New Technology of Pressure Relief Control in Soft Coal Roadways with Deep, Violent Mining and Large Deformation: A Key Study

Abstract

Previous studies have shown that the influence of deep dynamic pressure on the surrounding rock control of a coal roadway is one of the difficulties in mine roadway support. Based on the investigation of the headgate 11231 in a coal mine, this study analyzes the damage characteristics of coal roadway surrounding rock affected by deep dynamic pressure, expounds on the difficulties of controlling the roadway surrounding rock, and creatively proposes a cooperative control technology of external anchor–internal unloading for regulating large deformation of roadways. The vertical stress distribution and transfer law of surrounding rock with different hole-making depths, spacing, and lengths after roadway excavation were simulated and studied, and an appropriate parameter range of hole-making space in the stage without dynamic pressure influence was obtained. Considering the influence of mining dynamic pressure, the surrounding rock pressure relief effect of each optimized hole-making parameter was analyzed. In addition, the optimal hole-making parameters (the hole-making depth, spacing, and length were 8 m, 3.2 m, and 3 m, respectively) that can effectively reduce the high stress of roadway shallow surrounding rock in two stages (without and with dynamic pressure) and ensure integrity of the shallow surrounding rock were obtained. The actual field application shows that the new technology can reduce the higher rib deformation by approximately 850 mm and achieve a good surrounding rock control effect. The research and practice show that the pressure relief control for soft coal roadways with deep, violent mining and large deformation has achieved success, providing technical support for the maintenance of the same type of roadway.

1. Introduction

With the deep mining of coal resources gradually becoming the norm [1,2,3], the engineering response problems of discontinuous, uncoordinated large deformation and large-scale instability of roadway surrounding rock caused by the typical deep “three high” occurrence environment and coal mining dynamic pressure have become urgent engineering problems that need to be solved [4,5]. Research shows that the complex stress field [6], deformation brittle–ductile transition [7], continuous deterioration [4,8], and strong rheology of deep coal and rock mass are the major reasons for the continuous large deformation of deep roadway surrounding rock. In China, coal roadways excavated by underground coal mines account for approximately 80% of the total roadway excavation volume [9]. Coal science and technology workers have conducted a lot of research on the surrounding rock properties [10,11], stress environment [12,13], and roadway deformation characteristics [14,15] of deep coal roadways through field investigation, theoretical analysis, experimental research, and numerical simulation.

In recent years, a lot of work has been carried out to study the impact of mining on the rock mass and to recommend measures for reducing strain changes of rock mass and minimizing the stress on the surface. Since mining production has a significant effect on the stress-strain behavior of rock mass, the issues of reducing this influence are very relevant and scientists around the world are trying to minimize it. Adigamov, A.E., et al. [16] established the stress-strain behavior model of disturbed rock mass with regard to anisotropy and discontinuities, which can be used to calculate the strength of underground rock mass; Khayrutdinov, A.M., etc., [17] studied the change of stress-strain characteristics of rock mass after using different-strength backfill and carried out research on the stress-strain relationship of disturbed rock mass under different conditions. On the basis of studying the influence of mining activities on the stress-strain relationship of rock mass [18,19], scholars at home and abroad have developed many targeted coal roadway surrounding rock support technologies. High prestressed strong bolt support [20], high strength, high stiffness and high prestressed bolt support [21], and butted long bolt support [22] technologies can effectively control the generation and development of separation, sliding, and cracks in shallow surrounding rock as well as improve the post-peak strength of deformed surrounding rock and its mechanical parameters. Support technologies, such as the constant resistance and large deformation bolt support technology [23] and high-strength pressure relief anchor box beam support system [24] can release the uncontrollable deformation energy in the surrounding rock after the roadway support is completed, and then, give full play to the support capacity of the bolt, so that the bolt and the pressure relief surrounding rock can form a stable support bearing body. The anchor cable truss support technology [25] can simultaneously provide extrusion stresses in the horizontal and vertical directions of the roadway roof and rib, thus effectively reducing the maximum tensile stress and maximum shear stress of the surrounding rock in the anchorage zone. The anchor-grouting combined support technology [26] can re-condense the broken surrounding rock of the coal roadway, effectively improve the mechanical properties of the surrounding rock, and ensure the integrity and safety of the roadway surrounding rock. In addition, the active–passive coupling support technology [27] involves combining bolts (cables) with the passive support components to maintain stability of the coal roadway surrounding rock under special geological conditions and fully utilize the initial support resistance from the active support and the high deformation resistance of the passive support. To control the deformation of deep coal roadway surrounding rock, while seeking innovation of the support technology, it has become an important research direction for the surrounding rock control of deep coal roadways to realize the release or transfer of high concentrated stress in the coal roadway surrounding rock through stress control (surrounding rock pressure relief) to achieve stability of the surrounding rock. Currently, many roadway pressure relief control methods are available, such as arranging the roadway in the stress reduction zone [28], physical pressure relief of surrounding rock (e.g., by drilling [29] and blasting [30,31]), excavating the pressure relief roadway [32], and roof cutting pressure relief [33]. The pressure relief control technology can solve the problem of controlling the surrounding rock under various working conditions to a certain extent.

Analyzing the above, it can be noted that the surrounding rock control in soft coal roadways with deep, violent mining and large deformation is a very topical issue, but the existing research results have not made a big breakthrough in the surrounding rock control of this type of roadway. Given the above problems, this study creatively proposes a cooperative control technology of external anchor–internal unloading of surrounding rock in deep coal roadways. First, the roadway shallow surrounding rock is strengthened through an anchor-grouting combined support. Then, a hydraulic hole-making machine is used to create pressure relief space with appropriate spacing in a certain range of the deep coal roadway to transfer the high concentrated stress from the shallow to the deep rock and effectively improve the stress environment of the roadway surrounding rock. The drawings in Figure 1 show how by combining the characteristics of new and old pressure relief technologies, both conventional pressure relief drilling [29] and hole-making pressure relief can effectively realize the transfer of high concentrated stress in the shallow surrounding rock to the deep rock. However, the conventional pressure relief dense drilling makes the shallow surrounding rock more broken, reducing its strength. In contrast, the hole-making pressure relief technology uses steel pipes to effectively solidify the borehole surrounding rock and ensures the integrity and effectiveness of the shallow surrounding rock and support structure. The new cooperative control technology was first applied to the deep dynamic pressure mining roadway and achieved success, ensuring the roadway rib did not expand during the service period. This has important research value for the surrounding rock pressure relief support of a high stress and large deformation coal roadway.

2. Project Overview

2.1. Engineering Geology and Problems

The headgate 11231 of a coal mine is located in the 1100 southern mining area. In the south of the headgate is solid coal panel 11231, in the northern part is fault SF₈₆, in the eastern part is the −760 m horizontal main roadway, and in the western part is the concentrated roadway of the 1100 southern mining area. The average buried depth of the headgate is 740 m, and the roadway size is 5.0 × 3.5 m (width × height), with an anchor mesh cable combined support. The average thickness of the No. 2 coal seam is 4.5 m, and the average dip angle is 14°. Panel 11231 is a gangue backfilling panel, and the compressed ratio of the gangue after compaction is approximately 80%. A drawing of the location and columnar of panel 11231 is shown in Figure 2.

In the process of advancing each panel in the 1100 southern mining area, although the goaf is filled with gangue, the surrounding rock of the section roadway within 90–120 m in front of the panel still has large deformation owing to the mining dynamic pressure. As the mining progresses, the surrounding rock is continuously renovated by a team to ensure normal mining, as illustrated in Figure 3. According to statistics, during the service period of the 1100 southern mining area, the section roadways need to be renovated at least once, and some sections even require two to three renovations. The high frequency of renovation work has greatly increased the cost of roadway support and the labor intensity of workers. Moreover, it has seriously restricted the safe and efficient production of the mine. To solve the engineering problem caused by the repeated roadway renovation and improve the production efficiency of the mine, this study selects a reasonable position of headgate 11231 in a coal mine as a test roadway for applying the new support technology. The position of the test roadway is displayed in Figure 2.

As indicated in the curve chart in Figure 4, the accumulated approach of the two ribs is approximately 610 mm (no mining influence stage) between the completion of headgate 11231 and 9 months before the mining dynamic pressure. The influence range of the increased dynamic pressure of panel 11231 is approximately 90 m. Because of the influence of mining dynamic pressure on the panel, the deformation of the surrounding rock increases sharply. The data show that within the period (approximately 21 days) from the influence of dynamic pressure on the roadway surrounding rock to 30 m from the panel, the accumulated approach of the two ribs of the roadway surged to 1240 mm (mining influence stage). Then, the roadway was renovated, and the displacement monitoring was performed again. The accumulated approach of the two ribs of the renovated roadway to the completion of mining of the panel was approximately 520 mm (mining influence stage). Therefore, the total approach of the two ribs of the roadway is approximately 1760 mm. The continuous large-scale extrusion of the two ribs of headgate 11231 (Figure 4(II)-a–c) led to damage of the support structure (Figure 4(I)-a–c). The reduction in support strength of the roadway rib coal aggravated the continuous large deformation of the coal body, forming a vicious cycle of large deformation of the surrounding rock and damage of the support body, which posed a great threat to the safety of underground personnel and equipment.

2.2. Analysis of Control Difficulties of Roadway Surrounding Rock

Through investigation and analysis of the mining and production geological conditions of panel 11231, the reasons for the large deformation of headgate 11231 are as follows:

(1)

High in situ stress. The average buried depth of headgate 11231 is 740 m, and the primary rock stress reaches 20 MPa. After roadway excavation, the shallow surrounding rock changed from a three-dimensional to a two-dimensional stress state. Moreover, the stress was redistributed, which increased the surrounding rock stress by two to three times at a certain depth. The surrounding rock of the roadway deteriorates rapidly owing to the high in situ stress and stress state change (as depicted in Figure 5a, the roadway in this section (near panel 11231) experiences deformation of surrounding rock dominated by vertical stress, which is mainly manifested in large roof subsidence).

(2)

Complex tectonic stress. The geological structure of the 1100 southern mining area is complex, and the tectonic stress caused by tectonic movement has a great influence on the stability of the roadway surrounding rock. As illustrated in Figure 5b, this section of the roadway (near panel 11231) undergoes deformation of surrounding rock dominated by tectonic stress, which is mainly manifested by the large amount of movement of the two ribs getting closer.

(3)

The coal body is soft and broken. The strength of the coal body in a coal mine is low, owing to the loose and soft properties of the coal body. The borehole peep (Figure 5c-I) shows that the coal body is broken when the roadway rib depth is 3 m. It can be seen from Figure 5c-II that when the roadway rib has no large deformation, the soft coal body in the roadway rib peels off under an effective support. The low strength and weak self-supporting capacity of the coal body is the main reason for the large deformation of headgate 11231.

(4)

The roadway section is large. Headgate 11231 has a center height of 3.5 m and a roadway width of 5 m. Research shows that the larger the roadway section, the easier it is to cause greater stress concentration, and the more serious the deformation and damage of the roadway surrounding rock. In addition, headgate 11231 is a trapezoidal roadway driven along the coal seam roof, and the height of the roadway panel rib (higher rib) reaches 4.2 m. The irregular cross-sectional shape causes the surrounding rock of the roadway to be subjected to unbalanced pressure, and the surrounding rock of the roadway undergoes asymmetric deformation. The displacement monitoring data show that the deformation of the higher rib of the headgate is larger than that of the non-panel rib (lower rib). As displayed in the curve data in Figure 4, the deformation of the higher rib accounts for 65.6 and 67.0% of the accumulated approach of the two ribs in the stage without and with dynamic pressure, respectively.

(5)

Strong influence of mining. The average mining height of panel 11231 is 4.5 m. As indicated in Figure 5e, although the goaf is filled with gangue, owing to the low compressed ratio and large mining height, the overburden movement is strong after mining, and the strong disturbance range of the panel is up to 90 m. The broken coal body of the roadway surrounding rock within the disturbance range can easily undergo large deformation.

3. Principle and Key Technical Parameters of the Cooperative Control Technology of External Anchor–Internal Unloading of Coal Roadway Surrounding Rock

According to the above analysis, to solve the problem of repeated renovation of mining roadways caused by their large deformation, technical research should be conducted from the following two perspectives: (1) complex stress environment in coal roadways (difficulties 1, 2, and 5) and (2) soft and broken characteristics of surrounding rock (difficulty 3). Based on this, the cooperative control technology of external anchor–internal unloading of coal roadway surrounding rock is proposed in this study.

3.1. Principle of Cooperative Control Technology of External Anchor–Internal Unloading of Coal Roadway Surrounding Rock

The cooperative control technology of external anchor–internal unloading refers to strengthening the shallow surrounding rock of the roadway through the combined strategy of anchor-grouting and then using physical means to make pressure relief holes with reasonable spacing in a certain range of the deep part of the roadway to improve the stress environment of the roadway surrounding rock. The pressure relief holes can also provide a large compensation space for the deep coal body to transfer to the roadway space and effectively block the intermediate source of large deformation of roadway surrounding rock. In addition, the strengthening of the surrounding rock in the shallow part of the roadway can restrict the shallow coal body from moving to the pressure relief space. Generally, the internal pressure relief method involves making large-diameter holes with reasonable spacing in a certain range in the deep part of the roadway by using hydraulic hole-making equipment. The method mainly includes determining reasonable technical parameters using geological steel pipes with an appropriate diameter to effectively support the ordinary boreholes in the shallow part of the roadway and creating large-diameter holes in the deep part of the roadway.

Figure 6 illustrates the principle of cooperative control technology of external anchor–internal unloading. This technology mainly includes two aspects, namely “external anchor” and “internal unloading”. As mentioned above, the external anchor is a high-efficiency and strong pre-tightening support structure of anchor cable truss beams formed by strong anchor cables and a channel steel or steel belt beam. The shallow coal roadway is grouted to form an anchor-grouting reinforced bearing body of shallow surrounding rock of the roadway (Figure 6A), which creates a good surrounding rock environment for internal hole-making and pressure relief. Internal unloading mainly includes two objectives. (1) The pressure relief space in the deep part of the roadway makes the peak area of the abutment stress caused by the roadway rib substantially transfer to the deep part (Figure 6B,C), which reduces continuous damage (due to high concentrated stress) to the shallow coal and rock mass and improves its stress environment. (2) Continuous large-diameter holes provide a large compensation space for the transfer of deep coal body to the roadway space and effectively block the intermediate source of large deformation of roadway surrounding rock (Figure 6D). It should be pointed out that the structural integrity of the surrounding rock in the shallow anchorage zone should not be damaged during internal hole-making and pressure relief, and the pipe-fixing method can be used to effectively support the shallow ordinary drilling area to ensure that the strength of the coal and rock mass in this area is not reduced by the hole-making.

3.2. Key Technical Parameters of Internal Hole-Making and Pressure Relief

Effective implementation of the internal hole-making and pressure relief technology mainly involves the reasonable selection of three technical parameters, as shown in Figure 7.

(1)

Hole-making depth L₁. The hole-making depth affects the abutment stress distribution of the roadway rib and the anchorage range of the shallow surrounding rock after the completion of roadway excavation. When the hole-making depth is extremely large, it cannot effectively relieve the pressure, and even more seriously, it may increase the stress peak in the shallow anchorage zone and affect the stability of the shallow surrounding rock. On the other hand, when the hole-making depth is extremely small, the stress transfer effect is not significant, and the hole-making space can easily cause damage to the shallow anchorage zone.

(2)

Hole-making spacing L₂. After the hole-making space is created, the coal body outside the hole-making space migrates to this space, forming a certain range of surrounding rock loose areas. Appropriate spacing of hole-makings can connect the loose areas of two adjacent holes-making along the axial direction of the roadway, and the loose areas of continuous hole-making connect with each other in the axial direction of the roadway to form a pressure relief continuous zone (Figure 6E). When the spacing is extremely large, the pressure relief continuous zone cannot be formed. In addition, the coal body between the hole-making spaces is still in a state of high concentrated stress, and the surrounding rock pressure relief effect is poor. In contrast, when the spacing is extremely small, dense ordinary boreholes cause great damage to the shallow surrounding rock, which affects its integrity, and at the same time increases the labor of workers and reduces construction efficiency.

(3)

Hole-making length L₃. The hole-making length has a significant influence in terms of two aspects: (1) the greater the hole quantity and length, the greater the coal output and the more obvious the compensation effect of the hole-making space on deep coal; (2) pressure relief amplitude, namely, the transfer distance of highly concentrated stress. In general, when the hole-making depth is appropriate, the greater the hole-making length, the greater the transfer distance of the peak value of the roadway rib abutment stress to the deep rock. When the hole-making length is extremely small, the pressure relief range is small, the pressure relief effect is not sufficient, and the pressure relief hole-making is closed in a short time, preventing its blocking effect on the transfer of deep coal body to the roadway space. On the other hand, when the hole-making length is extremely large, the deeper hole-making space has little effect on the pressure relief of the shallow surrounding rock of the roadway.

Owing to the large dip angle of the coal seam and limited by the technical capability of the hydraulic hole-making equipment, the coal output effect of the internal hole-making space in the lower rib of the headgate 11231 test section is poor and cannot meet the technical requirements. Meanwhile, comparing the displacement monitoring data of headgate 11231, the higher rib deformation of the headgate accounts for more than 65% of the total displacement of the two ribs of the roadway. Therefore, effectively limiting the deformation of the higher rib surrounding rock of the headgate can greatly relieve the renovation pressure of headgate 11231 and meet the engineering requirements of the panel.

4. Study on Key Technical Parameters of Pressure Relief by Internal Hole-Making

4.1. Establishment of Numerical Model and Research Ideas

To determine the key technical parameters of pressure relief by internal hole-making in deep coal roadways affected by dynamic pressure, a numerical simulation study was carried out according to the actual situation on-site and the technical principle of pressure relief by internal hole-making combined with the FLAC3D finite element software. The numerical model is depicted in Figure 8. The numerical model size is 130 × 70 × 90 m. The length of panel 11231 is 59 m. The mining height is 4.5 m, and the dip angle of the coal and rock strata is 14°. Considering that panel 11231 is a gangue backfilling panel, the final compressed ratio is approximately 80%. It is considered that the equivalent mining height of the panel is 0.9 m. The mining roadways are arranged along the coal seam roof. The roadway width is 5 m, the center height of the roadway is 3.5 m, the distance from the borehole to the roadway floor is 1.5 m, and the upward angle of the hole-making in different schemes ranges from 3.0 to 8.5°. Each coal and rock stratum in the numerical model adopts the Mohr-Coulomb constitutive model. The left and right boundaries of the model are fixed with horizontal displacement along the x-direction, the front and rear boundaries of the model are fixed with horizontal displacement along the y-direction, and the bottom boundary of the model is fixed with z-displacement along the vertical direction. Moreover, a load of 17.115 MPa is applied to the top boundary of the model to simulate the overburden weight. The coefficient of lateral pressure of the model is 1.2. Based on the basic mechanical parameters of coal rock mass measured in the laboratory and data from the literature, the mechanical parameters were calculated [34,35,36].

Table 1. Mechanical parameters of coal and rock strata.

Rock Strata	$\mathit{K}$/GPa	$\mathit{G}$/GPa	${\mathit{C}}_{\mathit{m}}$/MPa	${\mathbf{\sigma }}_{\mathit{tm}}$/MPa	${\mathbf{\phi }}_{\mathbf{m}}$/(°)	$\mathit{D}$/(kg·m−3)
No. 1 coal seam	2.60	1.60	1.00	1.32	25.0	1.40
Siltstone	4.77	4.59	3.60	3.15	32.5	2.65
Sandy mudstone	4.87	4.18	3.20	3.10	35.0	2.40
Medium grained sandstone	4.50	4.10	3.15	3.05	33.0	2.65
No. 2 coal seam	2.60	1.60	1.00	1.32	25.0	1.40
Mudstone	4.20	4.10	2.00	2.00	28.0	2.40
No. 2 lower coal seam	2.60	1.60	1.00	1.32	25.0	1.40
Medium fine sandstone	4.77	4.88	4.10	3.80	33.0	2.65
Overlying strata	4.87	4.18	3.20	3.10	35.0	2.40
Lower strata	4.77	4.59	3.60	3.15	32.5	2.65

presents the physical and mechanical parameters of each coal and rock stratum.

Given the key technical parameters (L₁, L₂, and L₃) of pressure relief by internal hole-making of headgate 11231 in a coal mine, a variety of numerical simulation schemes were set up by using a control variable method. The goals were to study the vertical stress distribution law of the surrounding rock under different hole-making parameters and determine the final hole-making parameters of the roadway. As presented in

Table 2. Numerical simulation schemes.

Scheme Number	Technical Parameter of Hole-Making
Schemes 1–5	L2/m	L3/m	L1/m
	4.0	3.0	4.0	6.0	8.0	10.0	12.0
Schemes 6–10	L1/m	L3/m	L2/m
	8.0	3.0	2.4	3.2	4.0	4.8	5.6
Schemes 11–15	L1/m	L2/m	L3/m
	8.0	4.0	1.0	2.0	3.0	4.0	5.0

, there are 15 numerical simulation schemes in total (the technical parameters of Schemes 3, 8, and 13 are the same).

According to the field investigation, the surrounding rock deformation of headgate 11231 can be divided into two stages: the no mining influence stage and the mining influence stage. Based on the displacement monitoring data in Figure 4, the displacement of the two ribs and deformation of the higher rib, respectively, account for 34.1 and 33.9% of the total deformation in the no mining influence stage. That is, the deformation of the two ribs of the coal roadway in the no mining influence stage accounts for approximately 1/3 of the total deformation, and the roadway first goes through the no mining influence stage after the support is completed. In this stage, the surrounding rock of the deep coal roadway is mainly affected by the high abutment stress of the roadway rib formed after roadway excavation. Therefore, it is necessary to study the pressure relief law of the coal roadway surrounding rock under different hole-making parameters in the no mining influence stage after roadway excavation to determine the appropriate hole-making parameters and reduce the deformation of the roadway rib at this stage. The abutment pressure caused by coal mining has changed the original stress distribution state of the roadway surrounding rock. The superposition of the advanced abutment pressure and the original high abutment stress of the roadway rib makes the stress of the roadway rib (panel rib) complex and changeable. Based on the key technical parameters of pressure relief by hole-making determined in the stage without mining influence, the parameters of pressure relief by hole-making are continuously optimized so that it can effectively relieve the surrounding rock pressure and reduce the influence of the superimposed high stress on the roadway rib on the surrounding rock in the mining influence stage.

4.2. Key Technical Parameters of Pressure Relief by Internal Hole-Making in the No Mining Influence Stage

(1)

Hole-making depth

The vertical stress distributions in the higher rib of the roadway with different hole-making depths are illustrated in Figure 9b–f. When the hole-making depths are 4, 6, and 8 m, the peak stress of the higher rib of the roadway is located in the deeper part of the hole-making space. When the hole-making depth increases from 8 m to 10 m and 12 m, a new high stress peak zone is formed between the roadway space and the hole-making space, which is not conducive to the stability of the roadway surrounding rock. The stress monitoring lines are arranged at the higher rib of the roadway along the direction of hole-making, as displayed in Figure 9(A₁-A₁) to (F₁-F₁). According to the stress distribution value of the higher rib of the roadway under the conditions of no hole-making and different hole-making depths, a comparison diagram of the stress curve of the roadway rib is drawn, as indicated in Figure 10.

To effectively evaluate the pressure relief effect of different hole-making depths on the surrounding rock, two evaluation indices were selected according to the stress transfer law. First is the transfer amplitude of the high concentrated stress, which is the transfer distance from the original stress peak position to the deep part of the roadway. Second is the reduction in the high concentrated stress in the shallow surrounding rock of the roadway, that is, the pressure relief effect on the surrounding rock in the high abutment stress zone of the roadway rib after roadway excavation. The pressure relief effect from high to low is categorized as excellent, good, fair, no, and poor. As shown in Figure 10a, when the hole-making depth is 4 m, the transfer distance from the original stress peak position to the deep part is 0.5 m. The pressure relief space only reduces the stress in the low stress zone of the shallow roadway, which cannot achieve the purpose of inward movement of the original high concentrated stress of the roadway rib, and there is almost no pressure relief effect. As depicted in Figure 10b, when the hole-making depth is 6 m, the transfer distance is 1.5 m. The pressure relief space makes the stress in the original stress peak zone of the roadway rib decrease significantly: the stress at the peak position decreases by 7.6 MPa, and the high stress zone transfers to the deep part. Thus, the pressure relief effect is evident. As illustrated in Figure 10c, when the hole-making depth is 8 m, the transfer distance is 3.0 m. The pressure relief space makes the stress in the original stress peak zone of the roadway rib decrease overall: the stress at the peak position decreases by 17.3 MPa, and the high stress zone transfers to the deep part considerably. Therefore, the pressure relief effect is remarkable. As displayed in Figure 10d,e, when the hole-making depths are 10 and 12 m, the stress distributions on the original stress peak zone and shallower surrounding rock are not changed significantly after pressure relief. Furthermore, the original stress peak position is not moved inward effectively, and even the shallow abutment stress increases locally, aggravating the damage on the surrounding rock.

(2)

Hole-making spacing

The vertical stress distributions on the higher rib of the roadway with different values of hole-making spacing are indicated in Figure 11a–e. When the hole-making spacing is 2.4 and 3.2 m, a good pressure relief zone can be formed between the hole-making spaces, so that multiple holes form a continuous pressure relief zone along the axis of the roadway. Moreover, the high concentrated stress in the shallow part of the surrounding rock can be uniformly transferred to the deep surrounding rock. Thus, the pressure relief effect is good. When the hole-making spacing is increased to 4.0 m, the transfer effect of the high concentrated stress of the coal mass between the two holes to the deep surrounding rock becomes worse, and a stress concentration zone (Zones A and B) near the inner and outer ends of the hole-making area appears. When the hole-making spacing is 4.8 m, Zones A and B are connected, and the stress of the coal mass between the two holes is restored to the original stress state, resulting in no pressure relief effect. When the hole-making spacing continues to increase to 5.6 m, the high stress moving inward to the deep part accumulates in the coal mass between the two holes, forming a stress-increasing area, which not only has no pressure relief effect, but also causes great damage to the coal mass.

To accurately evaluate the pressure relief effect of different values of hole-making spacing on the surrounding rock, the two evaluation indices proposed above were used. It can be seen from Figure 11 that the coal mass between the two holes (the position with the worst pressure relief effect) has the greatest impact on the stress change of the surrounding rock with different values of hole-making spacing. Therefore, the vertical stress monitoring lines ((F2-F2) in Figure 9 and (A2-A2) to (E2-E2) in Figure 11) were arranged at the center of the two holes and parallel to the holes in the different hole-making spacing schemes. Figure 12 presents the stress curve comparison diagram of the roadway rib under different values of hole-making spacing.

As shown in Figure 12a, when the hole-making spacing is 2.4 m, the transfer distance from the original stress peak position to the deep part is 3.0 m. After pressure relief, the stress in the original stress peak zone decreases significantly, and the stress at the peak position decreases by 13.5 MPa. Thus, the pressure relief effect is excellent. As depicted in Figure 12b, when the hole-making spacing is 3.2 m, the transfer distance is 2.5 m. After pressure relief, the stress at the original stress peak position decreases by 6.0 MPa, and the pressure relief effect is good. When the hole-making spacing is increased to 4.0 m, the stress in the peak zone of the original stress does not decrease significantly after pressure relief but increases slightly in local areas, and the pressure relief effect is generally fair. As illustrated in Figure 12d,e, when the hole-making spacing values are 4.8 m and 5.6 m, the stress in the original stress peak zone increases significantly after pressure relief. The stress at the peak position increases by 3.8 MPa and 4.5 MPa, respectively, and the transfer distances are 2.0 m and 1.0 m, respectively. Hence, the pressure relief effect is fair.

(3)

Hole-making length

The vertical stress distributions of the higher rib of the roadway with different hole-making lengths are indicated in Figure 13b–f. With an increase in the hole-making length, the transfer distance from the peak stress zone of the roadway rib to the deep part gradually increases, and the range of the stress peak zone of the roadway rib gradually decreases. When the hole-making length increases to 3 m, the range of the stress peak zone of the increased hole-making length does not change significantly.

The layout of stress monitoring lines is shown in Figure 13(A₃-A₃) to (F₃-F₃), and Figure 14 presents a comparison of the roadway rib stress curves with different hole-making lengths. The two indices proposed above were used to evaluate the pressure relief effect of the surrounding rock with different hole-making lengths. In addition, to analyze the influence of the different hole-making lengths on the range of the peak stress zone, the stress of 25 MPa was defined as the stress boundary value to measure the range of the stress peak zone. Thus, the reduction coefficient K of the surrounding rock stress peak zone under different hole-making lengths is

$$k=\frac{{{\displaystyle s}}_i}{{{\displaystyle s}}_0}\times 100\%$$

(1)

where S_i is the area of the stress peak zone (greater than 25 MPa) when the hole-making length is 1–5 m, i = 1–5; and S₀ is the area of the stress peak zone (greater than 25 MPa) without hole-making. By defining the area of the original stress peak zone (greater than 25 MPa) as 1, measuring Figure 13, and combining with Equation (1), we obtain k_i = 1.90, 0.90, 0.56, 0.51, and 0.54 (i = 1–5). Analysis shows that the smaller the value of k, the greater the pressure relief effect.

As depicted in Figure 14a–e, when the hole-making lengths are 1, 2, 3, 4, and 5 m, the transfer distances from the original stress peak position to the deep part are 1.5, 2.0, 3.0, 3.5, and 5.0 m, respectively, and the stress reduction values at the peak position are 4.8, 14.8, 17.3, 19.31, and 20.3 MPa, respectively. From the stress curve, it can be observed that the different hole-making lengths have good pressure relief effects. To effectively evaluate the cost performance K of the different hole-making lengths in transferring the original stress peak point to the deep part, the ratio of the transfer distance from the original stress peak position to the deep part to the hole-making length (which can represent the construction cost, that is, the greater the hole-making length, the greater the construction cost) is calculated as follows:

$$K=\frac{{{\displaystyle l}}_i}{{{\displaystyle L}}_{2-i}}\times 100\%$$

(2)

where l_i is the transfer distance (in m) from the original stress peak position to the deep part, i = 1–5; and L_2−i is the hole-making length (in m), i = 1–5. Substituting the known data into Equation (2), we obtain K_i = 1.5, 1.0, 1.0, 0.875, and 1.0, respectively. It can be observed from the analysis that the greater the K value, the higher the cost performance of hole-making and pressure relief.

(4)

Research and analysis of key technical parameters

According to the above analysis, the pressure relief effect of the different hole-making schemes was evaluated with the vertical stress as the main index. The comprehensive evaluation results of each key technical parameter evaluation index on the pressure relief effect of the surrounding rock of different hole-making schemes are listed in

Table 3. Evaluation results of surrounding rock pressure relief for different hole-making schemes.

Evaluation Index		Scheme Number (Hole-making depth)
		Scheme 1	Scheme 2	Scheme 3	Scheme 4	Scheme 5
1	Pressure relief effect	No	Good	Excellent	No	Poor
2	Transfer distance to deep/m	0.5	1.5	3.0	−1.0	1.0
Evaluation index		Scheme number (Hole-making spacing)
		Scheme 6	Scheme 7	Scheme 8	Scheme 9	Scheme 10
1	Pressure relief effect	Excellent	Good	Fair+	Fair	Fair
2	Transfer distance to deep/m	3.0	2.5	2.0	2.0	1.0
Evaluation index		Scheme number (Hole-making length)
		Scheme 11	Scheme 12	Scheme 13	Scheme 14	Scheme 15
1	Pressure relief effect	Fair	Good	Good	Good	Good
2	Transfer distance to deep/m	1.5	2.0	3.0	3.5	5.0
3	k	1.90	0.90	0.56	0.51	0.54
4	K	1.5	1.0	1.0	0.875	1.0

. According to the analysis, Schemes 2 and 3 can be selected with respect to the hole-making depth, that is, the hole-making depths are 6 and 8 m, respectively. Schemes 6, 7, and 8 can be selected in terms of the hole-making spacing; thus, the hole-making spacing values are 2.4, 3.2, and 4.0 m, respectively. When the hole-making length is 1 m, the reduction coefficient of the range of the stress peak zone k₁ ˃ 1. In addition, the transfer distance from the original stress peak position to the deep part is small, so this scheme should be excluded. For the other hole-making length schemes (Schemes 12–15), it is not appropriate to analyze the pressure relief effect simply from the perspective of stress, as it should be analyzed in combination with other factors.

4.3. Optimization Analysis of Key Technical Parameters of Pressure Relief by Internal Hole-Making in the Mining Influence Stage

Panel 11231 adopts step-by-step mining with an interval of 5 m. Figure 15 shows the nephogram of abutment stress distribution and the stress monitoring curve in front and behind the panel near the rib of the headgate. By analyzing Figure 15, the following can be inferred: (1) The stress reduction zone is from the coal rib of the panel to 4 m in front of the panel. Moreover, the stress rise zone is 4 m in front of the panel and further away, and the distance between the stress peak position and the coal rib of the panel is 12 m. (2) Affected by the dynamic pressure of coal mining, the stress value in the higher rib of the headgate increases sharply. The vertical stress values of the coal body at depths ≥ 6 m are greater than the primary rock stress, forming a high concentrated stress zone. The stress peak zone of the roadway rib shifts deeper under the influence of mining, with a transfer distance of 4–5 m. (3) The advanced abutment pressure caused by coal mining and the high abutment stress of the roadway rib are superimposed on each other, which jeopardizes the stability of the roadway rib coal in the area affected by dynamic pressure. To summarize, transferring the high concentrated stress in the shallow coal body of the roadway using hole-making and reducing the continuous damage caused by the high stress to the shallow coal and rock are essential for maintaining the stability of the surrounding rock of the coal roadways affected by mining.

Based on the key technical parameters of hole-making determined at the stage without dynamic pressure influence and in combination with the dynamic pressure disturbance law of panel 11231, we continued to optimize the hole-making parameters and arranged stress monitoring lines at reasonable positions in the stress rise zone in front of the panel to analyze the stress distribution law of surrounding rock with different hole-making schemes under the influence of superimposed stress.

(1)

Hole-making depth

As illustrated in Figure 16, when the hole-making depth is 6 m or 8 m, the stress decreases in the area where the stress on the higher rib of the panel increases during the dynamic pressure influence stage. Comparing Figure 15a and Figure 16b, the following can be observed: (1) When the hole-making depth is 8 m, the range of the stress reduction zone and the pressure relief degree are larger than that when the hole-making depth is 6 m. (2) When the hole-making depth is 8 m, the pressure relief space transfers the peak zone of the superimposed stress of the roadway rib to the deeper part, and the pressure relief effect is good. (3) When the hole-making depth is 6 m, the hole-making space causes great damage to the coal body at the roadway depth of 5 m, which affects the stability of the rock mass of bolt-grouting coal in the shallow part of the roadway. Based on the above analysis, Scheme 3 is preferred with respect to the hole-making depth of headgate 11231; that is, the hole-making depth is 8 m.

(2)

Hole-making spacing

Figure 17(a-1), (b-1), and (c-1) show the vertical stress nephogram of the higher rib of the roadway with different values of hole-making spacing in the advanced dynamic pressure zone. The center positions of the two holes are respectively selected to monitor their stress values. The corresponding stress curves are displayed in Figure 17(a-2), (b-2), and (c-2). Figure 17(a-2), (b-2) indicate that the pressure relief zone is still formed at the center of the two holes. When the hole-making spacing increases to 4.0 m, stress concentration between the two holes occurs, and the hole-making space threatens the stability of the roadway rib coal body, which is not conducive to the stability of the roadway rib surrounding rock in the advanced dynamic pressure zone. The hole-making spacing is extremely small, and the boreholes are dense. The dense boreholes cause great damage to the coal body in the shallow anchorage zone of the roadway, which is not conducive to the stability of the roadway surrounding rock. Through comprehensive comparison and analysis, it is determined that the optimal hole-making spacing is 3.2 m.

(3)

Hole-making length

As shown in Figure 18, with an increase in the hole-making length, the pressure relief range increases, the peak position of the superimposed stress shifts deeper into the surrounding rock, and the pressure relief effect becomes more evident. However, considering the construction capacity of hole-making machines and the use efficiency of the hole-making space, the hole-making length cannot be increased without limit. Comparing Figure 18a–d, the hole-making lengths of 3 and 4 m can meet the pressure relief requirements of the surrounding rock in the mining influence stage. As depicted in Figure 18b,c, when the hole-making lengths are 3 and 4 m, the effective pressure relief ranges of the surrounding rock are 4.5 m and 5.0 m, respectively. As the hole-making length increases by 1 m, the effective pressure relief range does not increase significantly. In combination with equation (2), K₃ = 1.0 ˃ K₄ = 0.875. Comparing the pressure relief effect and construction quantities of the two schemes, the hole-making length of 3 m can better meet the pressure relief requirements, reduce the construction quantities, and obtain the optimal solution of hole-making pressure relief and efficient construction.

(4)

Determination of key technical parameters

According to the stress distribution law of the surrounding rock with different hole-making schemes in the advanced dynamic pressure zone of panel 11231 and the comprehensive analysis of factors such as construction benefit, cost, and construction period, the key parameters for pressure relief of higher rib by hole-making in headgate 11231 were determined as follows: the hole-making depth, spacing, and length are 8 m, 3.2 m, and 3 m, respectively.

5. Cooperative Control Technology of External Anchor–Internal Unloading of Surrounding Rock in Deep Coal Roadways

5.1. Technical Parameters of External Anchor–Internal Unloading of Surrounding Rock in the Test Section of Headgate 11231

The cooperative control technology of external anchor–internal unloading of surrounding rock in the test section of headgate 11231 was implemented in two stages, namely, strengthening of the shallow surrounding rock and pressure relief by hole-making in the deep part. As illustrated in Figure 19I-a–c, reinforcement of the shallow surrounding rock is first carried out for the test section of the headgate, and three steel strand anchor cables with dimensions of φ21.8 × 10,500 mm and a row spacing of 2.0 × 1.6 m are added to the roof. Moreover, three steel strand anchor cables with dimensions of φ21.8 × 4500 mm are added to the two ribs. The row spacing between the higher ribs is 1.3 × 1.6 m, whereas that between the lower ribs is 1.1 × 1.6 m. Each anchor cable uses three resin anchor agents of model Z2360 for each hole. The roof and rib anchor cables are connected by H-type double steel belt beams supporting large square pallets (400 × 400 mm) and small pallets (200 × 200 mm) to form the roof and rib anchor cable truss beam structure, which can resist the overall outward heave of the two ribs of soft coal in the roadway. The pre-tightening force of the anchor cables is not less than 250 kN. After completing the anchor cables, shotcrete was applied on the higher rib surface of the roadway in the test section, and C20 concrete was sprayed with a thickness of 30 mm. After completion of the surrounding rock shotcrete, coal grouting was conducted on the roadway rib. Three grouting holes were arranged on the higher rib of the roadway with a spacing of 1.2 m, and two grouting holes were arranged on the lower rib with a spacing of 1.4 m. The grouting material was cement slurry and soluble silicate, in which the water–cement ratio was 1.3:1–1.4:1 (mass ratio), and the ratio of the cement slurry to soluble silicate was 1:0.5–1:1.0 (volume ratio). The cement used was P.O42.5 normal Portland cement, and the Baume density of soluble silicate (liquid sodium silicate) was 35° Bé. The depth of the grouting hole was 3 m, the grouting pressure was not less than 2.5 MPa, and the row distance of the grouting hole was 3.0 m.

After completing the bolt-grouting support of the shallow surrounding rock of the roadway, deep coal hole-making was performed to relieve the pressure, as indicated in Figure 19. The distance from the borehole to the roadway floor is 1.5 m, and the upward angle is 6.5°. The diameter of the shallow drilling hole is 100 mm. To protect the integrity of the shallow surrounding rock and reduce the damage caused by hole-making to the shallow coal, steel pipes were imbedded in shallow boreholes (Figure 19(Ⅱ)-a), and grouting was conducted for pipe fixing. The deep part is the hole-making space, and its key parameters are as described above: the hole-making depth, spacing, and length are 8 m, 3.2 m, and 3 m, respectively.

5.2. Analysis on the Effect of Cooperative Control Technology of External Anchor–Internal Unloading in Roadway Surrounding Rock

(1)

Analysis of the prestressed field of the surrounding rock reinforced anchor cables in the coal roadway test section

According to the support parameters of the original bolts and reinforced anchor cables in the test section of headgate 11231, the numerical simulation software was used to simulate the prestressed field formed by the roadway shallow surrounding rock and hole-making space, and the effective control range and control principle of the cooperative control technology of external anchor–internal unloading were comprehensively analyzed. As shown in Figure 20, two supporting bodies (anchor coal mass and hole-making space, as depicted in Figure 20a) and three zones (external anchor zone, buffer protection zone for anchorage zone, and internal unloading zone, as illustrated in Figure 20b) are formed in the roadway surrounding rock after adopting the cooperative control technology of external anchor–internal unloading. After implementing the anchor-grouting support in the roadway shallow surrounding rock, anchored coal mass is formed in the range of 0–5 m depth of the surrounding rock. The strength of the surrounding rock is improved effectively, which provides a basic protection environment for resisting large deformation of the surrounding rock caused by deep high ground pressure and strong mining. After the hole-making space is formed, a pressure relief deterioration zone is created within 7–12 m depth of the surrounding rock, which transfers the high stress in the shallow surrounding rock to the deep part, alleviating the continuous damage due to the high stress in the shallow surrounding rock. At the same time, the large-diameter hole-making space can effectively absorb the continuous deformation caused by the transfer of the deeper coal body to the roadway space due to the high horizontal stress. The depth of the surrounding rock (5–7 m) is the buffer protection space, whose function is to avoid damage to the shallow anchored coal mass caused by the hole space.

(2)

Observation results and analysis of roadway surrounding rock displacement

The roadway surrounding rock displacements in the test section (using the new technology) and in the non-test section were monitored on-site. The results are displayed in Figure 21. The values of the deformation of the higher rib of the roadway in the non-test section are 400 mm and 780 mm in the no mining and mining influence stages, respectively. The deformation of the higher rib of the roadway in the test section is 190 mm after hole-making, and the accumulated reduction in the higher rib deformation of the roadway is approximately 850 mm. As indicated in Figure 22, after the new support technology is adopted, the available width of the roadway is not less than 4.2 m, which can always meet the ventilation and transportation requirements of the roadway. Furthermore, there is no need to renovate the roadway rib. This technology effectively limits the continuous deformation of the rib surrounding rock in the test section of headgate 11231 and ensures the stability of the surrounding rock of the mining roadway affected by the deep dynamic pressure.

6. Conclusions

(1)

Given the surrounding rock control problem of a large deformation mining roadway disturbed by deep dynamic pressure in a coal mine, the cooperative control technology of external anchor–internal unloading of surrounding rock of a large deformation coal roadway was proposed.

(2)

A numerical model conforming to the production of the geological conditions of panel 11231 of the coal mine was constructed, and the vertical stress distribution and transfer rule of surrounding rock under the conditions of five schemes for three key hole-making parameters were studied. The pressure relief degree and effect of each scheme’s hole-making space in the no dynamic pressure influence stage were analyzed using a variety of evaluation indices, and the reasonable parameter range of the surrounding rock hole-making space in this stage was obtained.

(3)

The pressure relief effect of each optimized scheme under the mining dynamic pressure condition of panel 11231 was studied, and the effective pressure relief range of each scheme was compared and a comprehensive evaluation of factors, such as construction benefit, cost, and construction period, was performed. Based on the results, the major technical parameters for pressure relief during hole-making were finally determined as follows: the hole-making depth, spacing, and length were 8 m, 3.2 m, and 3 m, respectively.

(4)

The field practice was carried out in the test section of headgate 11231. The monitoring results showed that the displacement of the hole-making rib was reduced by 850 mm, making the available width of the roadway no less than 4.2 m, which meets the ventilation and transportation requirements of the roadway, and eliminates roadway rib renovation, which ensures the stability of the surrounding rock. Thus, the new technology is of great significance for the further development of strategies for controlling the surrounding rock of deep coal roadways affected by dynamic pressure.