1. Introduction
With the increasing demand for coal resources, and the gradual deepening of coal mining depths, traditional mining methods are facing several challenges, including resource wastage, tight mining face succession, and the need for improved roadway safety performance. Gob-side entry retaining (GER) technology enables pillarless mining, providing significant improvements in coal recovery rates, alleviating succession pressure, and reducing roadway excavation volumes. However, GER technology remains constrained by complex strata pressure, roof–floor collapse, floor heave, and rib spalling during the entry retaining process. In summary, conducting further research on strata behavior laws and surrounding rock control for GER technology has become an emerging trend. Many scholars have conducted in-depth studies on strata behavior laws in roof-cutting pressure relief working faces and achieved a series of important research outcomes [1,2,3,4,5].
Jiang et al. [6,7] proposed a directional roof-cutting coordinated pressure relief control method, revealing its mechanism of roadway stabilization through reducing working face stress, surrounding rock stress, and roof–floor deformation. Yang et al. [8,9,10] developed a presplit blasting roof-cutting pressure relief method for adjacent roadways based on the short cantilever beam theory; through intercepting advanced mining stress transmission paths, it achieved surrounding rock stress environment regulation, reducing hydraulic support resistance peaks by 24.9% and decreasing roadway roof–floor and rib deformations by 34.9–50.1%. Zhu et al. [11,12] introduced a dense pressure relief hole roof weakening coordinated control method, and engineering tests demonstrated its capability to replace conventional blasting techniques, achieving safe and efficient pressure relief for roadway protection, thereby establishing a “mechanical regulation-pressure relief roadway protection”-coordinated technical system. Gong et al. [13] established a deep/shallow hole coordinated presplit blasting parameter system; through advanced slot cutting, grouting reinforcement, and flexible gangue blocking, they achieved coordinated surrounding rock control. Field measurements showed fracture development rates of 85%/90% for deep/shallow holes, respectively, with roof subsidence reduced by 46.3%, forming a “directional slotting-structure reconstruction-active roadway protection” hard roof control system that breaks through conventional strong support concepts. Habib et al. [14,15] investigated soundless cracking demolition agent (SCDA) chemical expansion rock-breaking technology; through borehole optimization and uniaxial auxiliary loading experiments, they revealed that increasing the borehole diameter reduces the hard rock fracture time to 7 h, with an optimal diameter-spacing ratio of 12.8–14.6, establishing a “chemical-mechanical” coordinated rock-breaking system that provides green mining solutions without detonation and with low vibration. Dmytro et al. [16,17,18] innovatively developed an underground gangue in situ backfilling and coal gangue quality-controlled coordinated mining process, reducing raw coal ash content to 15.2% while increasing the energy equivalent by 7.4% (9.6 TJ); they created a coal quality–energy dynamic prediction algorithm, breaking through traditional process limitations in energy efficiency/coal quality coordinated optimization. Wang et al. [19,20,21,22,23,24] conducted theoretical calculations and numerical simulations based on deep-hole blasting roof-cutting pressure relief principles and technologies; the results showed significant reduction in the surrounding rock deformation of retained roadways after roof cutting, verifying the effectiveness of advanced deep-hole presplit blasting technology in improving the surrounding rock deformation of gob-side entry retaining in thick coal seams with soft floors. Liu et al. [25,26,27] addressed the challenge of generating initial stress fields with principal stresses, oblique to boundaries, in FLAC3D models; they proposed determining boundary stress directions using elastic theory combined with the Mohr’s circle pole and force equilibrium methods, employing quasi-stress boundary control to generate initial stress fields, and verification showed high consistency between model principal stress values/directions and actual conditions, confirming the method’s effectiveness and providing technical references for similar projects. Liu et al. [28,29,30] developed a hydraulic fracturing vertical roof-cutting technology based on additional tensile stress roof bending regulation; using mechanical models to precisely locate maximum additional horizontal tensile stress zones for fracturing implementation, they achieved vertical slot cutting and coordinated control of the hard roof and surrounding rock, establishing a “mechanical criterion-fracturing positioning-roof cutting pressure relief” technical system.
Currently, pillarless mining technology has established a relatively comprehensive technical system in thin-coal-seam mining and has been successfully implemented in multiple mining areas. However, practical applications and research under medium–thick- and thick-coal-seam conditions remain insufficient. Therefore, we conducted field observations of roadway roof subsidence, floor heave, rib convergence, and working face hydraulic support loading in this study, in order to analyze the strata behavior characteristics of gob-side entry retaining with presplit blasting roof-cutting pressure relief in the headgate of Panel 81308 at Huayang Mine No. 2. The findings provide theoretical support for a support design of gob-side entry retaining in mines with similar geological conditions.
2. Project Overview
2.1. Engineering Background
The mining engineering plan of Panel 81308 in Huayang Mine No. 2 is shown in Figure 1. The panel is located in the 13th mining district of coal seam #8 at level 470, with panel elevation ranging from 487 to 541 m, surface elevation between 846 and 1063 m, and burial depth varying from 305 to 576 m. The panel has a strike length of 1460 m and a dip length of 240 m, covering a total area of 350,400 m2. The proposed mining method employs strike longwall retreat with comprehensive mechanization, with extraction following the coal seam strike direction. The coal seam dip angle ranges from 1° to 8°, averaging 4°, and the seam demonstrates stable development.
2.2. Roadway Support Method
The headgate of Panel 81308 employs a rectangular cross-section with an excavated width of 5.2 m and height of 3.6 m, providing a clear width of 5.0 m and a height of 3.4 m, developed along the coal seam roof. Figure 2 shows both the original and reinforced support schemes for the retained roadway. The original support system consists of cable bolts, combined with steel straps and metal wire mesh. The reinforced support system uses four rows of high-strength cable bolts (Φ21.8 mm × 11.2 m). The first row is installed 0.9 m from the rib and 0.4 m from the cutting slot, with 0.8 m spacing; the second row is arranged 1.2 m from the first row, with 0.8 m spacing; the third row is placed 1.6 m from the second row, with 1.6 m spacing; and the fourth row is positioned 1.4 m from the third row, with 1.6 m spacing in a staggered arrangement relative to the third row.
2.3. Roof Cutting Technical Parameters
Based on the engineering background of roof-cutting pressure relief automatic roadway formation pillarless mining technology, the current application of pillarless roof cutting in gob-side entry retaining has its critical success factors lying in the advanced roof-cutting technique and its effectiveness. Panel 81308 of Huayang Mine No. 2 employs directional drilling and blasting for advanced roof-cutting operations, with a vertical borehole depth of 1275 mm and a dip angle of 15°. Specific parameters are shown in Table 1.
2.4. Ground Pressure-Monitoring Scheme for Gob-Side Entry Retaining with Roof Cutting
After implementing the pillarless mining technology of gob-side entry retaining with roof cutting and pressure relief, in order to facilitate the formulation of subsequent mining measures for Panel 81308, it was necessary to monitor the stress conditions and deformation of the roadway and working face. The surrounding KJ1100 coal mine rock dynamic monitoring system for gob-side entry retaining (excavation) was adopted; this system consists of a central station computer, a KZG18 intrinsically safe mine signal converter, a GMY500 intrinsically safe mine bolt (cable) stress transmitter, a GUD2000 intrinsically safe mine displacement sensor, and other equipment. The monitoring system flowchart is shown in Figure 3. The main working principle is as follows: the data collected from each sensor are transmitted to the central station computer through the signal converter, and the central computer performs information storage and analysis. The system monitors parameters such as the working resistance of powered supports and the shrinkage of powered support legs in real time; these data are used to evaluate the rationality of support parameters in the working face and roadway, as well as to study the movement laws of the roof strata.
In this section, we detail the monitoring scheme designs for both the roadway and working face. The specific arrangements are described below.
For the retained roadway section of the headgate of Panel 81308, starting from 68 m behind the setup room to the stopping line, one monitoring station was installed every 50 m. Each station consisted of one roof separation monitor and one constant-resistance cable bolt stress monitor. The system collected data every 30 min. The monitoring equipment is shown in Figure 4.
Within working face 81308, one pressure-monitoring point was installed every 30 m, totaling seven pressure-monitoring points. The system collected data every 30 min. The layout schematic of the monitoring stations and points is shown in Figure 5.
3. Simulation Study on Roof-Cutting Effectiveness
In this study, based on the roof–floor rock mass structural characteristics of Panel 81308 and the pillarless roof-cutting gob-side entry retaining (GER) design, we established a numerical model (200 m × 10 m × 30 m) using FLAC3D5.0 simulation software. Given the average burial depth of 440 m for coal seam #8 in Panel 81308 of Mine No. 2, and assuming an average overburden density of 2500 kg/m3, the calculated vertical stress is approximately 11 MPa. Thus, an 11 MPa vertical stress (downward) and a 5 MPa horizontal stress were applied to the model’s upper boundary to simulate in situ strata stress conditions. The model consists of seven vertically stratified layers. The constructed model is shown in Figure 6.
For the simulation, we employed the Mohr–Coulomb failure criterion for rock mass assignment and the Mohr–Coulomb slip model for joint characterization; this combination accurately reflects geomechanical failure mechanisms, enabling reliable simulation of rock mass failure and joint slip behavior. Physico-mechanical parameters of coal–rock masses were determined through uniaxial compression tests, Brazilian splitting tests, and subsequent calibration, with the finalized mechanical parameters presented in Table 2.
The simulation results, depicted in Figure 7, show that, without roof-cutting measures, the peak residual stress in the solid coal rib reached 19.8 MPa, with a cumulative vertical displacement of 26.5 mm. After implementing roof-cutting pressure relief, the peak residual stress reduced to 14.5 MPa, with the corresponding roof subsidence decreasing to 18.6 mm, demonstrating 26.8% stress concentration reduction and 29.8% deformation control improvement. The simulation verifies that roof cutting achieves the mechanical effects of stress redistribution and energy dissipation through reconstructing roof stress transmission paths. The fundamental mechanisms for reducing stress concentration and roof subsidence through deep-hole blasting roof cutting include the following: (1). Severing mechanical continuity between goaf and roadway roof beams, thereby reducing pressure on GER support structures; (2). Promoting timely goaf caving and effective waste rock filling, transferring overburden loads to collapsed strata to relieve GER support pressure; (3). Minimizing roof beam rotation angles (typically <15°) after cutting in the headgate of Panel 81308, thus reducing the predefined deformation of GER roof beams; (4). Creating artificial weak zones to control roof fracture locations.
4. Ground Pressure Monitoring Data Analysis
4.1. Analysis of Surrounding Rock Displacement Evolution
The ground pressure monitoring stations in the headgate of Panel 81308 were categorized into three groups (advance, central, and rear monitoring sections). Average values from each station group were calculated to plot the characteristic surrounding rock deformation patterns versus distance from the working face, as shown in Figure 8.
Analysis of Figure 7 and Table 3 reveals the following: (1). The average total roof-to-floor deformation measures 607 mm, including 443 mm (73%) of floor heave. Rib deformation averages 170 mm, predominantly occurring at the pillar side. Both roof–floor and rib deformations exhibit similar evolutionary patterns: rapid initial growth, followed by gradual stabilization. The roof–floor deformation rate significantly exceeds the rib deformation rate, with floor heave dominating roof subsidence, and pillar-side deformation being primary in rib convergence. (2). Maximum roof–floor convergence occurs within 0–88 m behind the face (primary deformation zone), decreasing through 88–142 m, and stabilizing beyond 178 m. The initial deformation mechanism is governed by composite roof structure movement, while later-stage deformation manifests as progressive settlement from goaf fracture zone compaction. Pillarless roof-cut roadways demonstrate “wide disturbance range and prolonged residual deformation” characteristics under mining-induced stresses.
4.2. Cable Bolt Load Analysis
The load-monitoring data of roof cable bolts in the retained roadway section were collected, with the average load curve plotted in Figure 9. Monitoring results indicate that cable bolt stress begins to significantly increase approximately 30 m ahead of the face due to front abutment pressure. The stress stabilizes at about 185 m behind the face, indicating that the rock surrounding the roadway reaches relative stability.
Load analysis reveals that intense deformation occurs within 0–70 m behind the face (caving zone influence area). High-strength cable bolts in this zone provide active support, effectively controlling roof deformation and ensuring stability.
4.3. Roof Separation Analysis
Field monitoring data indicate that the roof separation development process can be divided into the three following characteristic zones: the initial development stage, within 15 m ahead of the working face; the core development area, extending from 15 m ahead to 85 m behind the working face; and the zone beyond 85 m behind the face to the stress stabilization boundary, where the roof exhibits composite beam-type integral rotational subsidence, reaching a maximum separation of 27.8 mm. Monitoring results demonstrate that, when the lag distance exceeds 140 m, the separation deformation stabilizes, with the final separation magnitude maintaining at approximately 39.6 mm, as shown in Figure 10.
4.4. Working Face Pressure Variation
Based on the hydraulic support load-monitoring data from Panel 81308 (as shown in Figure 11), the roof-cutting pressure-relief technology demonstrates significant stress regulation effects: the measured load curves of the cutting-side supports show a 12.7–18.4% reduction in peak bearing capacity compared to under untreated conditions, indicating that the roof-cutting blasting technique has achieved the expected results.
5. Field Application Effectiveness
Post-blasting borehole inspection (Figure 12) reveals well-developed fracture networks near the borehole collar in roof-cutting holes. The external roof-cutting measures play decisive roles in gob-side entry retaining. Field practice at Panel 81308 confirms that this technology successfully interrupts roof stress transmission, effectively ensuring entry stability. The field application results are shown in Figure 13.
6. Conclusions
(1). The surrounding rock deformation in roof cutting for pressure relief in gob-side entry retaining exhibits the following three-phase evolution: ① the strong disturbance phase (0–88 m behind face), dominated by composite roof structure destabilization, showing peak deformation rates; ② the weak disturbance phase (88–142 m), controlled by goaf waste rock compaction, with decaying deformation rates; ③ and the stabilization phase (>178 m), in which stress redistribution is completed, maintaining deformation rates below 0.5 mm/m. Floor heave constitutes 72% of roof–floor convergence, with pillar-side deformation being primary in rib movements.
(2). Front abutment pressure induces significant cable bolt load increase, starting 30 m ahead of the face. The intense deformation zone spans 0–70 m behind the face (caving influence area), with stress stabilization occurring at ~185 m behind it, revealing asynchronous evolution between support systems and the surrounding rock deformation.
(3). Numerical modeling and field monitoring jointly validate the mechanical optimization mechanism. Artificially created weak planes from pre-splitting blasting effectively intercept the stress transmission between the roadway and goaf rooves, inducing “voussoir beam-waste rock” composite structures in overlying strata and achieving effective deformation control that meets safe production requirements.
Conceptualization, B.H.; methodology, T.L.; software, C.S.; validation, R.Z.; writing—original draft preparation, R.Z.; writing—review and editing, B.H.; funding acquisition, B.H. All authors have read and agreed to the published version of the manuscript.
Not applicable.
Not applicable.
The data presented in this study are available on request from the corresponding author.
The authors declare no conflicts of interest.
Footnotes
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Figure 1 Mining layout plan of coal seam #8 in Panel 81308.
Figure 2 Original and reinforced support systems for the retained roadway.
Figure 3 Monitoring system flowchart.
Figure 4 Monitoring equipment diagram.
Figure 5 Layout of ground pressure monitoring stations in Panel 81308.
Figure 6 Numerical simulation base model.
Figure 7 Comparison of roof-cutting vs. conventional conditions in the headgate of Panel 81308.
Figure 8 Characteristics of surrounding rock deformation vs. distance from working face in retained roadway.
Figure 9 Cable bolt stress-monitoring curves in the headgate of Panel 81308.
Figure 10 Roof separation monitoring curves in the headgate of Panel 81308.
Figure 11 Three-dimensional visualization of hydraulic support pressure variation at working face.
Figure 12 Borehole imaging results.
Figure 13 Field application results of gob-side entry retaining.
Design parameters of advanced roof pre-splitting drilling and blasting.
| Parameter | Value | Remarks and Explanations |
|---|---|---|
| Borehole position | 200–500 mm | Distance from mining rib to cutting seam holes in the headgate of Panel 81308: 200–500 mm |
| Cutting seam dip angle | 15° | Angle between borehole and vertical direction |
| Borehole inclined length | 13 m | Inclined length of cutting seam holes |
| Borehole diameter | 50 mm | — |
| Energy-gathering tube | BTC35-1 500 | Outer diameter: 42 mm; inner diameter: 36.5 mm; charge diameter: Φ35 mm; single section: 1.5 m |
| Emulsion explosive | Grade III | Specification: Φ35 × 200 mm; single cartridge weight: 200 ± 10 g |
| Borehole spacing | 500 mm | Determined based on field test results |
| Charge structure | 5 + 4 + 4 + 3 + 2 + 1 | Single hole uses six energy-gathering tubes; stemming length: 4.0 m; 19 cartridges per hole |
| Maximum simultaneous detonation holes | 15 pieces | Post-blast gas concentration < 0.5%; CO concentration < 0.0024% (24 ppm) |
Rock mass mechanical parameters for numerical calculation.
| Stratum Name | Thickness (m) | Density (kg/m3) | Tensile Strength (MPa) | Friction Angle (°) | Cohesion (MPa) | Elastic Modulus (GPa) | Poisson’s Ratio |
|---|---|---|---|---|---|---|---|
| Overburden Strata | 6.00 | 2500 | 4.90 | 45.50 | 3.80 | 8.50 | 0.20 |
| Fine-grained Sandstone | 5.50 | 2630 | 7.37 | 42.92 | 3.11 | 14.94 | 0.21 |
| Sandy Mudstone | 4.37 | 2590 | 4.74 | 40.03 | 2.94 | 12.12 | 0.23 |
| Mudstone | 3.47 | 2570 | 2.71 | 39.35 | 2.32 | 2.45 | 0.26 |
| Coal Seam #8 | 3.50 | 1450 | 2.07 | 36.87 | 1.74 | 2.22 | 0.25 |
| Fine-grained Sandstone | 3.19 | 2630 | 8.34 | 41.34 | 3.51 | 14.64 | 0.22 |
| Sandy Mudstone | 3.97 | 2590 | 4.56 | 39.95 | 2.84 | 11.29 | 0.24 |
Surrounding rock deformation data at monitoring stations in retained roadway.
| Monitoring Station Name | Roof–Floor Convergence (mm) | Floor Heave (mm) | Rib-to-Rib Convergence (mm) |
|---|---|---|---|
| Advance | 640 | 460 | 200 |
| Central monitoring section | 580 | 390 | 70 |
| Rear monitoring section | 600 | 480 | 240 |
| Average | 607 | 443 | 170 |
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