Ground Control Method of Using Roof Cutting Pressure Release and Energy-Absorbing Reinforcement for Roadway with Extra-Thick Hard Roof

Abstract

Coal mining in the western region of China is often faced with extra-thick coal seams and extra-thick hard roof, where the roof is difficult to collapse in time, resulting in large-suspended area in the gob and the frequent occurrence of dynamic disasters such as mine earthquakes. To this end, we propose an integrated ground control method of roof cutting pressure release and energy-absorbing reinforcement for roadway with hard roof. We take the #122108 working face of the Caojiatan coal mine as the engineering example to apply this new method. The stress reduction rate is used to analyze the pressure relief effect. The influence of pressure relief parameters on the roadway is analyzed through numerical experiments. Three-dimensional stress evolution of the surrounding rock using the optimized parameters is further studied, which verifies the advantages of the new method, as evidenced by the reduction of stress up to 40.87%. Moreover, we conduct physical model test of the new method and reveal the control mechanism on the surrounding rock. The deformation of the roof and floor of roadway with roof cutting is lower than that of the roadway without roof cutting and the maximum deformation is reduced by 52.26%. Subsequently, we design the field roof cutting parameters for pressure release and energy-absorbing reinforcement, and evaluate the parameters of roof cutting, strengthen blasting, and energy-absorbing cables. Field monitored data show that the average deformation of the roadway floor using the new method is 201.8 mm, which is 66.37% smaller than that of roadway without using the new method, and the peak stress at the working face is 27.75% less than that without using the new method. The validity of the physical model test results and the rationality of the design parameters for pressure relief and reinforcement is verified through the field data.

Highlights

• An integrated ground control method of roof cutting pressure release and energy-absorbing reinforcement for roadway with hard roof is proposed.

• The design basis and control mechanism of roof cutting pressure release and energy-absorbing reinforcement parameters for thick hard roof roadway are clarified by numerical and large-scale 3D geological model tests.

• The design and field application of core parameters of directional roof cutting pressure release and energyabsorbing reinforcement have been successfully carried out.

1 Introduction

With the gradual depletion of coal resources in the eastern and central regions of China, coal mining has shifted to the western region. The western region has abundant coal resources with thick coal seams (Bin 2016; Chen et al. 2017; Wei et al. 2022; Wojtecki et al. 2022), which has become an important production region in China. Coal mining in this region is often faced with hard roof conditions and extra-thick coal seams, of which the extra-thick hard roof is of good integrity and high strength. However, under this condition, the roof is difficult to collapse in time and the gob is suspended in a large area (Wang et al. 2015; Zhao et al. 2017). During coal mining, roadways are affected by the mining of adjacent working faces (Li et al. 2023; Mao et al. 2023). Especially, when the mining height is large and the roof is hard and easy to form a stress transmission structure, it is easy for dynamic phenomena to occur such as rockbursts when the rock stratum collapses. For roadways where extra-thick coal seams and extremely thick hard roof conditions coexist, the mining-induced stress is more intense and the influence range is wider (Gong et al. 2021). Traditional roof reinforcement materials are prone to fracture failure (Jiang et al. 2023; Li et al. 2016; Wang et al. 2021a), which can no longer meet the ground control requirements of such roadways. Aiming at solving the problems of dynamic pressure and rock deformation control in roadways, scholars have conducted extensive studies on pressure relief and support control.

In terms of roadway pressure relief, Gao et al. (2013) adopted the method of forced caving by blasting to reduce the additional stress of the filling side next to the roadway and to prevent the filling side of the hard roof of the deep shaft retained along the gob from being crushed when the roof collapses. Yang and Zheng (2020) studied the overburden displacement of the thick coal seam under fully mechanized caving mining, and proposed a hydraulic fracturing impact prevention method for hard roof, which can reduce the roof weighting step distance, dynamic load coefficient, and maximum pressure. Wang et al. (2021b) developed a method of high-strength anchor grouting, roof cutting, and self-formed roadway. This method uses high-strength bolt grouting to improve the integrity of the roadway roof, and uses the directional roof pre-splitting to cut off the stress transfer path between roofs. The experimental results show that the rock stress after this treatment is on average 20.8% lower than that of the gob side entry, and the rock deformation is 45.1% that of the latter. The studies described above show that pressure relief technologies such as forced caving by deep hole blasting, hydraulic fracturing, and roof pre-splitting and coal seam cutting can change the roof stress transfer path and reduce the stress above the roadway. The pressure relief technology of roof pre-splitting and coal seam cutting greatly reduces the stress concentration magnitude and range, which can completely cut off the physical connection between the gob and the roadway roof along the strike direction of the roadway (He et al. 2015; Wang et al. 2020a, b).

In terms of roadway support, Kang et al. (2018) proposed to use high-strength, high-toughness bolts to safely control the surrounding rock of the roadway. Based on the excellent performance of NPR (negative Poisson's ratio) structure in impact resistance, energy absorption, and yield, He et al. (2014) developed a constant-resistance energy-absorbing anchor cable with NPR structure. The anchor cable allows for the large deformation of the support material under constant resistance conditions and has good energy absorption and impact resistance. Wang et al. (2022a) established a constant-resistance energy-absorbing bolt with high strength, high elongation, and high pre-tension, which has good impact resistance and overall deformation capacity. In short, the use of energy-absorbing reinforcement can effectively absorb the energy accumulated in the surrounding rock of the roadway (Wang et al. 2022b), improve the erosion resistance of the reinforcement system, and help control the rock deformation caused by the collapse of the thick hard roof.

In this work, we propose a ground control method using roof cutting for pressure releasing and energy-absorbing reinforcement for extra-thick hard roof roadway. Furthermore, we conduct numerical and physical models to compare the results, and discuss the advantages of roof cutting pressure relief and energy-absorbing reinforcement in roadway ground control. Finally, this method has been successfully applied to the field and monitored results suggest the effectiveness and rationality of the new method.

2 Ground Control for Roadway with Extra-Thick Hard Roof

2.1 Principles

The reasons for the difficulty in controlling the surrounding rock of roadway with extra-thick hard roof are: (1) The extra-thick hard roof above the working face is of high strength and is difficult to collapse, and the large suspended roof is prone to form stress concentration and energy accumulation in the rock. (2) Under the condition of extra-thick coal seam, the mining disturbance is severe, and the collapse of the large suspended roof has a greater impact on the dynamic pressure. (3) Under the influence of large dynamic stress, the traditional roadway reinforcing materials are prone to failure due to insufficient elongation and poor energy-absorption capacity, and the roadway faces the risk of dynamic disasters such as mine earthquakes.

To this end, we propose the integrated concept of roof cutting for pressure releasing and energy-absorption reinforcement: (1) Before the mining of the working face, directional roof cutting is applied to cut off the stress transfer path between the roadway roof and the gob roof for stress release. (2) At the same time, blasting is applied onto the roadway roof to accelerate the collapse of the hard roof while strengthen roof cutting and pressure relief, and reducing the impact of the collapse on the roadway. (3) After the comprehensive pressure relief described above, the constant-resistance energy-absorbing reinforcement is implemented. The method allows the extra-thick and hard roof above the roadway to form an integrated structure, absorb the strain energy when the thick and hard roof in the gob collapses, so as to ensure the safety and stability of the roadway.

2.2 Control Method for Roadway with Extra-Thick Hard Roof

Based on the principles, the control method of roof cutting for pressure releasing and energy-absorption reinforcement in hard roof roadway is proposed, as shown in Fig. 1.

Fig. 1

Principles of roof cutting pressure releasing and energy-absorbing reinforcement

The technical principles of this method are:

• 1. The roof of the roadway near the gob side is directionally cut by using the characteristics of low-tension resistance of rocks. It makes roof blastholes create concentrated tension in presplitting direction. When multiple adjacent blastholes are simultaneously blasted, the directional crack generates and penetrates the surrounding rocks to form a directional roof presplitting surface. In this way, the stress transfer path of the roadway and the overlying roof of the gob can be cut off, so that the gob roof can collapse directionally along the cutting surface under the action of the mining-induced stress and the gravity of the rock stratum, as shown in Part I of Fig. 1.

• 2. After directional cutting, the gob roof is blasted to collapse. In addition to strengthen the roof cutting for pressure release, the blasting accelerates the collapse of the hard roof, thereby reducing the impact of large gob roof collapse on the roadway.

• 3. Constant-resistance energy-absorbing anchor cables are used to reinforce the roadway roof and absorb the strain energy of the rock in the event when the extra-thick roof collapses in the gob, as shown in Part II of Fig. 1. The combined energy-absorption reinforcement and pressure releasing measures constitute an integrated ground control system, which ensures the safety and stability of the surrounding rock of the roadway when the thick and hard roof collapses in the gob during mining.

2.3 New Type Constant-Resistance Energy-Absorbing Anchor Cable

The constant-resistance energy-absorbing anchor cable is the key reinforcing material for ground control, and its main body consists of a cable body and a constant resistance device, as shown in Fig. 2. When the stress of the anchor cable is lower than the designed constant resistance, the elastic deformation of the cable body will absorb the energy of the rock. When the stress of the anchor cable is greater than or equal to the designed constant resistance, the cable body slides along the inner side of the sleeve, and the anchor cable is deformed to absorb the rock energy (Wang et al. 2022b). The performance of the constant-resistance energy-absorbing anchor cable under tensile load is shown in Fig. 2.

• 1. The constant-resistance energy-absorbing anchor cable exhibits elastic stage and constant resistance stage. (a) Elastic stage (section 0 → A): load and elongation are approximately linear; (b) Constant resistance stage (section A → B): after the load reaches 368.2kN (point A), the constant-resistance body slides along the inner side of the constant-resistance sleeve, and the constant load fluctuates in the range of 313.1–381.6 kN.

• 2. The maximum elongation of the anchor cable is 25.6%, and the energy absorbed per unit length is 12.7 × 10⁴ J. The maximum elongation of traditional anchor cables can reach about 3–7%, and the energy absorbed per unit length is 0.8 × 10⁴–1.8 × 10⁴ J (Li et al. 2023). The absorbed energy of the former is 7.1–15.9 times that of the latter. It can be seen that the constant-resistance energy-absorbing anchor cable has larger energy-absorption capacity while maintaining higher constant resistance.

Fig. 2

Performance of the constant-resistance energy-absorbing anchor cable under tension

3 Numerical Test of Roof Cutting for Pressure Release Control

3.1 Engineering Background

The Caojiatan Coal Mine is located in western China. The #122108 working face of the mine is buried 300–363 m, the length along the strike is 5,910 m, the length along the dip is 280 m, the average thickness is 10.60 m, the mining height is 5.8 m, and the caving height is 5–6 m. The roof above the working face in the ventilation roadway is mainly sandstone, which is 2.22 m thick fine sandstone, 4.20 m thick siltstone, and 21.20 m thick medium sandstone from the bottom to the top. The total thickness of sandstone is 27.62 m, which belongs to a typical extra-thick and hard roof. Among them, the medium sandstone is the key rock stratum, with a compressive strength of 52.1 MPa and a tensile strength of 7.8 MPa. In the mining stage of the working face, the mining-induced stress is high, the floor and sides are severely deformed, and the reinforcing system is failed in some locations, as shown in Fig. 3.

Fig. 3

Borehole histogram and layout of the working face

We take this working face as the engineering background to conduct numerical tests on directional roof cutting for pressure release and discuss the advantages of this method in the ground control of such roadways.

3.2 Conceptual Design

The selection of pressure relief parameters is crucial to the effect of ground control (Wang et al. 2022c; He et al. 2021, 2018). The designed parameters include roof cutting parameters and strengthen blasting parameters. Roof cutting parameters include Type A scheme (different roof cutting angles) and Type B scheme (different roof cutting heights). Strengthen blasting parameters are in Type C scheme (different blasting angles).

For Type A to C schemes, a FLAC^3D model with the same field dimension, geomechanical parameters, and reinforcing parameters is established. The roadway section size, rock reinforcing strength, and coal seam thickness of the Caojiatan Coal Mine are taken as constant, while the roof cutting angle, roof cutting height, and blasting angle are variables. The schemes are described as follows.

Type A scheme (different roof cutting angles): Roof cutting angles, (Ai, i = 1–5), are 0°, 10°, 20°, 30° and 40°. According to the field condition, the roof cutting height in this scheme is 30 m, and the details are provided in Table 1.

Table 1 Type A scheme for different cutting angles

Type B scheme (different roof cutting heights): Top cutting height, (Bi, i = 1–5), are 5 m, 10 m, 20 m, 30 m and 40 m. The roof cutting angle is constant, which is taken according to the optimal roof cutting angle in Type A scheme, and the details for this scheme are listed in Table 2.

Table 2 Type B scheme for different cutting heights

Type C scheme (different blasting angles): the blasting angle is used as a variable to study the promotion effect of strengthen blasting on the roof cutting pressure relief. The blasting angle in this section is defined as the deviation from the roof cutting direction. The blasting angle scheme is represented by Ci, i = 1–5, and the corresponding blasting angles are 0°, 5°, 10°, 15° and 20°, respectively. In Type C scheme, the cutting angle and height are selected according to the optimal parameters determined after the comparative analysis of Type A and Type B schemes. And the blasting height is selected according to the roof cutting height of the best ground control in Type B scheme. The specific details are provided in Table 3. The physical and mechanical parameters of the rock mass are shown in Table 4. In simulating the weaking effect due to blasting, the mechanical parameters of the rock mass in the fragmentation area are reduced to 1/5 of the original values (Xiong and He 2006; Zhu et al. 2021).

Table 3 Type C scheme for different blasting angles

Table 4 Physical and mechanical parameters of the rock mass

3.3 Establishment of Model and Evaluation Index

3.3.1 Model Establishment

According to the field geological conditions, we build a numerical model with a dimension of 330 × 144 × 180 m (width × length × thickness). The Mohr Coulomb criterion is used as the constitutive model. The bottom boundary of the model is fixed in the vertical direction, the front, back, left, and right boundaries are fixed in the horizontal direction, and the upper boundary is applied with the overburden stress. The monitoring section is set at 90 m from the front boundary of the model, and the monitoring points are set at the roadway roof, floor, and solid coal rib side, represented by R_w, F_w, and C_w, respectively, w = 1–4. The interval between the measurement points is 5 m, as shown in Fig. 4.

Fig. 4

Schematic diagram of the model and monitoring scheme

3.3.2 Establishment of Evaluation Index

By calculating the mean stress of the monitoring points at the same distance from the roadway boundary, we analyze the rock within 5–20 m of the roadway roof, floor, and rib side. For example, the average rock stress at 5 m from the roadway boundary is $\overline{{\text{S} }_{1}}\text{=}\left({\text{R}}_{1}\text{+}{\text{C}}_{1}\text{+}{\text{F}}_{1}\right)\text{/3}$, and the average rock stress at 10 m from the roadway boundary is $\overline{{\text{S} }_{2}}$.

Next, the evolution of rock stress in different schemes will be compared and analyzed to examine the effect of pressure relief. The quantitative evaluation index ${\delta }_{{\text{R}}_{\text{ij}}}$ of stress reduction rate is established to analyze the effect of pressure relief and control, which is defined as the percentage of the average stress reduction of the rock with roof cutting versus that without roof cutting,

$$\delta_{{R_{ij} }} = (\overline{R}_{0} - \overline{R}_{ij} )/\overline{R}_{0} \times 100\% ,$$

(1)

where ${\delta }_{{\text{R}}_{\text{ij}}}$—Average stress reduction rate of the rock in scheme ij. The higher the ${\delta }_{{\text{R}}_{\text{ij}}}$, the more significant the impact of roof cutting. i represents the number of different types of schemes, i = A–C, j represents the sub-schemes under the general scheme, and j = 1–5. For example, ${\updelta }_{{\text{R}}_{\text{A1}}}$ represents the average stress reduction rate of the rock in A1 scheme, %.

${\overline{R} }_{0}$—Average stress of the rock without roof cutting, MPa.

${\overline{R} }_{\text{ij}}$—Average stress of the rock in ij scheme, MPa.

Based on the stress reduction rate of the rock, the types of schemes are compared and analyzed, and the sub-scheme with the largest stress reduction rate in each scheme is taken as the optimal scheme. In addition, the stress reduction rate of the optimal scheme is taken as the benchmark and its first integer value as the limited range, which is used as the reference interval for comparing the pressure relief effect of each sub-scheme. For example, if the maximum stress reduction rate $\delta_{{\text{R}}}$ in each scheme is 24%, then 20% < $\delta_{{\text{R}}}$ < 24% is the reference range for the comparison and selection of the pressure relief effect of each sub-scheme, and all sub-schemes with stress reduction rate within this range can be regarded as reasonable schemes.

3.4 Result Analysis

3.4.1 Analysis of Different Pressure Relief Parameters

• 1. Analysis of different top cutting angles

  Figure 5 shows the stress distribution and evaluation index of the roadway rock under different roof cutting angles (A1–A5 schemes).

  • 1. In the different cutting angle schemes (A1–A5 schemes) of the roadway, the rock stress shows a trend of increasing first and then stabilizing as a function of distance away from the roadway surface, while the stress of the solid coal rib side shows a trend of decreasing first and then stabilizing. Affected by directional roof cutting, the average rock stress within 5–20 m from the roadway surface is generally lower than that of the rock without roof cutting, as shown in Fig. 5a–c.

  • 2. The maximum average rock stress of the roadway without roof cutting appears at 10 m from the roadway surface, while the maximum average stress of the roadway with roof cutting appears at 15 m or even 20 m from the roadway surface. The rock stress with roof cutting is significantly lower than of the rock without cutting, as shown in Fig. 5d.

  • 3. With the increase of the roof cutting angle, the stress reduction rate increases first and then decreases. In the 30° roof cutting scheme (A4), the stress reduction rate of the rock reaches the maximum of 22.31%. Therefore, this roof cutting angle has the best effect on the stress release, as shown in Fig. 5d. The stress reduction rate in A3 scheme and A4 scheme is in the range of 20% < $\delta_{{\text{R}}}$<22.31%. Therefore, the reasonable range of the roof cutting angle is 20°–30°.

  Fig. 5

  

  Stress evolution and evaluation index of roadway with different roof cutting angles

  In short, when the roof cutting angle is 30°, the optimal cutting control effect can be obtained for the roadway under this condition. Therefore, according to these optimized parameters, B type is used in the numerical comparison test to further study the control effect of different roof cutting heights on the roadway.

• 2. Analysis of different roof cutting height schemes

  Figure 6 shows the stress distribution and evaluation index of the roadway in different roof cutting height schemes (B1–B5 schemes).

  • 1. The stress of the roadway in each roof cutting scheme is smaller than that of the roadway without roof cutting. With the increase of the cutting height, the stress of the roadway roof and solid coal rib side decreases, and the stress of the roadway floor increases, as shown in Fig. 6a–c. The increase of the cutting height increases the range and weight of the cantilever beam of the roadway roof, which may lead to an increase in the stress of the roadway floor.

  • 2. The maximum stress of the rock with roof cutting occurs at a distance 15 m or even 20 m away from the roadway surface. The stress of the rock with roof cutting is significantly lower than that of the rock without cutting, which is similar to the effect of the roof cutting angle.

  Fig. 6

  

  Stress evolution and evaluation index of the roadway with different roof cutting heights

  With the increase of cutting height, the stress reduction rate of the rock first increases rapidly and then slowly increases. In the 40 m cutting height scheme (B5), the rock stress reduction rate reaches the maximum of 23.08%. Therefore, 40 m is the cutting height that can achieve the best stress release effect. The schemes with reduction rate in the range of 20% < $\delta_{{\text{R}}}$< 23.08% are the B3–B5 scheme. In this range, as the roof cutting height increases, the increment of stress reduction rate gradually decreases. Considering field conditions, the cutting height of the roof r should not be too large (more than 30 m), and the reasonable cutting height is 20–30 m.

  According to the above optimized parameters, the C type numerical comparison test was used to study the facilitation effect of different strengthen blasting angles on the pressure relief of the roadway. The strengthen blasting height is constant, and it is set the same as the optimal cutting height.

• 3. Analysis of schemes with different blasting angles

  Figure 7 shows the stress distribution and evaluation index of roadway rock in different blasting angle schemes (C1–C5 schemes).

  • 1. In the scheme of different strengthen blasting angles (C1–C5 schemes). With the increase of the blasting angle (from 5° to 20°), the stress of the roadway floor decreases first and then remains basically unchanged, while the stresses of the roof and the rib side do not change much, as shown in Fig. 7a–c. The results show that the change of blasting angle has a greater influence on the roadway floor than other segments of the roadway.

  • 2. With the increase of the blasting angle, the rate of rock stress reduction first increases and then decreases. The stress reduction rate reaches the maximum of 34.15% at the blasting angle of 10° (C3 scheme). Therefore, the 10° blasting angle has the best auxiliary pressure relief effect on the rock stress of the roof-cutting roadway, as shown in Fig. 7d. The schemes within the range of stress reduction rate of 30% < $\delta_{{\text{R}}}$ < 34.15% are C2–C4 schemes, therefore, the reasonable range of strengthen blasting angle is 5°–15°.

  Fig. 7

  

  Stress evolution and evaluation index of the roadway in different roof blasting angles

  According to the above parameter analysis, it can be concluded that: in view of the extra-thick and hard roof roadway, the roof cutting height of 20–30 m and the cutting angle of 20–30° are the optimal range with significant pressure relief effect. Moreover, the strengthen blasting angle of 5°–15° away from the cutting direction is the optimal range, which enhances the release effect significantly.

3.4.2 Analysis of Stress Evolution

The directional roof cutting and strengthen blasting parameters of the roadway are designed according to the reasonable selection range of the above parameters and in combination with the thickness of the roof. Specifically, the cutting height (vertical height) of the roof stratum is 28 m, the height (vertical height) of the strengthen blasting is 25 m, the angle of directional roof cutting is 30°, and the angle of the strengthen blasting is 10° different from the angle of the directional roof cutting.

The monitoring section is set in the middle of the strike length of the working face, as shown in Fig. 8a. The three-dimensional stress evolution of the surrounding rock is shown in Fig. 8b. X-axis represents the length of the working face in the direction of inclination, (m); Y-axis represents the distance between the monitoring section and the excavation position of the working face, (m), and "-" indicates that the monitoring section is located before the excavation position of the working face; and Z-axis represents the vertical stress in the surrounding rock, (MPa).

• 1. The mining-induced stress at roadway position with original reinforcement without roof cutting on the left side of the working face is 22.14 MPa, which is close to that of the coal rib side of the roadway. However, the mining-induced stress at the working face at the right roof-cutting roadway is small, and a large range of low-stress area appears on the coal rib side. The pressure of the same magnitude as that of the left roadway occurs about 27 m away from the coal rib side of the right roadway. This shows that under the combined action of roof cutting pressure release and strengthen blasting, the right roadway exhibits obvious pressure relief effect.

• 2. Comparing the mining-induced stresses of the roof without cutting on the left side and of the roof with cutting on the right side of the working face, it can be seen that when the working face is behind the monitoring section (Y: 0–90 m), the size and evolution of the lateral bearing pressure of the solid coal rib side in the two types of roadways are similar. However, when the working face is ahead of the monitoring section (Y: − 90 to 0 m), there are large differences between the lateral bearing pressure of the solid coal rib side of the two roadways and the mining-induced stress at the coal side of the working face.

• 3. The maximum mining-induced stress of the solid coal rib side is 17.23 MPa on the right side of the working face with roof cutting, while it is 27.73 MPa on the left side of the working face without roof cutting. The former is 37.87% lower than the latter, indicating that under the combined action of roof cutting and strengthen blasting, the pressure relief effect on the solid coal rib side in the roadway is significant.

Fig. 8

Three-dimensional stress evolution of the roadway

4 Physical Model Test

To verify the rationality and effectiveness of the numerical results in the previous section, we conduct corresponding physical model tests to examine roof cutting pressure relief and energy-absorption reinforcement for roadway with extra-thick hard roof. Subsequently, we compare the stress changes between numerical and physical models.

4.1 Experimental Apparatus

The three-dimensional physical model apparatus developed by the authors’ group is used to carry out the model tests. The system consists of a hydraulic reaction loading system, a high-precision real-time monitoring system, an automatic coal mining and roadway driving system, and a directional roof cutting simulation system, as shown in Fig. 9. The automatic coal mining and roadway driving system includes coal mining simulation device, roadway driving simulation device and power stretching device. The first two devices are used for coal mining simulation and roadway driving simulation, and the power stretching device is designed to accurately control them.

Fig. 9

Physical model test system

4.2 Experimental Design

4.2.1 Determination of Similarity Scale

The geometric similarity scale is C_L = 60, the unit weight similarity scale is C_γ = 1.2, and the stress similarity scale is C_f = C_L × C_γ = 72. The field geological range of 330 m × 144 m × 180 m is selected for simulation, and the converted model size is 5500 mm × 2400 mm × 3000 mm.

The dimension of the model roadway is 93 mm × 72.5 mm (width × height), which is used to simulate the actual roadway size of 5.6 m × 4.35 m (width × height). According to the numerical results and the thickness of the actual roof stratum, the designed directional roof cutting height (vertical height) in the physical model is 467 mm, which corresponds to the actual height of 28 m. The designed strengthen blasting height (vertical height) in the model is 417 mm, corresponding to the actual height of 25 m. The directional roof cutting angle is 30°, and the strengthen blasting angle deviates from the directional roof cutting angle by 10°.

4.2.2 Test Schemes

The test sections of the roadways on both sides of the working face are selected for comparative analysis. The two test sections are represented by M1 and M2. Traditional reinforcement design was used in the test section M1 on the left side (actual field design), and pressure relief and energy-absorbing reinforcement was implemented in the test section M2 on the right side. The test schemes are shown in Fig. 10 and Table 5.

Fig. 10

Schematic diagram of the test schemes

Table 5 Scheme design of the physical model test

• 1. Loading and excavation scheme

  Through the hydraulic reaction loading system, the vertical stress of the physical model is loaded to 0.114 MPa and the horizontal stress is loaded to 0.057 MPa. After the loading is completed, the excavation of the model body starts. The excavation sequence includes the following four stages: left roadway excavation Ai (i = 1–60, 60 steps, 50 mm each step) → right roadway excavation Bi (i = 1–60, 60 steps, 50 mm each step) → directional presplitting roof cutting Ci (i = 1–12, the directional roof cutting system works 12 steps ahead of the working face, 50 mm each step, 600 mm in total) → excavating the working face Di, (i = 1–60, 60 steps, 50 mm each step). The directional roof cutting is completed when the directional roof cutting system is fully drawn out. The strengthen blasting is realized by pre-buried mica sheets in the roadway roof according to the designed height and angle.

• 2. Monitoring scheme

  In the interior of the model body, a monitoring section 325 mm away from the front surface is placed along the strike direction of the simulated working face, which is used to monitor the stress and deformation of the traditionally reinforced section (M₁ section) and the roof cutting pressure relief energy-absorbing reinforced section (M₂ section). The monitoring points, represented by R_i and F_j respectively, are arranged in the roadway roof and floor coal bodies. The positions of these monitoring points correspond to those of the monitoring points converted by geometric similarity scale in the previous numerical test.

4.3 Test Process

The model body is loaded according to the designed stresses. After the loading is stable, the model body is excavated, as shown in Fig. 11.

• 1. Roadway excavation stage on the left side of the working face: after the model is loaded, the roadway forming device embedded on the left side of the model is pulled at a constant speed, as shown in Step I of Fig. 11. The entire excavation process is divided into 60 steps, and excavation continues after each step is completed.

• 2. Roadway excavation stage on the right side of the working face: after the left roadway is excavated and stabilized, the right roadway forming device is pulled at a constant speed, as shown in Step II of Fig. 11. The excavation process is consistent with that of the left roadway.

• 3. Roof cutting stage of the right roadway: after the right roadway is excavated and stabilized, the directional roof cutting system of the right roadway is pulled at a constant speed to simulate the roof cutting and pressure relief of the roadway, as shown in Step III of Fig. 11. The process is divided into 12 steps within 600 mm ahead of the right roadway, with each step pulling 50 mm.

• 4. Working face excavation stage: after the right roadway is 600 mm ahead of the directional roof cutting location and stabilized for a period of time, the mining sliding device of the working face and the directional roof cutting device of the right roadway are pulled at a constant speed. At the same time of mining the working face, the right roadway is subject to advanced directional roof cutting synchronously, as shown in Step IV of Fig. 11.

Fig. 11

Diagram of the test process

4.4 Analysis of Test Results

4.4.1 Data Validation

In this section, the roadway without roof cutting is compared with the roadway with roof cutting, and the stress change of the roadway at 4.5 m ahead of the working face is analyzed. The comparison of stress distribution in roadway roof and floor in the two types of tests is obtained, as shown in Figs. 12 and 13.

• 1. In the numerical simulation and physical model test, the stress evolutions in roadway roof and floor without and with roof cutting are similar. The stresses of the roadway roof and floor with roof cutting are lower than those of the roadway without roof cutting, indicating that the directional roof cutting pressure relief has advantages in the pressure relief of the roadway.

• 2. After stress conversion using similarity scale, the stresses of roadway roof and floor in the physical model are consistent with the field stresses in the numerical simulation. And the average stress difference is 11.44–26.87%.

Fig. 12

Comparison of stress distribution in roadway roof

Fig. 13

Comparison of stress distribution in roadway floor

4.4.2 Comparison of Stress Evolution

Data of roadway roof monitoring points (R₁–R₂) and floor monitoring points (F₁–F₂) in the monitoring section are recorded. Stresses of roof and floor before the working face reaches the monitoring section (advanced working face, the distance is 0 to − 325 mm) and after the working face passes the monitoring section (lagging working face, the distance is 0–325 mm) along the mining direction are analyzed. Figures 14 and 15 show the stress evolutions of roadway roof and floor without roof cutting and under traditional reinforcement and with roof cutting with strengthen blasting and energy-absorbing reinforcement are obtained.

• 1. For the roadway after directional roof cutting, the stress at the shallow monitoring point (R₁) of the roof is always low, with the maximum not exceeding 0.068 MPa, which is 40.87% lower than the maximum stress (0.115 MPa) at the shallow monitoring point of the roof of the roadway without roof cutting. The shallow monitoring point of the floor (F₁ 5 m from the roadway floor surface) is in a low stress state during the advanced and the lagging working face stages relative to the monitoring section, which is mostly not affected by the mining of the working face.

• 2. The stresses of the roof and floor of the roadway with directional roof cutting are lower than that of the roadway without roof cutting. Further, the stress rapidly reduces to a low stress state after the working face passes the monitoring section. However, the stresses of the roof and floor of the roadway without roof cutting are in a large stress state in the stage when the monitoring section is ahead of the working face. And the stress starts to slowly reduce in the stage when the monitoring section is behind the working face.

• 3. In the roadway treated by directional roof cutting combined with strengthen blasting and energy-absorbing reinforcement, the stress of the roof and floor is reduced to a certain degree. The average stresses of the roof and floor of the roadway with roof cutting are 53.74% and 26.26% lower than that of the roadway without roof cutting, and the peak stresses are 46.02% and 49.61% lower, respectively. This indicates that directional roof cutting has a good pressure relief effect on surrounding rock of the roadway roof and floor.

Fig. 14

Stress variation of roadway roof

Fig. 15

Stress variation of roadway floor

4.4.3 Comparison of Deformation Evolution

We analyze the displacement data of R₁ monitoring point of the roadway roof and F₁ monitoring point of the floor in the monitoring section, as shown in Fig. 16.

• 1. The rock deformation of the roadway floor in the advanced working face is generally greater than that of the roadway roof, which suggests that the roadway floor is more affected by the mining-induced stress during mining the working face due to the influence of the extra-thick hard roof, and the deformation of the floor without reinforcement is significantly larger.

• 2. Compared with the traditionally reinforced roadway without roof cutting, the rock deformation of the roadway with directional roof cutting combined with strengthen blasting and energy-absorbing reinforcement is lower than that of roadway without roof cutting. The maximum roof deformation of the advanced working face is 3.43 mm, and the floor deformation is 4.01 mm. The maximum roof and floor displacements of the roadway are decreased by 54.47%, which implies that the method of directional roof cutting combined with strengthen blasting and energy-absorbing reinforcement has good reinforcing effect on the deformation of the roadway roof and floor.

Fig. 16

Deformation evolution of the roadway roof and floor

4.4.4 Characteristics of Overburden Collapse at the Working Face

With the mining of the working face, the roof strata of the roadways on both sides have collapsed to varying degrees. The characteristics of the overburden collapse at the working face are shown in Fig. 17.

• 1. Roadway without roof cutting on the left side of the working face: when the working face is mined for a certain distance, the hard roof near the gob on the left side of the roadway form a suspended roof within a certain range, as shown in the left schematic of Fig. 17a. When the area of the suspended roof reaches a certain extent, with the continuous mining of the working face, the suspended roof suddenly breaks, rotates, and sinks. One end is connected with the roadway roof, and the other end is connected with the collapsed roof of the gob, as shown in the right schematic of Fig. 17a.

• 2. Roadway with directional roof cutting on the right side of the working face: when the working face is mined for a short distance, the hard roof near the gob on the right side of the roadway will collapse along the directional cutting seam, as shown in the left schematic of Fig. 17b. With the continuous mining of the working face, the hard roof in the gob near the roadway continues to collapse to varying degrees. However, the surrounding rock of the roadway is basically not affected by the collapse of the roof of the whole working face, as shown in the right schematic of Fig. 17b.

• 3. Comparison of roadways on both sides: the mining distance when the roof of the working face on the right side collapses for the first time is far less than that of the roadway without roof cutting on the left side. This indicates that the physical connection between the roadway roof with directional roof cutting and the roof of the working face is cut off. Under the action of deep hole directional roof cutting, the roof of the gob can collapse rapidly, which solves the issue of large roof suspension in the gob of the hard roof roadway. The sudden collapse of a large suspended roof near the left roadway without roof cutting has a great impact on the roadway, and the large stress in the gob is transmitted to the roadway through the roof stress transmission structure.

Fig. 17

Comparison of roof collapses of roadways and working faces

5 Field Application

5.1 Roof Cutting Parameters and Reinforcement Design

Combined with the results of numerical simulation and physical model test, the parameters of directional roof cutting and energy-absorbing reinforcement were designed and applied in the ventilation roadway of the Caojiatan #122108 working face. The directional cutting height is 28 m, and the cutting angle is 30°, the strengthen blasting height is 25 m, and the strengthen blasting angle is 10°. The constant-resistance energy-absorbing anchor cable is located on the side of the roof cutting, its model is Φ22 × 9000 mm and its row spacing is 2000 mm. Specific directional pressure relief parameters and field reinforcement design are shown in Fig. 18.

Fig. 18

Pressure relief parameters and reinforcement design

5.2 Analysis of Monitoring Results

5.2.1 Monitoring and Analysis of Roadway Rock Deformation

With the new method applied, six deformation monitoring stations are arranged within 0–100 m of the test section, with an interval of 20 m. Two monitoring sections (I and II) are arranged in front of and behind each monitoring station, numbered w-I and w-II, with w = 1–6. With the mining of the working face, the change of roadway floor deformation is shown in Fig. 19.

• 1. The average deformation of each monitoring section within 100 m of the mining face is 201.8 mm, which is 74.8% lower than the field actual deformation. This suggests that the comprehensive control method of directional roof cutting combined with strengthen blasting and energy-absorbing reinforcement is effective in ground control.

• 2. The floor deformation at the same position of the physical model in the previous section is 4.01 mm, which is converted into the field actual deformation of 240.6 mm by the geometric similarity scale of 1:60. The field measured result is slightly smaller than the physical model result, and the difference between the two is 16.13%, which verifies the rationality and effectiveness of the physical model result.

Fig. 19

Deformation variation of the roadway floor

5.2.2 Stress Analysis of the Roadway Reinforcement

As the working face is being mined, the stress of the advanced reinforcement at the #122108 working face in the ventilation roadway is monitored in real time. Figure 20 shows the stress evolution of the advanced reinforcement along with the mining of the working face.

Fig. 20

Stress evolution of the roadway ahead of the working face

It can be seen from Fig. 20 that the stress of the roadway after directional roof cutting and strengthen blasting exhibits a stepwise descending trend.

• 1. Without blasting, the peak stress of the reinforcing system reaches 40 MPa, and the average stress is 27.1 MPa.

• 2. When the reinforcing system enters the test section from the start of blasting, the peak stress of the reinforcing system in the roadway decreases to 36.1 MPa, 8.25% less than that without blasting, and the average stress is 26.6 MPa, 1.84% less than that without blasting.

• 3. Between the reinforcing system enters the test section and the working face enters the test section, the peak stress decreases to 32.9 MPa, 17.75% less than that without blasting.

• 4. After the working face enters the test section, the peak stress of the reinforcing system in the roadway decreases to 28.9 MPa, 27.75% less than that without blasting, and the maximum reduction is 11.1 MPa.

It can be seen from the stress changes of the reinforcing system in the four stages described above that the stress decreases continuously with the mining of the working face after the comprehensive control method of directional roof cutting in combination with strengthen blasting and energy-absorbing reinforcement is implemented.

6 Conclusions

• 1. We proposed an integrated ground control method of roof cutting pressure relief and energy-absorbing reinforcement to tackle ground control challenges in extra-thick coal seams with extra-thick hard roof. Through directional roof cutting to cut off the stress transfer path between the roadway and the gob roof, the impact of hard roof collapse is reduced by strengthen blasting, and the constant-resistance energy-absorbing anchor cables are used to ensure the safety and stability of the roadway.

• 2. We conducted numerical simulations on different pressure relief parameters for roadway with thick hard roof. We analyzed the impacts of directional cutting and strengthen blasting parameters and examined three-dimensional stress evolution of the surrounding rock of the roadway. The advantages of roof cutting pressure relief and energy-absorbing reinforcement in the stability of the roadway are verified.

• 3. The model comparison test of pressure relief and energy absorption control of thick and hard roof roadway was carried out, and the difference of stress and control effect of roadway surrounding rock before and after the safety control method of roof cutting pressure relief and energy absorption support was obtained. Using the new ground control method, the average stress of the surrounding rock is reduced by 40.1%, the maximum deformation is reduced by 74.8%. The control mechanism of roof cutting pressure relief and support energy absorption in roadway with thick hard roof is clarified.

• 4. We carried out a field design and application of the new method based on the laboratory experimental results. Field monitored data shows that the average deformation of the roadway floor with the new control method is 201.8 mm, which is 66.37% less than that of the roadway without using the method, and the peak stress of the reinforcing system is 27.75% less than that without using the method. The fidelity of the physical model test data and the rationality of the design method of pressure relief reinforcing parameters are verified.