Study on the rational position of externally offset mining roadway in the lower split during layered mining of a thick coal seam

Abstract

To improve coal recovery rates and address the challenge of determining optimal locations for the external offset of lower split mining roadways in thick coal seams, a targeted study was conducted using the 52,202 working face mining roadway at the Wulansetai Coal Mine in Shenmu City, Shaanxi Province, China, as the research context. First, a systematic computational analysis was conducted on the strength of the wide coal pillar in the 60-meter section of the upper stratum and the load it bears. The results show that the strength of this coal pillar is 15.7 MPa, which is significantly greater than the 6.7 MPa load it bears under the influence of the goaf; Based on the theory of limit equilibrium, further calculations show that the width of the limit equilibrium zone for the wide coal pillar in the upper stratum section is 13.08 m, with its elastic zone extending from 13.08 m to 46.92 m from the right edge of the coal pillar. Second, using the finite difference numerical simulation software FLAC3D, we simulated and analyzed the evolution of the stress field and displacement field in the lower-level mining roadway under different external offset distances. Simulation results indicate that when the external offset distance for a stratified mining roadway beneath a thick coal seam is set to 24 m, the stress distribution in the surrounding rock is relatively uniform, deformation is minimized, and the difficulty of roadway maintenance is lowest, making this the most feasible option. Finally, this optimized design was implemented in the 52,202 working face mining roadway at the Wulansetai Coal Mine. Field results confirmed that the 24 m external offset distance for the lower-level mining roadway fully meets the mine’s requirements for safe and efficient production. The findings of this study provide scientific guidance and engineering insights for the rational layout of stratified mining roadways beneath thick coal seams under similar geological conditions.

Introduction

With the continuous advancement of mining technologies and mechanical equipment, China has made remarkable progress in the extraction of medium-thick coal seams. Many intelligent mines adopt fully mechanized top coal caving or full-seam mining technologies to mitigate the inefficiencies, operational difficulties, and resource waste associated with traditional slicing mining in thick coal seams^1–4. However, layering mining remains widely used in small-scale mines. For downward mining of thick coal seams, the lower partitioned coal body is affected by the mining of the upper partitioned seam and the residual coal pillars, resulting in a complex stress environment and severe surrounding rock damage. Therefore, understanding the stress distribution characteristics and damage patterns of surrounding rock under residual coal pillars, and determining a reasonable spatial layout for the lower layered roadways, is particularly crucial for ensuring the safe mining of thick coal seam lower layers.

Many experts and scholars have conducted research on the stress field characteristics and damage patterns of the floor caused by coal pillars in upper stratification zones. Yin、Wang et al^5,6. studied the stress expressions of the floor and the roof of the lower layered roadway under the upper layered section coal pillar using theoretical analysis and numerical simulation, as well as the deviatoric stress and vertical stress peak lines of the roadway after mining the upper coal seam. They also determined the disturbance range of the roadway under the sectional coal pillar and proposed a combined support technology of support and unloading under the sectional coal pillar. Liu Xiaoming et al.⁷ studied the development patterns of fractures and the distribution characteristics of stress in the floor coal and rock layers after the mining of the overlying coal seam, under conditions of close spacing between coal seam variations, using theoretical analysis, field measurements, and numerical simulation methods. Li Zhenhua et al.⁸ took Zhao Gu No.2 Mine as the engineering background and systematically carried out research on the reasonable layout of roadways for working faces with coordinated mining of seam pillars and lower coal seams. Wang Xiaohe et al.⁹ using Ta Mountain Coal Mine as the background, employed numerical simulation to analyze the connectivity between the roadway plastic zone and the coal pillar plastic zone, and proposed arranging the mining roadway at a position relatively far from the section coal pillar.

Traditional lower layering mining roadways are often arranged in an internal manner within the stress-relief zone beneath the upper layering goaf^10–14. However, this approach results in excessive coal pillar retention, high resource loss, and significant floor damage. After mining the upper split-level working face, the depth of floor damage is relatively large. When the interlayer spacing is small, the fractured roof of the upper split goaf hinders effective bolt and cable support, severely impacting production efficiency. Overlapping roadway layouts face similar challenges, as roadways are subjected to high stress concentrations in coal pillars, leading to severe deformation¹⁵. When an external staggered arrangement is used, the roadway will be affected by the concentrated stress of the coal pillar, but by strengthening the support and improving the bearing capacity of the roadway, the stability of the roadway can be maintained, thereby reducing the size of the coal pillars in the lower stratified sections and greatly increasing the coal recovery rate.

Ren Heng et al.¹⁶ analyzed the lateral abutment pressure from upper-seam mining and simulated plastic failure, vertical stress distribution, and roof subsidence in externally staggered roadways using FLAC3D. Li Qunhang, et al.¹⁷ using Xiahezui Coal Mine as the background, studied the problem of roadway layout for mining thin coal seams at extremely close distances under the influence of concentrated stress. Liu Chao et al.¹⁸ proposed a mining roadway layout method combining internal and external staggered arrangements for close-distance coal seams. Wang Zhiqiang et al.¹⁹ established a mechanical model of unconventional coal pillars with staggered layers, considering the distribution characteristics of elastic strain energy density along the bottom roadways and coal pillars under different external offsets, and determined a reasonable external offset. Most of the above studies focus on relatively thin coal seams and coal pillars of normal size, while there is less research on thick coal seams with stratification, arranging the return mining roadways in a staggered manner outside the extracted section under coal pillars with a width of 60 m.

The 5 − 2 coal seam at the Wulansetai Coal Mine has an average thickness of 7.8 m. Mining of the upper sub-seam has now been completed. To improve the recovery rate of coal resources, the Wulansetai Coal Mine has designed an external offset layout for the lower sub-seam mining roadways. Taking the mining roadway of the 52,202 working face as the subject of study, this research employs a combined approach of theoretical analysis, numerical simulation, and field testing to conduct an in-depth analysis of the stress, deformation, and failure characteristics of mining roadways with different external offset positions, thereby providing a basis for determining optimal roadway locations.

Engineering overview

The Wulansetai Coal Mine mainly extracts the 5 − 2 coal seam. In the southern area, the seam splits into the upper 5^− 2 seam (4 m thick) and the lower 5^− 2 seam (3.8 m thick), with an interlayer spacing of 20 m to 28 m. In the northern area, the seams merge into a single 5^− 2 seam with a spacing of 20 m to 1 m. The upper layering of the 5^− 2 seam in the northern area (workfaces 52113 and 52115) has been fully extracted, leaving a 60 m wide coal pillar. The 52,202 mining roadway is designed beneath this pillar, with its positional relationship illustrated in Fig. 1.

Fig. 1.

Comparison diagram of position relation of upper and lower strata roadway.

Workface 52,202 is the first mining workface in the lower sub-seam of Coal Seam 5 − 2 in the northern wing of the Wulansetai Coal Mine. Its surface elevation ranges from + 994.544 m to + 999.003 m, with a burial depth of -170.3 m to -193.36 m. The coal seam dip ranges from 1° to 3°, making it a near-horizontal seam. The overall stratigraphy exhibits a monocline structure trending northeast-southwest with a dip of less than 1°. No faults with a drop greater than 30 m or obvious fold structures have been identified, nor is there any evidence of magmatic activity. The area is characterized only by localized, gentle undulations of varying widths, indicating a simple structural setting. The immediate roof comprises siltstone, the main roof consists of medium-grained sandstone, and the floor includes siltstone and fine-grained sandstone. The composite stratigraphic column is shown in Fig. 2.

Fig. 2.

Composite geological column map of coal strata.

Theoretical analysis of wide coal pillar loads

Theoretical study on the loading of the wide coal pillar

During the layered mining under thick coal seam, in order to understand the impact of the section wide coal column in the middle after the mining of the upper layered working surface, the column strength and stress of the section wide coal column need to be analyzed^10,20, and then the magnitude of the concentrated load applied to the section wide coal column by the superposition of the pressure of the overlying rock layer and the concentrated load of the adjacent goaf area on the section wide coal pillar is calculated. Whether the wide coal pillar undergoes plastic damage under the superposition of overlying rock formation pressure and concentrated load in goaf depends on the concentrated load under the coal pillar and the magnitude of its own cylinder strength. If the concentrated load exceeds the strength of the coal pillar, the coal pillar will undergo plastic deformation in part or even overall.

There are four determinants of the strength of the coal pillar in the section, namely the physical and mechanical properties of the coal seam, the width and height of the coal column and the internal mining situation.

The coal pillar strength was calculated using the Obert-Duvall/Wang empirical formula as follows²¹:

1

where f is the critical uniaxial compressive strength (3.84 MPa), B is the pillar width (60 m), and h is the pillar height (4 m).

Substituting the parameters into Formula (1) yields = 15.7 MPa. To ensure its accuracy, the Bien-iawski empirical formula is used for calculation²¹:

2

where is the strength of the critical dimension column, is the average uniaxial compressive strength of the coal column sample, and 17.16 MPa.

Bring each parameter into the above formula (2) to obtain =24.43 MPa. The smaller of the two calculated values is taken, resulting in a coal pillar strength of 15.7 MPa of the leave in the middle of two workfaces.

According to the theory of mine pressure and rock formation control, the loads of the coal columns in the section reserved for the overlying coal seam mining are generated by the joint action of the overlying rock formation and the adjacent goaf formation. Taking into account the depth effect of stress transfer in the coal column, a mechanical model of the coal column affected by the uniformly distributed loads of the goaf and the overlying rock formation are constructed as shown in Fig. 3. The calculation formula of the loads of the wide coal pillar is²¹:

Fig. 3.

Mechanical model of coal pillar under the combined action of overlying strata and goaf.

3

where: D is the width of the goaf, take 195 m; H is the buried depth of the coal seam, take 193 m; is the thickness of the overlying rock layer, take 4 m; is the collapse angle of the overlying rock layer, take 30°. K is the load coefficient, when the workface on both sides of the coal pillar on the section are mined, it is generally taken 1/3; is the average bulk weight of the overlying rock layer, takes 25 kN/m³.

Uniform load along the width direction of the coal pillar in the section:

4

Substitute the above parameters into the above formula (3) (4) and obtain , ,and the uniform distribution load on the section wide coal column is much smaller than the strength of the section coal column. Therefore, after the working faces of the 5 − 2 coal layered 52,113 and 52,115 are restored, the section wide coal pillars left in the middle can maintain good stability.

Stress partitioning in wide coal pillar

After the mining of the 52,113 and 52,115 working faces on the 5 − 2 coal stratified in Wulansetai Coal Mine was completed, the original rock stress of the overlying rock was broken, and the stress was redistributed. The stress on the working face was transferred to the coal pillar in the section, and the lateral support pressure distribution curve formed on the coal pillar in the section was approximately in the shape of a “bell-shaped”, as shown in Fig. 4.The stress partition of coal pillars can generally be divided into the ultimate equilibrium zone (rupture zone and plastic zone) and the elastic zone²², with a peak stress of 8.4 MPa and a stress concentration coefficient of 1.79.When the length of the coal pillar is greater than the length of the limit balance zone on both sides, there is an elastic area in the middle of the coal pillar that can effectively carry the load of the overlying rock formation. The Wulansetai Coal Mine is located 40 km south of Daliuta Town, Shenmu City of Shaanxi province, China. The mining area belongs to Shenfu Coal Field. The coal quality is excellent. Considering the actual situation of safe and efficient production on site, in order to ensure the yield rate of coal resources, the mine adopts an externally erroneous arrangement of the lower layered mining roadway, and arranges the lower layered mining roadway under the elastic area of the wide coal pillar in the upper layered section.

Fig. 4.

Lateral support pressure distribution curve and zoning diagram.

Assume that the length of the limit equilibrium area (rupture area and plastic area) is , since both sides of the coal column are goaf areas, the length of the elastic area is. According to the limit equilibrium theory²³, there are:

5

where: m is the thickness of the coal seam, take 3.8 m; is the side pressure coefficient, take 0.59; is the internal friction angle of the coal seam interface, take 27.2°; is the bonding force of the coal seam interface, takes 11 MPa; K is the stress concentration coefficient, takes 1.79; is the average rock layer bulk weight, 25 kN/m³; H is the burial depth of the mining roadway 193 m.

By bringing the above parameters into formula (5), getting the width of the limit balance zone is 13.08 m, therefore, the elastic zone is located within 13.08 m to 46.92 m from the right edge of the coal pillar. The optimal offset position for the lower-seam mining road should be located between 13.08 m and 39.52 m from the right edge of the coal pillar. When the outer offset is 39.52 m, the left side of the mining roadway is located at the edge of the limit equilibrium zone.

Numerical simulation of roadway layout

Model setup

To fully understand the stress, displacement, and failure characteristics of the mining road at different external offset positions, the finite difference numerical simulation software FLAC3D was used to establish a numerical model based on the geological conditions and the coal-rock stratigraphic profile of the 52,202 working face. The above layered 60 m wide coal pillar and minimum layer spacing 1 m are the basic conditions, and the reasonable outer staggered distance of the 52,202 roadway under the upper layered wide coal pillar. In order to facilitate calculation, the model size X×Y×Z = 180 m×1 m×59.5 m was established, and a total of 20,520 units and 41,876 nodes were divided. The bottom and left and right boundary horizontal directions of the model are both fixed boundary conditions. The rock formations on the upper surface of the model are uniformly distributed according to the weight of the overlying rock formation. After the model is established, the initial stress field balance of the surrounding rock is first carried out. During the model calculation process, the Mohr-Coulumb criterion is used as the basis for discriminating the overlying rock failure. The numerical model diagram is shown in Fig. 5. Rock mechanical parameters are listed in Table 1.

Fig. 5.

Numerical model diagram.

Table 1.

Rock physical and mechanical parameters of each rock formation.

Coal rock strata	Density (kg/m3)	Bulk modulus (GPa)	Shear modulus (GPa)	Inner friction angle (°)	Cohesion (MPa)	Tensile strength (MPa)
coal	1293	2.09	0.90	27.20	11.00	0.19
Siltstone	2482	5.64	4.64	35.00	9.52	2.39
Fine-grained sandstone	2471	5.15	4.40	23.85	39.15	1.97
Medium-grain sandstone	2481	8.02	6.08	20.70	20.96	2.65

Simulation solution

When the current layered mining roadway is arranged under the wide coal pillar of the upper layered section in an external offset method, it can be seen from the above theoretical calculation results that the external offset distance should be more than 13 m. Since both of the two working faces in the upper part layer mining are engaged in mining operations, according to the symmetry principle, the maximum external distance can reach 30 m, which is half of the coal pillar. To facilitate analysis, the external offset of the simulation scheme is set 0 m to 30 m, with a gradient of 6 m. Set up six layout configurations for the side-to-side offset between the right wall of the mining roadway in the 52,202 working face and the left wall of the mining roadway in the 52,115 working face above, with offsets of 0 m, 6 m, 12 m, 18 m, 24 m, and 30 m²⁴.

Among them, the offsets of 0 m and 6 m were located below the limit equilibrium zone of the wide coal pillar as the control group. The distribution laws of the stress field and displacement field of the surrounding rock of the 52,202 roadway under different simulation schemes were analyzed.

Results and analysis

Stress distribution

Figure 6(a) through 6(f) show the stress distribution contours of the 52,202 roadway at different offset distances. As shown in Fig. 6, When the upper and lower roadways are arranged with a 0 m lateral offset, the surrounding rock stress is minimal. This means that the mining roadway is located near the goaf, and the stress is in the pressure relief zone. When the external offset was 6 m, the roadway was still in the pressure relief zone, the stress on the right was 5.84 MPa, and the peak stress on the left was 9.43 MPa. Although the stress on both sides of the roadway is small, the surrounding rock is broken, the load-bearing capacity is weak, the roof plate is uneven, the horizontal shear stress is large, and the anchor cable has weak shear resistance, which is not conducive to anchor cable support. When the stratified mining roadway is arranged 12 m outward beneath the coal pillar of the section at present, the stress peak on the left side of the mining roadway reaches 11.61 MPa and the stress peak on the right side reaches 6.29 MPa. When the external offset 18 m layout is arranged, the stress peak on the left side is not much different than when the external offset 12 m layout is arranged, reaching 11.87 MPa, and the right side increases to 8.72 MPa. When the external offset distance is 24 m, the stress on the left side of the roadway gradually begins to shift to the right, and the peak stress on the left begins to drop to 11.43 MPa. The stress concentration phenomenon on the right is becoming more and more apparent, and the peak stress increases to 10.69 MPa, but the concentration degree is still smaller than the left side. When the external offset distance reaches 30 m, the stress concentration effect of the coal is completely transferred to the coal pillar on the right side of the roadway, with the stress reaching 11.1 MPa, while the peak stress on the left side drops to 9.45 MPa. The comparison chart of the peak stress of the surrounding rock of the roadway with different external offset distances is shown in Fig. 7.

Fig. 6.

Stress distribution cloud map of roadway surrounding rock arranged at different external fault distances. (a) external offset distance 0 m, (b) external offset distance 6 m, (c) external offset distance 12 m, (d) external offset distance 18 m, (e) external offset distance 24 m, (f) external offset distance 30 m.

Fig. 7.

Comparison diagram of peak stress of surrounding rock in mining roadways with different outward staggered distances.

In general, when the external offset distance of the mining roadway is 18 m, the stress on the surrounding rock on the left side of the roadway is the greatest, reaching 11.87 MPa and 8.72 MPa on the right side; When the external offset distance mining roadway is 24 m, the stress on the left side of the roadway begins to decrease, peaking at 11.048 MPa, while stress begins to concentrate on the coal pillar on the right side and shows an increasing trend, peaking at 10.5 MPa. Therefore, from a stress perspective, an external offset of 18 m to 24 m is optimal, as this ensures that the stress on the coal mass on both sides of the mining road tends to be evenly distributed, which contributes to the stability of the road.

Displacement distribution

The deformation of the surrounding rock of the roadway in the layered recovery roadway under different external offset distances is shown in Fig. 8. When the external offset distance of the mining roadway is 0 m, the maximum sinking amount of the roadway top plate is 198 mm. As the offset distance increases, the amount of roof subsidence shows a decreasing trend; the reduction in roof subsidence at an offset distance of 12 m is the greatest compared to that at an offset of 6 m, with a decrease of 42.6%. When the 52,202 mining roadway trough was arranged under the wide coal pillar of the upper layered section, the minimum sinking amount of the top plate was 42.8 mm, which was 78.4% lower than the maximum sinking amount of the roadway roof plate, indicating that increasing the distance between the coal pillars are conducive to reducing the sinking amount of the top plate. The amount of bulging of the bottom plate increases with the increase of the external offset distance, but the maximum value and the minimum value are not much different, and the difference is only 7 mm. The displacement of the left side of the roadway reaches a maximum value of 70.2 mm when the external offset distance is 18 m. When the external offset distance is 24 m, the displacement of the left side of the roadway decreases to 66.4 mm, which is 5.4% lower than when the external offset distance is 18 m. As the external offset distance increases to 30 m, the displacement of the left side of the roadway shows an increase again. The displacement of the right part of the roadway shows a tendency to increase first and then slowly decrease with the increase of the external offset distance. When the external offset distance is 0 m, the upper right side of the roadway is an upper layered goaf area. The right part of the roadway is in the pressure relief area and is not affected by stress; therefore, the displacement of the right part of the roadway is only 11.5 mm. When the external offset distance increases to 6 m, the displacement of the right part of the roadway suddenly increases to 74 mm, and the increase reaches 84.46%. The reason is that the stress on the wide coal pillar in the outer section gradually begins to appear in a concentrated state, the stress begins to increase, and the displacement changes suddenly. However, when the external offset distance is greater than 12 m, as the external offset distance increases, the displacement of the right part of the roadway is generally showing a downward trend.

Fig. 8.

Variation law of roadway surrounding rock deformation with offset distance.

Taking into account the displacement of the surrounding rock, the stress in the surrounding rock, and the coal recovery rate, the comprehensive analysis concludes that, for the lower-seam mining roadways beneath wide coal pillars, an offset layout should avoid areas of stress concentration in the coal pillars and areas of maximum disturbance to the surrounding rock. Therefore, a reasonable offset distance of 24 m is recommended.

Field application

In order to verify the optimal layout plan for the 5 − 2 coal 52,202 mining roadway of Wulansetai Coal Mine and improve the recovery rate of coal resources, it was decided to arrange the 52,202 mining roadway under the wide coal pillar of the upper layered with an external offset distance of 24 m.

Design of roadway support parameters

(1) Top plate anchor support parameters. High-strength left rebar anchor bolt, diameter of Φ 22 mm, length of 2400 mm, number of anchor rods is 8, inter-row distance is 1000 mm, anchor force is not less than 100 KN, the pallet adopts a high-strength ball pallet with a thickness of 15 mm and a diameter of 300 mm, and uses 2 resin anchor agents, 1 K2335 type, and 1 Z2360 type.

(2) Support parameters of the anchor rod. Using cut-to-length fiberglass-reinforced plastic anchor rods, with a diameter of Φ22 mm, a length of 2200 mm, there are 4 anchor rods, a distance of 1000 mm, an anchor force of no less than 100 KN, a matching pallet of Φ125mm, 2 resin anchor agents, 1 K2335 type, and 1 Z2360 type.

(3) Top plate anchor cable support parameters. High-strength low-relaxation steel strands are used, with a diameter of Φ17.8 mm, a length of 6800 mm, there are 3 anchor cables, a distance of 2000 mm, and an anchoring force of no less than 320 KN. The pallet is made of a high-strength ball tray with a thickness of 15 mm and a diameter of 300 mm, and two resin anchoring agents are used, one K2335 type, and one Z2360 type. The schematic diagram of the support of the mining roadway is shown in Fig. 9.

Fig. 9.

Schematic diagram of mining roadway support. (a)Schematic diagram of the support section of the mining roadway, (b) Schematic diagram of roof anchor bolt and cable arrangement for support in mining roadway, (c) Schematic diagram of bolt arrangement and support for the side of the mining roadway.

Analysis of mine pressure manifestation in mining roadways

To verify the validity of the theoretical and numerical analysis results, displacement measurement points were set up on the mining roadway surface using a cross-pattern layout, as shown in Fig. 10. On-site monitoring was conducted to measure the displacement of the mining roadway roof, floor, and both walls, and the characteristics of the surrounding rock’s deformation and deformation rate were analyzed. For the 52,202 mining roadway, which is offset 24 m outward beneath a 60-meter-wide coal pillar, a total of three monitoring stations were established at distances of 100 m, 150 m, and 200 m from the cutting face along the mining roadway. A 30-day on-site monitoring of rock mass displacement was conducted, yielding the rock mass displacement curve shown in Fig. 11 and the rock mass deformation average rate curve shown in Fig. 12.

Fig. 10.

Layout of surface displacement measurement points in the roadway.

Fig. 11.

Curve of surrounding rock displacement change at measuring points.

Fig. 12.

The average approach rate curve of the surrounding rock displacement at the measurement point.

As shown in Figs. 11 and 12, rock mass deformation in the longwall face increased sharply after the face was fully excavated. Deformation along the two sides stabilized after approximately 20 days, with the maximum displacement reaching 133.8 mm and the average displacement rate decreasing from 24.33 mm/d to 0 mm/d. The convergence of the top and bottom plates slowed down around the 23d and gradually began to stabilize, with the convergence rate decreasing from 14.47 mm/d to 0.03 mm/d, and the maximum convergence reaching 62 mm. Figure 13 shows a rendering of the application of the external offset for the mining roadway (Fig. 13).

Fig. 13.

Application effect diagram of the outward staggered arrangement of mining roadways.

ß

As demonstrated by the industrial field tests described above, it is feasible to arrange the 24 m offset-type sub-level mining roadways in the lower coal seam beneath the wide coal pillar in the upper sub-level section. This approach significantly improves coal recovery rates and aligns with the principles of safe and efficient production.

Conclusions

(1) Based on the actual engineering geological conditions of the Wulansetai Coal Mine, a computational analysis was conducted on the strength of the 60-meter-wide coal pillar reserved in the upper layer of the 5 − 2 coal seam and the loads acting upon it. The results showed that the strength of the wide coal pillar was 15.7 MPa, which is significantly greater than the 6.7 MPa load it bears, indicating that the coal pillar is highly stable. Using the theory of limit equilibrium, an analysis was conducted on the stress distribution zones of the 60-meter-long coal pillar in the upper seam. The calculation determined that the width of the limit equilibrium zone is 13.08 m, with the elastic zone extending from 13.08 m to 46.92 m from the right edge of the coal pillar. The mining road for the lower seam should be located within the elastic zone.

(2) Numerical simulation analysis shows that as the offset distance increases, the stress distribution in the surrounding rock of the tunnel gradually shifts from asymmetric to uniform and symmetric. When the offset is 24 m, the peak stresses on both sides of the roadway are 10.05 MPa and 11.04 MPa, respectively, representing the optimal balance in stress distribution. As the distance from the external offset increased, the deformation of the roadway surrounding rock exhibited a pattern of initially decreasing and then stabilizing when the external offset reached 24 m; the convergence of the roof and floor reached 54.8 mm, and that of the two side walls reached 121.9 mm, indicating the best control of the surrounding rock.

(3) Field industrial trials confirmed that, with a 24 m offset layout and the specified support parameters, the deformation of the surrounding rock stabilized within 23 days after excavation. The displacement of the roof, floor, and both sides remained within safe and controllable limits, and the stability of the tunnel met the requirements for safe and efficient mine production.

Author contributions

Yucheng LIU: Writing and editing, numerical simulation, data processing, theoretical calculation, etc. Xiaoxiang CHEN: Data curation, Writing-original draft, Funding acquisition. This manuscript has been read and approved by all the authors before submission, and all the authors agreed to submit this manuscript to the journal. All authors actively contributed to the writing of this manuscript and agreed to submit the final version.

Funding

This research was supported by the National Natural Science Foundation of China (project number: U1504515, 51304068), the Guiding Research Project of China Coal Industry Association (MTKJ2015-251), and the Project for Cultivating Young Backbone Teachers of Henan Province Universities (2017GGJS055).

Data availability

The data are available from the corresponding author upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.