Gob-side Entry Research Articles

Long-Distance and High-Efficiency Temporary Support Technology in Dynamic Pressure Bearing Area of Gob-Side Entry Retaining

Long-Distance and High-Efficiency Temporary Support Technology in Dynamic Pressure Bearing Area of Gob-Side Entry Retaining

Mitigating Coal Spontaneous Combustion Risk within Goaf of Gob-Side Entry Retaining by Roof Cutting: Investigation of Air Leakage Characteristics and Effective Plugging Techniques

Relative to conventional coal pillar retention mining technology (the 121 mining method), gob-side entry retaining by cutting roof (the 110 mining method), a non-pillar mining technique, efficiently addresses issues like poor coal resource recovery and significant rock burst damage. Nonetheless, the open-type goaf created by 110 mining techniques suffers from complex and significant air leaks, increasing the likelihood of coal spontaneous combustion (CSC) within the gob area. To address the CSC problem caused by complex air leakage within the goaf of gob-side entry retaining by roof cutting, this study takes the 17202 working face of Dongrong Second Coal Mine as the object of study. Field tests and simulation calculations are conducted to research the features of air leakage and the distribution of the oxidation zone within the goaf. Subsequently, plugging technology with varying plugging lengths is proposed and implemented. The tests and simulations reveal that the airflow migration within the goaf follows an L-shaped pattern, while air leakage primarily originates from gaps found in the gob-side entry retaining wall. The amount of air leaking into the gob-side entry retaining section is 171.59 m3/min, which represents 7.3% of the overall airflow. The maximum oxidation zone within the goaf ranges from 58.7 m to 151.8 m. After the air leakage is blocked, the airflow migration route within the goaf is transformed into a U-shaped distribution, and the maximum oxidation zone range changes from 42.8 m to 80.7 m. Engineering practice demonstrates that after air leakage plugging, the total air leakage volume within the gob-side entry retaining section significantly reduces to 20.59 m3/min, representing only 0.78% of the total airflow volume. This research provides reference on how to prevent the occurrence of CSC in similar mine goafs.

Evolutionary law and regulatory technology of roof migration on gob-side entry retaining

In order to study the evolutionary law of roof migration on Gob-Side Entry Retaining, this paper takes the gob-side entry retaining in the comprehensive mining face of the Ningtiaota coal mine as the engineering background, and analyzes the evolutionary law of the overlying rock layer on the roof at different locations during the roadway stay and the stress distribution around the roadway through numerical simulation software, which shows that there is a concentration of stress inside the Flexible formwork concrete wall, and therefore the maximum settlement of the roof on the side of Flexible formwork concrete wall is 35.35 mm, due to the existence of “arch-shaped” decompression area from the working face. Therefore, the maximum settlement of the roof slab on the side of flexible formwork concrete wall was 35.35 mm. Due to the existence of “arch-shaped” decompression area on the roof and floor of roadway, the settlement of the roof slab on both sides of the roadway gradually increased when it was from − 20 to 10 m away from the working face, and the central position had the following pattern of firstly decreasing and then gradually increasing, and then exceeding the top of the roadway. After decreasing and then gradually increasing, after 10 m ahead of the working face, the two sides of the roadway roof subsidence law and the central part of the roadway to maintain the same; the use of cutting the top of the flexible mold concrete wall support technology as a means of controlling the top of the roof along the empty roadway subsidence, the analysis shows that the roof after roof cutting of the amount of subsidence have been reduced, the maximum difference in the rate of change of the displacement is 0.011%, and the maximum difference in the amount of subsidence of 4.98 mm; through the field monitoring data analysis of the pressure of mining The peak value of the influence curve of the working face is located at 19 m of the working face, 9 m of the lagging working face and 19 m of the roadway outside the working face are less affected by the additional mining stress field, comparing the fracture brokenness of the roadway roof before and after the roof cutting, the fracture area in the uncut section is much larger than that in the section of the roof cutting.

Research and Development of Steel Fiber Reinforced Concrete Filling Material and Its Application in Gob-Side Entry Retaining Technology in Deep Mines

Against the background of the prevailing green development paradigm, numerous coal mines have embraced the adoption of gob-side entry retaining mining technology. The most commonly employed form of gob-side entry retaining involves building an artificial wall along the edge of the goaf behind the working face to maintain the roadway. The pivotal challenge in gob-side entry retaining lies in the roadside support. Currently, commonplace concrete serves as the predominant material for the roadside filling body. Nevertheless, traditional concrete exhibits drawbacks, including inadequate tensile strength and poor toughness, leading to wall cracks or even collapses in the retaining wall. Steel fiber, a frequently employed reinforcement and toughening agent in concrete, has found widespread application in the construction sector and other fields. However, its use as a roadside filling material in underground coal mines remains infrequent. Therefore, in this paper, the flow and mechanical properties of steel fiber concrete were tested and analyzed, and field industrial tests were conducted. Results of indoor experiments show that steel fibers reduce the slump of concrete. The addition of steel fibers shifted the pore compacting stage, linear elasticity stage, and destabilization stage forward and improved the post-peak bearing capacity. The addition of steel fibers makes the concrete compressive and tensile strength show a “first increase and then decrease” trend; both peaked at 1.5%, and the increase in tensile strength is more pronounced. Steel fibers enhance the strength of compressive strength of concrete at an early age, weaker at a late age, and tensile strength inversely. The addition of steel fiber can change the concrete matrix from tensile damage to shear damage, and the toughness index shows the trend of “first increase and then decrease”, and reaches the peak value when the dosage is 1.5%. Industrial test results show that steel fiber concrete as a roadside filling body can reduce the surrounding rock surface displacement and bolt (cable) force.

Study on Pressure Relief, Pre-Splitting, Softening and Gas Extraction in Highly Tough and Extra-Thick Outburst Coal Seams with Hard Roofs: A Case Study

ABSTRACT With Yushutian Coal Mine as the engineering background, the feasibility of pressure relief, pre-splitting and softening (PPS) by adjacent seam mining and its impact on top coal caving and gas extraction were studied, and the technology was applied on site. Through theoretical analysis, numerical simulation and onsite investigation, the feasibility of using gob-side entry retaining technology to achieve continuous mining of adjacent seam (Lower 4# coal seam) for PPS of underlying coal rock was demonstrated. The onsite application result shows that after the mining of the Lower 4# coal seam, the periodic weighting step of the 110,504 working face is shortened by half; the goaf basically caves along with mining; and the maximum monthly mining rate rises to 97.3%. After high-level directional long boreholes gas extraction, the gas concentration in return air of the 110,504 working face declines to 0.3–0.35%. Through the application of PPS by adjacent seam mining, the mining rate of the working face jumps significantly, and the gas concentration drops notably. Meanwhile, the coal and gas outburst coal seam is recovered safely and efficiently. The above research serves as a valuable reference for the safe and efficient mining of coal mines and enhance gas energy utilization with similar geological conditions.

Key parameters of gob-side entry retaining by roof cutting in close-distance seam group

During the process of close-distance seam group mining, the coal pillar in the upper coal seam is a stress-concentrated area, which leads to a loss of stability of the roadway during mining of the lower coal seam. This lack of stability introduces great safety hazards to coal mines. To solve the problem of stress concentration of coal pillars, the method of gob-side entry retaining by roof cutting is proposed to remove the coal pillar. In this study, FLAC3D was used to analyze the depth and angle of pre-split blasting. LS-DYNA was used to analyze the spacing of the blasthole. Using the methods of theoretical analysis and numerical simulation, we determined that the optimal depth of the pre-split blasting was 6 m, the optimal angle for pre-split blasting was 15°, and the optimal spacing of the blasthole was 500 mm. A field test was carried out in the 1010201 ventilation roadway of the Yuwang Coal Mine, China. The on-site peeping results showed that when the spacing of the blasthole is 500 mm, connecting cracks can form under the action of blasting stress. After the working surface is mined, the roof strata could collapse and fill the gob over time when the depth and angle of the pre-split blasting are 6 m and 15°, respectively.

Analysis on Failure Mechanism of Rock-Coal Two-Body Structure with Weak Cemented Rock and the Condition of a Gob Side Entry without Coal Pillar

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.

Roof control mechanism and partition compensation support technology of gob-side entry formed by N00 mining method in thin coal seam

Roof control mechanism and partition compensation support technology of gob-side entry formed by N00 mining method in thin coal seam

Large Section of Soft Rock Gob-Side Entry Retaining Surrounding Rock Control

When cutting roof and pressure relief, the impact of blasting is especially obvious on the integrity of the roof of a large-section soft-rock roadway. The reasonable parameters of blasting roof cutting were studied through using Xiaohezui coal mine -12110 roadway as the engineering background. The LS-Dyna simulation results show that: Interval blasting is beneficial to the expansion of interhole cracks and the damage to the perimeter of the hole is smaller than that of adjacent blasting, the lower the charge uncoupling factor, the larger the perimeter of the hole fragmentation zone. The mechanical model of the large-section soft-rock roadway after retrieval was established, the support resistance equation of the roadway side support and the gangue in the extraction area was derived, and the minimum support force required for the roadway side support was solved. According to the site situation, the roadway support scheme for different areas was proposed, and after continuous delamination monitoring of the roof plate, it was found that the amount of roof-plate delamination was finally stabilized within 180 mm after the working face was advanced, and the effect of leaving the roadway was good.

Stability analysis and control countermeasures of surrounding rock of gob-side entry formed by roof-cutting in inclined coal seams

The stability of the surrounding rock in the inclined coal seam stope is crucial for the safety of the gob-side entry. In this article, we systematically analyze the surrounding rock stability of the gob-side entry formed by roof-cutting (GEFR) in an inclined coal seam. We identify three forms of roof structure evolution in the formation process of GEFR and analyze the two main modules affecting the deformation of the surrounding rock structure. We determine the critical elements of surrounding rock deformation in different stages of the GEFR retaining process. Further, we propose a classification of the deformation of the surrounding rock structure of GEFR into stress and structural types during stabilization. We analyze the length of the gangue side's bearing area and calculate the lateral gangue's impact energy. We establish three instability modes of the roof based on the fractured state of the roof structure of the GEFR and provide a mechanical criterion of instability. Finally, based on the results of our theoretical analysis, we propose a control principle for the surrounding rock deformation of GEFR and establish countermeasures for controlling the roof-cutting angle and an asymmetric coupling support system. A field case study was conducted under conditions involving an inclined thick coal seam to investigate the surrounding rock deformation and overburden stress evolution using field monitoring data. The field test results confirm that the control measures of the control roof-cutting angle and the asymmetric coupling support system positively impact the stability of the surrounding rock of the roof-cutting entry in an inclined coal seam.

Experimental and numerical investigations of goaf roof failure and bulking characteristics based on gob-side entry retaining by roof cutting

The stability control of gob-side entry retaining (GSER) has consistently been an important topic of interest within the field of no-pillar mining. This paper presents a novel discontinuous similar material for conducting physical model experiments, in conjunction with discrete element numerical simulation, to investigate the goaf roof failure and bulking characteristics based on gob-side entry retaining by roof cutting. The results indicate that the application of roof cutting technology (RCT) can significantly expand the failure range of the goaf roof vicinity to the roof cutting line. This expansion leads to a 20% increase in the volume of broken roof rock, which in turn facilitates the formation of effective support and stress compensation for the overlying strata, and the possibility of failure and displacement in the overlying strata is reduced. RCT can decrease the length of the cantilever beam located on the gob-side roadway, which is beneficial for maintaining the stability of the roadway. This paper presents a rational approach for determining an appropriate roof cutting height and angle. The in-situ application results show that the deformation of the surrounding rock of GSER by RCT is small, indicating a high level of integrity, fully meeting the requirements of serving the production of the subsequent working face. This paper serves as a reference for future research on the stability control of gob-side roadways based on GSER approach.

Research on the Control Technology and Key Parameters of External Anchor-Internal Unloading of Surrounding Rock During Gob-Side Entry Driving Under Severe Mining of 1000-m-Deep Mine

Research on the Control Technology and Key Parameters of External Anchor-Internal Unloading of Surrounding Rock During Gob-Side Entry Driving Under Severe Mining of 1000-m-Deep Mine

Research on the technology of gob-side entry retaining by pouring support beside the roadway in “three soft” coal seam: A case study

This paper's goal is to investigate if a gob-side entry retention technique combined with a surrounding rock support system is feasible in three soft coal seams. Field engineering confirmed the results of numerical simulation tests and similar simulation tests, which were conducted in accordance with the actual geological conditions of Zhaojiazhai Mine. The following conclusions are reached after studying the technology and process parameter of the gob-side entry retaining in three soft coal seams in conjunction with theoretical calculations: the coal seam of Zhaojiazhai Coal Mine's 12 209 working face is a part of the soft coal seam, and its loose circle is approximately 1.8 m. The expansion roadway size is 3.5 m, and the potential loose circle range is 1.32 m, according to the same model and numerical simulation test. The support scheme after the expansion of the road working face is determined to be the “anchor rod + anchor cable + hydraulic lifting shed” support method. Furthermore, this article suggests a building method for the reinforcement and enlargement of gob-side entry retaining in three-soft thick coal seam by theoretical analysis and numerical simulation. Roadway shotcrete, advance grouting, building of a large deformation anchor cable and continuous resistance, single column lifting shed, hydraulic lifting shed, and roadway enlargement in advance are all steps in the procedure. Furthermore, an analysis is conducted on the deformation features of the surrounding rock in gob-side entry retention. The study highlights the significance of actively supporting the surrounding rock, fortifying the roof support, guaranteeing the stiffness compatibility between the shoulder filling body and the surrounding rock on the roof, boosting the wall's strength and stability, and enhancing the roadway's stability.

Application of Roof Cutting Gob-side Entry Retaining Technology in Soft Coal Seam with Large Mining Height in Liangbei Mine

Application of Roof Cutting Gob-side Entry Retaining Technology in Soft Coal Seam with Large Mining Height in Liangbei Mine

Stability mechanism and countermeasure of the solid coal rib in deep gob-side entry retaining: Insights from theoretical analysis numerical simulation

The stability and integrity of the solid coal rib in deep gob-side entry retaining (GER) can be compromised due to the cyclic loading and unloading caused by mining-induced stress. This can lead to failure of the deep GER during depressurized mining operations. In this study, we focus on a specific case at the 94103 working face in Qishan Coal Mine of Xuzhou Mining Bureau. We establish an engineering model that describes the interaction between the solid coal rib and the main roof in GER, aiming to elucidate the characteristics of main roof failure and instability throughout the entire GER process. this model particularly emphasizes the mechanical properties of the solid coal rib as a contributing factor. Additionally, developed a limit stress state model for evaluating bolt-supported plastic solid coal ribs, which helps determine appropriate support resistance levels to prevent two common forms of failure in these ribs. Furthermore, created a numerical calculation model to investigate different bolt conditions' impact on solid coal rib failure mechanisms. Finally, based on field monitoring data validation, we propose control measures for reinforcing solid coal ribs along with suggestions for roof support design and filling body construction schemes under similar geological conditions. These research findings offer valuable guidance for developing effective reinforcement strategies for filling bodies.

Numerical studies of floor heave control by the rock bolts reinforcement technology in retained gob-side gateroad

The effectiveness of modified rock bolts reinforcement technology for floor heave control in gob-side entry retaining was studied in this paper. A finite-element numerical simulation was used to analyse stress-strain state of surrounding rocks before and after immediate floor reinforcement with six floor support schemes. It was found that after immediate floor reinforcement with rock bolts with diameter 32 mm and 96 mm, floor heave in retained gob-side gateroad reduces by 2.94 – 3.6 times respectively. The modification of rock bolt reinforcement scheme by the additional installation of piles in the entry corners was proposed. Piles installation significantly reduces the zone of horizontal and vertical post-peak strains in the gateroad floor. The best variant among compared ones is a floor support scheme with two corner piles, 1 m and 2 m long. With this scheme, floor heave decreases by 3.05 times. The comparison of the materials costs of the floor support schemes were performed. As a result, the most effective floor support scheme was proposed and prospects for the further progress of floor reinforcing technologies were outlined.

Surrounding Rock Failure Mechanism and Control Technology of Gob-Side Entry with Triangle Coal Pillar at Island Longwall Panel in 15 m Extra-Thick Coal Seams

Surrounding Rock Failure Mechanism and Control Technology of Gob-Side Entry with Triangle Coal Pillar at Island Longwall Panel in 15 m Extra-Thick Coal Seams

An exploration of improving the stability of mining roadways constructed in soft rock by roof cutting and stress transfer: A case study

In the confinement of traditional supports, reinforced supports are often used to control the deformation of roadways constructed in soft rock. Due to the failure to weaken the factors that induce deformation at the source, the roadway remains damaged after the restoration period. To this end, taking inspiration from roof cutting and pressure relief concepts, the protection mechanism of stress transfer in mining roadways constructed in soft rock of longwall remaining coal pillars was studied in the context of a typical coal mine. Unlike the technical scheme of gob-side entry retaining, no components are added in this study. The roof cutting and coal mining progress are consistent to ensure the integrity of the roadway in front of the longwall coal mining face (LCMF). The effect of the roof-cutting parameter on the roadway is studied via numerical simulations, and the optimal parameters are subsequently applied to similar material simulation experiments. It was found that roof cutting relieved the mechanical connection between the gob roof and the roadway roof, and the high-stress area was transferred from the roadway roof to the front of the LCMF. Roof cutting diminished the abutment stress in the surrounding rock in the area of intense dynamic pressure in front of the LCMF (average reduction of 12.57 %), which effectively improved the stability of the roadway. The roof cutting impaired the traction effect of the roadway roof deformation behind the LCMF on the strata ahead and effectively reduced the roadway deformation (27.3 % reduction) and coal pillar deformation (15.7 % reduction). On-site practice has demonstrated the effectiveness of this research, and the results of this research provide a new idea for preventing the destruction of roadways constructed in soft rock.

Research on the impact and control techniques of gangue rib in hard roof

Blocking gangue support is a key link in gob-side entry retaining by roof cutting and pressure release (GERRC), and its stability is related to the success or failure of the retained roadways. Due to the severe dynamic pressure and high hardness of the gangue during hard roof collapse, serious bending and toppling instability of the section steel can easily occur. Since potential safety hazards exist in gangue ribs, the 1105 panel of the Qiuji Coal Mine is used as the engineering background, a mechanical model of gangue impact is constructed based on four forms of gangue motions, and the failure of a blocking gangue support structure under hard roof conditions is explored with ABAQUS software. The results indicate the following: (1) Section steel experiences transient bending by gangue impact, and will further bending failure or toppling instability occurs under roadway roof subsidence and gangue compression. (2) This impact causes the steel bar mesh to generate a significant transient lateral force, which couples with gangue compression, resulting in instability and slip of the section steel. To this end, three optimized components have been proposed, sliding yield I-steel, reinforced bolts and link components, and the optimal parameters are obtained. The results of the numerical simulation and field tests suggest that the new blocking gangue support structure has a good support effect, and the field deformation of the gangue rib is reduced by 86%. No failure of I-steel occurs with the new blocking gangue support structure. This study can provide a reference for the control of gangue ribs in GERRC under similar geological conditions.

Study on Dynamic Crack Expansion and Size Effect of Back–Filling Concrete under Uniaxial Compression

With the continuous expansion of the application range of gob–side entry retaining technology, the depth, height, and advancing speed of coal seams also increase, which brings great problems to the stability control of surrounding rock structures of gob–side entry retaining. As one of the main bearing structures of the surrounding rock, the stability of the roadway–side support body is a key factor for the success of gob–side entry retaining. In order to study the deformation characteristics and instability mechanism of roadway-side support body, based on the roadway–side support materials of gob-side entry retaining, the dynamic expansion test of back–filling concrete cracks under uniaxial compression was carried out. The YOLOv5 algorithm was applied to establish the fine identification and quantitative characterization method of macroscopic cracks of the samples, and the dynamic expansion rule of roadway-side support body cracks and its dimensional effect were revealed by combining the fractal theory. The results show that the F1 value and average precision mean of the intelligent dynamic crack identification model reached 75% and 71%, respectively, the GIoU loss value tends to fit around 0.038, and the model reached the overall optimal solution. During the uniaxial compression process, micro cracks on the surface of the back–filling concrete first initiated at the end, and after reaching the yield stress, the macroscopic cracks developed significantly. Moreover, several secondary cracks expanded, pooled, and connected from the middle of the specimen to the two ends, inducing the overall instability of the specimen. The surface crack expansion rate, density, and fractal dimension all show stage change characteristics with the increase in stress, and the main crack expansion rate has obvious precursor characteristics. With the increase in the size, the decrease in crack density after back–filling concrete failures gradually decreases from 93.19% to 4.08%, the surface crack network develops from complex to simple, and the failure mode transits from tensile failure to shear failure. The above research results provide a basic experimental basis for design optimization and instability prediction of a roadway–side support body for engineering-scale applications.