Unloading Technology and Application Research of Variable Diameter Drilling in Dynamic Pressure Roadway

Abstract

Theoretical analysis and numerical simulation are used to study the influence of different parameters of variable diameter borehole pressure relief technology on the surrounding rock and support. A strain-softening model was established to analyze the intrinsic connection between the parameters of variable diameter boreholes and the evolution of surrounding rock stress, deformation law, and support strength. The results show that: (1) With the increase in shallow borehole diameter, it is easy to cause anchor de-anchoring phenomenon. (2) After the deep borehole diameter is more than 250 mm, it transfers the peak of the shallow vertical stress to the deep surrounding rock (about 16 m away from the coal wall). (3) If the position of the variable borehole aperture is set between the anchorage zone and the stress peak of the roadway, the stress transfer effect is better, and the influence and effective binding force on the surrounding rock is smaller. (4) When the spacing is 1.0 m~2.0 m, the vertical stress starts to transfer to the deep surrounding rock, the deformation of the surrounding rock is smaller, and the reduction in the effective binding force of the anchors is smaller. The result can provide a reference for similar production conditions.

1. Introduction

With the increase in depth and intensity of coal mining, disasters such as mine earthquakes and rock bursts become more and more serious [1]. The local pressure relief measures can be divided into the coal seam pressure relief measures to control the energy storage, the dynamic load reduction technology to control the sudden release of roof energy, and the floor pressure relief method to improve the floor stress environment. Various types of decompression and pressure relief measures have a limited scope of action, and the main purpose is to reduce the stress level of coal and rock layers around the roadway, form a “rupture circle”, and weaken the severe dynamic load [2]. Coal seam pressure relief measures generally include drilling pressure relief, coal blasting, water injection softening, and so on. Among them, the large-diameter drilling pressure relief technology has the characteristics of simple operation, low construction cost, and strong applicability, which is widely used in rock burst mines [3,4,5,6,7]. The essence of the coal seam drilling pressure relief is to actively release the part of the energy accumulated in the surrounding rock of the roadway due to the rebalancing of the original rock stress in the process of the roadway after the excavation. To avoid the ruck burst and other disasters that are produced by the violent release due to the excessive energy accumulation in the surrounding rock and to ensure the safety of coal production [8,9]. Many scholars at home and abroad have mainly researched the mechanism and drilling arrangement parameters of drilling pressure relief and impact prevention [10,11,12].

As for the research on the mechanism of pressure relief boreholes to prevent rock bursts. The action mechanism of pressure relief in drill holes and the redistribution process of surrounding rock pressure are studied by different numerical simulation software (FLAC3D, version 5.0, Itasca Consulting, Inc., Minneapolis, MN, USA; 3DEC, version 5.0, Itasca Consulting, Inc., Minneapolis, MN, USA), and the results are applied to industrial tests. The results show that the mechanism of pressure relief from drilling holes is that the drilling holes actively change the integrity of the surrounding rock and form plastic zones so that the stress is transferred to the deeper part of the surrounding rock, which reduces the energy aggregation of the coal and rock seam around the roadway and reduces the risk of rock burst [13,14,15]. Sanfirov et al. [16] divided the surrounding rock of the borehole into three parts through the degree of destruction of the coal around the borehole: the residual strength zone, the plastic zone, and the elastic deformation zone. The widths of the three zones were calculated and used as a guide to determine the drilling parameters. Li et al. [17] analyzed the elastic-plastic state of the coal seam borehole under the ideal state of elastic strain softening; Tambovtsev [18] established a similar analytical mechanical model to analyze the energy input required to be able to produce macroscopic cracks under different drilling diameters; Zhai et al. [19] used a three-axis loading experimental system to simulate the lateral stresses on the coal, using acoustic emission (AE) to monitor acoustic emission events during different drilling processes and record their characteristics; Zhao et al. [20] applied physical modeling and acoustic emission techniques to study the fracture evolution of rock in prefabricated circular boreholes and found that tensile splitting cracks were produced in the direction parallel to the loading direction, and compression cracks were produced at both sides of the borehole.

In the research of drilling arrangement parameters, some foreign scholars such as Williams and Johnson [21], Lempp et al. [22], and Paraschiv-Munteanu and Cristescu [23] took the lead in implementing the coal seam drilling pressure relief measures in mine production for the prevention and control of rock burst and wrote it as an industry standard into the regulations and stipulated that the pressure relief borehole method as the preconstruction must be carried out before the mining; based on a large number of engineering practices, they proposed the optimal borehole diameter and spacing and summarized the formula of independent borehole rupture radius with the underground measurement data to get good results in preventing and controlling the rock burst. On this basis, research on the design of drilling unloading and preventing rock burst parameters has been started one after another. Zhao et al. [24] studied the influence of the drilling arrangement on the mechanical properties of the coal model through uniaxial compression tests and determined the intrinsic connection between the drilling diameter, the number of rows, and the energy evolution. Geng et al. [25] determined the optimal drilling diameter for preventing rock bursts. Wang et al. [26] used similar materials to design different physical tests with different numbers of boreholes and arrangements and obtained that the effect of pressure relief is positively proportional to the borehole diameter, and the reduction in hole spacing and the increase in borehole depth will enhance the effect of pressure relief. Brady and Brown [27], and Wang and Park [28] used Particle Flow Code (PFC) numerical simulation to obtain the occurrence, expansion, and penetration of cracks around the boreholes, to achieve the goal of pressure relief of coal seams. The increase in the diameter of the boreholes and in the depth of the boreholes will strengthen the effect of pressure relief. Zhang et al. [29] studied the generation and development of local cracks around the borehole and concluded that the higher the density of the borehole, the more cracks are developed, the more energy is released, and the effect of pressure relief is better. Wu et al. [30] studied the influence of the shape of the borehole on the mechanical properties and fracture characteristics of rock-containing holes under the action of uniaxial load, analyzed the crack development and expansion of different types of specimens and the distribution of stress, and finally obtained the stability order of the borehole with different shapes as follows: circle > inverted u-shape > trapezoid > square > rectangle. Lin et al. [31] studied the crack initiation, agglomeration mechanism, and damage behavior of granite specimens with different prefabricated borehole diameters, distributions, and spacings.

The above research results have played a powerful role in promoting the development of drilling pressure relief technology for dynamic pressure roadways in working faces. The force state of anchor bolts and anchor cables changes under the influence of dynamic pressure. Wang constructed a 2D different element method (DEM) model of a deep tunnel in an underground coal mine and comprehensively evaluated the effects of yielding (D-bolt and Roofex) and conventional anchors (fully resin-grouted steel bars) on controlling the self-initiated strain burst. Tahmasebinia conducted 36 static tests and 576 dynamic tests to examine the effects of bolt diameter, steel yield and ultimate strength, dynamic loading rate, and dynamic loading mass on cable bolt displacement, shear, and energy absorption capacity; however, the interrelationship between support and unloading borehole parameters was not considered [32,33]. It is known that the diameter of the drill hole has the greatest influence on the decompression effect [34]. However, conventional pressure relief drilling will weaken the strength of the roadway support structure while transferring the high stress of the surrounding rock. The deformation of the roadway surrounding rock increases, and even the support structure fails, and it is difficult to coordinate between the transfer of surrounding rock stress and the control of surrounding rock deformation. Moreover, most of the rock burst mines have the problem of insufficient pressure relief (rock burst still occurs after pressure relief). Based on this, this paper puts forward the method of variable diameter borehole pressure relief and provides the principle of variable diameter borehole pressure relief. Through numerical simulation, a strain softening model is established to analyze the effect of drilling parameters on the evolution of surrounding rock stress, deformation law, and support structure. While not causing damage to the roadway support as much as possible, the pressure relief effect is improved and the rock burst danger of the working face is reduced.

2. The Pressure Relief Principle of Variable Diameter Borehole

In the high-stress environment, the larger horizontal stress can affect the stability of the borehole wall, fissure evolution, and the probability of rock impact, so it can be used to reduce the horizontal stress on the stability of the surrounding rock of the dynamic pressure tunnel through the variable borehole unloading technology [35]. The variable diameter borehole pressure unloading principle can be summarized as “joint support in the shallow part and full energy release in the deep part”. After excavation of the roadway, the original rock stress is redistributed. The stress peak appears in the shallow surrounding rock of the roadway, such as σp in Figure 1. To reduce the influence of the drilling pressure relief on the roadway support structure and support strength, small-diameter drilling holes are constructed in the shallow surrounding rock. The stresses in the shallow surrounding rock are released by plastic deformation, and another part is transferred to the deep surrounding rock. Large-diameter drilling holes are constructed outside the anchorage zone to the range of the stress peak of the roadway surrounding rock; during the influence of the dynamic load, the drilling holes are plastically damaged, cracks continue to develop, the plastic zones of multiple drill holes are connected, the same stress transfer occurs, and a new stress peak σp’ is formed at the bottom of the drill holes. The internal space of the variable borehole is continuously compacted under the action of stress, and the accumulated elastic energy is fully released through plastic deformation. Thus, the purpose of reducing the surrounding rock stress and protecting the roadway support anchorage zone is achieved.

3. Materials and Methods

(1)

Model parameters and boundary conditions

The size of the model is X × Y × Z = 80 m × 30 m × 60 m, and the roadway was tunneled along the Y-axis. The volume weight of the overburden is 25 kN/m³, and displacement constraints are imposed on the lateral and lower surfaces of the model. Combined with the ground stress test data, a uniform load of 7.85 MPa was applied to the top of the model, and the lateral stress concentration coefficients were both 0.8. To eliminate the influence of the model boundary on the simulation results, a boundary coal pillar of about 22 m was reserved on each side [36]. Variable diameter pressure relief boreholes have more influencing factors than conventional pressure relief boreholes, mainly including shallow diameter, deep diameter, variable diameter locations, and spacing. To study the influence of the above four key parameters on the stability of the roadway’s surrounding rock, FLAC3D (version 5.0, Itasca Consulting, Inc., Minneapolis, MN, USA) numerical simulation software was used to analyze the influence of different parameters on the vertical stress of the surrounding rock, surrounding rock deformation, and anchor support. In the simulation, Rayleigh damping is used to simulate the effect of dynamic load on the roadway surrounding rock, the minimum critical damping ratio is 0.005, and the minimum center frequency is 3.33 Hz [33,37,38,39]. The model diagram of variable aperture borehole is shown in Figure 2.

(2)

The constitutive model and mechanical parameters

The intrinsic model selects the strain softening model that can better describe the mechanical properties of coal rock, which is different from the Mohr–Coulomb model. In the strain softening model into the plastic yielding stage, the cohesion, internal friction angle, shear expansion angle, and tensile strength of the material will decrease with the plastic strain. The shear yield function is as follows:

$$F^s={\sigma }_1-{\sigma }_3{N}_{\phi }+2c\sqrt{N_{\phi }}$$

(1)

where $N_{\phi }={\displaystyle \frac{1+\sin \phi }{1-\sin \phi }}$, φ is the internal friction angle, °; c is the cohesive force, MPa; σ₁ and σ₃ are the maximum and minimum horizontal principal stresses, MPa.

The tensile yield function is as follows:

$$F^t={\sigma }_1-{\sigma }_3$$

(2)

When F^s < 0, the material undergoes shear damage; when F^t < 0, the material undergoes tensile damage; when F^s > 0 and F^t > 0, the material does not undergo damage. In the numerical simulation of FLAC3D, it is agreed that the compressive stress is negative and the tensile stress is positive in the above formula.

The numerical model was imported into FLAC3D software for material assignment, and the mechanical parameters of coal rock were converted according to the laboratory test results. Relevant studies show that the uniaxial compressive strength and stiffness in the numerical model should be 0.284 and 0.469 of the laboratory test values, respectively; the modulus of elasticity, cohesion, and tensile strength should be 0.1~0.25 of the test values, and the Poisson’s ratio should be 1.2~1.4 times of the measured values. Based on the above study, the mechanical parameters of coal rock formation materials in the model were obtained as listed in

Table 1. Mechanical parameters of coal rock materials.

Rock Layer	Density/kg·m−3	Thickness/m	Bulk Modulus/GPa	Shear Modulus/GPa	Cohesion/MPa	Internal Friction Angle/°	Tensile Strength/MPa
Mudstone	2750	2.97	4.24	5.13	3.69	36	1.38
Sandy mudstone	2515	10.16	8.58	8.58	4.96	34	5.16
Medium coarse sandstone	2667	3.62	10.43	5.67	12.23	38	5.20
Carbonaceous mudstone	2606	3.38	3.77	4.07	2.82	36	1.29
Medium coarse sandstone	2667	2.48	10.43	5.67	12.23	38	5.20
Sandy mudstone	2515	8.29	8.58	8.58	4.96	34	5.16
Carbonaceous mudstone	2606	2.67	3.77	4.07	2.82	36	1.29
9# Coal	1380	14.35	2.2	2.6	1.56	35	2.01
Sandy mudstone	2515	2.71	8.58	8.58	4.96	34	5.16
Mudstone	2750	3.68	4.24	5.13	3.69	36	1.38
11# Coal	1380	5.69	2.2	2.6	1.56	35	2.01

.

When exceeding the load that the coal rock can bear, the internal cracks begin to expand, and the deformation of the coal rock enters the elasticity stage into the elastoplasticity stage from the elasticity stage, and at this time, the strength of the coal rock itself is reduced. Combined with the post-peak morphology of the rock stress–strain curve during the loading process, the cohesion and internal friction angle of the rock layer near the coal seam [40,41,42,43] were reassigned to achieve the adjustment of the modulus of elasticity and Poisson’s ratio, and the specific strain softening parameters are given in

Table 2. Parameter setting of strain softening.

Lithology		Cumulative Plastic Shear Strain Value	Internal Friction Angle/°	Cohesion/MPa
Floor	Sandy mudstone	0	34	4.78
		0.05	32	3.86
		0.1	28	2.53
Coal	9# Coal	0	35	1.56
		0.05	31	1.21
		0.1	28	0.97
Roof	Carbonaceous mudstone	0	36	2.82
		0.05	32	2.45
		0.1	24	1.87
	Medium grain sandstone	0	38	12.23
		0.05	33	10.46
		0.1	26	9.77

. The anchor parameters in the model are listed in

Table 3. Parameters of the anchor simulated in the model.

Emod/MPa	ytension/MPa	xcarea/m2	gr_coh/N	gr_k/N·m−1	gr_per/m
200	0.31	3.8 × 10−4	4.37 × 105	2 × 107	0.785

.

The control variable method was used to study the influence law of different variable diameter borehole parameters on the stability of the roadway surrounding rock, and the simulation scheme is shown in

Table 4. Numerical simulation scheme table.

Number	Shallow Borehole Diameter/mm	Deep Borehole Diameter/mm	Variable Aperture Position/m	Borehole Spacing/m
1	80	250	4	1.0
2	120	250	4	1.0
3	160	250	4	1.0
4	200	250	4	1.0
5	250	250	4	1.0
6	80	80	4	1.0
7	80	150	4	1.0
8	80	200	4	1.0
9	80	250	4	1.0
10	80	300	4	1.0
11	80	250	0	1.0
12	80	250	2	1.0
13	80	250	4	1.0
14	80	250	6	1.0
15	80	250	8	1.0
16	80	250	16	1.0
17	80	250	4	0.5
18	80	250	4	1.0
19	80	250	4	1.5
20	80	250	4	2.0.
21	80	250	4	2.5

.

4. Results and Discussion

4.1. Influence of Different Parameters on Vertical Stress Distribution in the Surrounding Rock

(1)

Shallow borehole diameter

From Figure 3, it can be seen that when there is no pressure relief borehole in the roadway, the vertical stress peak of the roadway side surrounding rock is 15.3 MPa, which is located on the roadway side at about 6.5 m, and the stress concentration area is larger. With the increase in the diameter of the shallow borehole (80 mm → 300 mm), the peak stress is transferred to the deep part (16.5 m → 16.9 m from the roadway side), the peak vertical stress shows an increasing trend (15.9 MPa → 17.6 MPa), and the vertical stress at the position of the original peak vertical stress shows a decreasing trend (8.6 MPa → 7.6 MPa). The reduction in vertical stress at the original peak location is limited. It shows that the shallow borehole diameter has less influence on the vertical stress of the roadway surrounding rock, mainly because the shallow borehole depth is 4 m, which is in front of the stress peak of the roadway, and has little influence on the stress transfer effect.

(2)

Deep borehole diameter

From Figure 4, it can be seen that as the diameter of deep borehole increases (80 mm → 300 mm), the stress peak is transferred to the deeper part (6.6 m → 15.9 m from the roadway side), and the vertical stress peak now shows an increasing trend (14.6 MPa → 16.9 MPa), and the vertical stress at the location of the original vertical stress peak shows a decreasing trend (14.3 MPa → 8.5 MPa). After the diameter of the deep borehole exceeds 250 mm, the reduction in vertical stress at the original vertical stress peak is limited, indicating that the pressure relief zones formed by the borehole have penetrated each other, and the stress reduction effect has reached the limit. The main reason why the diameter of the deep borehole has a greater influence on the vertical stress of the surrounding rock is that the location of the variable borehole diameter is at 4 m of the roadway side, and the deep borehole is in the stress concentration area of the roadway surrounding rock. The larger the diameter of the deep borehole, the larger the plastic zone is formed, and the greater the influence on the effect of the stress transfer on the surrounding rock.

(3)

Variable aperture position

From Figure 5, with the increase in the distance between the variable aperture position and the roadway side (0 m → 16 m), the position of the stress peak is gradually closer to the roadway side (16.9 m → 6.6 m), and the present peak vertical stress shows a decreasing trend (17.6 MPa → 14.6 MPa), and the vertical stress at the original peak vertical stress position shows an increasing trend (7.6 MPa → 14.3 MPa).

When the position of the variable borehole is located in the roadway before the peak vertical stress without pressure relief (≤4 m), the original peak vertical stress is reduced to 8.6 MPa, with a reduction of 44.4%. When the variable aperture position is located around the peak vertical stresses in the roadway without pressure relief (6 m), there are two vertical stress concentration zones, and the original peak vertical stress is reduced to 11.9 MPa, with a reduction of 22.2%. When the position of the variable borehole is located after the peak vertical stress position of the roadway (≥8 m), the vertical stress at the original peak position is reduced to 13.8 MPa, with a reduction of 9.8%. The main reason is that the shallow boreholes are located in the stress concentration area, which produces a smaller plastic zone. Combined with the previous section, when there is without pressure relief borehole, the peak of the vertical stress position appears at about 6.5 m from the side of the roadway, which shows that the interaction between the stress field of the pressure relief borehole and the stress field of the surrounding rock leads to a reduction in the peak stress. At the same time, this realizes the transfer of the stress to the deeper part of the roadway.

(4)

Borehole spacing

From Figure 6, it can be seen that with the reduction in the spacing of the variable diameter boreholes (2.5 m → 0.5 m), the location of the peak stress is gradually far away from the roadway side (6.7 m → 16.6 m), and the peak vertical stress is increasing (14.9 MPa → 18.4 MPa), and the vertical stress at the location of the original peak vertical stress is decreasing (14.9 MPa → 7.7 MPa).

When the borehole spacing is greater than 2.0 m, the vertical stress peak value of the roadway surrounding rock is basically not transferred, and the stress peak value is slightly reduced. When the borehole spacing is between 1.0 m and 2.0 m, the vertical stress peak value of the surrounding rock begins to transfer, the original stress peak value decreases greatly, and the present stress peak value begins to increase, mainly because the pressure relief zone generated by the boreholes begins to penetrate each other. The main reason is that the pressure relief zones generated by the boreholes start to penetrate each other. When the distance between the boreholes is less than 1.0 m, the position of the surrounding rock vertical stress peak is unchanged, the reduction in the original stress peak tends to be stable, and the present stress peak increases and the boreholes have the pressure relief effect for the whole length of the borehole.

4.2. Influence of Different Parameters on the Deformation of Surrounding Rock

(1)

Shallow borehole diameter

From Figure 7, it can be seen that when there is without pressure relief borehole in the roadway, the maximum deformation of the two sides is 287.5 mm, and the maximum deformation of the roof is 330.7 mm. The shallow borehole diameter and the deformation of the two sides show a positive correlation, and the roof and floor convergence is also positively correlated.

When the diameter of shallow boreholes is between 0 and 120 mm, the displacement of two side coal walls is slightly reduced, mainly because the boreholes provide deformation space for two sides of the surrounding rock. After the diameter of shallow boreholes is larger than 120 mm, the deformation of roadway sides begins to increase, and the growth rate increases with the increase in diameter of shallow boreholes. The deformation of the roof-to-floor convergence of the roadway increases with the increase in the diameter of the shallow borehole, and the rate of increase is unchanged.

(2)

Deep Borehole Diameter

As can be seen from Figure 8, the connection between the change of the deep borehole diameter and the surface deformation of the roadway surrounding rock is weaker; although it shows a positive correlation, the increase is small. From this, it can be seen that the influence of deep borehole diameter on the roadway surface deformation is small, mainly because the location of the variable borehole diameter is outside the anchorage zone of the roadway, and the diameter of the boreholes in the anchorage zone is small, which has a small influence on the weakening of the strength of the surrounding rock. It also explains that the main factor affecting the surface deformation of the roadway is the boreholes in the anchorage zone.

(3)

Variable aperture position

As shown in Figure 9, the deformation of roadway sides is reduced by 103.6 mm, 168.6 mm, 174.7 mm, 184.2 mm, and 191.0 mm, respectively, when compared with the position of variable borehole is 0 m. When the position of the variable borehole is in the anchorage area, the influence on the deformation of the roadway is larger. Therefore, the position of variable borehole diameter should be located outside the anchorage zone.

(4)

Borehole spacing

As shown in Figure 10, as the spacing of the variable diameter boreholes increases, the deformation of the roadway surrounding rock decreases, indicating that the damage to the surrounding rock is smaller when the spacing of the boreholes is larger. In the case of spacing of 0.5 m, the increase in the surrounding rock deformation is larger, indicating that in this state, the overall support structure of the roadway surrounding rock is damaged, resulting in a significant increase in deformation.

4.3. Influence of Different Parameters on the Support Structure

(1)

Shallow borehole diameter

In this paper, the average value of normal stress of the surrounding rock near the anchors is taken as the effective binding force [44]. As can be seen from Figure 11, the effective binding force of the roadway side anchor and roof anchor is 6.12 MPa and 6.31 MPa, respectively, when pressure is without relief. With the increase in shallow borehole diameter, the effective binding force of the roadway side and roof anchor both continue to decrease, but the decrease in the roadway side anchor (6.01 MPa → 4.53 MPa) is larger than that of the roof anchor (6.25 MPa → 5.92 MPa), which shows that the deformation of the roadway surrounding rock is significantly enhanced under this condition. From this, it can be seen that the influence of boreholes on the roof anchors is weaker. As the diameter of the shallow borehole increases, the effective binding force of the anchor decreases, and de-anchoring is easy to occur when subjected to dynamic pressure.

(2)

Deep borehole diameter

As can be seen from Figure 12, with the increase in deep borehole diameter, the effective binding force of roadway side and roof anchors is continuously reduced, but the reduction in roadway side anchors (6.10 MPa → 5.88 MPa) is significantly larger than that of roof anchors (6.30 MPa → 6.20 MPa). The effective binding force of anchors can still be maintained above 5 MPa, and the location variable aperture is located outside the anchorage zone, so it has less influence on the support structure.

(3)

Variable aperture position

As can be seen from Figure 13, with the increase in the distance between the variable aperture position and the roadway side, the effective binding force of the roadway side and the roof anchors both continue to increase, but the growth rate of the roadway side anchor (4.53 MPa → 6.10 MPa) is significantly larger than that of the roof anchor (5.92 MPa → 6.30 MPa).

The closer the location of the variable aperture is to the roadway side, the smaller the effective binding force of anchors. When the location of variable aperture is inside the anchorage area, the effective binding force of the anchor decreases more, and when the location of variable aperture is outside the anchorage area, the effective binding force of the anchor is unaffected. Therefore, to ensure the strength of the support structure, the position of the variable aperture should be located outside the anchorage area.

(4)

Borehole spacing

As can be seen from Figure 14, with the increase in borehole spacing, the effective binding force of roadway side and roof anchors is continuously reduced, but the growth of side anchors (6.11 MPa → 4.96 MPa) is significantly larger than that of roof anchors (6.31 MPa → 6.13 MPa), and the reduction is largest when the borehole spacing is 0.5 m.

In summary, it can be seen that the shallow borehole diameter has the most obvious effect on the deformation amount of the roadway and anchor support, to ensure the strength of the support structure, the optimal shallow borehole diameter is 120 mm; the deep borehole diameter has the most obvious effect on the stress transfer of the roadway surrounding rock, to improve the overall pressure relief effect, the optimal deep borehole diameter of 200 mm~300 mm; the position of the variable borehole diameter has the most obvious effect on the original stress peak, it can be seen that the stress peak can be controlled by the variable diameter of the borehole, to improve the overall pressure relief effect. To control the peak stress at the bottom of the variable diameter borehole, the optimal variable diameter borehole is located at 4 m~6 m from the roadway side. The spacing of the borehole has a significant effect on the transfer of stress to the surrounding rock, the deformation of the roadway, and the impact of the anchor support. To achieve reasonable control of the support structure strength and the deformation of the surrounding rock, the optimal variable diameter borehole spacing is in the range of 1.0 m~2.0 m.

5. Industrial Testing

5.1. The Variable Diameter Borehole Program

The variable diameter boreholes were drilled within 200 m of the working face in the 19,111 auxiliary transport roadway to relieve pressure. CMQSI-450/5.2S pneumatic drilling truck equipment (Shandong Xin Coal Mining Equipment Group Company Limited, Jining, China) was used for the site construction, connecting the through-hole drilling rods and reducer joints through the hydraulic variable-diameter reaming drill bit, and driven by the pneumatic drilling truck. The boreholes were arranged in a single row, 1.5 m from the floor, with a spacing of 2.0 m. The depth of the boreholes on the coal wall side was 16 m, and the depth of the boreholes on the coal pillar side was 12 m. The shallow boreholes had a diameter of 100 mm, and the deep boreholes had a diameter of 250 mm. The location of variable diameter boreholes was located at a point 4 m away from the roadway side, and after 4 m of drilling, the water pressure was increased and the alloy blades inside the drilling head were pushed away from the drill bits so that the diameter of the drill holes was changed from 100 mm to 250 mm. The plan and section of the drilling arrangement are shown in Figure 15.

5.2. Monitoring by the Method of Drilling Bits

A group of drilling bits monitoring holes is constructed every 30 m in the test area of 19,111 auxiliary transport roadway, and a total of 7 groups of drilling bits monitoring holes are constructed, 2 holes in each group, and the amount of drilling cuttings is recorded once every 1 m of drilling. The early warning indicators of pulverized coal quantity are shown in

Table 5. The early warning indicators of pulverized coal quantity.

Depth/m		1−5	5−10	10−15
Hazard criteria	Quality of pulverized coal	3.6 kg/m	5.5 kg/m	7.5 kg/m
	Dynamic phenomenon	Stuck drill, suction drill, against drill, strange noise, impact in the hole

, and the pulverized coal quantity is shown in Figure 16. It can be obtained that the amount of drilling bits on both sides of the roadway is less than the warning value, and the impact danger is small.

5.3. Stress On-Line Monitoring

Deep and shallow stress meters are arranged on both sides of the roadway to realize real-time online monitoring of the internal stress of the coal body. Specific monitoring program: In the 19,111 auxiliary transport roadway from the cutting eye 50 m from the two sides of the roadway, the first group of stress measurement points is set up and then a group every 25 m, the depth of the holes is 8 m and 15 m, the spacing of the holes is 1.0~1.5 m, a total of 10 groups of arrangements. When the distance between the measuring point and the working face is less than 30 m, it starts to retreat, and as the working face advances, the measuring point moves forward in turn, always keeping the monitoring range of 300 m in front of the coal wall of the working face until the end of the mining. The basis of early warning judgment of stress online monitoring is shown in

Table 6. Stress online monitoring and early warning criteria.

Measuring Point Depth	Warning Level	Warning Value
Shallow hole (8 m)	Intermediate warning	10~12 MPa
	Advanced warning	≥12 MPa
Deep hole (15 m)	Intermediate warning	12~14 MPa
	Advanced warning	≥14 MPa

.

The borehole stress meters were arranged in the middle of the variable diameter boreholes at depths of 8 m and 15 m. From Figure 17, it can be seen that the stress change at 8 m between the solid coal side and the coal pillar side is small, with a small increase and a maximum value of about 7.10 MPa, which indicates that the pressure relief effect is good in the area. About 15 m has a larger change in stress, with a peak value of 11.5 MPa on the solid coal side on 15 December 2022, and a rapid decrease in stress to 5.1 MPa thereafter. The stress at 8 m is similar to that of the solid coal side, with a limited increase and the maximum value is about 7.11 MPa, which indicates that the pressure relief effect is better in this area; the coal pillar side reaches the peak value of 11.8 MPa on 17 December 2021, and then the stress decreases rapidly to 5.5 MPa, which indicates that the borehole collapses due to the high stress, and the coal undergoes yielding damage, and the high stresses are released.

From the above analysis, it can be seen that after pressure relief of variable diameter drilling in the 19,111 auxiliary transport roadway, the stress concentration in the shallow surrounding rock was greatly reduced, and the stress was transferred to the deep rock layer, which verified the principle of the pressure relief of the variable diameter drilling and realized the safety and stability of the roadway.

6. Conclusions

FLAC3D numerical simulation was used to study the influence of vertical stress, deformation, and support structure under different parameters of variable diameter pressure relief boreholes, and the following main conclusions were drawn:

(1)

Shallow borehole diameter has less influence on stress transfer and more influence on roadway deformation and support structure. When the shallow borehole diameter increases, the deformation of the roadway becomes larger, and the effective binding force of the anchor decreases, which is easy to cause the de-anchoring phenomenon.

(2)

The deep borehole diameter has a greater influence on stress transfer and a smaller influence on roadway deformation and support structure. The larger the deep borehole diameter, the smaller the change of surrounding rock stress and effective binding force of the anchor, but the pressure relief zone formed in the deep part of the borehole continues to increase. And the effect of roadway stress transfer is better.

(3)

The influence of variable borehole diameter position on stress transfer, roadway deformation, and anchor support is larger. With variable aperture position in the anchorage area, the stress transfer effect is better, but the roadway deformation is larger, and the effective binding force of the anchor is smaller. With variable aperture position between the anchorage area and the peak stress position of the roadway side, the stress transfer effect is better, and the deformation of the roadway is smaller, the effective binding force of the anchor is reduced to a lesser extent. With variable aperture position outside the peak stress position of the roadway side, although the deformation of the roadway is reduced, the effective binding force of the anchor is also small, but the stress transfer effect is better. Although the deformation of the roadway is reduced, the change of the effective binding force of the anchors is also small, but the effect of stress transfer is poor.

(4)

Borehole spacing has a significant effect on the stress transfer to the surrounding rock, the deformation of the roadway, and the impact of anchor support. When the borehole spacing is larger than 2.0 m, the effect of vertical stress transfer is poor. When the borehole spacing is between 1.0 m and 2.0 m, the vertical stress begins to transfer to the depth of the surrounding rock, the deformation of the roadway is small, and the effective constraint force of the anchors can be maintained at the normal level. When the borehole spacing is smaller than 1.0 m, the deformation of the roadway is larger, and the effective binding force of the anchors decreases sharply.

(5)

The variable diameter drilling program is given, and variable diameter boreholes are constructed within 200 m of the working face. The boreholes are arranged in a single row, 1.5 m away from the floor, with a spacing of 2.0 m. The depth of the holes on the coal wall side is 16 m, the depth of the holes on the coal pillar side is 12 m, the diameter of the shallow boreholes is 100 mm, and the diameter of the deep boreholes is 250 mm; the position of the variable diameter boreholes is located in the place of 4 m away from roadway side. The industrial test was carried out in the 19,111 auxiliary transport roadway of the Pingshuo No.1 Coal Mine. Through the method of drilling bits and the monitoring of the drilling stress gauge, the stress concentration of the surrounding rock in the roadway is reduced, which proves that the variable diameter drilling pressure relief program is reasonable and feasible.