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The successful retaining of the gob‐side entry under a thick and hard roof stratum is difficult because of the high pressure present and complex construction technology commonly used. To solve this problem, a new type of gob‐side entry supporting system is proposed in this paper. This system is mainly composed of concrete‐filled steel tubular columns (CSTCs), flexible cushion, and gob isolation structures. This new supporting system combines the high‐strength support of CSTCs with the flexible support of cushion bodies and is simple to construct, enabling fast and efficient gob‐side entry retaining under a thick and hard roof stratum. The range of the roof strata controlled by gob‐side support structures is determined for a case study, and a calculation formula for the gob‐side support resistance is established. Through theoretical and experimental research, a reasonable calculation formula for the choice of CSTC is also established. The CSTC structure ultimately selects Φ194 × 8 mm hollow steel tubes and C40 grade concrete for use in a field application, which can provide 4814 kN of supporting force. A simple on‐site construction process is designed for the field application, and the time required for entry retaining per meter is only approximately 40‐45 minutes. This application shows that the new technology controls the deformation of the retained entry very well; the final deformation stabilizes at 412 mm, which meets the engineering requirements. In this work, a new type of gob‐side supporting system based on the concrete‐filled steel tubular columns (CSTCs) was proposed. It was proven that the new type of gob‐side entry retaining technology based on the CSTC structure is more suitable for entry retaining engineering under high‐stress conditions and clearly exhibits the advantages of a high supporting resistance, low supporting cost, and simple construction process.

Studies regarding the reuse of a gob‐side roadway formed by the N00 longwall mining method are scarce. To investigate the mine pressure behavior characteristics during roadway reuse, the anchor cable force, roof‐to‐floor deformation, and working resistance of the hydraulic support obtained from the verification working face were analyzed. Test results show that the influence range of the mining‐induced pressure was 0‐90 m in front of the working face. The ratios of the first and periodic weighting steps between the entry‐retaining and non‐roof‐cutting sides were 20.25 and 1.94, respectively. However, the ratios of the ground pressure strengths at the two sides were 1.07 and 1.31, respectively. The results indicate that the monitored roadway can be classified into three deformation zones based on the severity of the mine pressure behavior: large, intermediate, and creep. The main roof between the adjacent working faces exhibited a failure type comprising the “O‐X” and “O‐Y” states, which resulted in an unconventional mine pressure behavior. This study provides insights into the N00 longwall mining method as well as important guidance for tackling similar geological conditions when using this mining method. The mine pressure behavior characteristics of a gob‐side roadway formed by the N00 longwall mining method in the stage of reusing are analyzed based on a field test. The automatically formed roadway can be divided into three deformation zones according to the severity of the mine pressure behavior: large, intermediate, and creep. The fracture form of the main roof above the adjacent working faces during the reusing stage is the composite state of “O‐X” and “O‐Y,” resulting in an unconventional phenomenon of mine pressure behavior.

Aiming at the roof control challenges of gob‐side entry retaining in fully mechanized top‐coal caving faces, traditional concrete‐filled steel tube (CFST) support technology exhibits limitations such as slow strength development and complex construction equipment. This study, based on the 1311 fully mechanized caving face in Shanxi Runhong Coal Mine, proposes a gob‐side entry retaining technology integrating roof cutting for pressure relief with support by grouted steel tubes. Through physical experiments, numerical simulations, and field tests, the mechanical properties of sulphoaluminate cement‐based backfill and grouted steel tube structures were analyzed. The results indicate that the sulphoaluminate cement‐based slurry with a water‐cement ratio of 0.4 exhibits remarkable early‐strength characteristics, achieving compressive strengths of 30.85 and 31.26 MPa at 1 and 28 days, respectively. The support system attains the 28‐day bearing capacity of traditional CFST within 24 h, effectively resolving the timeliness inadequacy of gob‐side support. Increasing the steel tube wall thickness enhances the bearing capacity more significantly than material strength improvement, with notable stress concentration and bending deformation observed at the middle region during loading. These findings provide a theoretical basis for support parameter optimization. Field applications demonstrate that this technology ensures surrounding rock stability while offering advantages such as simplified construction and reduced labor costs. This study provides theoretical support for the development of gob‐side entry retaining technology and offers practical guidance for its application in fully mechanized caving faces.

Gob-side entry retaining (GER) is a popular non-pillar mining technique regarding how to reserve a gateroad for the use of next panel mining. When used in thick coal seams, the conventional entry retaining method requires a huge amount of filling materials and may cause entry (gateroad) accidents. Thus, an innovative non-pillar longwall mining approach is introduced. First, structural and mechanical models were built to explore the mechanism of the new approach. The modeling results indicate that effective bulking of the gob roof and reasonable support of the entry roof were key governing factors in improving entry stabilities and reducing roof deformations. Accordingly, a directional roof fracturing technique was proposed to contribute to gob roof caving, and a constant resistance and large deformation anchor (CRLDA) cable was used to stabilize the entry roof. Subsequently, the evolutionary laws of the roof structure and stresses were explored using numerical simulation. It was found that the structure of the surrounding rocks around the retained entry changed significantly after roof fracturing. The stress-bearing center was transferred to the gob area, and the entry roof was in a low stress environment after adopting the approach. Finally, the approach was tested on a thick coal seam longwall mining panel. Field monitoring indicates that the retained entry was in a stable state and the index of the retained entry met the requirement of the next mining panel. This work provides an effective and economical approach to non-pillar longwall mining in thick coal seams.

Though the real tri-axial testing simulated by FLAC3D, the failure mechanism and deformation process of rock-coal “two-body” structure in the in-situ area, named crucial point, which bore the ultimate lateral abutment pressure and broke in the beginning to lead to the whole damage of the “three soft” ground-rock around the gob-side entry retaining was obtained, and the relationship between the ultimate strength and cohesion of rock was obtained. By field observation the condition of gob side entry without coal pillar was obtained: top old roof as the bearing body, when the height of direct roof was larger than 1.309m1 and soft enough, the half-arch structure could made by the upper strata of the direct roof, as a weak layer, which gave a force straight up against to the old roof overturning and sinking. That was the condition of gob side entry retaining without coal pillar.

Generation, propagation, and coalescence of the shear and tensile cracks in the gob-side filling wall are significantly affected by the location of the fracture of the critical rock block. The Universal Discrete Element Code software was used to investigate crack evolution characteristics in a gob-side filling wall and the parameter calibration process for various strata and the filling wall was clearly illustrated. The cracks in both the filling wall and the coal wall propagate inward in a V-shape pattern with dominant shear cracks generated initially. As the distance between the fracture and the filling wall decreases, the number of cracks in the filling wall decreases, and the stability of the filling wall gradually improves; thus, by splitting the roof rock at the optimal location, the filling wall can be maintained in a stable state. Additionally, we conducted a sensitivity analysis that demonstrated that the higher the coal seam strength, the fewer cracks occur in both the filling wall and the coal wall, and the less failure they experience. With the main roof fracturing into a cantilever structure, the higher the immediate roof strength, the fewer cracks are in the filling wall. With the critical rock block fracturing above the roadway, an optimal strength of the immediate roof can be found that will stabilize the filling wall. This study presents a theoretical investigation into stabilization of the filling wall, demonstrating the significance of pre-splitting the roof rock at a desirable location.

With the increase of coal seam mining thickness, the caving height of stope roof, the mining-induced stress increase, and the control difficulty of gob-side entry retaining increase. To apply the gob-side entry retaining (GER) technology in thick coal seams, optimize the support method and reduce the deformation, based on directional energy-gathering blasting technology and roadside filling technology, gob-side entry retaining with synergistic roof cutting and roadside filling (GER-RCRF) is proposed. Through theoretical analysis, numerical simulation and field experiments, the mechanism and effect of the method are analyzed. The results show that due to the stress relief of roof cutting, the pressure of the solid coal rib and the roadside filling of the GER-RCRF is reduced, and the stress concentration area shifts to the deep rock mass. Under the condition of roof cutting, the stress of roadside filling is distributed in a “single peak”, and with the width increases, the peak stress first increases and then decreases, and then increases and then decreases. Meanwhile, the roof cutting height should be based on the broken and expansion effect of the rock strata, considering the key stratum effect of the rock layer. The stress of the surrounding rock is further reduced when the overlying key strata is cut off. Finally, the GER-RCRF has successfully reduced the stress of roadside filling, roof and solid coal rib, optimized the stress environment of surrounding rock, and successfully realized the purpose of gob-side entry retaining. The field test verified the effectiveness of GER-RCRF. The stress of roadside filling and the deformation of surrounding rock are significantly reduced. The research results provide a certain degree of scientific basis for the successful application of gob-side entry retaining in thick coal seam.HighlightsGob-side entry retaining with synergistic roof cutting and roadside filling (GER-RCRF) method is proposed based on directional energy-gathering blasting technology and roadside filling technology.Roof cutting can reduce the pressure of roadway surrounding rock, and roadside filling can support roof. The synergistic effect of the two can optimize the stress environment and reduce the deformation of surrounding rock.Comprehensive monitoring showed that GER-RCRF method effectively realizes the gob-side entry retaining in thick coal seam, meanwhile, reduces the deformation of surrounding rock and the stress of roadside filling.

Gob-side entry retaining (GER) is a technique in non-pillar mining, which maintains the original mining roadway along the edge of gob and retains it as a mining roadway for the subsequent working face. This technique offers significant advantages such as a high coal mining rate and cost-effective roadway retention. This paper focus on the GER implementation in 52605 panel of Daliuta Coal Mine and introduce an innovative technique involving the utilization of flexible formwork concrete wall (FFCW). To verify the feasibility of this technique, a numerical model was established. Furthermore, the stability mechanism of surrounding rock during the mining process of 52605 panel was thoroughly examined. Simulation results indicate that during the mining, the roadside backfill body (RBB) gradually bears load, causing peak stress transfer from gob side towards solid coal side. Moreover, plastic zone of roof and solid coal exhibited a noticeable increase, leading to a combined tensile-shear failure. Based on the stress and plastic zone evolution characteristics of surrounding rock during the mining process of the working face, control techniques were proposed and industrial experiment was successfully carried out. Ultimately, on-site monitoring results show that the deformation control effect of surrounding rock was good, and there was no obvious pressure manifestation in the working face.

To facilitate the reuse of the gob-side retained entry as a long-term return airway, alleviate mining-excavation pressure, and reduce roadway development costs, this paper proposes a secondary gob-side entry retaining technique. The study examines the movement behavior of overlying strata throughout the entire process of secondary gob-side entry retaining, introduces a roof support concept based on “major and minor structural zones,” establishes a mechanical model of the roof structure for secondary retention, derives a design formula for the roadside backfill, analyzes the main factors influencing the stability of the overlying strata structure, and proposes an integrated “four-in-one” surrounding rock control technology for secondary gob-side entry retaining. The results indicate that: (1) The “major structural zone” of the overlying strata stabilizes only after experiencing three mining disturbances. (2) The coordinated load-bearing behavior of the “minor structural zone” in roof support is crucial to surrounding rock stability. A “four-in-one” control strategy is proposed, integrating the roadside packing bodies on both sides, the roof bolting-cable system, floor reinforcement, and internal roadway support to form a stable load-bearing structure. (3) Appropriately reducing the roadway width, the widths of the two packing bodies, and the cantilever length of the main roof on the goaf side can enhance the support capacity of the coal rib during the initial retention stage. This reduces the load on the packing bodies during the secondary gob-side entry retaining stage, thereby alleviating surrounding rock stress. The proposed approach has yielded favorable outcomes in engineering practice, demonstrating both theoretical relevance and practical significance for supporting roadways under similar conditions.

Longwall top coal caving with gob-side entry retaining is frequently encountered in thick coal seams. Under such geological conditions, significant deformation, support system failure, and the inability to retract supports, caused by intense mining-induced stress disturbances and weak surrounding rock, have become prominent factors hindering safe and efficient coal production. To ensure gob-side entry retaining stability, a combined method integrating pressure relief (PR) and roadside filling support (RFS) is adopted. Through theoretical analysis and numerical simulation, the combination mechanism and pressure relief effect of PR and RFS are analyzed. The results show that PR efficaciously cuts off the suspended roof structure on the goaf side, effectively utilizing the broken expansion effect and supporting effect of the collapsed gangue. Meanwhile, RFS provides high-strength resistance for the roof cutting short-wall beam. The stress distribution of gob-side entry retaining combined pressure relief and roadside filling support (GER-PRRFS) has obvious zoning characteristics. Furthermore, the stress concentration area is greatly transferred to the deep part of coal-rock mass, accompanied by a substantial reduction in the peak stress value, thereby realizing the load-bearing homogenization of the coal-rock mass. Finally, a ‘low-level support - high-level strata pressure relief - upper-level strata load bearing’ method for stability control of GER-PRRFS is proposed. Field tests demonstrates that the deformation of surrounding rock and the disturbance intensity of mining-induced stress are significantly reduced.