With the increase in the mining depth of coal mines in China, the problem of large deformation of roadways owing to high-ground pressure has become prominent even under enhanced support systems. To reduce the high pressure on the surrounding rock, this study investigates a pressure-relief method for deep roadways using drilling borehole groups. Based on a deep roadway in the Huainan mining area of China, the influences of drilling parameters, such as borehole diameter, length, and arrangement were investigated. The results indicate that the fan-shaped arrangement of the borehole group can compensate for the dilatancy deformation of the surrounding rock. The peak stress of the surrounding rock is reduced and transferred to the inner part of the surrounding rock. Furthermore, a field experiment was conducted on an experimental roadway. The deformation of the roadway was monitored and compared with that of an adjacent roadway that did not apply the pressure-relief method. The monitoring results indicated that the deformation of the experimental roadway was significantly reduced.

In China, coal is a major energy resource, which plays a dominant role in energy systems. China’s coal reserves are approximately 597 trillion tons, out of which approximately 53% are buried in the deep stratum (exceeding the depth of 1000 m) [1-3]. With the increase in the coal mining depth in recent years, an increasing number of roadways suffer from high geo-stress, which induces large deformation and failure and poses serious threats to mining safety [4-6]. Therefore, preventing large deformations of deep roadways with high geo-stress has become an important issue in coal mining. To prevent large deformation problems in deep roadways, engineers usually enhance the support system. For example, the use of high-strength and super-long bolts [6, 7], a method of bolting and shotcreting, U-steel support, grouting and floor bolting casting [3-5], and the reduction of the interval spacing of the supporting structures [4, 5]. Although some development has been achieved, large roadway deformations still frequently occur under high-ground pressure. The resistance offered by the supporting structure is extremely limited. Repeated repair is difficult and results in significant economic losses. Another approach to prevent large deformations of roadways is to release the high-ground pressure around the roadway using special measures. For example, floor grooving releases the high stress that accumulates on the floor, which is conducive to treating floor heaves [7, 8]. The rock mass within the range of pressure relief is destroyed by using high-pressure water injection softening and blasting pressure-relief methods, which reduces the elastic modulus and strength of the rock mass [5-7]. Drilling boreholes [6-9] in coal seams is beneficial for preventing coal and gas outburst accidents. Consequently, the accumulated energy on the surrounding rock surface decreases, leading to the release of deformation caused by rock mass swelling. The stress state of the rock mass around the roadway is changed from the state of high in situ stress to the state of low in situ stress. However, research on pressure-relief theory and technology is far from mature, and research on pressure relief for deep soft rock roadways is scarce [9-12].

In recent years, many scholars have studied drilling pressure-relief technology using various methods. German researchers [13] observed through the diameter of 95, 145, and 200 mm drilling tests that the minimum diameter of drilling is 95 mm and drilling spacing cannot be higher than 10 m. The drilling depth is three times the mining height at the mining face and four times the mining height at the side of the roadway. The former Soviet Union researchers conducted a lot of research on drilling pressure relief [14], and the test showed that when the drilling diameter was 300 mm and the hole spacing was 1.5–2 m, the pressure-relief effect of coal seam was good. When the hole spacing is 3 m, the pressure-relief effect decreases. Wang et al. [15, 16] studied the mechanism of borehole pressure relief through numerical simulations and analyzed the determination method of borehole parameters and the evaluation basis of the pressure-relief effect. They proposed the design of “pressure-relief-support” for the deep roadway. Wang et al. [17] found that different boreholes have different failure modes, and that their collapse can significantly reduce the energy accumulated in the rock. Li et al. [18] investigated the pressure-releasing process during borehole collapse, which provided guidance for the timing of supporting measures. Lu et al. [19] found that the pressure-relief effect was better when the boreholes were parallel to the intermediate principal stress. Yuan et al. [20] studied the influence of borehole diameter on the mechanical properties of the surrounding rock in deep roadways by establishing model experiments and obtained a formula for a reasonable value of hole spacing. Mishra et al. [21] proposed an on-site monitored method of borehole pressure relief for evaluating a single excavation roadway. Liu et al. [22] used numerical simulations to analyze damage to the surrounding rock structure and high-stress transfer caused by different arrangements of boreholes. Lan et al. [23] and Guo et al [24] simulated and analyzed the effect of borehole diameter, borehole spacing, and coal stress on pressure relief. Both Ma et al. [25] and Song et al. [26] studied the combined support mechanism of the combined support of roadway pressure-relief holes and bolts.

However, few studies have focused on field experiments on pressure-relief methods for deep coal mine roadways using borehole groups [27-29]. In this study, the influence of borehole groups on the stability of deep roadways was investigated through numerical simulations and field experiments. A fan-shaped arrangement of borehole groups is proposed. Furthermore, an application experiment was conducted on an experimental roadway. The deformation of the experimental roadway was monitored and compared with that of the adjacent roadways, which do not apply the pressure-relief method.

The Guqiao coal mine in the Huainan mining area is located in Anhui Province, China (see Figure 1). It is designed for an annual production capacity of 10 Mt. The buried depth of the eastern wing roadways exceeds 1000 m. The maximum tested in situ stress is approximately 32 MPa.

The rock mass is mainly composed of soft rocks, such as mudstone, sandy mudstone, siltstone, and coal seams. Therefore, massive roadway deformation often occurs despite the adoption of an enhanced support method. Serious floor heaves and roof sags frequently occur and require repeated repairs. In this study, an east-wing belt roadway was chosen as the experimental roadway for the pressure-relief method using borehole groups. The experimental section was approximately 60 m long (see Figure 2). The burial depth of the experimental roadway was approximately 1030 m. Coal seam 11-2 is overlying and coal seam 8 is underlying in the test roadway, and there is no mining activity in coal seam 11-2 and coal seam 8. The dip angle of overlying coal seam 11-2 and underlying coal seam 8 in this section is slow, and the dip angle of coal layers is about ∠2–4°. The hydrogeological conditions are simple, the main water damage is sandstone fracture water, and the normal water inflow of 1–3 m³/h.

Monitoring sites 1# and 2# were arranged in the experimental roadway, whereas Monitoring sites A1# and A2# were arranged in the rail roadway, which is adjacent to the belt roadway and does not implement a pressure-relief scheme (see Figure 2) They exhibit similar geological conditions and support systems.

The shape of the roadway is a straight wall semi-circular arch. The excavation method adopts the method of artificial air hammer drilling and stratified blasting. The roadway section was 5.6 m wide and 4.6 m high. The rock strata were mainly composed of sandy mudstone, siltstone, and fine silty sandstone. The dip angle of the rock strata was approximately 3–8°. The previously excavated roadways with similar geological conditions were surfed from severe deformation, although the enhanced support method was adopted, and the maximum displacement of the roadway surface exceeded 500 mm within 3 month after excavation (see Figure 3).

3.1 Prediction of Plastic Zone Around Single Borehole

A stress reduction zone was produced around the surrounding rock of the roadway after the pressure relief (see Figure 4). The peak stress induced by excavation was assumed to be reduced after pressure relief, promoting the deep transfer of high stress to the surrounding rock and reducing the risk of large deformation and failure of the surrounding rock. It not only provides compensation space for the radial deformation of the surrounding rock of the high-stress roadway but also effectively controls the rate of deformation of the surrounding rock on both sides, which is conducive to the support and production of the surrounding rock.

The drilling mechanics model was established using the elastic–plastic mechanics theory(see Figure 5). To facilitate the analysis, it was assumed to be a plane-strain problem. The polar coordinate system was used to simplify the ability of the inner wall of the borehole to resist deformation into a uniformly distributed radial stress P1. The stress equilibrium equation can be expressed as [30, 31]:

σrr+σrσθr=0
(1)

where, σr and σθ represent the radial and circumferential stresses, respectively, and r is the polar radius.

Assuming that both the axial and circumferential displacements are equal to zero, the equation can be written as [30, 31]:

d2udr2+1rdudrur2=0
(2)

where u denotes the displacement.

The radial stress component and circumferential stress were obtained using equations (1) and (2), as [30, 31]:

{σr=AB1r2σθ=A+B1r2
(3)
A=2C1(λ+G),B=2C2G
(4)

where C1 and C2 are the constants, G is the shear modulus of the rock, λ=νE1+ν1-2ν , ν is the Poisson’s ratio, and E is the elastic modulus of rock.

σθ > σr and the following equation can be derived. (See equations (1)–(7) in the online Supplementary Material 1 for comprehensive derivation process.):

σθσr=2Br2=2(Peσ0)r12r22r2(r22r12)=σs
(5)

where σs is the yield stress of rock mass, Pe is the equivalent self-bearing pressure on the shallow part of the borehole, σ0 is the radial in situ stress in the rock mass, r1 is the borehole radius, r2 is the limit disturbance radius of the borehole induced stress, and σs is the yield stress of the rock mass.

At r = rs, the radius of the plastic zone around the borehole can be written as (see equations (8)–(10) in the online Supplementary Material 1 for comprehensive derivation process):

rs=2σ0-Per12r22σsr22-r12
(6)

3.2 Pressure-Relief Drilling Design

Presently, the drill diameter of mining geological drill in the coal mines is 65–153 mm. The commonly used types in the Huainan mining area are Φ94, Φ113, Φ133, and Φ153 mm. The Φ153 mm large-diameter drill bit is used for shallow reaming; its drilling rate is slow, and it is not suitable for deep drilling. Therefore, a drill bit with a diameter of approximately 133 mm was selected for this field experiment.

The unloading disturbance range of the high-stress roadway excavation was approximately five times the borehole diameter. In addition, to ensure the integrity of the roof arch, boreholes were arranged in the range from the shoulder to the corner of the roadway to induce lateral pressure relief, and the stress state of the roof was improved synchronously.

By comparing the current common horizontal drilling layout with proposed fan-shaped layout, although the horizontal drilling scheme is parallel in space, the operation process is simple, convenient for construction, and the surrounding rock can achieve uniform pressure relief. However, it is not conducive to the full release of energy in the stress concentration area, and the rig moves frequently. The fan-shaped arrangement means that each row of boreholes is arranged in a radial fan shape in space, so that the boreholes are dense at the opening borehole, but the rig displacement frequency during the construction process is low, and the pressure-relief strength in the stress concentration area is increased. Considering the expected pressure-relief effect and difficulty of implementation, we choose a fan-shaped arrangement method for the pressure-relief borehole groups, which include 32 boreholes distributed symmetrically on both sides of the roadway (see Figure 6).

Each row of boreholes was arranged in a fan or radial shape. The relevant parameters have been determined (see Figure 7). Thus, the rig shift does not need to be moved frequently. The length of the boreholes was 25 m and their diameter was 133 mm.

4.1 Model Establishment

A three-dimensional numerical model of the belt roadway was established using the FLAC3D software, as shown in Figure 8. The model dimensions were 80 m × 60 m × 60 m. The roadway was 5.6 m wide and 4.6 m high. The bottom and horizontal boundaries were fixed. The vertical stress σzz = 23 MPa vertical stress, horizontal stress σxx = 32 MPa, σyy = 25 MPa. Y = 20 m was selected as the center to arrange the 32 combined pressure-relief boreholes. The rock mass obeys the Mohr–Coulomb criterion. The mechanical parameters of the rock strata are listed (see Table 1). The drilling arrangement is illustrated (see Figures 8 and 9).

4.2 Result Analysis

4.2.1 Research on the Influence of Different Parameters on Pressure Relief Effect

In order to research on the influence of different borehole diameters on pressure-relief effect, pressure-relief boreholes of different diameters (94, 113, and 133 mm) were selected for numerical simulation of multi-hole pressure relief. The resulting vertical stress contour under different borehole diameters were observed (see Figure 10).

The results of numerical simulation show that the vertical stress of surrounding rock decreases more with the increase of borehole diameter. The larger the diameter, the better the pressure-relief effect.

In order to research on the influence of different borehole lengths on pressure-relief effect, drilling holes with different lengths (20, 25, and 30 m) were selected. The vertical stress contours of roadway surrounding rock under different length parameters were compared (see Figure 11).

Numerical simulation results have been observed. There is a significant change in the degree of reduction of the vertical stress in the surrounding rock as the drilling length increases. However, this reduction becomes relatively stable beyond this point and exceeds the stress concentration area at 25 m. Therefore, it is recommended to use large-diameter boreholes for pressure-relief purposes. It would be more appropriate to select a large-diameter relief borehole of approximately 25 m in length.

4.2.2. The distribution of vertical stress after pressure relief using the final borehole groups is shown (see Figures 9 and 12)

The multi-row boreholes formed a stress-unloading surface in the surrounding rock (see Figure 12(a)). By connecting the plastic zone of the borehole, the failure of the stress-bearing zone is maximized, the range of pressure relief becomes larger, and pressure-relief effect improves (see Figure 12(b)). The peak vertical stress at the cutting plane (Y = 30 m) was 13.65%, which was 15.62% lower than that before pressure relief, and the concentrated stress on the side was significantly reduced (see Figure 12(c)).

A horizontal monitoring line was selected at a height of 1 m on the right side of the roadway, and measurement points with depths of 0, 2, 5, 7, 9, 12, 14, 16, 18, and 20 m were selected to observe the vertical stress of the shallow and deep parts of the surrounding rock (see 12,13,Figures 12(b) and 13).

The vertical stress along the monitoring line is illustrated (see Figure 13). The peak value of the vertical stress is reduced from 44.67 to 38.89 MPa before and after pressure relief, and there is a tendency to transfer to the deeper part of the surrounding rock.

The initial deformation of the roadway was characterized by peripheral contraction (see Figure 14). After the implementation of pressure-relief drilling, the displacements of the roof, floor heave, and sidewalls were significantly reduced. The roof subsidence of the roadway was reduced by 36.08%. The floor heave was reduced by 37.51%. The displacements of the left- and right-sided walls were reduced by 38.25% and 32.65%, respectively. The results indicate that the drilling design scheme reduces the peak stress of the rock surrounding the roadway, transfers it to the deeper part of the surrounding rock, and aids in controlling roadway stability.

5.1 Implementation of Pressure-Relief Drilling

Based on the designed scheme, pressure-relief borehole groups were implemented in the experiment. Monitoring site 2# implements a pressure-relief scheme (see Figure 2). All monitoring sites exhibit similar geological conditions and support systems. The total length of the experimental section was approximately 60 m with three borehole groups. Each group consisted of 32 boreholes. The borehole groups in this field are shown (see Figure 15).

The surface displacement measurement of the roadway includes the displacement of two sides of the roadway, the roof sag, and the floor heave. The monitoring sites are measured by the method of cross-measurement during monitoring period.

5.2 Monitoring Results of Surface Displacement

The monitoring results for the surface displacement are shown (see Figure 16.). During the period of 0–40 days, the displacements of the side, floor, and roof of monitoring site 1# increased rapidly, with a speed of approximately 2.8 and 2.6 mm/day, respectively. After 40 days, the displacement of the side slowed down after pressure relief, with a deformation rate of approximately 0.49 mm/day; the deformation rate of the roof was approximately 0.94 mm/day; the deformation rate of the floor was approximately 0.84 mm/day. The displacements of the roof and floor were higher than those of the side. The maximum two-sided displacement, roof sag, and floor heave at monitoring site 1# and monitoring site A1#, monitoring site A2 #, and monitoring site 2 # were reduced by 40.54%, 36.3%, and 41.37% respectively compared with monitoring site 1# and monitoring site A1#, monitoring site A2# and monitoring site 2#. Compared with the minimum two-sided displacement, roof sag, and floor heave at Measuring points A1#, A2#, and 2#, the minimum two-sided displacement, roof sag, and floor heave were reduced by 30.52%, 30.08%, and 30.32% (see Table 2). The deformations of the roof and floor of the roadway and surrounding rock on the two sides were controlled.

5.3 Inner Displacement of Surrounding Rock

Multipoint displacement meters were installed on the roof near Monitoring site 1# in the pressure-relief area and Monitoring site 2# in the nonpressure-relief area. The measurement points were 4, 6, 8, and 10 m deep. The inner displacements of the roof are shown (see Figure 17).

Before pressure relief, the displacements of the deep and shallow base points of the roof gradually increased, and the displacement rate of the deep displacement of the roof was high. After 55 days, the displacement rate gradually decreased. The deformation stability values of surrounding rock at 4 and 6 m base points in the shallow part of the pressure-relief zone were 29 and 24 mm, which were 27.5% and 22.58% lower than those in the nonpressure-relief zone. The deformation of surrounding rock at 8 and 10 m base points in the deep part was stable at 17 and 13 mm, which was reduced by 26.09% and 13.33% (see Figure 17). A figure shows an image of the experimental roadway 200 days after the pressure relief (see Figure 18). The drilling pressure-relief scheme could effectively reduce the high stress and control the radial displacement of the deep surrounding rock.

5.4 Discussion on Mechanism of Hole Collapse After Drilling

The strength of the surrounding rock mass decreases after drilling and forming of roadway surrounding rock mass. The rock mass around the borehole can maintain its stability briefly at the initial stage of the completion of the borehole. Under the influence of pressure-relief stress and time effect, the rock mass of the borehole wall will collapse and block the borehole [23].

Research into the collapse conditions of real boreholes indicates that the hole effect is formed in the original rock layer after the borehole is formed, resulting in a redistribution of stress around the borehole. The original stress of the rock layer is balanced and unbalanced. The surrounding rock mass along the central axis of the borehole can be divided into four zones: crushing, plastic, elastic-plastic, and elastic. During the drilling process, the drilling tool disturbs the rock surrounding the borehole wall, resulting in a fractured zone where the rock is mostly distributed in blocks. The surrounding fracture develops into the most unstable state due to the pressure-relief effect of the borehole. Due to the stress of the surrounding rock on the roadway and the effects of time, the rock mass in the fractured area may fall or collapse, resulting in blockage and plugging of boreholes.

In summary, drilling holes into the surrounding rock can create a pressure-relief zone that disrupts the stress balance of the original rock layer and redistributes stress around the hole. The drilling process can damage the rock mass, reducing its strength and causing cracks to develop around the hole. This can lead to poor stability of the coal and rock bodies in the crushing area and collapse of the rock mass in the hole wall, which can block the hole under pressure-relief stress and ultimately achieve the goal of stress consumption. The purpose of drilling at a specified angle is to reduce the stress in the longitudinal stress concentration area and to facilitate the construction of the drilling rig.

The influence of borehole groups on the stability of deep roadways was studied through numerical simulations and field experiments. A fan-shaped arrangement of the borehole group was proposed and applied to an experimental roadway in the Guqiao coal mine. The main conclusions are as follows:

  1. The contradiction between high stress and low strength is the key reason for large deformations of deep roadways. Numerical simulations and field monitoring showed that reasonable borehole pressure-relief measures could effectively reduce the high stress of the rock surrounding the roadway.

  2. The results of the numerical simulation and field test showed that the design scheme of the pressure-relief borehole proposed in this study could effectively slow the large deformation rate and reduce the deformation of the rock surrounding the roadway. The field comparative monitoring results showed that the deformations of the two sides of the surrounding rock could be reduced by approximately 35.53%, roof sag could be reduced by approximately 33.19%, and floor heave could be reduced by approximately 35.85%.

  3. Reasonable borehole pressure relief could effectively reduce the concentrated stress. This can be used as an auxiliary means to prevent and control large deformation disasters in deep high-stress weak surrounding rocks.

Data supporting the results can be found in figures and tables within the manuscript.

We declare that we have no conflicts of interest.

This paper received financial supports from the National Natural Science Foundation of China (grant numbers:42277170). Our thanks go to the Huainan Mining Industry (Group) Limited Liability Company for their support in data collection and rock observation.

The derivation of the equation for solving the radius of the plastic zone of drilling is in the Supplementary Material. In the file, the expressions of axial stress and axial strain of borehole are presented. The stress boundary conditions around the borehole are determined. Then the expressions of σθ, σr and σL are derived. The critical yield stress formula of the borehole is derived by obtaining the formula of the stress around the borehole and combining with the relevant Tresca yield criterion. Finally, the theoretical formula of borehole plastic zone is obtained.

Supplementary Materials Available: **[ The derivation of the equation for solving the radius of the plastic zone of drilling is in the Supplementary Material. In the file, the expressions of axial stress and axial strain of borehole are presented. The stress boundary conditions around the borehole are determined. Then the expressions of σθ, σr and σL are derived. The critical yield stress formula of the borehole is derived by the formula of the stress around the borehole and combining with the relevant Tresca yield criterion. Finally, the theoretical formula of borehole plastic zone is obtained.]**.

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Supplementary data