), the coal pillar is damaged and located in the plastic deformation stage under the influence of front abutment pressure, at this time, the coal pillar could be referred to as yield pillar.The decrease of bearing capacity of coal pillar (see Figure 8 σ′ B ) leads to the transfer of the front abutment pressure to one side of the solid coal, and the increase of the crushing range of solid coal leads to the transfer of the stress peak to the depth, and stress concentration degree increases (see Figure 8 σ B ).After the roadway in the rear of longwall face (see stage C in Figure 8), under the effect of the given deformation pressure of the overburden fracture block, strong rock pressure appears in NCPER.At this time, due to the coal pillar into yield state, most mining-induced stress is transferred to the solid coal, a small amount of stress is borne by compaction gangue in gob.With the increase of coal pillar compaction, its bearing capacity decrease further (see Figure 8 σ′ C ), and the stress on the solid coal increases significantly (see Figure 8 σ C ).It is found in the test that the amount of the roadway deformation in the rear of the longwall face is very large, accounting for more than 80% of the total roadway deformation, accompanied by evident roof subsidence, rib spalling, and severe floor heave.It is the key stage to determine NCPER success.
It should be indicated that after longwall face 1 mining, the remained roadway 4 will also be affected by the front abutment pressure of the working face 2. However, according to the existing successful experience of GSER, if the roadway roof stability can be maintained after experiencing the mining influence of panel 1, the roadway reuse can be realized.Therefore, the focus of roadway control should be on the mining influence stage of panel 1 for NCPER.According to the borehole columnar section near tailgate #110505 (borehole #9-2 in Figure 9), the roadway roof is revealed to be predominantly sandstone, specifically including siltstone, fine sandstone, medium sandstone, and gritstone, and the firmness coefficient f is 2.84-6.10.The detailed strata structure and parameters of coal seam and roof are depicted in Table 1.

| Engineering background
The key stratum determination method given in the literature 41 is used to determine the key stratum, and the judgment process is omitted due to space limitation.The determination result is that the key stratum which is located 20.85 m away from the coal seam is fine sandstone with a thickness of 9.95 m.Since the 3DEC discrete element analysis software needs to presuppose the rock size, the size parameters of key block B need to be computed before establishing the numerical model.Using the data given in Table 1, plug in formulas (1) and (2), the fracture position of block B x 1 and the cantilever beam length x 2 , respectively, are 9.96 and 10.06 m, and the total length of block B is 20.02 m.

| The establishment of a numerical model
On the basis of the computation in Section 3.1, the thickness of key block B is 9.95 m and the length is   The physical and mechanical parameters of coal seam and rock strata in the model are determined through an iterative method.First, the physical and mechanical parameters of coal and rock samples are tested by using a rock mechanics testing machine in the laboratory.Then, the RocLab software is used to transform the laboratory coal and rock sample parameters into rock mass parameters, and the model is initially assigned parameters.Taking the excavation of the 110503 headgate as an example, the physical and mechanical parameters of coal and rock mass of the numerical model are inverted through roadway deformation verification.The verification results are depicted in Figure 11, and the inversion parameters are depicted in Table 1.Notably, the joint parameters of coal and rock strata in the numerical model are determined based on lithology equal proportion, with a value of 0.5 times the mechanical parameters of coal rock mass.

| Numerical simulation scheme
It is found that roof cutting and pressure relief adopted can effectively improve the GSE stress environment and reduce the roadway deformation. 48,49Thus, these measures were used for several GSEs in China.Using the established numerical model, the regulation effect of the roof-cutting and pressure relief technology on the GSE stress environment is studied, and the roof-cutting depth and angle are determined.On this basis, the influence of coal pillar width on GSE stability after roof cutting and pressure relief is studied, the reasonable coal pillar width is selected, the difficult area of GSE is determined, and the cooperative control technology of NCPER with "roof cutting and pressure relief, coal pillar optimization, and active support" as the core is given.The flow of the numerical simulation scheme is depicted in Figure 12.

| Analysis of numerical simulation result
Roof-cutting depth Figure 13 a 1 -a 6 depicts the vertical stress distribution curves inside the coal rib with the roof-cutting angle of 10°and the roof-cutting depth of 0, 10, 20, 30, 40, and 50 m, respectively.As seen from the figures, the peak value of stress inside the coal rib is approximately 19.09 MPa, which is approximately 19 m away from the coal rib surface; with the increase of the roof-cutting depth, the concentration degree of vertical stress inside the coal rib decreases gradually.When the roof-cutting depth is 10 m, the effect on improving coal rib stress is not evident; when the roof-cutting depth increases from 10 to 30 m, the concentration degree of vertical stress is effectively alleviated, the peak value decreases from 18.72 to 16.66 MPa, and the peak position moves approximately 1 m towards the coal rib, indicating that the crushing range of the coal rib decreases.However, once the cutting depth exceeds 40 m, the vertical stress inside the coal rib decreases sharply, the peak stress decreases rapidly from 16.66 to 8.94 MPa, predominantly because the key stratum is cut off by roof-cutting borehole, which relieves the given deformation force generated by the key block B on the coal rock mass below.With the further increase of roof-cutting depth, the degree of stress concentration will also be alleviated, but the improvement effect will gradually wane.Considering the engineering amount and construction costs of the roof-cutting borehole, it is generally recommended to cut off the key stratum in the field.

Roof-cutting angle
The roof-cutting angle has influence on the cantilever beam length of key block B (see Figure 6) and affects the force on the coal rib.Thus, the roof-cutting angle and the cantilever beam length of block B are evaluated together.Figure 14 a  | 1399 roof-cutting angle, the cantilever beam length of the key block B also decreases, the stress concentration inside the coal rib is effectively alleviated, and the crushing range of the coal rib is gradually reduced.When the cantilever beam length of key block B is 5 m (roof-cutting angle is 8°), the stress distribution curve of the coal rib is approximately the same as that of the cantilever beam length of key block B at 0 m (the roof-cutting angle is 0°).This indicates that the stress of the coal rib gradually becomes stable after the cantilever beam length of key block B is reduced to a certain extent.On the basis of the above analysis, it can be obtained that the reasonable roof-cutting angle of Panel #110503 is 5-27°.Notably, the roof-cutting angle determined on the project site is generally 0-15°, of which 10°is commonly used, mainly because it can not only achieve a good pressure relief effect, but also facilitate the work of the roof cutting when choosing 10°.

Coal pillar width
In Sections "Roof-cutting depth" and "Roof-cutting angle", reasonable selection of roof-cutting depth and angle can effectively improve the GSE stress environment.However, the change in stress environment will inevitably affect the determination of the chain pillar width.Figure 15 depicts the stress distribution and deformation of GSE with the chain pillar width of 3-8 m under the condition that the roof-cutting depth is 40 m and the roof-cutting angle is 10°.The left side in Figure 15 gives a comparison diagram without roof cutting.As you can see in the figure, the stress concentration in the narrow coal pillar and solid coal wall of the GSE is relatively large without cutting the roof, and the peak stress of 3-8 m coal pillar is 16.66, 17.77, 18.67, 19.11, 18.41, and 19.63 MPa, respectively, and the stress concentration coefficient exceeds 3. The high stress concentration in the coal pillar and rib leads to a huge displacement of the surrounding rock, especially the two ribs of the roadway.Considering the rib in the main deformation area as an example, the rib displacement with diverse coal pillar widths towards the roadway is more than 1500 mm, and the residual section of the roadway is only 21.47%-24.93%.
However, after roof cutting is adopted in the roadway, the stress concentration degree of the GSE is alleviated, the stress concentration degree of the coal pillar gradually increases with the increase in the coal pillar width, and the stress transferred to the coal rib gradually decreases with the increase of the coal pillar bear capacity.When the coal pillar width increases from 3 to 8 m, the peak value of vertical stress in the coal pillar increases from 4.66 to 8.27 MPa, and the peak value of stress in the coal rib decreases from 15.13 to 14.08 MPa.The effective improvement of the stress environment of mining significantly reduces the large deformation of the surrounding rock of GSE, the deformation of the coal pillar and solid coal side decreases to 400-500 mm, and the residual section of the roadway increases to 72.11%-74.88%,which is 3.00-3.36times of that without roof cutting.
To sum up, for NCPER, reasonable roofcutting parameters can improve the GSE stress environment.With the roof cutting and pressure relief technology, the chain pillar width can be reduced effectively, the recovery rate of coal resources can be increased, deformation of the surrounding rock of roadway can be controlled, and the maintenance cost of roadway can be reduced.However, it should be noted that when determining the chain pillar width on the project site, in addition to considering the roadway deformation, the chain pillar should also be optimized according to the requirement of mine disaster prevention and control.For example, for the mine without gas, groundwater, coal spontaneous combustion, and other disasters, the coal pillar width only needs to be determined from the deformation control perspective.Once the mine has the above disaster risk, the coal pillar width needs to be increased to avoid the occurrence of associated disasters caused by the fissure connected on both sides of the coal pillar.As depicted in Figure 15, for tailgate #110505 of Yushuling Mine, if the roof-cutting technology is adopted, the reasonable width of coal pillar for chain pillar can be controlled within 3-5 m.NCPER technology is adopted in tailgate #110505, and the stability control of roadway should be considered from three aspects: roof cutting and pressure relief, coal pillar optimization, and active support.Among them, roof cutting and pressure relief should be conducted in headgate #110503, tailgate #110505 should be excavated in advance, and the roof cutting and pressure relief should be conducted after tailgate #110505 excavation.Therefore, when determining the width of coal pillar and support technology in tailgate #110505, the parameters of roof cutting and pressure relief should be determined on priority, because they affect the design of coal pillar width and support parameters.After the roof cutting and pressure relief parameters are determined, the optimal width of coal pillar and the supporting technology of NCPER are determined.Then, according to the order of roof cutting and pressure relief, optimizing coal pillar width and roadway support, the cooperative control technology of NCPER is expounded.

| Control technology of NCPER (1) Roof cutting and pressure relief
On the basis of the data in Sections "Roof-cutting depth" and "Roof-cutting angle", in considering cutting off key block B and reducing the cantilever beam length, the optimal roof-cutting depth and angle are 40 m and 10°, respectively.However, according to Table 1, the actual distance from the key stratum of Panel #110503 to the roof is only 26.85 m, Considering the construction difficulty of boreholes, the optimal roof-cutting depth is determined to be 35 m.The roof-cutting depth of Yushuling Mine in the early stage is 25 m, and after improving the charge structure, the deep hole precracking blasting operation with a depth of 35 m can be realized.For this purpose, the deep hole preblasting method is used on site to cut the roof and relieve the pressure.The operation site is located in the headgate #110503, 30-60 m in front of the longwall face.The borehole depth is 35 m, the tilt of 10°towards the gob, the borehole spacing is 0.5 m, the detonation approach is blasting every other borehole.Roof-cutting operation and charge structure are depicted in Figure 16.
(2) Determination of the optimal coal pillar width The determination of roof-cutting parameters affects the design of coal pillar width.Under the premise that the roof-cutting depth is 35 m and the angle is 10°, the reasonable coal pillar size is 3-5 m, and a 4 m coal pillar is used in field application.Notably, according to the study in Section "Coal pillar width", when the chain pillar is 4 m, the deformation of the coal pillar is much greater than that of the solid coal rib under the given deformation pressure of the roof.Therefore, when a 4 m coal pillar is adopted, necessary reinforcement for the coal pillar should be focused and carried out, and asymmetric subsidence of the roof should be controlled.
(3) Support technology of NCPER After determining the roof-cutting parameters and the coal pillar width, this section will introduce the roadway support technology.The study demonstrates that the control focus should be on the coal pillar rib and the roof for NCPER.Thus, based on the conventional support of the roadway, the roof is reinforced by grouting anchor cable joist (see Figures 16A and 17A), and the coal pillar is reinforced by bidirectional grouting anchor cable (Figures 16A and 17B).As depicted in Figure 18, the conventional support including bolt (anchor cable) support is adopted in the roadway.The bolt specification of rib is Φ16 × L1800 mm, the row and column spacing is 750 × 800 mm; the bolt specification of roof is Φ20 × L2200 mm, and the row and column spacing is 850 × 850 mm.The roof adopts anchor cable reinforcement support, whose specifications are Φ22 × L8300 mm, and the row and column spacing is 2000 × 2500 mm.A metal net is laid between the bolt (anchor cable) tray and roof and rib.The metal net is welded with 6 mm rebar, and the mesh size is 100 × 100 mm.
To control the stability of the coal pillar, the author's team invented the bidirectional grouting anchor cable, 50 the reinforce of coal pillar is depicted in Figure 19, the specification of bidirectional grouting anchor cable is Φ22 × L5000 mm, and the row and column spacing is 1200 × 2400 mm.Simultaneously, a row of anchor cable joist is constructed along the side of coal pillar on the roadway roof, the anchor cable specification is Φ22 × L8300 mm, the spacing is 2000 mm, the joist is 11# I-beam and the length is 2500 mm, and two anchor cables are installed in every anchor cable joist.

| The effect analysis of field application
According to the control ideas and NCPER technologies that had been proposed in Section 4.1, the field industrial test is conducted in tailgate #110505, and the rock pressure in the roadway is monitored.After the roof cutting, the effect of roof cutting is observed using the borehole imaging method, and the result is depicted in Figure 19.After precrack blasting of the roadway, the observation borehole and the precracking borehole are peeped.The results demonstrate that evident crack expansion can be seen in the observation borehole, and the roof-cutting effect is good.
Figure 20 depicts the curves of roadway deformation and the position of the longwall face.As can be seen from the figure, when the NCPER is 60 m away from the front of the longwall face, the deformation begins to increase, but the deformation is far less than that behind the longwall face.The main deformation and failure area is 0-100 m behind the longwall face.When the roadway lags behind the longwall face by 100 m, the deformation of surrounding rock gradually becomes stable.The roadway deformation in the three periods of roadway excavating, front of the longwall face, and behind the longwall face are depicted in Figure 21, and the three stages contribute 12.72%, 21.75%, and 65.53% to the total roadway deformation, respectively.After the roadway becomes stable, the residual section of the roadway is approximately 92.67% of the original section.
Figure 22 depicts the monitoring figure of the roof separation in tailgate #110505.It can be seen from the figure that asymmetric roof subsidence is effectively controlled after adopting reasonable roofcutting parameters, coal pillar width, and support technology.During the observation period, no evident separation is observed in tailgate #110505, and the roof control effect is good.The 110505 tailgate realizes the reuse and saves the longwall face preparation time of 3-4 months and obtains remarkable economic and social benefits.

| DISCUSSION
A new layout of the GSE in the longwall face was discussed in the study, which is mainly used to improve the recovery ratio of coal mine and ease the tension of mining and excavated.In this method, the tailgate of the next longwall face is excavated by the method of narrow coal pillars retained before the previous longwall face mined, and left to serve the next longwall face.
Narrow coal pillars entry retained will be influenced by the mining of two longwall faces, and the deformation stage is roughly the same as that of GSER.If the maintenance is not effective, it is difficult to ensure the reuse of narrow coal pillars entry retained.The researches show that reasonable roof cutting and pressure relief have a positive effect on coal pillar design and surrounding rock stability control, and a good cutting effect can appropriately decrease the width of coal pillar.Therefore, on the basis of optimizing the pressure relief parameters and ensuring the relief effect, the numerical simulation was adopted to optimize the coal pillar width of the test roadway.
The successful application of NCPER shows that improving surrounding rock stress is an effective means to maintain roadway stability.Therefore, the pressure relief effect of roadway should be evaluated actively when using narrow coal pillar to stay roadway.A good degree of pressure relief can significantly reduce the width of coal pillar.When the pressure relief effect is not effective or the measures are lacking, the roadway stability should be maintained by increasing the width and carrying capacity of coal pillar.

| CONCLUSIONS
Adopting GSE can significantly increase the coal recovery rate, but it is also easy to strain the relationship between mining and excavating.To overcome this technical problem, the authors' team invented a new type of roadway layout -NCPER, based on the successful experience of GSER and RAACF.Before the mining of the previous longwall face, the tailgate of the next longwall face is excavated by leaving narrow coal pillar, and retained to continue to serve the next longwall face.On the basis of the engineering background of Yushuling Coal Mine tailgate #110505, the effect of roof cutting and pressure relief on coal and rock stress, coal pillar design, and roadway deformation control from the perspective of improving roadway stress environment is studied in the research.It is pointed out that the reasonable roof-cutting depth should be to cut off the key block, and the roof-cutting angle should not be too large (generally 5°-15°), otherwise it is difficult to reduce the cantilever beam length of the key block.Roof-cutting effect affects the design of coal pillar, based on the optimal roof-cutting parameters, the recommended value  of 4 m is chosen for the reasonable width of coal pillar in the test roadway.Finally, the coal pillar and the roof are taken as the key control areas of NCPER, meanwhile, the bidirectional grouting anchor cable and grouting anchor cable beam were used to strengthen, respectively.Field application shows that after the test roadway is reused, the deformation of roof, ribs, and floor was less than 200 mm, and the residual section of the roadway was about 92.67% of the original section, and a good roadway control effect was obtained.

F
I G U R E 2 (A) Building filling body entry retained layout mode.(B) A-A section map.F I G U R E 3 (A) Roof cutting and pressure relief entry retained layout mode.(B) A-A section map.

F
I G U R E 5 (A) Roadway driven along gob-side and heading for adjacent advancing coal face layout mode.(B) A-A section map.F I G U R E 4 (A) Entry driven along the gob-side layout mode.(B) A-A section map.

F
I G U R E 6 (A) Narrow coal pillar entry retained layout mode.(B) A-A section map.

F
I G U R E 7 (A) Key stratum "O-X" fracture of the longwall face roof.(B) A-A section map.
On the basis of the background of tailgate in Panel #110505 of Yushuling Mine in Xinjiang, the stability control technology of the surrounding rock in NCPER is studied.Panel #110505 is mining 5# coal seam with an average coal thickness of 9.5 m, as depicted in Figure9.Panel #110505 adopted a single-wing arrangement.Its west is adjacent to the boundary of the well field.Its east is the coal transporting rise of the district.The north is adjacent Panel #110503, and the south is the unmined district.The roadway is excavated along the coal seam floor, with a length of approximately 1250 m and a buried depth of approximately 200-220 m.The section of the roadway is rectangular, and the size is 4.8 m × 3.4 m (width × height).Presently, the commissioning work of Panel #110503 has been completed and is ready for mining.Considering the problem of the strained relationship between mining and excavating of Panel #110505, tailgate #110505 is planned to be excavated in advance.Panel #110503 has arranged the barrier coal pillar of 15 m length.To improve the coal recovery rate, Panel #110505 is scheduled to adopt the NCPER layout mode.Tailgate #110505 was excavated and retained after experiencing the mining influence of Panel #110503 and continued to be used as Panel #110505's tailgate.

F
I G U R E 8 (A) Stress distribution of narrow coal pillar entry retained.(B) I-I section map.F I G U R E 9 Mining and excavating engineering plan of Yushuling Mine.T A B L E 1 Physical and mechanical parameters of coal and rock mass in the numerical model.m −3 ) h (m) K (GPa) G (GPa) c (MPa) φ (°) t (MPa)

20 .
02 m.On the basis of these parameters, the numerical computation model as depicted in Figure10is established, and headgate #110503, 110505 tailgate, barrier coal pillar, roof-cutting borehole, and part of 110503 and Panel #110505 are included.The size of the model is length × width × height (X × Y × Z) = 150 m × 1 m × 92 m, and it is divided into 9,768,000 units.Considering that coal seam and roadway are the major research objects of the model, grid encryption is conducted on them to ensure the continuity of stress transmission.The displacement of the horizontal and bottom boundary of the model is fixed, and the vertical stress of 3.70 MPa is applied to the top boundary to simulate the buried depth of 148 m.According to the ground stress test results, the horizontal stress is assigned, and the lateral pressure coefficient is 2. In the model, the strain softening constitutive model is adopted for coal seam and the Mohr-Coulomb constitutive model for rock strata.

F
I G U R E 10 Numerical calculation model.F I G U R E 11 Deformation verification curve of 110503 headgate.
1 -a 6 shows the vertical stress distribution curves of the coal rib when diverse cantilever beam lengths of key block B are 0, 5, 10, 15, 20, and 25 m (corresponding to roof-cutting angles of 0°, 8°, 18°, 27°, 34°, and 41°) at the roof-cutting depth of 40 m.It can be seen from the figures that the vertical stress concentration degree of the coal rib gradually decreases with the decrease in cantilever beam length (roof-cutting angle).When the cantilever beam length is 20 and 25 m, the vertical stress zoning curve of the coal rib is approximately the same.Currently, the roof-cutting line is located in the key block C, and it is difficult to reduce the coal rib stress because the cantilever beam length of block B is unchanged.With the further reduction of the F I G U R E 12 Numerical simulation scheme.NCPER, narrow coal pillar entry retained.WANG ET AL.

F
I G U R E 13 Effect of roof-cutting depth on coal wall stress.

F
I G U R E 14 Effect of roof-cutting angle on coal wall stress.WANG ET AL. | 1401 F I G U R E 15 Vertical stress and horizontal displacement curves of different coal pillar widths.

F
I G U R E 16 Diagrammatic sketch of roof-cutting operation and charge structure.(A) Construction location, (B) charge structure, and (C) construction diagrammatic sketch.F I G U R E 17 Reinforcement support of roof and coal pillar.(A) Reinforced support of the roof and (B) reinforced support of the yield pillar.F I G U R E 18 The 110505 tailgate supporting section.F I G U R E 19 Precrack blasting effect.

F
I G U R E 20 Rock pressure in mine data.WANG ET AL.| 1407

F
I G U R E 21 Roadway deformation at diverse stages.

F
I G U R E 22 Roof separation effect.