Non-pillar Mining Research Articles

Research on Roof Cutting and Pressure Releasing Technology of Cumulative Blasting in Deep and High Stress Roadway

Affected by high in-situ stress and mining activities, In the original support scheme, entry of guotun coal mine is seriously deformed. The side of the head entry bulges out seriously and the roof sinks seriously. In order to solve the problem of large deformation of roadway, guotun coal mine adopts the technology of a nonpillar mining method with entry automatically retained in 4306 mining face. First, taking method of theoretical analysis, the stress model of blasthole under the deep condition is established, the mechanical conditions of crack propagation under the action of directional detonation wave and high surrounding rock binding force are obtained theoretically. Next, the mechanical model of roof cutting and pressure relief is established. Afterwards, The mechanical model of directional blasting under the condition of stress constraint is established by using LS-DYNA numerical calculation software, the evolution characteristics of effective stress and the law of crack growth are analyzed. In the end, the field test was carried out in 4306 mining face, and the blasting effect was observed by drilling peep, and the deformation of the head entry before and after pressure relief was monitored and analyzed. The results show that directional fracture blasting can produce a continuous crack, and the effect of pressure relief is remarkable, effectively controlling the deformation of surrounding rock under high in-situ stress.

Surrounding rock deformation and supporting measures on small structure in non-pillar mining with gob-side entry retaining

This study proposes a “large and small structure” concept of cutting cantilever based on the technical principle of gob-side entry retaining in a non-pillar. A mechanical structure model of the “small structure” concept of cutting cantilever is established by analyzing the stress transfer mechanism of the overlying strata in the gob-side entry. The calculation formula of the roof sinking is derived. The deformation law of the surrounding rock and the roof subsidence characteristics in the small structure are studied using a numerical analysis method. Moreover, the surface displacement curve is obtained by combination with the field measurement. The results show that the cutting structural plane generated by pre-splitting blasting effectively changes the stress transfer between the roof of the gob-side entry and the working face, which is beneficial to the rapid formation of the large structure with a self-stabilization bearing. The deformation of the surrounding rock of a “small structure” roadway is significantly controlled and combined with the rigid support system comprising constant-resistance and large-deformation anchor cables, gangue prevention structures, and a pier-beam unit. The gangues in the goaf interact with the large and small structures formed by the surrounding rock of the gob-side entry retaining, thereby forming a synergistic support structure for effectively increasing roadway stability.

О совершенствовании бесцеликовых способов охраны повторно используемых выемочных выработок

The key problem of underground mining of mineral deposits is to ensure the stable condition of the rock mass that contains the mine workings when conducting second working and preparatory works. To do this, it is necessary to create conditions that dictate strict requirements for maintaining the working section of workings. Active and often uncontrolled displacement of rocks in the interface zones leads not only to changes, but also to a significant deterioration in the operational condition of the workings, reducing the efficiency and safety of production processes. Known methods and means of securing workings have a limited scope of application due to the high cost, labor intensity of work or insufficient efficiency. Given the current state of the coal industry and the lack of financial support from the state, the main feasible ways to solve the problem of reducing production costs in the conditions of deep mines in the Eastern Donbass should include increasing the level of spatial concentration of work, as well as the use of less material-intensive and cheaper types of support for excavation workings. To solve the issue of maintaining the development workings of the extraction area and the possibility of non-pillar coal seam mining, a method of securing is proposed. It involves the placement of a rigid cast strip in a transport working and a rigid reusable security structure (multiplier) at the interface between the second and development workings. The structure is installed behind the sections of the mechanized and individual lining or in combination with other security structures (wood chocks and organ timbering, bollards of reinforced concrete blocks), with or without them, but with a mandatory combination of a multiplier with individual support stands.

Gob-Side Entry Retaining Involving Bag Filling Material for Support Wall Construction

Gob-side entry retaining, also termed as non-pillar mining, plays an important role in saving coal resources, high production and efficiency, extending the service life of mine and improving the investment benefit. Herein, a gob-side entry retaining method involving the use of bag filling material for wall construction is proposed based on the thin seam mining characteristics. First, a gob-side entry retaining mechanical model is established, and the side support resistance of the 8101 working face is calculated. The mechanical properties of the bag material are investigated through experiments, and the construction technology of the gob-side entry retaining approach involving the use of bag filling material for wall construction is introduced. The deformation on the two sides, the roof and floor of the roadway, are simulated via numerical methods and monitored during field tests. The results show a small control range for the deformations and a good roadway retention effect, thereby proving the feasibility of the bag filling material for wall construction. This study provides a reference for the development of gob-side entry retaining mining for thin coal seams.

An Innovative Non-Pillar Coal-Mining Technology with Automatically Formed Entry: A Case Study

• Innovative approach of non-pillar coal mining is accompanied by the automatically formed entry. • The mutual cooperation of three key techniques achieves non-pillar coal mining engineering. • A field test verified the feasibility and effectiveness of the non-pillar coal mining technology. • The proposed non-pillar coal mining technology is supposed to popularize on a large scale. A non-pillar coal-mining technology with an automatically formed entry is proposed, which reduces the waste of coal resources and the underground entry drivage workload. Three key techniques in this technology cooperate to achieve automatic formation and retaining of the gob-side entry, and to realize non-pillar mining. Constant-resistance large deformation (CRLD) support ensures the stability of the entry roof; directional presplitting blasting (DPB) separates the entry roof and the gob roof; and a blocking-gangue support system (BGSS) integrates the caved rock material as an effective entry rib. An industrial test was conducted to verify the engineering effects of these key techniques. The field application results showed that the retained entry was under the pressure-relief zone due to the broken-expansion nature of the caved rock mass within the DPB height. After going through a provisional dynamic pressure-bearing zone, the retained entry entered the stability zone. The final stable entry meets the requirements of safety and production. The research results demonstrate the good engineering applicability of this technology. By taking the framework of the technology design principles into consideration and adjusting the measures according to different site conditions, it is expected that the proposed non-pillar coal-mining technology can be popularized on a large scale.

Acoustic Emission Characteristics of Coal Samples under Different Stress Paths Corresponding to Different Mining Layouts

Research on the mining-induced mechanical behavior and microcrack evolution of deep-mined coal has become increasingly important with the sharp increase in mining depth. For rock units in front of the working face, the microcrack evolution characteristics, structural characteristics, and stress state correspond well to mining layouts and depths under deep mining. The acoustic emission (AE) characteristics of typical coal under deep mining were obtained by conducting laboratory experiments to simulate mining-induced behavior and utilizing AE techniques to capture the variation in AE temporal and spatial parameters in real time, which provide an important basis for studying the rupture mechanisms and mechanical behavior of deep-mined coal. The findings were as follows: (1) AE activity under deep mining was characterized by three stages, corresponding to crack initiation, crack stable propagation, and crack unstable propagation. As the three stages proceeded, the AE counting rate and AE energy rate presented stronger clustering characteristics, and the cumulative AE counting and cumulative AE energy exhibited a sharp increase by an order of magnitude. (2) The crack initiation and the main stages of crack propagation were determined by characteristic points of variation curves in the AE parameters over time. In the main crack propagation stage, the number of cumulative AE events and the cumulative AE counts were similar among the three mining conditions, while coal samples under coal pillar mining released the largest amount of AE energy. The amount of accumulated AE energy released by coal samples increased by one order of magnitude according to the sequence of protective coal-seam mining, top-coal caving mining, and nonpillar mining. (3) Fractal technology was applied to quantitatively analyze the AE spatial evolution process, showing that the fractal dimension of the AE location decreased as the peak stress increased, corresponding to protective seam mining, caving-coal mining, and nonpillar mining. The above results showed that the deformation and fracture characteristics of coal under deep mining followed a general law, but were affected by different mining conditions. The crack initiation and main rupture activity of coal occurred earlier under the conditions of protective seam mining, top-coal caving mining, and nonpillar mining, successively. Moreover, nonpillar mining induced the strongest and highest degree of unstable rupture of the coal body in front of the working face.

Size and spatial fractal distributions of coal fracture networks under different mining-induced stress conditions

Coal permeability is a key issue in CO2 injection and enhanced coalbed methane (CBM) recovery, and it is determined by the fracture network, which is strongly influenced by mining-induced stress evolutions. For a comprehensive understanding of the size and spatial distribution characteristics of coal fracture networks under different mining-induced stress conditions, a series of laboratory experiments, computed tomography (CT) scans and image analyses focusing on 3D coal fracture systems have been conducted considering the stress conditions induced by three typical mining layouts, i.e., top-coal caving mining (TCM), non-pillar mining (NM) and protective coal-seam mining (PCM). The size and spatial distributions of mining-induced coal microfractures and the anisotropic tortuosity characteristics of coal fracture networks have been quantitatively determined based on fractal theory. The results show that the size distributions of microfracture geometries, the fractal dimensions of fracture size distribution and the tortuosity of the mining-induced coal fracture networks vary according to the mining-induced stress conditions, resulting in differences in the seepage capacity of coal masses. The coal samples subjected to PCM conditions have the highest percentage of large microfractures, and those exposed to NM conditions have the lowest percentage. The fractal dimensions (Da and Dd) of the microfracture size distributions of the typical coal specimens decrease in the order of PCM, NM and TCM conditions, and the Da and Dd of coal specimens without pre-mining unloading-expansion simulation (PUES) are much lower than those of specimens with PUES. The tortuosity fractal dimensions (δ) of the mining-induced fracture network are spatially anisotropic. The ranges of δ for the coal masses exposed to PCM, NM and TCM conditions are 1.0–1.2, 1.2–1.3 and 1.25–1.4, respectively. Using fractal theory and the measured fracture network data, the anisotropic spatial distribution of coal permeability can be theoretically estimated.

Stability characteristics of a fractured high roof under nonpillar mining with an automatically formed roadway by using a visualized discrimination approach

AbstractNonpillar mining (NPM) with an automatically formed roadway is an emerging mining technology that does not use coal pillar retention and drivage. This technology can be used to actively control the caving height of the roof by directional cutting technology and can promote the rapid equilibrium of a fractured high roof under the support of the caved gangue. Therefore, when analyzing the stability of a high roof under NPM, not only the interaction force of the fractured rock blocks but also the effects of the support provided by the gangue, and the coal body should be considered. In addition, if necessary, the roof support in the roadway should also be considered. Based on this idea, in this paper, we analyzed the stability characteristics of a fractured high roof and suggested the stability conditions of the fractured rock of a high roof, including actual stability, pseudostability, and instability and provided the criteria for determining these three stability conditions. Furthermore, a visualized approach for the determination of roof stability was proposed. The core idea of the proposed approach is to transform the parameters that cannot be measured in the abovementioned conditions into visualized data that can be directly or indirectly obtained in the field, allowing for the detection and occasional assessment of the stability condition of the fractured rock of a high roof. This visualized determination approach can be used to assess the roof stability condition in a timely manner during the production process and to ensure production safety. The abovementioned research results were successfully used in the S1201‐II mining face of the Ningtiaota Coal Mine, indicating that the proposed approach is feasible and can be used under similar conditions.

On the excavation-induced stress drop in damaged coal considering a coupled yield and failure criterion

Investigating the stress drop of abutment pressure is the key to a deep quantitative analysis of the discontinuous stress redistribution under mining. In the present study, uniaxial and triaxial compression tests are carried out separately to determine the bulk and shear moduli, the cohesion, and the internal friction angle of the coal samples. By extending the meaning of Mohr’s circle referring to yield stress instead of the maximum principal stress, a yield line is introduced to illustrate the stress drop of Mohr’s circle referring to yield stress instead of the maximum principal stress at the elastoplastic boundary. Furthermore, a theoretical solution of the stress drop as a function of the damage is proposed to investigate how the abutment pressure differs considering the yield line and failure line. In addition, applying the stress drop at the yield line in non-pillar mining, top coal mining, and protective coal mining shows that the damage has a nonlinearly positive influence on the stress drop. The results shows that the bulk modulus and internal friction angle have a more sensitive influence on the stress drop than do the shear modulus and cohesion. Finally, the stress drop is divided into a discontinuous stress drop at the yield line and a structural stress drop at the failure line. The stress drop is effective in describing the discontinuous stress redistribution and shows a clear difference in the movement direction of Mohr’s circle considering the unloading pressure.

Application of Roof-Cutting and Pressure-Release Entry Retaining in Inclined Coal Seams: A Case Study

The technology of gob-side entry retaining as an important method to realize the sustainable development of coal resource has been widely applied in China’s coal industry. However, when used in inclined thick coal seams, some disadvantages, such as complex strata pressure and gangue downslide, may cause serious entry accidents. To solve this issue, this study adopted a novel roof-cutting and pressure-release entry retaining (RPER) technology in inclined thick coal seams. First, the principle of RPER and the stability mechanism of gob-side entry surrounding rock were analyzed. Subsequently, the key technologies of the novel method including a new constant resistance anchor cable and a new type of directional cumulative blasting technique were proposed to improve the stress environment of retained entry, and an entry-side support system that guaranteed the effect of entry are described in detail. Finally, an industrial test of RPER in inclined thick coal seam was performed. Field monitoring data show that the entry retained by new method RPER was in a stable state, and the section of the retained entry after can meet the production needs of next panel. This work provides a safe and efficient nonpillar mining method in inclined thick coal seams, and it has significant significance for guiding other mines to carry out nonpillar mining in inclined thick seam.

Energy Evolution Pattern and Roof Control Strategy in Non-Pillar Mining Method of Goaf-Side Entry Retaining by Roof Cutting—A Case Study

This article focuses on the energy density alteration during non-pillar mining method of goaf-side entry retaining by roof cutting (GERRC) and adjacent working face mining. We also studied the support control strategy of goaf-side roadway. Numerical calculation model is established, and the parameters of the model are verified by the measured advance abutment pressure and numerical solution. Based on the numerical model, the energy density during mining is studied. It is found that the whole energy evolution pattern of the goaf side entry during the two adjacent working face mining includes: the original rock energy, the advance energy of the current working face, the dynamic lateral abutment energy caused by strata movement, the lateral abutment energy of the adjacent working face. The support body failure and surrounding rock large deformation phenomenon often occur in goaf side roadway, which is influenced by multiple energy disturbances. Research shows that strong stress disturbance of surrounding rock generates in front of the working face 23 m and behind of working face 60 m in GERRC method. In the second goaf-side entry retaining, the range is in front of the working face 47 m. The evolution law of energy field puts forward the strategy of using the high constant resistance and large deformation (CRLD) anchor cable and procured preferable effect.

Mechanical Model and Control Technology of Roadside Gangues in Gob-Side Entry Retaining by Roof Cutting

Gob-side entry retaining by roof cutting (GERRC) is a new technique regarding a non-pillar mining method based on the “cutting cantilever beam theory” proposed in recent years. In this technology, the composition, formation mechanism, load transfer mode and stability mechanism of roadways are quite different from those of traditional coal mining methods. Based on the technical principle of the GERRC, the formation mechanism of the sidewall composed by collapsed gangues is clarified. Also, on basis of the observation of engineering site, the movement characteristics of the gangues in goaf are analyzed, and the movement processes are mainly divided into two stages, i.e. the rapid collapse stage and the slow compaction stage. Then, the interaction between the gangues and retaining structures in different movement stages is revealed. Meanwhile, a mechanical model of the roadside gangues was established, and the mechanical characteristics of the retaining structures were analyzed. Besides, a control concept, considering both dynamic and static pressure in the lateral direction and making it extensible to release pressure in the axial direction, was put forward to support the gangues in goaf based on the interaction characteristics of the gangues and the retaining structures. At last, three different retaining structures for the gangues were proposed in view of different mining heights, and the roadside gangues control system was established, which has been proved to be of good practicality in the engineering site. The research results can provide technical guidances for the control of roadside gangues in the GERRC and accelerate the process of popularization and application of the GERRC, which is of positive significance.

Study on Three-Dimensional Stress Field of Gob-Side Entry Retaining by Roof Cutting without Pillar under Near-Group Coal Seam Mining

In order to explore the distribution law of stress field under the mining mode of gob-side entry retaining by roof cutting without pillar (GERRCP) under goaf, based on the engineering background of 8102 and 9101 working faces in Xiashanmao coal mine, the stress field distribution of GERRCP and traditional remaining pillar was studied by means of theoretical analysis and numerical simulation. The simulation results showed that: (1) in the front of the working face, the vertical peak stress of non-pillar mining was smaller than that of the remaining pillar mining, and it could effectively control stress concentration in surrounding rock of the mining roadway; the trend of horizontal stress distribution of the two was the same, and the area, span and peak stress of stress the rise zone were the largest in large pillar mining and the minimum in non-pillar mining. (2) On the left side of the working face, the vertical stress presented increasing-decreasing characteristics under non-pillar mining mode and saddle-shaped distribution characteristics under the remaining pillar mining mode respectively. Among them, the peak stress was the smallest under non-pillar mining, and compared with the mining of the large pillar and small pillar, non-pillar mining decreased by 12–21% and 3–10% respectively. The position of peak stress of the former was closer to the mining roadway, indicating that the width of the plastic zone of the surrounding rock of the non-pillar mining was smaller and bearing capacity was higher. In the mining of the large and small pillar, the horizontal stress formed a high stress concentration in the pillar and 9102 working face respectively. In non-pillar mining, the horizontal stress concentration appeared in solid coal, but the concentration area was small.

Comparison of the macroscopical stress field distribution characteristics between a novel non-pillar mining technique and two other current methods

Recently, an original innovative non-pillar coal mining technology was developed and widely implemented in China’s coal production based on this background. To explore the difference of stress field distributions between a novel non-pillar mining technique and two other current methods, different numerical models, that is, mining with gob-side entry retained by filling (GERFM), mining with roof cutting and pressure relief (RCPRM), and non-coal pillar mining with roadway formed automatically (RFANM), had been built based on the first industrial test of RFANM. The difference of the stress field distributions in those three types of mining patterns was studied and analyzed. The results showed that in front of the mining face, the influence of gob-side entry retaining method on stress distribution had a limited range. Due to the impact of roof cutting, the vertical stress of RCPRM was the minimum within the range of 0–10 m from the roadway, and it was the maximum within the range of 10–80 m. In the mining pattern of GERFM and RCPRM, the abutment stress was highly concentrated around the gate entry, and the stress-concentrated position of RCPRM was further than GERFM under the influence of roof cutting. However, there was no stress concentration in RFANM because the gate entry was canceled in this new mining pattern. At the side of mining face, the stress-concentrated position of GERFM was always located in the coal seam and was close to the roadway, whereas the stress-concentrated position of RFANM and RCPRM was located in the top layer which was deeper and higher than GERFM and was far away from the roadway. While the roadway was stable, the relationship between the peak value of the surrounding rock stress was GERFM > RCPRM and RFANM. According to the above laws, it was obtained that canceling early excavation was helpful to reduce the advance pressure in front of the working face, and roof cutting could cause the stress concentration zone to move away from the roadway. Thus, the stress of surrounding rock near the roadway was reduced, which was more beneficial to ensure the stability of the roadway.

Meso- and macroeffects of roof split blasting on the stability of gateroad surroundings in an innovative nonpillar mining method

The 110 mining method, an emerging and innovative nonpillar longwall mining method, can dramatically increase coal recovery rates and reduce entry accidents. One of the core techniques of this method is roof splitting, the operation that is directly related to the stability of the retained entry. In this work, a directional roof split blasting (DRSB) technique was introduced, and the effects of DRSB on the stability of the entry surroundings were comprehensively studied at the meso- and macrolevels. First, a micromechanical damage model considering the heterogeneity of the roof rock was developed using the finite element method (FEM), and the blasting-induced damage evolution in the roof rock was numerically explored using the FEM model. Subsequently, the macroeffects of DRSB were studied using the finite difference method (FDM). A meticulously validated FDM numerical model incorporating a double-yield model for the gob materials and calibrated parameters was developed to investigate the effects of roof splitting on the stabilities of the entry surroundings. The numerical simulation results were finally verified using on-site monitoring data. These results indicate that the DRSB technique could effectively control crack propagation in the roof rock and protect the entry roof from being fragmented or damaged. Macroscopic analysis revealed that applications of DRSB affected the stress distributions and failure states of the entry surroundings. As the roof splitting effects were enhanced, more vertical stresses were transferred to the gob area, causing the stress concentrations in the entry surroundings to be mitigated. Consequently, the deformations in the entry surroundings were reduced, and the stability of the retained entry was improved. The proposed models and obtained results potentially produce reasonable values for the applications of pillarless mining in similar projects.

Overview of Solid Backfilling Technology Based on Coal-Waste Underground Separation in China

China is the world’s largest coal producer country. However, large-scale coal mining has led to severe environmental pollution issues such as surface subsidence and gangue piling up. The gangue discharging amount has ranked the first in the world and coal mine enterprises are facing enormous discharging reduction pressure. This paper summarizes the research progress of the solid backfilling mining technology and then illustrates the realistic demands and significance of implementing underground coal-waste separation. It also focuses on the technical principles, systems and key equipment of the common underground coal-waste separation methods, such as the selective crushing method, the dense medium shallow groove method, the vibro-assisted jigging method and full-size water separation method and ray identification method. In addition, the selection steps of underground coal-waste separation method, the design process of large section separation chamber and the design principle of separation and backfilling system are proposed, finally, the mining-separating-backfilling + X for coal mining is put forward. By combining the technology of mining-separating-backfilling with other technologies, such as gob-side entry retaining with non-pillar mining, gas extraction, solid waste treatment, water protection mining, mining under buildings, railways and water bodies, the integrated mining methods, mining-separating-backfilling + setting pillars, gas drainage, treatment, protection and prevention methods are formed. It also introduced the ‘mining-separating-backfilling + gas extraction’ technology’s whole idea, system arrangement, separation equipment and practical engineering application effects based on the specific engineering case of pingmei no. 12 coal mine. The results indicate that the integration of underground coal-waste separation and solid backfilling technology could achieve gangue discharging reduction, underground washing and surface subsidence control. It is effective at realizing green mining.

Comparative analysis of the mine pressure at non-pillar longwall mining by roof cutting and traditional longwall mining

Abstract Non-pillar coal mining has been developed and implemented in the recent decades in China's coal mining industry. The non-pillar longwall mining by roof cutting without pre-excavated entry (N00 mining method) is one of the latest non-pillar mining methods and this method has the advantages of reduced roadway drivage ratio and increased resource recovery ratio. Previous studies show that the mining pressure during the working face advancing is one of the main factors that affect the stability of underground structures and the safety production. However, there is no evaluation or analysis of the mining pressure at the mining face using entry retaining with roof pre-cutting and an absence of pre-excavated tail entry. In this paper, both field monitoring and numerical simulation approaches are employed in the analysis of the mining pressure distribution characteristics within a range of the whole working face during the face advancing. The results are compared with the field data and simulation results from the traditional mining method performed in the same coal mine. Results supported the idea that the N00 mining method can generate a low-stress area for the retained entry. The stability of the working face and retained entry can be well maintained due to the mine pressure optimization. This paper can aid in the understanding of structural mechanic modeling and mine pressure distribution features, structural mechanic analysis and mine pressure distribution features of the N00 mining method.

A Case Study on Optimization and Control Techniques for Entry Stability in Non-Pillar Longwall Mining

In order to reduce large deformation failure occurrences in non-pillar longwall mining entries due to roof weighting behaviors, a case study in Halagou coal mine was conducted on optimization and control techniques for entry stability in non-pillar longwall mining. The Universal Discrete Element Code (UDEC) modeling was adopted to study entry stability in non-pillar mining, and the characteristics of deformation and stress and crack propagation were revealed. The large deformation transmission between the entry-immediate roof and the gob-immediate roof could be eliminated by optimizing the entry roof structure through a directional roof-cutting method. The localized tensile stresses generated in the entry-surrounding rock caused the generation of coalescent macroscopic fractures, which resulted in the instability of the entry. The tensile stress state could be inhibited by an active flexible support system through enhancing the confining pressure on the surrounding rock. Serious rotation subsidence occurs in the entry roof due to periodic weighting of the main roof, which could be greatly reduced by a passive rigid support pattern. The numerical and field test results both showed that the roof weighting pressure was offloaded by the technique and that the deformation of the entry surrounding the rock in non-pillar mining was quite small. Thus, the technique can effectively ensure the stability of the gob-side entry, which can provide references for entry stability control in non-pillar longwall mining.

Distribution and Evolution Characteristics of Macroscopic Stress Field in Gob-Side Entry Retaining by Roof Cutting

Non-pillar mining technology plays an important role in sustainable exploitation of coal resources. Gob-side entry retaining by roof cutting (GERRC) is a new technique regarding a non-pillar mining method based on the “cutting cantilever beam theory”. In order to study the distribution and evolution characteristics of macroscopic stress field of surrounding rocks in GERRC, and find out differences from traditional pillar retained mining method (PRMM), some key issues about abutment pressure and stress concentration shell were analyzed by numerical simulations. Results show that: (1) distribution characteristics of lead abutment pressure for the two mining methods are basically the same during the primary mining stage; (2) during the secondary mining stage, peak stress in front of mining face of the two modes is close, while the abutment pressure can be transferred more evenly to coal and rock mass far away from goaf when using GERRC, and that is more likely to cause high stress concentration in the coal mass near the goaf when mining with PRMM; (3) for the PRMM, lateral abutment pressure will produce a higher stress concentration in section coal pillar and a high residual stress will be maintained in it when coal masses on both sides of the pillar are extracted; (4) adjacent goafs can be connected to form a wide range of pressure relief region when using GERRC, and shape of distribution of the maximum principal stress appears a semi-space ellipsoidal shell in three-dimensional space, and it is a single arch shape in the section perpendicularly to the mining direction, however, it looks like a “m” shape in that section when mining with PRMM because of the high stress concentration in the coal pillar.

Surrounding rock control of an innovative gob-side entry retaining with energy-absorbing supporting in deep mining

In case of deep mining engineering, the conventional gob-side entry retaining (GER) method encounters inevitable difficulties, due to high stress, high dynamic disturbance and large deformation issues. Recently, an innovative no-pillar mining approach of GER is proposed and applied in several realistic cases. According to the innovative approach, two novel techniques, the pre-fracturing on the roof before mining activities, and a novel support system with a good energy-absorbing capacity, make it possible to effectively restrain the large deformation in the deep surrounding rock. In this study, the innovative mining approach with the corresponding energy-absorbing support systems in deep mining is systematically presented. The pre-fracturing effect, taking into account the novel support system, is demonstrated in a numerical approach, and the numerical results are compared with those of the conventional method. Finally, some real monitoring data from the Chengjiao coal mine are given to further prove the advantages of the innovative mining method with energy-absorbing support systems. This work provides an effective and economical method to non-pillar mining in deep coal mine.