Advances in Civil Engineering
Volume 2019 (2019), Article ID 7540749, 12 pages
https://doi.org/10.1155/2019/7540749

Research Article

The Stability of Gob-Side Entry Retaining in a High-Gas-Risk Mine

Junwen Zhang1,2 and Yulin Li³

¹College of Resource and Safety Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China
²State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology, Xuzhou 221116, Jiangsu, China
³School of Mechanics and Civil Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China

Correspondence should be addressed to Junwen Zhang; zhangjunwen1977@163.com

Received 28 August 2018; Accepted 19 December 2018; Published 13 January 2019

Guest Editor: Dengke Wang

Copyright © 2019 Junwen Zhang and Yulin Li. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

There are series of problems faced by most of the coal mines in China, ranging from low-coal recovery rate and strained replacement of working faces to gas accumulation in the upper corner of coalfaces. Based on the gob-side entry retaining at the No. 18205 working face in a coal mine in Shanxi Province, theoretical analysis, numerical simulation, and engineering practice were comprehensively used to study the mechanical characteristics of the influence of the width of the filling body beside the roadway and the stability of surrounding rock in a high-gas-risk mine. The rational width of the filling body beside the roadway was determined, and a concrete roadway-side support with a headed reinforcement-integrated strengthening technique was proposed, which have been applied in engineering practice. The stability of the filling body beside the roadway is mainly influenced by the movement of the overlying rock strata, and the stability of the surrounding rock can be improved effectively by rationally determining the width of the filling body beside the roadway. When the width of the roadway-side filling body is 2.5 m, the surrounding rock convergence of the gob-side entry retaining is relatively small at only 5% of the convergence ratio. It has been shown that the figure for roof separation is relatively low, and strata behaviors are relatively alleviated and gas density do not exceed the limit, which are the best results of gob-side entry retaining. The results of this research can provide theoretical guidance for excavation of coal mines with similar geological conditions and have some referential significance to safety and efficient production in coal mines.

1. Introduction

Most of the coal mines in China are identified as high-gas-risk mines. The traditional U-type ventilation can easily cause a problem of gas accumulation in the upper corner of coalfaces. As the depth of coal mining increases, in situ rock stress increases dramatically and the coal pillar width becomes increasingly large. However, the relatively wide coal pillar size cannot ensure good maintenance effects of roadways, which may result in frequent occurrence of gas accumulation and gas overrun. In recent years, with the advancement of bolt supporting technology and mechanization level, the technology of gob-side entry retaining has been widely applied in thin- and medium-thick seams with better conditions [1–3]. The gob-side entry retaining support technology, as a kind of advanced roadway for replacing the pillar method, needs to preserve the mining roadways at the existing working face along the edge of the gob, which can achieve Y-type ventilation. This technology can contribute to reducing air leakage in the gob areas and gas emission quantity. It can also help to avoid floating pollution of coal dust at the working face and effectively improve working conditions. In addition, there is no need to establish stage coal pillar at the working face of gob-side entry retaining, which can raise coal recovery rate and prolong the service life of the mine. The difference between traditional mining method and mining method of gob-side entry retaining is shown in Figure 1.

(a)

(b)

(c)

(d)

Figure 1: The traditional mining method and mining method of gob-side entry retaining.

The gob-side entry retaining would undergo mining influence twice, especially influence caused by both in situ rock stress and strong mining stress after the coal extraction of the working face, which can lead to strong deformation of roadways and difficult problems of roadway maintenance [4, 5]. Extensive research has been conducted on the law of mining pressure of gob-side entry retaining technology and on surrounding rock deformation of roadways [6–10]. Xie carried out research on the displacement characteristics of surrounding rock and evolution characteristics of stress in the coal sidewalls and the filling body and used numerical simulation software to study the role of the filling body beside the roadway in gob-side entry retaining, proposing that a rational selection of the filling body parameters is key to the successful implementation of gob-side entry retaining technology [11, 12]; Bai et al. studied the fracture behaviour of overlying strata above the roadway and analyzed the stress conditions in the filling body beside the roadway from the perspective of the surrounding rock structure [13–15]; Zhang used numerical simulation and similarity simulation methods to analyze the movement of the roof under the condition of the filling body with different widths and determined the width of the filling body beside the roadway [16, 17]. On the basis of key strata control theory, Ma et al. [18, 19] studied the filling technology of roadway along the goaf, established the mechanical model of the surrounding rock structure of the roadway of in situ filling, and made an in-depth analysis of the interaction mechanism between the surrounding rocks and the backfill materials in the roadway. Zhang et al. [20] analyzed the deformation characteristics of surrounding rocks along the roadway and the mechanism of lateral support and deduced the formula for calculating the width of the lateral support according to the movement rule of overburden for mining with solid compacted filling. Chen et al. [21] analyzed the relation between support and surrounding rock deformation and stress distribution by numerical simulation and revealed the supporting mechanism of the roadway. Tan et al. [22] analyzed the dynamic characteristics of hard strata and derived the adaptability principle of lateral support in the roadway along the goaf, which includes load adaptability and deformation adaptability. Whether the gob-side entry retaining technology was successful or not mainly depended on whether or not the parameters of the filling body and entry-in support were reasonable. When the selection of parameters was more reasonable, the stability of the roadway was better. Most previous studies aimed at small cross-sectional coal roadways and mainly focused on roadway-side supporting resistance and supporting materials, while limited research was undertook on the rock structure of gob-side entry retaining for large cross-sectional coal roadways and on the deformation characteristics of such roadways. In particular, so far, there have been no uniform methods used to determine the filling body width which is considered as a key issue for the gob-side entry retaining. Therefore, it is necessary to further research the influence of the filling body beside the roadway on mechanical characteristics and stability of surrounding rock under the condition of large cross-sectional coal roadways.

In this work, taking a gob-side entry retaining at the No. 18205 working face in a coal mine in Shanxi Province as the engineering background, a mechanical model of this gob-side entry retaining system was established and the influence of the filling body beside the roadway on the stability of surrounding rock was studied by using a method that combines theoretical analysis and numerical simulation. A rational width of the roadway-side filling body was further determined, and an “anchor with steel mesh” combined support technology was proposed. Finally, the effect of gob-side entry retaining technology was verified by the deformation of the filling body and the displacement of surrounding rock in the roadways.

2. Stability Analysis of Surrounding Rock in Gob-Side Entry Retaining

With the continuous advancement of working face in the process of applying the gob-side entry retaining technology, fracture and crevice are formed in the main roof strata when the main roof reaches its ultimate strength under the combined influence of its gravity and the gravity of the overlying strata [23, 24]. The fracture at the four edges of the plate continues to extend and run through the whole plate until it is shown in an O shape, and the bending moment in the central plate reaches the maximum under the action of force. When the stress exceeds the ultimate strength of the plate own, there will be a fracture formed in the central plate, which will eventually be connected with the fracture at the four edges of the plate and form X-type destruction, as is shown in Figure 2. As the working face advances continuously, the periodic fracture and caving are formed in the main roof strata above the stope. The fractured main roof touches the falling gangue in the gob and gradually begins to stabilize under the load-bearing effect of these gangue, while high stress caused by the main roof beside the gob will act on the immediate roof, top coal, substance coal, filling body, and floor, which may cause the stress of surrounding rock in the roadway to increase dramatically and cause the plastic zone and surrounding rock deformation to increase as well [25–28].

Figure 2: Diagram of destruction of the main roof in gob-side entry retaining.

The structure of a skew span beam caused by the destruction of the main roof is defined as a large structure of surrounding rock in gob-side entry retaining at the top coal caving face, while in a small region of surrounding rock in the roadway, the anchorage body formed collectively by the roadway roof supported by an anchor bolt (anchor cable), integrated coal beside the roadway and filling body, is defined as a small structure of surrounding rock in gob-side entry retaining at the top coal caving face[22]. The existence of the low stress zone of the gob-side entry retaining under the structure of the skew span beam makes it possible to implement the gob-side entry retaining technology. However, whether the gob-side entry retaining practice is successful or not depends on whether the small structure of surrounding rock is able to maintain good stability, while the stability of the small structure mainly depends on whether the parameters of the filling body and entry-in support are reasonable or not. As a result, by determining the rational width and strength of the filling body beside the roadway, the main roof strata can be cut off to form a stable structure in a relatively short time, which can result in the decrease in the deformation amount and deformation duration of surrounding rock of entry retaining and good stability of surrounding rock of gob-side entry retaining.

2.1. Engineering Background

The No. 18205 working face is a gob-side entry retaining face with a relative gas emission rate of 8.28 m³/min and a gas pressure of 1.9 MPa, which is classified as high-gas mine. The length of the working face is 1592 m, and the width of it is 211 m. The average thickness of the coal seam is 3.29 m, and the dip angle of the coal seam is 5°. The Polodyakonov coefficient of the coal seam is between 2 and 2.5, which is classified as a stable seam. The immediate roof and the main roof are limestone and siltstone, respectively, with an average thickness of 2.4 m and 4.21 m, while the seam floor is the sandstone layer with an average thickness of 2.08 m.

The layout of Y-type ventilation is applied in the working face; that is, the No. 18205 belt tunnel and the No. 18205 track tunnel are used to intake air, while the No. 18207 track tunnel is used to return air. The No. 18205 track tunnel is gob-side retaining entry, and it adopts the rectangular section with a width of 5.5 m and a height of 3.5 m. The detailed layout of the coalface and tunnels is shown in Figure 3.

Figure 3: Layout diagram of the No. 18205 coalfaces and tunnels.

2.2. Theoretical Analysis of the Rational Width of Filling Body beside the Roadway

With the continuous advancement of the working face, the region of roof control constantly enlarges, which will result in the loss of stability and the occurrence of fracture of the main roof below. The roof-cutting resistance caused by the filling body beside the roadway enables the main roof along the outside of the filling body to reach its ultimate bending moment, thus cutting off the main roof. The mechanical computing model of roof-cutting resistance of gob-side entry retaining is shown in Figure 4.

Figure 4: Computing model of roof-cutting resistance of gob-side entry retaining.

The roof-cutting resistance can be solved by the following equation:

where Pq is the roof-cutting resistance of the filling body beside the roadway, mN/m; σy and x₀ are abutment pressure of coal seams beside the retaining entry, MPa, and the width of the stress limit equilibrium zone (m), respectively, which can be obtained by equations (2) and (3); c is the roadway width, m; d is the empirical value of the filling body width, m; M₀ is the residual bending moment of the A-end main roof, MN·m; ML is the ultimate bending moment of the main roof strata, MN·m; q₀ is the self-weight of the immediate roof per unit length, MN/m; q is the self-weight of the main roof and the weight of the soft rock strata above the main roof per unit length, MN/m; NC is the shearing force caused by the main roof beside the gob, MN; ΔSC is the deflection of C-end when AC rock being cut off, m; ΔSB is the deflection of B-end before the main roof caving, m; h is the thickness of the main roof strata, m; and l is the length of BC rock, m, which can be obtained by equation (4):

where C₀ and φ₀ are cohesion and internal friction angle of interface between coal seam and rock strata of the roof and floor, respectively, MPa (°); PX is the supporting resistance exerted by support on coal beside the roadway, MPa; M is the mining height, m; γ is the average unit density of overlying strata, N/m³; H is the mining depth, m; A is the side pressure coefficient; k is the stress concentration coefficient; b is the weighting interval of the main roof, m; and Lm is the length of the working face, m.

The thrust along with the direction of strata can be calculated by the following equation:

The width of the filling body beside the roadway, d, is mainly determined by roof-cutting support resistance and roadway-side support strength [23], which can be calculated by the equation:

where d is the roadway-side support width, m; K₁ is the safety coefficient, generally elected between 1.1 and 1.2; and σ is the strength of the filling body beside the roadway, MPa.

2.3. Compressive Tests on Roadway-Side Filling Body with Different Ratios

In order to ensure the establishment of the mining roadway at the working face and the safety of mining operations in the adjacent stope, the filling body used in gob-side entry retaining should have sufficient strength to meet the requirement of mechanical stability of surrounding rock in the retaining entry. The main supporting body of the filling body was the concrete pumped into a flexible framework. Consequently, the rational ratio and curing time of concrete materials played a crucial role in ensuring the quality of the filling body [29, 30].

The main concrete materials were cement, gravel, and sand. In order for the filling slurry made of concrete to meet the requirement of liquidity and strength, it was necessary to make a suitable adjustment to the ratio of cement, gravel, and sand because the different ratios of filling materials had a great influence on the mechanical property of the filling body. In order to identify the influence of material ratio and curing time on the mechanical property of the filling body, the uniaxial compressive strength of the filling body cured for 8 h, 24 h, and 3 d can be used as indicators.

2.3.1. The Preparation of Test Samples

After preparing, the concrete needed to be immediately poured into cubic plastic molds with 50 ∗ 50 ∗ 50 mm and properly tamped down with the stirring rod to remove air bubbles introduced in the process of stirring, as is shown in Figure 5. After that, the test samples were sealed with waterproof insulating belts and put into the environmental chamber for curing. Before filling with test samples, the plastic mold was drilled in a hole at the bottom, after which the hole was sealed with glue and inner wall of the plastic mold was uniformly daubed with lubes. When sampling, the glue used to seal the hole was firstly taken out, and then the test samples were blown out from the plastic mold with the help of the air pump, which could, to the greatest extent, avoid damaging the test samples in the process of sampling and ensure the integrity of the samples taken from the plastic molds.

Figure 5: The preparation of test samples.

2.3.2. The Tests on Uniaxial Compressive Strength

Twenty-four hours ahead of the test, the samples to be tested were demoulded. Before the test, the top of the samples was rubbed down with tools and the face of the samples was made parallel and perpendicular to the axes of samples. After demoulding, the samples were sealed with freshness protection package and continually put into the environmental chamber for curing until being ready for the test. In order to ensure the repeatability and reliability of the experimental results, each experiment was repeated at least three times. During the process of the test, the loading rate of the press machine was set at 0.5 mm/min.

2.3.3. Uniaxial Compressive Strength of Concrete

In the process of the test, two factors including material ratio and curing time were selected and analyzed and 27 experiments were conducted, as is shown in Figure 6. Given the filling cost and the liquidity of the concrete, it was determined ultimately that the ratios of concrete materials (cement: gravel: sand) were 1 : 1:2, 1 : 2:1, and 1 : 2:2, respectively. During the process of curing, the samples were taken every 8 hours and the total number of sampling was 9. Assuming that the peak value of the stress-strain curve was uniaxial compressive strength, the experimental results of samples are shown in Figure 7.

Figure 6: The test of compressive strength.

Figure 7: Compressive strength of the concrete under different schemes.

As is shown in Figure 7, the compressive strength of concrete samples was directly proportional to cement content, sand content, and curing time, and it was inversely proportional to gravel content. In particular, the influence of cement content on the strength of the filling body was the most dramatic, followed by that of sand content and curing time, while the influence of gravel content was the least significant. The cement was hydrated with water and bonded with aggregates together to form solids with certain strength which were the main bearing structure of the concrete. As the curing time increased gradually, on the one hand, the hydration of the cement consumed the water existing inside the concrete, but on the other hand, the products generated from the hydration could play a role in filling the pores, which could cause the pore structure to become more solid. As a result, the compressive strength of the concrete increased gradually, but, as the curing time increased continuously, the hydration weakened and the growth rate slowed down as well. With regard to the sample 1, when the curing time was 32 h, combining the data from ground pressure monitor, the strength of the concrete could meet performance requirements of gob-side entry retaining for the filling body. Therefore, based on the abovementioned factors, it was finally determined that the ratio of concrete materials (cement : gravel : sand) was 1 : 1 : 2 and the curing time was 32 h.

2.4. Theoretical Calculations and Analysis

Coal industry belongs to high-risk production industry in China. The situation of coal mine safety production is still serious due to the complicated environment, special conditions, and uncertain risk factors; thus, when the relevant mine design parameters is determined, the most dangerous situation must be considered; when the required roof-cutting of the gob-side entry retaining is determined, the roof has to be cut off in any case, even in the most dangerous situation, in order to be able to effectively avoid the security risk problems in the process of coal mining. So the basic parameters of the No. 18205 working face are shown in Table 1. Then, the parameters mentioned above were substituted in the equation, thus determining that the required roof-cutting resistance of gob-side entry retaining at the working face was 18.40 MN·m.

Table 1: The basic parameters of the No. 18205 working face.

Basic parameters
Value (unit)
Basic parameters
Value (unit)

ML
8 MN·m
γ
2.5 × 10⁴ N/m³

M₀
0 MN·m
H
340 m

q₀
5 × 10⁵ N/m
A
0.8

q
1 × 10⁶ N/m
k
2

C₀
2.05 MPa
Lm
210 m

φ₀
28°
b
23 m

PX
0.0375 MPa
c
5.5 m

h
4 m
d
2.5 m

M
3.0 m

Table 2: Material parameters.

Feature of rocks
Bulk modulus (GPa)
Shear modulus (GPa)
Cohesion (MPa)
Poisson ratio
Density (kg·m⁻³)
Tensile strength, st (MPa)

Medium sandstone
6.9
3.3
3.2
0.27
2690
2.2

Siltstone
8.8
4.8
3.3
0.23
2750
2.5

Limestone
3.6
2.1
2.0
0.22
2250
1.5

Coal
2.5
1.6
1.5
0.35
1350
1.1

Mudstone
3.7
2.1
2.1
0.22
2360
1.5

Medium sandstone
6.9
3.3
3.2
0.27
2690
2.2

Siltstone
8.8
4.8
3.3
0.23
2750
2.5

3. Numerical Simulation Scheme of the Stability of Gob-Side Entry Retaining

3.1. The Establishment of Numerical Models and Scheme Design

A three-dimensional FLAC3D numerical simulation model was built based on the geological conditions of the No. 18205 working face (Figure 8). The entire model size (length × width × height) simulated a measured volume of 520 m × 150 m × 50 m, with 154,650 cells and 174,127 nodes. The horizontal flank and vertical bottom surface movement in the model were limited. The equivalent uniform load (9.45 MPa) caused by the weight of overlying strata and topsoil layers was imposed on the upper boundary of the model. The Mohr–Coulomb was applied in this model, and the lateral pressure coefficient is set to 0.8. The detailed mechanical parameters of rock strata are shown in Table 2.

Figure 8: Diagram of the three-dimensional numerical simulation model.

According to the final material proportion and curing time determined in Section 2.3, the average strength of concrete materials is 9.0 MPa. By substituting equation (6) of Section 2.2, it can be obtained that the supporting width of the roadway side backfill body is 2.45 m. Considering other supports in the roadway, the width of the backfill is determined by theoretical calculation to be 2.5 m. Based on field research and experience of the gob-side entry retaining of the other coal mines, considering the stability and safety of the roadway, it is finally determined to simulate the filling width with the spacing 0.5 m, so on combining the mechanical calculations with the field engineering practice, three simulation schemes were proposed. The widths of the filling body beside the roadway in the gob-side track entry retaining at the No. 18205 working face were 2.0 m, 2.5 m, and 3.0 m, respectively. In addition, the filling body width in the roadway of the working face was set to 1 m, and the mining interval and filling interval of the working face both were set to 2 m, as is shown in Figure 9. In addition, the measuring points were set in the model to monitor the stress change and displacement change in the vertical direction.

Figure 9: The layout diagram of the No. 18205 working face and entry.

3.2. Numerical Simulation Results and Analysis

3.2.1. Relations between Vertical Stress Distribution of the Filling Body and the Distance to Panel Coal Wall

In terms of the different widths of the filling body beside the roadway used in gob-side entry retaining, the vertical stress distribution in the middle of the filling body is shown in Figure 10.

Figure 10: Relations between vertical stress of the filling body and distance to the panel coal wall.

As is shown in Figure 10, the changing trends of vertical stress caused by the filling body with different widths were basically consistent. To be specific, when the distance of the filling body beside the working face was about between 0 m and 30 m, the change of the added resistance accelerated and the support resistance increased after the filling body was built, which could effectively support the roof in the roadway and cut off the roof beside the gob. With the damage of the main roof, support resistance of the filling body, to a certain extent, decreased when the distance was between 40 m and 60 m. When the distance was beyond 60 m, support resistance of the filling body did not change any more. At that time, the filling body could maintain the balance of the roof that had been broken above the roadway. The filling body width was wider, the vertical stress was larger, and the bearing capacity was stronger. The vertical stress of the filling body increased with the increase of width and bearing capacity of the filling body. When the filling body width increased from 2.0 m to 2.5 m, the vertical stress increased from 8.1 MPa to 9.5 MPa, increasing by 17.2%; when the filling body width increased from 2.5 m to 3.0 m, the vertical stress rose from 9.5 MPa to 10.7 MPa, rising by 12.6%, which meant that the latter increment obviously decreased compared with the former one.

3.2.2. Relations between Vertical Stress Distribution and Filling Body Width

Considering that vertical stress of the filling body placed at a distance of 60 m behind the working face remained about the same, this place was selected for monitoring stress of the filling body. Taking the filling body beside the roadway as an origin, the vertical stress distribution of the filling body is shown in Figure 11 under the condition of different widths of the filling body beside the roadway in the gob-side entry retaining.

Figure 11: Relations between vertical stress and width of the filling body.

As is shown in Figure 11, the filling body had higher long-term strength, which could effectively prevent the filling body in the gob-side entry retaining from showing the rheological characteristics and prevent the filling body from being damaged by the advancing abutment pressure caused by extracting the working face. The filling body width had a larger influence on horizontal stress distribution of the filling body, and the horizontal stress reached the maximum in the middle of the filling body beside the roadway. However, the vertical stress distribution was approximately in a triangular shape. As the width of the filling body beside the roadway is continuously increasing, the range of the approximate triangle’s top angle gradually expanded and distribution pattern of the vertical stress changed from an approximate triangle to approximate ladder shape with the increase of the filling body width.

3.2.3. Relations between Filling Body Deformation and Width

Under the condition of different widths of the filling body beside the roadway in the gob-side entry retaining, the top surface displacement distribution of the filling body is shown in Figure 12.

Figure 12: Relations between top subsidence and width of the filling body.

As is shown in Figure 12, the changing trends of surface displacement of the filling body with different widths were basically consistent, which demonstrated that the existence of the filling body beside the roadway could not change basic movement laws of overlying strata. At the same time, with the advancement of the working face, the key block was subject to rotary deformation, so the subsidence of the filling body beside the roadway was obviously lower than that beside the gob. The filling body with certain yieldable capacity allowed the main roof to be subjected to certain deformation, which would enable the load to transfer to the gangue in the gob, thus decreasing the load imposed on the filling body. The top subsidence of the filling body decreased obviously with the continuous increase in the filling body width, yet the decrement was decreasing. Specifically, when the filling body width increased from 2.0 m to 2.5 m, the top subsidence of the filling body relatively decreased by 23 m. When the width increased from 2.5 m to 3.0 m, the top subsidence relatively dropped by 11 mm.

3.2.4. Relations between Surface Displacement of Surrounding Rock in the Roadway and Filling Body Width

Under the condition of different widths of the filling body beside the roadway in the gob-side entry retaining, the surrounding rock in the roadway placed at a distance of 30 m behind the working face was selected to monitor the surface displacement, by which the surface displacement distribution of surrounding rock that is shown in Figure 13 could be obtained.

Figure 13: Relations between surface displacement of surrounding rock in the roadway and filling body width.

As is shown in Figure 13, with the increase in the filling body width, the effects of entry retaining were better. The deformation of surrounding rock in the roadway continuously decreased, but decrement constantly dropped, and the deformation tended to be stable at a place where the distance to panel coal wall was 60 m. When the filling body width was 2.0 m, the roadway deformation was larger. When the width increased from 2.0 m to 2.5 m, subsidence of roof relatively decreased by 14 mm. When the width rose from 2.5 m to 3.0 m, subsidence of the filling body dropped by 65 mm.

To conclude, as the width of the filling body beside the roadway increases, the bearing capacity of the filling body could be improved. When the width of the filling body beside the roadway was 2.5 m, the subsidence of the filling body decreased and filling body was in a relatively stable condition, resulting in relatively lower support cost. Therefore, taking all kinds of factors into consideration, it was determined that the rational width of the filling body beside the roadway was 2.5 m.

4. Engineering Practice

4.1. Design of the Field Support Scheme

The support width of the gob-side entry retaining system in the No. 18205 working face track tunnel was 2.5 m, and the widths of the filling body in the gob and in the working face tunnel were 1.5 m and 1 m, respectively. The filling interval of the roadway-side filling body that was integrally cast with cement paste materials was 2.4 m. The template of the filling support underground mine is shown in Figure 14, and the effect of field application of the filling body in the gob-side entry retaining is shown in Figure 15.

Figure 14: Template of filling support.

Figure 15: Effect of field application.

4.1.1. The Reinforcement outside the Filling Body

Combined with the #8 metal mesh (the place where the wall was uneven was supported by the rhombic metal mesh) which was used as joint support, the filling body was reinforced by the Class IV ultrahigh-strength and special pretension steel bolts (20 mm diameter, 1600 mm length).The array space between bolts (interval × row) was 900 × 800 mm, and bisteel joists with 6 holes were used to connect bolts. The support parameters of the filling body are shown in Figure 16.

Figure 16: The support parameters of the filling body: (a) cubic view; (b) cross-sectional view; (c) lateral view.

4.1.2. The Reinforcement inside the Filling Body

In order to increase the overall stability and support strength of the filling body beside the roadway, the filling body was reinforced by wire-mesh reinforcement combined with thread steel. Each steel mesh was 2250 mm long and 3000 mm high with a grid size of 150 mm × 150 mm and a diameter of 8 mm. Five steel meshes were set in filling mold. Specifically, three were set perpendicular to the roadway roof and floor and both sidewalls of the roadway with an interval of 1000 mm. They were connected by thread steel (the diameter was 16 mm and the length was 2100 mm) in the middle of steel meshes and in a place with a distance of 300 mm from the roof and the floor. The other two were set parallel to both sidewalls of the roadway, and they were integrated with the former three ones by wires.

4.1.3. The Seal of Filling Body

On the basis of joint reinforcement with bolt belts and steel meshes, in order to prevent the surrounding rock from weathering, the concrete with a thickness of 50 mm was sprayed to seal the surface of support body, and the ratio of concrete (cement: gravel: sand) was 1 : 2 : 2. With the advancement of the working face, the local areas of the filling body may crack under the influence of many factors such as the roof pressure and the support body may be in the poor condition of roof contacting, which could result in gas leakage in the gob. Considering that kind of situation, the new quick-setting curing agent was used as a kind of sealing material for grouting reinforcement to deal with cracks existing in the walls and lapping zones. The parameters of sealing materials were as follows: the slurry concentration was between 60% and 70%, the compressive strength was more than 5 MPa, and the initial setting time was between 30 min and 40 min.

4.2. Effects of Field Application and Analysis

In order to verify the effect of the gob-side entry retaining in the No. 18205 working face track tunnel, the stress and deformation of the filling body, the displacement of surrounding rock in the roadway, and the roof separation were observed on-site and were analyzed together with the effect of the gas control.

4.2.1. Deformation Characteristic of Filling Body beside the Roadway

The deformation of the filling body beside the roadway was a reflection of the interaction between the filling body beside the roadway and surrounding rock with regard to the filling body. The deformation curve of the filling body which was closed to the roadway side is shown in Figure 17. The deformation of the filling body could be divided into three stages:

(1)The distance of the working face from 0 m to 10 m was termed “Stage 1,” at which the filling body did not yet bear larger load. The support body had larger early support resistance with lower longitudinal deformation, and the transverse deformation was almost zero.(2)The distance of the working face from 10 m to 40 m was termed “Stage 2.” In the process of the main roof forming structure, the overlying strata movement intensified with the rapid increase in the load imposed on the filling body beside the roadway and the deformation of the filling body. During this stage, the longitudinal and transverse central convergence velocities reached the maximum. When the longitudinal compressive deformation was larger, the transverse deformation synchronized with the longitudinal deformation, during which time the longitudinal deformation was greater than the transverse one.(3)The distance from the working face exceeding 40 m was termed “Stage 3,” at which the longitudinal and transverse deformation of the filling body tended to be stable as the development of surrounding rock movement reached a stable state. During the monitoring period, the maximum longitudinal deformation of the filling body was 123 mm, and the longitudinal compression ratio was 4.1%, which did not exceed the maximum compressive strength with a compression ratio of 5%. This situation was beneficial for the stability of surrounding rock in the gob-side entry retaining.

Figure 17: Relations between the deformation of the filling body beside the roadway and the distance from the working face.

4.2.2. Deformation Characteristics of the Surrounding Rock in the Roadway

The displacement curve of surrounding rock in the gob-side entry retaining is shown in Figure 18. After the deformation of surrounding rock is kept stable, the maximum convergence between the roof and the floor was 189 mm, and that at the both sidewalls was approximately 162 mm. The average roof separation was about 27 mm with a maximum of 41 mm. After the gob-side retaining entry reached the stable state, the displacements of the filling body and surrounding rock both were below 0.2 mm/d, which demonstrated that the surrounding rock in the gob-side retaining entry was relatively stable under the influence of mining and that strata behaviour appeared to be relative relieved, creating good conditions for the secondary recovery.

Figure 18: Displacement curve of the roadway.

At the same time, the gas accumulation in the upper corner of coalfaces was between 0.45% and 0.75%, which did not exceed the limit. After the gob-side retaining entry entered into the stable stage, the deformation of the filling body and the surrounding rock was lower. In summary, the rational width of the support body was able to play a better role in supporting the roof and maintaining the stability of the roadway.

5. Conclusion

Based on the gob-side entry retaining at the No. 18205 working face in a coal mine in Shanxi Province, in order to increase the recovery rate of the coal resources and to solve the problem of gas exceeding the limit, the theoretical analysis and numerical simulation were used to study the mechanical characteristics of surrounding rock in the gob-side entry retaining in a high-gas-risk mine. The main conclusions obtained from the research were as follows:

(1)The stability of the filling body beside the roadway was mainly influenced by the movement of overlying key strata, while the rational width and strength of the filling body beside the roadway could effectively improve the stability of surrounding rock.(2)With the increase of the width of the filling body beside the roadway, the bearing capacity of the filling body improved. Taking the cost of filling into consideration, it was eventually determined that the width of the support body beside the roadway at the No. 18205 working face was 2.5 m.(3)On the basis of the joint reinforcement by bolts and steel meshes, the engineering practice demonstrated that the cement paste materials were used to seal the roadway side, resulting in the lower roof separation and surrounding rock convergence, which could meet the requirement of safety production in a mine. In addition, this technology would not contribute to the gas exceeding the limit, which could guarantee the better application effect.

Abbreviations

Pq: | Roof-cutting resistance of the filling body beside the roadway, MN/m
σy: | Abutment pressure of coal seams beside the retaining entry, MPa
x₀: | The width of stress limit equilibrium zone, m
c: | The roadway width, m
d: | The empirical value of the filling body width, m
M₀: | Residual bending moment of the A-end main roof, MN·m
ML: | The ultimate bending moment of the main roof strata, MN·m
q₀: | The self-weight of the immediate roof per unit length, MN/m
q: | The self-weight of the main roof and the weight of the soft rock strata above the main roof per unit length, MN/m
NC: | The shearing force caused by the main roof beside the gob, MN
ΔSC: | The deflection of C-end when AC rock being cut off, m
ΔSB: | The deflection of B-end before the main roof caving, m
h: | The thickness of the main roof strata, m
l: | The length of BC rock, m
C₀: | Cohesion of interface between coal seam and rock strata of roof and floor, MPa
φ₀: | Internal friction angle of interface between coal seam and rock strata of roof and floor, °
PX: | The supporting resistance exerted by support on coal beside the roadway, MPa
M: | The mining height, m
γ: | The average unit density of overlying strata, N/m³
H: | The mining depth, m
A: | The side pressure coefficient
k: | The stress concentration coefficient
b: | The weighting interval of the main roof, m
Lm: | The length of the working face, m
K₁: | The safety coefficient, generally elected between 1.1 and 1.2
σ: | The strength of the filling body beside the roadway, MPa.

Data Availability

A part of data used to support the findings of this study are included within the article, and the other parts of data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research was supported by the National Key R&D Program of China (2018YFC0604703) and funded by the Research Fund of The State Key Laboratory of Coal Resources and Safe Mining, CUMT (SKLCRSM18KF021).

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