Research on the Deformation and Failure Characteristics and Control Technology of Mining Area Rises under the Influence of Mining Stress

Abstract

Affected by mining stress, roadways surrounding rock face problems such as serious deformation and failure and difficult support. In this study, with the II2 mining area rise in Taoyuan Coal Mine taken as the engineering background, the evolution laws of stress, deformation and plastic zone area of the mining area rises during the advance process of the working face were explored with the aid of FLAC^3D software. The results suggested that the stress, deformation and plastic zone area of the surrounding rock increase significantly when the distance between the working face and the track rise is less than 20 m. Based on this finding, it was further determined that the stopping line of the II8₂22 working face should be at least 20 m away from the track rise. Furthermore, in accordance with the deformation and failure characteristics of surrounding rock under the influence of mining stress, this paper conducted a simulation on four support schemes of mining area rises, and quantitatively analyzed the mechanical response of a roadway surrounding rock under these support schemes. The simulation results revealed that the support scheme of “bolt-mesh-spray-cable + grouting bolt” can effectively deal with the influence of mining stress on the working face. Meanwhile, an engineering application was carried out. By monitoring the surface displacement of the surrounding rock, it was found that the deformation of the roadway surrounding rock was effectively controlled, and a remarkable support effect was achieved. In short, the proposed support scheme greatly improved the stability and safety of surrounding rock in the mining area rise under the influence of mining stress.

1. Introduction

China has experienced multiple coal formation periods which have led to the extremely complex conditions of coal occurrence. During coal mine production, some roadways are arranged below the coal seam. Consequently, coal seam recovery inevitably disturbs floor roadways [1]. In the midst of mining, affected by the mining stress from the working face, roadways surrounding rock are characterized by surging stress [2], enhanced deformation [3] and severe failure. Meanwhile, as the working face advances, the stress conditions of floor roadways change constantly [4,5,6], and the deformation and failure of roadways surrounding rock demonstrate new characteristics. Such changes result in the difficulty in roadway support under the influence of mining stress [7], a substantial increase in support cost and the need for continuous maintenance and repair. Roadway stability control has become an engineering problem restricting the safe and efficient production of coal mines [8,9]. Therefore, it is crucial to study the deformation and failure characteristics and control technology of roadways under the influence of mining stress.

A clear understanding of the deformation and failure mechanism of floor roadways surrounding rock is the prerequisite for efficiently supporting the roadway and reducing its deformation and failure [10,11]. In recent years, a large number of scholars have conducted research on the deformation and failure characteristics of roadways from the aspects of theoretical analysis, numerical simulation and field engineering [12,13,14]. With the aid of FLAC^3D software, Guo et al. [15] simulated the dynamic evolution process of the plastic zone of floor roadways during working face advance, revealed the deformation and failure mechanism of roadways and put forward a targeted support scheme. By using the Kirsch equation, Xu et al. [16] studied the propagation and distribution rule of mining stress and analyzed the reasons for the changes in stress, displacement and plastic zone around roadways during coal seam mining. In tackling the problems of long deformation duration and serious mining-induced failure of deep roadways, Huang et al. [17] defined the concept of mining coefficients and disclosed the temporal and spatial evolution law of stress field and the large deformation and failure mechanism of deep roadways on the basis of the structural motion, degradation and rheological property of the surrounding rock. Gao et al. [18] explored the extrusion failure mechanism of roadways under the influence of mining stress by the UDEC Trigon method, and the simulation results reflected the process of roadway extrusion, roof collapse and bolt failure induced by mining stress. In addition, some scholars have investigated the mechanical response of roadways under mining disturbance from the perspectives of spatial position relationship between the working face and roadways [19,20], different stress levels [21,22], multi-seam mining [23,24] and mining sequence [25]. Corresponding support schemes have been proposed on the grounds of their research results, and remarkable effects have been achieved through engineering practice [26,27]. The research by these scholars indicates that the roadways surrounding rock present complex deformation and failure characteristics under the influence of mining stress, and the spatial position relationship between the end of the overlying coal seam working face and the floor roadways has a decisive impact on the stability of the roadways. Therefore, it is of great theoretical significance and engineering value to systematically analyze the deformation and failure evolution mechanism of roadways surrounding rock under the influence of mining stress and take effective support methods accordingly.

Against the engineering background of the II2 mining area rise in Taoyuan Coal Mine, this study analyzed the deformation and failure characteristics and control mechanism of the mining area rise under the influence of mining stress, put forward the control technology applicable to the mining-influenced roadway surrounding rock and carried out an engineering application. The research results are expected to provide both theoretical and practical reference to the effective control of roadway deformation and failure under the influence of mining stress.

2. Engineering Background

Taoyuan Coal Mine is a Permo-Carboniferous coalfield situated in the north of Sunan mining area in Huaibei Coalfield, Anhui Province, China. Four main rises, namely return-air rise, men-walking rise, haulage rise and track rise, have been excavated in the II2 mining area in the mine. All of the four main rises are located between the #8₂ coal seam floor and the #10 coal seam roof. The #8₂ coal seam and the #10 coal seam are the main mining coal seams in the II2 mining area. The former has a thickness of 0.63–2.51 m, an average thickness of 1.81 m, and an average coal seam dip angle of 22°, while the latter has a thickness of 0.54–4.04 m, an average thickness of 3.22 m, and an average coal seam dip angle of 35° (Figure 1).

Disturbed by mining of the lower #10 coal seam and the upper II8₂21 working face, the four main rises in the II2 mining area have deformed seriously. Moreover, mining of the II8₂22 working face will have a certain influence on the four main rises. The stopping line of the II8₂22 working face is expected to be in the middle-upper part of the men-walking rise and the haulage rise. Nevertheless, the haulage rise and the track rise are considerably disturbed during the mining process, which makes it difficult to ensure their stability. Thus, mastering the mine pressure behaviors of the main rises and the deformation and failure characteristics of the surrounding rock under the influence of mining stress is a necessity for determining a much safer stopping line and conducting reinforcements and supports for the four rises.

3. Research on the Deformation and Failure Characteristics and Control Technology of Mining Area Rises under the Influence of Mining Stress

3.1. Numerical Calculation Model

To investigate the influence of mining stress on the surrounding rock stress and deformation and failure of mining area rises during working face advance, this paper established a corresponding numerical calculation model with the aid of FLAC^3D software based on relevant geological data of the II2 mining area rises in Taoyuan Coal Mine. The size of the model was 350 m × 100 m × 250 m (length × width × height), and the rock failure followed the Mohr–Coulomb criterion. The four main rises, whose sections are all in the shape of a semi-circular arch with straight walls (width 3.6 m, straight wall height 1.5 m, arch radius 1.8 m), have a buried depth of 530–552 m and an average dip angle of 25°. Originally, they adopted the bolt-mesh-spray support whose parameters include bolt specification GM22/3000-490, row spacing 800 mm × 800 mm, concrete strength grade C20 and spraying thickness 100 mm. The bolts and the concrete spray layer were simulated by the cable structural unit and the shell structural unit, respectively. The physical and mechanical parameters of rock formation of the established model are listed in

Table 1. Physical and mechanical parameters of rock formation.

Lithology	E/GPa	μ	ρ/kg·m−3	C/MPa	ϕ/°
Fine sandstone	21.66	0.22	2650	12.76	36.6
Mudstone 1	8.13	0.25	2600	2.83	36.7
Coal	4.01	0.28	1400	0.8	33.6
Sandy Mudstone	15.88	0.24	2600	2.23	35.1
Medium Grained Sandstone	23.04	0.24	2650	11.75	38.6
Siltstone	18.93	0.25	2570	10.82	38.2
Mudstone 2	14.41	0.23	2570	3.35	35.6
C20 concrete	25.5	0.20	2400

. The boundary conditions of the model were as follows: the upper surface adopted a 9 MPa stress boundary, the four sides adopted a horizontal displacement constraint and the bottom adopted a vertical displacement constraint. The lateral pressure coefficient was set as 1.0. The established numerical calculation model is demonstrated in Figure 2.

The II2 mining area rises have been affected by mining of the underlying #10 coal seam and the overlying II8₂21 working face, and they are about to be disturbed by mining of the II8₂22 working face. Considering such a situation, this numerical simulation investigated the influences of mining of the #10 coal seam and the II8₂21 working face on the surrounding rock deformation and failure characteristics of the II2 mining area rises before exploring the influence of mining of the II8₂22 working face on the four rises. The position relationship between the working face and the mining area rises is demonstrated in Figure 3 where the horizontal distance between the stopping line of the II8₂21 working face and the return-air rise was 20 m, and the horizontal distance between the stopping line of the #10 coal seam and the return-air rise was 50 m.

3.2. Analysis on the Influence Law of Mining of the #10 Coal Seam and the II8₂21 Working Face on Deformation and the Failure of Mining Area Rises

3.2.1. Distribution of Stress on the Roadway Surrounding Rock

Figure 4 and

Table 2. Statistics of maximum vertical stress on the roadway surrounding rock before and after mining of the #10 coal seam and the II8₂21 working face.

Name of Rise	Before Mining		After Mining
	Maximum Vertical Stress/MPa	Stress Concentration Factor	Maximum Vertical Stress/MPa	Stress Concentration Factor
Return-air rise	17.17	1.37	20.66	1.65
Men-walking rise	17.04	1.36	20.02	1.60
Haulage rise	16.93	1.35	18.15	1.45
Track rise	16.85	1.35	17.07	1.37

give the distributions of vertical stress on the surrounding rock of the four rises and the statistics of maximum vertical stress on the surrounding rock before and after mining of the #10 coal seam and the II8₂21 working face, respectively.

It can be seen from Figure 4 and Table 2 that, before mining of the #10 coal seam and the II8₂21 working face, the rises are merely impacted by the initial stress field. In this case, the maximum stress on the position of 2–3 m from the two sides is around 17 MPa and the stress concentration factor ranges from 1.35 to 1.37. After mining, the stress states of the surrounding rock in four rises change to various extents, and the stresses of two sides increase differently. The maximum stresses rise to 20.66 MPa, 20.02 MPa, 18.15 MPa and 17.07 MPa, respectively, an increase of 21.33%, 17.45%, 7.23% and 1.29% from those before mining. The return-air rise and the men-walking rise are located 20–50 m in front of the working face, where the maximum stress of their surrounding rock shows a relatively large increase under the influence of mining stress. In contrast, in the haulage rise and the track rise, the surrounding rock is less affected on account of a relatively long distance from the working face.

3.2.2. Deformation of Roadway Surrounding Rock

Table 3. Roof-to-floor and side-to-side convergences of rises before and after mining of the #10 coal seam and the II8₂21 working face.

Name of Rise	Before Mining		After Mining
	Roof-to-Floor Convergence/mm	Side-to-Side Convergence/mm	Roof-to-Floor Convergence/mm	Side-to-Side Convergence/mm
Return-air rise	88	75	646	639
Men-walking rise	79	69	374	366
Haulage rise	68	68	218	203
Track rise	89	84	185	168

shows the deformation conditions of the roof, floor and two sides of the four rises before and after mining. Before mining, the roadway surrounding rock merely experiences simple self-deformation. At this time, the roof-to-floor convergence is 68–89 mm, and the side-to-side convergence is 69–84 mm. After mining, the four rises undergo significantly increased deformation of the roof, floor and two sides. Among them, as the return-air rise is the closest to the stopping line of the #10 coal seam and the II8₂21 working face, its roof-to-floor and side-to-side convergences amount to 646 mm and 639 mm, respectively, its roadway deformation being the most serious. The roof-to-floor and side-to-side convergences of the men-walking rise also rise to 374 mm and 366 mm, respectively, obviously affected by mining. The haulage rise and the track rise, which are far away from the II8₂21 working face, are relatively less affected by mining. The roof-to-floor and side-to-side convergences of the former increase to 218 mm and 203 mm, and those of the latter grow to 185 mm and 168 mm, respectively.

3.2.3. Plastic Zone Distribution of Roadway Surrounding Rock

Figure 5 presents the plastic zone distribution of surrounding rock of the four rises before mining of the #10 coal seam and the II8₂21 working face. Moreover, the areas of plastic zones of surrounding rock of the four rises were calculated on the ground of the data in Figure 5. Based on the calculation results, the changes in areas of plastic zones before and after mining were plotted (Figure 6).

As displayed in Figure 5 and Figure 6, before mining, plastic zones of shear failure occur in the roof, floor and two sides of the four rises, and their areas are 16.95 m², 14.9 m², 12.01 m² and 16.24 m², respectively. After mining, the ranges of plastic zones of surrounding rock of the four rises expand to various extents. Greatly affected by the mining stress, the return-air rise is severely damaged at its roof, floor and two sides, and the plastic zone expands to 39.9 m², an increase of 135.4% from that before mining. In the men-walking rise, the plastic zone area grows to 32.08 m² under the influence of mining stress, which 115.3% larger than that before mining. Being far away from the #10 coal seam and the II8₂21 working face, the haulage rise and the track rise are less affected by the mining stress. Their plastic zone ranges grow slightly by 56.2% and 47.17%, respectively.

3.3. Analysis on Influence Law of Mining of the II8₂22 Working Face on Deformation and Failure of Mining Area Rises

In this numerical calculation, the II8₂22 working face was mined step by step from right to left on the basis of the calculation results of mining of the #10 coal seam and the II8₂21 working face. The goaf was filled in the midst of working face advance. The process of roof caving and compaction was simulated by weakening rock parameters. The analysis started from the end of the II8₂22 working face that was 60 m away from the track rise, with a total advance distance of 120 m, 10 m each time. In this advance process, the dynamic changes in surrounding rock stress, deformation and failure of the four rises were simulated and analyzed. The diagram of the simulation scheme is exhibited in Figure 7.

3.3.1. Distribution of Stress on Roadway Surrounding Rock

Figure 8 demonstrates the distributions of vertical stress on surrounding rock of the four rises with different horizontal distances (i.e., 60 m, 40 m, 20 m, 0 m, −20 m, −40 m and −60 m) between the II8₂22 working face and the track rise. Figure 9 shows the variation curves of maximum vertical stress on the roadway surrounding rock in the advance process of the II8₂22 working face.

From Figure 8 and Figure 9, it can be seen that the return-air rise and the men-walking rise are less susceptible to mining of the II8₂22 working face, and their maximum vertical stresses on surrounding rock change mildly in the range of 20.3–21.3 MPa. When the distance from the working face to the track rise is less than 20 m, the vertical stress on the surrounding rock of the track rise experiences an accelerated growth. When the distance is 10 m, the stress on the track rise reaches its peak value, 21.27 MPa, and mining exerts the greatest influence on the track rise. As the working face advances, the horizontal distance further decreases, and the vertical stress on its surrounding rock plunges. The maximum stress drops to 14.82 MPa when the working face passes the track rise by 60 m, mainly because the track rise lies in the range of stress–relaxation area in the goaf at this time. The peak value (19.92 MPa) of vertical stress on the surrounding rock in the haulage rise appears when the working face passes the track rise by 20 m. The haulage rise lies within the influence range of the mining-induced abutment pressure. As the advance proceeds, the vertical stress on its surrounding rock finally falls to 17.60 MPa. Since the haulage rise is far away from the II8₂22 working face vertically, the vertical stress on it changes less notably compared with that on the track rise.

3.3.2. Deformation Characteristics of Roadway Surrounding Rock

Figure 10 shows the variation curves of roof-to-floor and side-to-side convergences of the four rises during the advance of the II8₂22 working face.

As can be observed from Figure 10, during the advance of the II8₂22 working face, the surrounding rock deformation in the four rises develops to varying extents. For the return-air rise that is farthest from the working face, the roof-to-floor convergence increases gradually from 651 mm to 665 mm, and the side-to-side convergence basically lies in the range of 648–658 mm. The roof-to-floor and side-to-side convergences of the men-walking rise change little, practically maintained in the ranges of 379–425 mm and 376–410 mm, respectively. The roof-to-floor and side-to-side convergences of the track rise soar when its distance from the working face is less than 20 m; they rise to 735 mm and 636 mm when the distance is −10 m; afterwards, they grow at decelerated rates, ultimately increasing slowly to 775 mm and 690 mm, which are 2.91 times and 3.22 times that before mining, respectively. When the haulage rise is far from the end of the working face, its surrounding rock deformation changes insignificantly. When the distance becomes about 20 m, the roof-to-floor and side-to-side convergences of the track rise begin to surge, finally reaching 534 mm and 507 mm which are marks of severe surrounding rock deformation. Among the four rises, the track rise is particularly affected by mining of the overlying II8₂22 working face. From the perspective of roadway deformation, the distance between the stopping line of the II8₂22 working face and the track rise should be larger than 20 m.

3.3.3. Plastic Zone Distribution of Roadway Surrounding Rock

Figure 11 depicts the plastic zone distributions of surrounding rock in the four rises when the distance from the II8₂22 working face to the track rise is 60 m, 40 m, 20 m, 0 m, −20 m, −40 m and −60 m, respectively. To facilitate the analysis on the development processes of plastic zones of surrounding rock in the four rises, the plastic zone areas of the four rises in Figure 11 were calculated. Based on the calculation results, the variation curves of their plastic zone areas during the advance of the II8₂22 working face were plotted (Figure 12).

It can be found from Figure 11 and Figure 12 that when the distance from the working face to the track rise is larger than 20 m, the working face is far away from the four rises. In this case, the rises are barely impacted, and the plastic zone areas do not change much. When the distance is smaller than 20 m, the plastic zone of surrounding rock of the track rise begins to expand rapidly under the influence of mining stress, especially those of the roof and the two sides. When the end of the working face is above the track rise, the plastic zone of surrounding rock further expands as a result of mining. When the working face passes the track rise by 20 m, affected by the mining stress, the plastic zone of the roof of the haulage rise begins to expand, and the plastic zone area is enlarged promptly. When the working face passes the track rise by 40 m, the track rise stands within the stress-relaxation range in the goaf. In this case, the bearing strength of surrounding rock is low, and the roof and the floor are seriously damaged. Under the influence of pressure relief, the plastic zone of surrounding rock of the haulage track also further expands. As the working face advance continues, the plastic zone area of the surrounding rock in the track rise finally increases to 52.74 m², a growth of 45.5% from that before mining of the II8₂22 working face. Meanwhile, the surrounding rock failure will seriously threaten the safety of production. The plastic zone area of surrounding rock of the haulage rise eventually rises to 42.16 m², which is 28.0% larger than that before mining. During the advance of the II8₂22 working face, the track rise and the haulage rise are apparently influenced by the mining stress, and their plastic zone areas jump. In contrast, the influences of mining stress on the return-air rise and the men-walking rise are limited. The plastic zone area of surrounding rock of the men-walking rise rises from 35 m² to 40 m², while that of the return-air rise is basically maintained at 41 m². It can be concluded that if the coal seam is mined according to the original II8₂22 working face stopping line, the track rise and the haulage rise will be damaged inevitably in a wide range. Hence, considering the factors such as support cost and economic benefit, the stopping line should be set at least 20 m away from the track rise.

4. Research on the Control Technology of the Surrounding Rock of Mining Area Rises under the Influence of Mining Stress

4.1. Support Scheme for Mining Area Rises

As can be known from the above-mentioned deformation and failure variation characteristics of the surrounding rock of mining area rises, the surrounding rock will suffer serious deformation and severe damage due to mining of the II8₂22 working face under original support conditions. The lack of effective reinforcement and support measures is about to cause instability and failure of roadways. Therefore, taking the track rise as an example, the following four representative roadway support models were established on the basis of the previous calculation model and the existing surrounding rock control technology. The optimal support scheme is concluded by comparing the deformation and failure characteristics of surrounding rock under the four schemes when the II8₂22 working face advances to 20 m away from the track rise. The four specific support schemes and numerical models are shown in Figure 13.

Scheme 1 adopts the original support method of the track rise, that is, the bolt-mesh-spray support (Figure 13a). Based on Scheme 1, Scheme 2 adds the U-shaped steel rib support which is simulated by the beam structural unit (Figure 13b). On the basis of Scheme 1, Scheme 3 arranges three prestressed anchor cables at the roof and the shoulder, which is simulated by the cable structural unit, with the pre-tightening force set to 120 kN (Figure 13c). Based on Scheme 3, Scheme 4 arranges grouting bolts at the roof and the sides. In these schemes, the simulation of grouting support is realized by means of improving the rock strength parameter within the grout diffusion range. The grout diffusion radius is set as 1.5 m, and the rock strength parameter is raised by 1.5 times [28,29,30,31] (Figure 13d). The parameters of the roadway surrounding the rock support and the parameters of support unit are given in

Table 4. Parameters of the roadway surrounding the rock support.

Support Material	Specification	Row Spacing/mm
U-Shaped steel	36U	800
Anchor bolt	GM22/3000	800 × 800
Anchor cable	YMS17.8/6250	1600 × 1600
Grouting bolt	Φ25/2500	1600 × 1600

and

Table 5. Parameters of the support unit in the numerical calculation models.

Mechanical Parameters	Value
Beam
Elastic Modulus/GPa	200
Poisson’s ratio	0.3
Yield strength/MPa	300
Cable
Elastic Modulus/GPa	200
Tensile yield strength/kN	345
Stiffness of the grout (N/m)	2 × 107
Cohesive capacity of the grout (N/m)	1 × 106

.

4.2. Analysis on Support Effect

4.2.1. Deformation Characteristics of the Roadway Surrounding Rock

Figure 14 demonstrates the deformation of surrounding rock of the track rise under different support schemes. According to the figure and the table, under the original conditions of bolt-mesh-spray support, the roof, floor and side deformations of the track rise are serious under the influence of mining stress. Specifically, the roof-to-floor convergence reaches 438 mm and the side-to-side convergence reaches 381 mm. In Scheme 2, with the addition of the U-shaped steel rib support, the roof-to-floor and side-to-side convergences decrease to 391 mm and 334 mm, which are merely reduced by 10.73% and 12.34% compared to that in Scheme 1, respectively. Such a reduction falls far short of what is required for support design. After adopting anchor cables in Scheme 3, the roof-to-floor and side-to-side convergences are reduced by 29.68% and 27.82%, respectively, compared to that in Scheme 1. This still fails to meet the design requirements, which makes it necessary to further improve the surrounding rock lithology and supporting strength. In Scheme 4, as the grouting bolts are applied for support, the roof-to-floor and side-to-side convergences are reduced to 87 mm and 84 mm, a drop of 80.13% and 77.95% from those under original support conditions, respectively. The result indicates that the support scheme of “bolt-mesh-spray-cable + grouting bolt” can achieve the purpose of controlling the deformation of surrounding rock of the roadway roof and sides.

4.2.2. Plastic Zone Characteristics of the Roadway Surrounding Rock

Figure 15 shows the plastic zone distribution of surrounding rock of the track rise under the influence of mining stress under the four support schemes. The plastic zone areas of roadway surrounding rock under different support schemes are illustrated in

Table 6. Plastic zone areas of the roadway surrounding rock under the four support schemes.

Support Scheme	Scheme 1	Scheme 2	Scheme 3	Scheme 4
Plastic zone area/m2	52.75	48.03	24.41	8.39

.

According to Figure 15 and Table 6, thanks to the continuous improvement of support technology, the plastic zone area of roadway surrounding rock shrinks continuously. The plastic zone area reaches 52.75 m² under the condition of bolt-mesh-spray support, and other support methods need to be applied for combined support. In Scheme 2, after adding the U-shaped steel rib support, the plastic zone area drops to 48.03 m² by 8.9%. The support effect is limited, and the plastic zone range remains large. In Scheme 3, with addition of the anchor cable support, the plastic zone area is further reduced to 24.41 m², a decrease of 53.7%. When the support method of Scheme 4 is adopted, the plastic zone area plunges to 8.39 m² by 84.1%. This indicates that the support method of “bolt-mesh-spray-cable + grouting bolt” can effectively improve the bearing capacity of surrounding rock and reduce its failure range, yielding a more remarkable support effect.

In this section, the numerical simulation results of the above four support schemes were analyzed, and the deformation and failure characteristics of surrounding rock in mining area rises under the influence of mining stress were synthetically studied. It was concluded that the support method of “bolt-mesh-spray-cable + grouting bolt” is capable of significantly reinforcing the surrounding rock and controlling the deformation and failure, so as to ensure long-term stability of mining area rises.

5. Field Application

The deformation and failure characteristics and control technology of mining area rises under the influence of mining stress were systematically analyzed by means of numerical simulation, and a combined support scheme of “bolt-mesh-spray-cable + grouting bolt” that was applicable to mining area rises was put forward. Under the specific engineering background of the II2 mining area rises in Taoyuan Coal Mine, a detailed support scheme was proposed for the four rises. Taking the track rise as an example, prestressed anchor cables with the specification of YMS17.8/6250 and grouting bolts with the specification of Φ25/2500 mm were added on the basis of the original bolt-mesh-spray support for reinforced support. With a row spacing of 1600 mm × 1600 mm, three anchor cables and six grouting bolts were arranged in each section and connected by steel band along the strike of the roadway. Figure 16 displays the detailed layout of track rise support.

During the advance of the working face, two monitoring stations were set at the reinforced section of the track rise to monitor the roadway surface displacement. The obtained variation curves of surrounding rock deformation of the track rise are illustrated in Figure 17.

On the first day of monitoring, the horizontal distance from the II8₂22 working face to the track rise is 80 m. At this time, the track rise is slightly affected by mining of the working face, and the roadway deforms insignificantly. On the 10th day, the horizontal distance is 60 m. At this time, the mining influence on track rise begins to show up, and the roadway deformation increases at an accelerated rate. As mining of the working face proceeds, the influence of mining on the track rise also climbs, and the same goes for the deformation rate. On the 30th day, the horizontal distance becomes 20 m. At this moment, mining of working face stops, but the roadway deformation still increases at a decelerated rate, which is indicative the gradually weakening influence of mining on the track rise. On the 50th day, mining of the working face has stopped for 20 days. The roadway deformation tends to be stable, and the roof-to-floor and side-to-side convergences stabilize at about 135 mm and about 120 mm, respectively, which satisfies the requirements of support design.

6. Conclusions

(1)

In this study, the distributions of stress, displacement and plastic zone area of surrounding rock of the four rises were systematically investigated by means of a numerical simulation. The following findings were obtained: Mining of #10 coal seam and the II8₂21 working face notably influences the return-air rise and the men-walking rise, and the stresses, deformations and plastic zone areas of their surrounding rock increase markedly. Mining of II8₂22 working face has a particularly strong impact on the track rise. During the advance of the working face, the stress, deformation and plastic zone area of surrounding rock of the track rise grow continuously. When the distance from the working face to the track rise is less than 20 m, the surrounding rock fails; its bearing capacity is reduced under the influence of mining stress; and the roof-to-floor and side-to-side convergences and the plastic zone area expand promptly. After the working face passes the track rise, the stress on the surrounding rock plunges, and the growth rates of deformation and plastic zone area both drop, eventually tending to stabilize. For the purpose of reducing the deformation and failure degree of surrounding rock of the track rise, the stopping line of the II8₂22 working face should be set at least 20 m away from the track rise.

(2)

Four support and reinforcement schemes were proposed according to the deformation rule and failure characteristics of surrounding rock of the mining area rises. Through a numerical simulation, the variation characteristics of the deformation and plastic zone area of roadway surrounding rock under different schemes were studied in depth. The results showed that the proposed modified support method of “bolt-mesh-spray-cable + grouting bolt” can effectively deal with the influence of mining stress of the working face, guaranteeing long-term stability of mining area rises.

(3)

The proposed support scheme was applied to the support projects of the four main rises of the II2 mining area in Taoyuan Coal Mine. The monitoring results of surface displacement of the track rise revealed that the deformation of roadway surrounding rock was effectively controlled after adopting the support method of “bolt-mesh-spray-cable + grouting bolt”, which remarkably improved roadway stability and safety.