Introduction

Rock burst has been recognized as one of the main hazards for underground coal mines for years, especially in China1,2,3. During the vast study and prevention practices in preventing coal mine rock burst, various methods have been proved to work well, such as pressure-relief borehole, deep-hole blasting and in some occasions, water injection4,5,6,7,8. However, as the mining depths keeps increasing which brings high ground pressure, the traditional methods we mentioned above are getting less effective to satisfy the challenges of rock burst control. Therefore, a novel method, pressure-relief roadway, was proposed recently9,10,11,12.

Pressure-relief roadway works in two respects. On one hand, it acts as a larger pressure-relief borehole, providing more deforming space to release energies stored in surrounding rock13,14,15. On the other hand, with its large space in size, a lot of engineering work can be conducted in this roadway, depending on specific demands of coal mines16,17,18,19,20.

Mengshan area is the center of gravity of coal resources exploitation in China. During the 13th Five-Year Plan period, the planned production capacity of the region accounts for nearly 1,2 of the national planned production capacity, and the modern mines21 under construction and to be built are high yield and high efficiency. However, in the actual mining process, it is found that Hujite mining area is affected by complex factors such as hard roof and high-intensity mining disturbance, and rock burst power disasters occur frequently, which has seriously restricted the safe and efficient production22 of the mine. In the early stage of construction of Hulusu Coal mine in Hujilt mining area, the “Shendong mode” of double-roadway excavation was adopted. During the mining process, different degrees of impact appeared, resulting in roadway damage and equipment damage, which seriously restricted the safety of mine production. At present, there are a lot of research results on the treatment technology of roof type rock burst and coal pillar type rock burst, but there are relatively few studies on the use of original roadway as relief roadway to prevent rock burst under such conditions23,24,25,26. The author takes 21,201 working face of Hulusu Coal mine as the research object, and puts forward the comprehensive control technology using relief roadway and other means. To provide reference for rock burst prevention and control under similar conditions.

The design of coal pillar dimensions and system configurations is critical for coal burst prevention. This study investigates a novel pillar system that employs pressure relief roadways to alter roof movement patterns in goaf areas and redistribute stress within coal pillars, thereby achieving protective effects on target roadways.

Project overview

Hulusu Coal mine is the only intelligent mine of 10 million tons with the first 2-1 coal seam in Hujite mining area. The buried depth is about 640 m, which is divided into four levels and six disk areas. At present, the mine is mining a level 2-1 coal seam and 2-1 middle coal seam, of which 2-1 coal is the top coal seam of the pan area, with an average thickness of 2.8 m; the middle coal of 2–2 is about 30 m below the coal of 2-1, with an average thickness of 3.0 m.

The 21,201 working face of Hulusu Coal mine is located in the east of the second pan area of 2-1 coal mine. The working face is 293 m wide before entering the pressure relief roadway, 278 m wide after entering the pressure relief roadway, and 3965 m long. Affected by fault, fold, scour zone, goaf and the impact tendency of coal and rock mass, 21,201 faces has the risk of medium rock burst during mining. Among them, the width of the coal pillar between the pressure relief roadway and the goaf of 21,102 working face is 30 m, the width of the pressure relief roadway is 5 m, and the width of the coal pillar between the pressure relief roadway and the air return roadway of 21,201 working face is 10 m.The schematic diagram of the roadway position of the 21,201 working face is shown in Fig. 1. The shape and size of the pressure relief roadway are set the same with other roadways, with its width and height at 5.2 × 4.7 m.The pressure relief roadway is located within the coal seam.

Fig. 1
Fig. 1
Full size image

Schematic diagram of the roadway position of 21,201 working face.

According to the research, the rock strata affecting the occurrence of rock burst are the rock strata within 100 m above the coal seam, in which the sandstone layer with large rock strength and thickness plays the main role. The 21,201 working face is distributed in five boreholes, HK14, HK25, HK35, HK43 and H27. Combined with the comprehensive column chart and coring situation, the overlying strata profile is drawn, as shown in Fig. 2.

Fig. 2
Fig. 2
Full size image

Profile of overlying strata on 21,201 working face.

Analysis of anti-shock mechanism of pressure relief roadway

Reducing stress concentration is one of the methods to prevent and control rock burst, mainly from the stability of coal pillar, excavating pressure relief roadway to promote the stress redistribution, so that the protected roadway is in the stress reduction area formed by the excavation of pressure relief roadway to analyze the anti-impact mechanism of pressure relief roadway.

Stability analysis of coal pillar

The research shows that the coal pillar size belongs to the critical type when the width, height ratio (W, H) is between 5 and 10.Before the 21,201 working face enters the pressure relief roadway, the section coal pillar width is 30 m and the width, height ratio is 10.71, which is close to the critical type coal pillar width, height ratio 10, and the stress concentration is high, which is not conducive to the roadway maintenance and the prevention of coal pillar rock burst. After the 21,201 working face enters the pressure relief roadway, the width of coal pillar in the section is 45 m, and the width to height ratio of coal pillar reaches 16.07, which is much higher than the critical ratio of coal pillar to width to height 10, which is more conducive to the control of roadway deformation and the prevention and control of rock burst risk. The width-to-height ratio of 10 m coal pillar near 21,201 working face is about 3.57, which is a typical yield coal pillar with low stress. At the sometime, the width to height ratio of 30 m wide coal pillar faraway from 21,201 air return roadway is 10.71, which belongs to the high-stress bearing coal pillar, but because it is far away from 21,201 air return roadway, it has little influence on the pressure of 21,201 Air return roadway, as shown in Fig. 3.

Fig. 3
Fig. 3
Full size image

Relation between W, H of composite coal pillar at 21,201.

Pressure relief mechanism of pressure relief roadway

The principle of depression-relief roadway is to dig a depression-relief roadway in the surrounding rock of the protected roadway and chamber so that the protected roadway and chamber are in the stress reduction zone, so as to improve the stability of surrounding rock and reduce the deformation of surrounding rock. The essence of the pressure relief method using pressure relief roadway is to dig a roadway or chamber specially used for pressure relief near the protected roadway, transfer the mining influence of nearby coal seam mining, promote the redistribution27 of stress caused by mining, and finally make the protected roadway in the stress reduction zone formed by the excavation pressure relief roadway, as shown in Fig. 4. Principle diagram of excavating pressure relief roadway inside coal pillar.

For the composite coal pillar at the side of the air return roadway of 21,201 working face, the coal pillar is divided into 10 m narrow coal pillar and 30 m wide coal pillar in the relief roadway, resulting in the stress distribution around the roadway, as Fig. 5.The 10 m narrow coal pillar located between 21,201 air return roadway and relief roadway is called yielding coal pillar, while the 30 m wide coal pillar located between relief roadway and goaf is called bearing coal pillar. Due to the isolation effect of the relief roadway, the high concentrated stress on the side of the goaf is applied to the bearing coal pillar faraway from 21,201 air return roadway, and only a small part of the stress acts on the 10 m yield coal pillar near 21,201 air return roadway. At this time, 21,201 air return roadway is already in the stress reduction zone. At the same time, a large amount of deformation of the bearing coal pillar under the action of high stress is also absorbed by the depressurized roadway, as shown in 6, which greatly reduces the stress concentration during the mining of the working face of 21,201.

Fig. 4
Fig. 4
Full size image

Principle diagram of excavating pressure relief roadway inside coal pillar. 1-Protected roadway; 2-relief roadway; 3-yield pillar; 4-abutment coal pillar.

Fig. 5
Fig. 5
Full size image

Schematic diagram of stress distribution and relief effect of coal pillar.

Numerical simulation before and after the implementation of pressure relief roadway

Establishment of numerical model

Based on the geological conditions of Hulusu Coal Mine, models are constructed for both scenarios: without and with the implementation of stress-relief entries. Without stress-relief entries, the coal pillar width between the 21,102 and 21,201 working faces is 30 m.With stress-relief entries, the width of the stress-relief entry is 5 m, there is a 10 m coal pillar between the 21,201 air return way and the stress-relief entry, and a 30 m coal pillar is left between the stress-relief entry and the 20,102 working face. The model dimensions are set to 700 m×1200 m×100 m (x×y×z), as shown in Fig. 6.

Fig. 6
Fig. 6Fig. 6
Full size image

Construction of numerical analysis mode, created with FLAC3D6.0 (http://www.itascacg.com/software/flac).

According to the comprehensive column chart of strata of the working face 21,201, the overlying rock layer of the built model is 542 m, and the average volume force of the overlying rock layer is 27kN, m3. Therefore, the vertical load of 14.65 MPa is applied to the upper part of the model to simulate the self-weight of the overlying rock layer. The boundary conditions around and at the bottom of the model are fixed constraints. In order to ensure the reliability of the initial in-situ stress field existing before the simulated mining, the in-situ stress of 2-1 coal seam is applied according to the measured insitu stress, that is, σxx = 24.74MPa, σyy = 13.36MPa, σzz = 16.31 MPa. The Initial command was used to set the initial stress for the horizontal initial stress, and the More-Coulomb failure criterion was used to calculate the initial stress. The simulated roof subsidence of approximately 500 mm aligns closely with field-measured data, thereby validating the model’s accuracy. The specific parameters in numerical simulation are shown in Table 1.

Table 1 Numerical simulation of rock mechanics parameters.
Full size table

This paper mainly simulates the stress field and the change of plastic zone of roadway surrounding rock before and after 21,201 working face enters pressure relief roadway. Therefore, in the process of simulation calculation, according to the actual mining order on site, the 21,102 working face is excavated first, then the 21,201 working face of the unimplemented depressurized roadway is excavated, and the 21,201 working face of the depressurized roadway is excavated finally.

Evolution law of abutment pressure of 21,201 working face

Figure 7a shows the three-dimensional view of vertical stress distribution of coal seam in the working face before entering the pressure relief roadway. Affected by the superimposed stress, vertical stress concentration is generated at the 45 m coal pillar side, and the stress is obviously higher than the other three sides of the goaf. The vertical stress distribution above the coal pillar presents a single-peak slope type feature, and the peak value of vertical stress is at the end of the working face, and then the peak value gradually decreases.

As shown in Fig. 7b, the 3D view of vertical stress distribution of coal seam in the working face after entering the pressure relief roadway shows that the vertical stress in front of the 21,201 working face is significantly reduced due to the existence of the pressure relief roadway, and the pressure relief roadway achieves better depressurization Effect. In order to more accurately compare and analyze the pressure relief effect of pressure relief roadway on the working face during mining, the stress variation law of 21,201 working face of pressure relief roadway and non-pressure relief roadway was extracted. Monitoring lines were arranged on the solid side of the air return roadway of the working face, and the vertical stress of the front position of 21,201 working face along the Y-axis direction (strike) was extracted, and the generated curve was shown in Fig. 8.

Fig. 7
Fig. 7
Full size image

3D view of vertical stress distribution in coal seam, created with Surfer 2023 (http://www.goldensoftware.com).

Fig. 8
Fig. 8
Full size image

Vertical stress distribution diagram in front of 21,201 working face.

As can be seen from Fig. 9, when no relief roadway is arranged, under the superposition of the front abutment pressure and the lateral abutment pressure in the gob area of the adjacent working face in the air return roadway of the working face 21,201,the peak vertical stress in the coal wall in front of the working face reaches about 55 MPa, which is 3.4 times of the original rock stress, within 100 m advance when the working face advances 300 m.When the relief roadway is arranged, the vertical stress within 100 m in front of the air return roadway on the working face of 21,201 is 38 MPa, which is 2.3 times of the stress in the original rock. Compared with the situation without pressure relief roadway, it can be seen that the vertical stress is significantly reduced, the peak value is reduced by 17 MPa, and the overall average is reduced by about 10 MPa.The vertical stress curve shows the same stress distribution law, the peak value and its affected area no longer increase, but continue to move forward with the advance of the working face, and the pressure relief effect is remarkable.

Stress distribution characteristics of surrounding rock at 21,201 face

In order to comprehensively analyze the stress distribution of surrounding rock before and after 21,201 working face entering pressure relief roadway, the axial principal stress of solid side of roadway in front of the working face is extracted during the process of simulating 21,201 working face advancing from the cut hole position, and the ratio and difference curves28 of principal stress are generated, as shown in Figs. 9 and 10.

Fig. 9
Fig. 9
Full size image

Before and after pressure relief roadway and air return roadway. Three-stage distribution diagram of axial principal stress ratio.

Fig. 10
Fig. 10
Full size image

Before and after pressure relief roadway. Three-stage distribution diagram of axial principal stress difference.

According to the comprehensive analysis of the change distribution law of the principal stress difference and the ratio curve of the principal stress, it can be seen that the change characteristics of the axial principal stress difference and the ratio of the air return roadway in front of the coal wall of the working face show the same three stages: the rapid decline stage, the stable decline again.

Through the analysis of the distribution characteristics of the principal stress curve, it can be seen that the existence of pressure relief roadway significantly reduces the principal stress of surrounding rock in front of 21,201 face, and the peak value of the principal stress ratio is reduced to 1.7, which plays a good pressure relief effect on the surrounding rock of 21,201 face.

Comprehensive prevention and control technology system

Hard roof active pre-cracking technology

According to the overlying strata profile on the working face of 21,201, there are thick layers of sandstone in the range of 20–40 m above the coal seam. Combined with the results of numerical simulation analysis, the stress concentration area is still within 100 m of the advance working face after entering the pressure relief roadway area. Therefore, pre-cracking measures are taken to reduce the stress concentration on the hard overlying roof in order to reduce the impact risk of the working face.

For the solid coal side roof of 21,201 air return roadway, the vertical pre-cracking pressure relief is implemented by the combination of high-low blasting pre-cracking drilling. One group of drilling holes is arranged every 20 m.The parameters of high drilling are as follows: hole depth 46 m, elevation Angle 60°; Low drilling parameters are as follows: hole depth 45 m, elevation Angle 45°; The aperture is 75 mm, and the azimuth is 270°. The main purpose is to weaken the key roof layer and shorten the working face period to press the step. The scheme is shown in Fig. 11. The parameters of blasting drilling are shown in Table 2.

Fig. 11
Fig. 11
Full size image

Profile of 21,201 return-air roadway roof pre-split blasting.

Table 2 Construction parameters of 21,201 air return roadway roof pre-splitting blasting.
Full size table

High strength coal pre-pressure relief technology

In view of the phenomenon of stress concentration in the coal body of 21,201 working face, the pressure relief drilling pre-relief in the coal seam construction of the mining wall of the air return roadway is implemented according to the strong, medium and weak impact danger areas, the spacing is 1 m, 2 m and 3 m, the aperture is 150 mm and the hole depth is 15 m. Through the construction of coal seam pressure relief drilling, the stress concentration of coal and rock mass in the area around the drilling is reduced, and the high stress is transferred to the depth of coal and rock mass, so as to achieve the purpose of local coal body pressure relief and play the role of pre-pressure relief. The specific construction parameters are shown in Table 3.

Table 3 Construction parameters of large diameter drilling of coal seam in 21,201 working face.
Full size table

Strong supporting system of roadway

Construction of roadway roof double bearing layer

Research on low-density and high-efficiency support technology for coal roadway in 21,201 air return roadway has been carried out. Based on the continuous beam theory, combined with the group anchor effect of large-row distance anchor bolt cable support and the principle of anchoring superposition, a new high-pressure long-anchorage cross-boundary support technology with high-strength anchor cable replacing anchor bolt has been proposed, as shown in Fig. 12.A row of bolt and a row of cable are arranged at intervals, and the row distance between the adjacent bolt and cable is 1100–1200 mm, so as to give full play to their respective supporting capabilities and avoid the interference caused by the high pretension cable on the bolt. At the same time, the “shallow foundation bearing layer” and “deep strengthened bearing layer” is formed on the stratum of the roadway roof at 2.5 m and 4.3 m respectively, so as to avoid the concentrated development of the separation layer on the support interface when the anchorage layer is uniformed.

Fig. 12
Fig. 12
Full size image

Double bearing layer of roadway roof.

Active support scheme of open roadway with hard roof

On the basis of the “big bolt support” technology, the “multi-layer flexible” support technology29 is optimized for the roadway along the goaf coal pillar. Each row of roof support adopts 5 screw steel bolts with a diameter of Ф22 × 2800 mm and 4 bolts with a diameter of Ф21.8 × 4300 mm anchor cable in each row. The row distance is adjusted to 1000 mm, and the area of roof separation and surrounding rock fissure development range is larger than the length of roof support is reinforced, and the diameter of the reinforced anchor cable in the middle of the roof is Ф21.8 × 7300 mm. Vertically arranged near the anchor rodin the way of 1-1-1-1, and the row distance is 2000 mm.

Solid coal side support selection Ф22 × 2800 mm rebar steel bolt + 8# galvanized rhombohedral barbed wire support, row distance of 1000 mm,3 in each row, spacing from top to bottom in order of 300–1000 mm- 1000 mm–500 mm.

On the basis of the short support of rigid bolt, two flexible bolts are constructed in every row of rebar bolton the side of the coal pillar in the roadway. The specifications of the flexible bolt are Ф22 × 4300 mm, and the spacing between them is 1800 × 1000 mm. They are arranged in the middle position of therebar bolt. Such as Figs. 13, 14 and 15.

Fig. 13
Fig. 13
Full size image

Plan of roof support of 21201 air return roadway.

Fig. 14
Fig. 14
Full size image

Section view of stopping wall support of 21201 air return roadway.

Fig. 15
Fig. 15
Full size image

Cross-section of coal pillar wall support of 21,201 air return roadway.

Front support scheme of hard roof open-air roadway

21,201 air return roadway adopts 18 ZQL15500, 18, 32D advanced hydraulic supports with a length of 120 m to support the roof in a single row, as shown in Fig. 16. The supporting force of the support body under the unit length of the strike is 2325KN, which is much higher than the top and bottom moving force of 463KN, and can achieve the impact resistance.

Fig. 16
Fig. 16
Full size image

Front support of 21,201 air return roadway.

Evaluation of pressure relief effect

In order to better test the comprehensive prevention and control effect, micro seismic monitoring, deformation observation of surrounding rock and active CT detection of vibration wave are adopted to carryout comprehensive monitoring and analysis.

Micro seismic monitoring

The average micro seismic frequency, energy and time frequency of large energy in the stable production stage before the working face 21,201 enters the depressurized roadway and the stable production stage after entering the depressurized roadway are selected for comparative analysis, as shown in Fig. 18 and Table 4.

Table 4 Comparative statistical table of micro seismic data of 21,201 working face before and after entering pressure relief roadway.
Full size table

As can be seen from Table 3; Fig. 17, after the working face enters the pressure relief roadway, the comprehensive prevention and control technology is implemented on this basis, and the micro seismic frequency is more but the energy level is relatively small, and there is no sudden increase phenomenon, which is relatively stable; The daily energy release is reduced by 80.8%, the large energy events are reduced by 90% on average, and no large energy events over 104 J occur.

Fig. 17
Fig. 17
Full size image

Comparison of micro seismic distribution of 21,201 working face before and after entering depressurized roadway.

Deformation observation of surrounding rock

Four groups of surrounding rock deformation observation points were arranged in the air return roadway before and after the 21,201 working face entered the relief roadway for data collection, and the data of the displacement of mining wall, roof subsidence and floor heave of the air return roadway were analyzed and compared respectively. The specific change curves are shown in Figs. 18, 19, 20.

Fig. 18
Fig. 18
Full size image

Displacement curve of stopping wall in air return roadway before and after entering pressure relief roadway.

Fig. 19
Fig. 19
Full size image

Change curve of roof subsidence in air return roadway before and after entering pressure relief roadway.

Fig. 20
Fig. 20
Full size image

Change curve of floor heave of air return roadway before and after entering relief roadway.

According to the comparative analysis of Figs. 19, 20 and 21, it can be seen that before the working face enters the depressurizing roadway, the maximum displacement of the mining wall and the maximum subsidence of the roof in the air return roadway are 680 mm and 780 mm respectively. After the working face enters the pressure relief roadway, the roof subsidence and floor heave are significantly improved, among which the maximum displacement of the mining wall is 510 mm, the maximum subsidence of the roof is 75 mm, and the floor heave is 200 mm, and most of the displacements are larger.

Fig. 21
Fig. 21
Full size image

Actual layout of observation point and shooting point.

At present, when the working face is pushed to the observation point, the deformation of the stopping wall is reduced by 25%, the subsidence of the roof is reduced by 90.3%, and the floor heave is reduced by 77.9%.

Vibration wave active CT detection

The vibration wave active CT detection equipment is installed in 21,201 transport roadway, and the shooting points are placed in 21,201 air return roadway respectively. The probe spacing is 10 m, the shooting point spacing is 10 m, and the charge capacity is 250 g30 per hole. As shown in Fig. 21, determine the number of probes 30 (R7-R36), 30 shooting points (B1 B30). The GeoWave software was used to mark the arrival time of the first wave for each waveform.

According to the relationship between the distance and the arrival time of the first wave and the positive anomaly of the p-wave wave velocity, the stress state and stress concentration degree in the coal seam are determined, and the distribution diagram of burst rock pressure on the 21201 working face is finally drawn, as shown in Fig. 22.There are two weak burst danger areas, and the remaining 75% areas are all non-burst danger areas.

Fig. 22
Fig. 22
Full size image

Danger distribution diagram of rock burst on 21,201 working face.

Conclusions

  1. (1)

    The stability of coal pillars of different widths and the anti-shock mechanism before and after the arrangement of pressure relief roadway are fully analyzed; the evolution law of abutment pressure and stress distribution law of surrounding rock are obtained by using numerical simulation software, which fully affirms the important pressure relief significance of relief roadway.

  2. (2)

    Put forward the hard roof active pre-cracking measures, coal high strength pressure relief measures, surrounding rock multi-layer support pre-system, the use of advanced support and other comprehensive prevention and control technology schemes.

  3. (3)

    After the implementation of the comprehensive prevention and control technology, the frequency of micro earthquakes is basically flat, the daily energy release is reduced by 80.8%, the average reduction of large energy events is 90%,and no large energy events over 104 J occur; The observed stopping wall deformation of surrounding rock was reduced by 25%,the roof subsidence by 90.3%, and the floor heave by 77.9%;There are only two weak impact danger areas on the vibration wave active CT detection working face, and the remaining 75% area is no impact danger area, and the pressure relief effect is remarkable.