Roadway Support Research Articles

Research on the optimal design of roadway support for fully mechanized coal seam working face

Abstract For the optimal design of surrounding rock deformation and support in the shallow buried coal seam of large-scale fully mechanized mining face, this paper took the surrounding rock of the roadway as the research object, used theoretical calculation and numerical simulation to establish the mechanical model of the deformation of the surrounding rock structure, and proposed and analyzed the reasonable roadway support scheme of the shallow buried coal seam. The numerical simulation results showed that the top anchor rod with Φ18×2200 mm and the inter-row distance of 1100×1000 mm, the anchor cable with Φ15.24×6300 mm and the inter-row spacing of 2200×2000 mm, the upper part anchor of Φ16×2200 mm, and the C30 concrete with a thickness of 200 mm for laying the bottom plate are selected, which was certified as the optimal support scheme. In practical engineering applications, the deformation of surrounding rock has been effectively controlled, and the supporting effect of surrounding rock was obvious, which was of great significance to ensure the safe production of coal mines.

Asymmetric Dynamic Support for Roadways in Subvertical Coal Seams

Asymmetric Dynamic Support for Roadways in Subvertical Coal Seams

Обоснование параметров поэтапного анкерного крепления для повышения скорости проходки выемочных выработок на угольных шахтах

Introduction. This article substantiates the prospect of introducing the staged bolting support for roadways in Russian mines in order to increase the rate of preparatory operations. Purpose. Justification of a staged bolting support scheme in order to accelerate roadway development to insure timely preparation of new long wall panels. Methods. The development of a staged roadway support method for mining gently inclined flat-lying coal seams is presented. In the initial phase of bolting support, the reduced number of bolts are installed, which are necessary to support the roadway during its construction. In the subsequent stage, with a certain delay from the face of the roadway, but prior to the initiation of long wall advancement, the full complement of bolts is installed to ensure the stability of the roadway throughout its operational lifespan. Results of numerical studies. Numerical studies allowed to estimate the distribution of stresses in the plane of the formation and the zone of limit state formed in the ribs of the roadway, both at the first stage of support, outside the long wall affected zone, and after the end of the second stage in the long wall affected zone. Based on the results of these studies, the necessity of ribs support of the roadway at the first stage of support are shown. Results of industrial research. A comparative analysis of the work cycle employed at one of the Kuzbass mines with the work cycle utilizing staged bolting support demonstrates that the rate of roadway development can be accelerated by up to 25 % in comparison with the conventional support scheme. Discussion. 1. The proposed staged bolting support scheme is recommended for the development of gently inclined flat-lying coal seams, as it will facilitate intensive underground coal mining. 2. The finite element method can be employed to successfully estimate the limit state zones forming and developing in the ribs of a roadway at different stages of its lifespan. 3. A comparative analysis of work cycles applied at one of the Kuzbass mines and the proposed patented staged support scheme revealed that the implementation of staged bolting support can potentially enhance the rate of roadway development by 25 %. Conclusion. 1. The article outlines the importance of timely preparation of long wall panels in leading coal-mining countries and presents a rationale for the introduction of staged bolting support of roadways at coal mines in Russia. 2. The method of staged roadway support, as developed and described in the article, represents a promising approach for the preparation of long wall panels in the conditions typical of Kuzbass mines. 3. In order to provide further details on the parameters of staged bolting support a physical simulation of the roadway stability be conducted.

Experimental investigation of the influence of mine water on mechanical properties of resin grout

Resin grout is widely used in geotechnical and underground engineering, and is often affected by different mine water solutions. This study considered the effects of different mine water solutions and soaking times on resin grout. Soaking tests and uniaxial compression tests were conducted to investigate the changes in the solution pH, relative specimen mass, and uniaxial compressive strength (UCS), and the deterioration of the resin grout’s mechanical properties caused by the mine water solution was analyzed. The corrosion mechanism of resin grout under the action of different mine water solutions was investigated through scanning electron microscopy tests. The results reveal that the pH value of the solution and the relative mass of the specimen gradually stabilized as the soaking time was extended, and the final solution was weakly alkaline. The increase in the acidity and alkalinity of the solution and the extension of the soaking time led to a gradual decrease in the UCS and elastic modulus of resin grout under the action of mine water. As the soaking was prolonged, the resin grout properties deteriorated to different degrees and Poisson’s ratio increased. Moreover, owing to the different types and degrees of mine water action on resin grout in different mine water environments, the changes in the resin grout microstructure were also different. The defined damage parameters can express the damage process of the resin grout’s UCS quantitatively under the action of mine water solution. Finally, beneficial engineering application countermeasures are proposed for different resin grout types used in roadway support applications in coal mines.

Experimental study on the bearing mechanical characteristics and deformation failure mechanism of damaged sandy mudstone

AbstractStudying the mechanical properties and failure mechanisms of damaged rocks holds practical significance in guiding tunnel or roadway support and assessing the stability of engineering rock masses. To uncover the failure mechanisms of sandy mudstone with varying degrees of damage, we conducted cyclic loading‐unloading tests to create specimens at different damage levels. Subsequently, uniaxial loading tests were performed to investigate the fundamental mechanical properties, wave velocity, and acoustic emission (AE) parameter evolution characteristics of the damaged specimens. The classification of damaged rocks was based on the energy evolution characteristics of damaged sandy mudstone. Finally, we discuss the damage and deterioration mechanisms of different types of sandy mudstone. The findings contribute to establishing a theoretical foundation for evaluating the degree of rock damage.

Investigation on the pullout bearing properties and stress characteristics of fully grouted bolts in layered rock mass

AbstractCoal‐measure strata is a typical layered structure, a better understanding of the bearing characteristics of fully grouted bolts in layered rock mass is of great significance for roadway support. However, limited studies have been conducted to investigate the bearing performance of fully grouted bolts in layered rock mass. In this paper, a numerical model of fully grouted bolt in layered rock mass was established, and a tri‐linear bond–slip model of the anchorage interface was imported into the numerical model by the user‐defined Fish language. The effects of strata sequence, strata thickness, and strata number in layered rock mass on the bearing performance of fully grouted bolts were systematically analyzed. The results show that fully grouted bolts in soft–hard type (SH‐type) layered rock mass has higher bearing capacity. The greater the thickness of the soft rock in the layered rock mass, the more significant the influence of the strata sequence on the bearing performance of fully grouted bolts. The bearing capacity of fully grouted bolts decreases with the increase of the thickness ratio of soft rock to hard rock. The greater the thickness of the hard rock in the layered rock mass, the more beneficial to the bearing performance of fully grouted bolts. With the increase of the strata number, the bearing capacity of fully grouted bolts gradually decreases. For bolt support of layered rock mass, the deep anchoring foundation of fully grouted bolts should be arranged in hard rock or elastic zone rock as much as possible, and rock grouting can be adopted to reduce the difference in mechanical properties between layered rock mass and improve the durability of fully grouted bolts. The research results may provide useful guidance and reference for the support design of fully grouted bolts.

Analysis of cracks development and damage evolution in red sandstone under dry-wet cycles based on temporal and frequency characteristics of acoustic emission

Utilizing the goafs of underground reservoirs (underground buildings) in western coal mines as storage spaces plays a crucial role in ecological environment protection and energy transition in mining areas. Hence, it is crucial to recognize the repeated erosion of reservoir water levels on the deformation and damage mechanisms of the roof and floor rock masses in the underground building dams. This paper intends to study the instability failure mechanisms and precursor responses of red sandstone samples with 0–3 dry-wet cycles during uniaxial loading-unloading under acoustic emission (AE) monitoring. The AE time-frequency features were analyzed, and the crack three-dimensional spatial extension and tensile-shear crack evolution were explored. The b-value and entropy value of AE events were presented for evaluating crack expansion damage status during loading-unloading. The findings suggest that the peak strength, elastic modulus, and crack thresholds of red sandstone exhibit a positive exponential decline with growing dry-wet cycles. Conversely, the peak strain and strain-softening modulus exhibit an opposite trend. With growing dry-wet cycles, AE signals from low-frequency, high-amplitude, and large-scale cracks decline, exhibiting a multi-frequency simultaneous response effect before sample instability. Meanwhile, the proportion and concentration of localization events with energy exceeding 104 aJ rise, and the failure instability gradually shifts from tensile-shear failure to tensile cracks. The sudden decline in b-value and the significant fluctuation in entropy value serve as short-term precursors to imminent rock instability failure, with the system energy following an ordered-disordered-ordered pattern. The research findings offer certain experimental foundation and novel judgment indicators for rock structure safety warning, including the stability of underground building dams, support for water-rich roadways, water hazard prevention, and utilization of abandoned mine spaces.

A study on the reasonable width of narrow coal pillars in the section of hard primary roof hewing along the air excavation roadway

AbstractAiming at the reasonable width of the narrow coal pillar of a fully mechanized caving face and the safety support of roadway, taking the coal pillar in the section between 110503 and 110505 face of the Yushuling Coal mine as the research background, a model of the hard basic roof fracture structure of fully mechanized caving face is established through theoretical analysis, and the roadway with narrow coal pillar is analyzed mechanically. Combined with the geological conditions of the working face, it is concluded that the low‐stress area is less than 3.29 m. When the internal stress field of the low‐stress environment is considered in the roadway layout, the influence of mining and the essential roof hardness should be considered. The reasonable size of the narrow coal pillar is 3 ~ 6 m, thinking that the load borne by the coal pillar is less than the ultimate strength of the coal pillar. The limit equilibrium theory calculates that the reasonable width of a coal pillar is at least 4 m. The stress and displacement of coal pillars with different widths of 3, 4, 5 and 6 m are analyzed by numerical simulation, and the 4 m narrow coal pillars are simulated and verified. Field industrial tests show that coal pillar and roadway surrounding rock deformation are small under asymmetric surrounding rock control. The research results have been successfully applied to engineering practice and can provide a reference for the research method of narrow coal pillar width under a hard basic roof.

Research on the Positioning Method of Steel Belt Anchor Holes Applied in Coal Mine Underground

In order to improve the automation and safety of underground steel belt support in coal mines, a method for the intelligent identification and positioning of steel belt anchor holes in roadway support using inductive sensors is proposed. Using STM32F407ZGT6 as the main control chip, tasks such as data acquisition and processing, motor motion control, etc., are assigned based on the real-time operating system FreeRTOS. Using the XY mobile platform equipped with inductive sensors to detect steel belts, The collected data includes coordinate values and voltage values. Adaptive threshold generation and correction strategies are used for threshold segmentation and extraction of anchor hole boundary points. The principle of Hough circle transformation is used to fit the extracted boundary points into circles. The results show that this method can perform anchor hole positioning with a positioning error of within 5 mm, meeting the design requirements.

Study on Support Technology of Truss Anchor Cable Beam in Deep Soft Rock Roadway Excavation

Aiming at the difficult problem of soft rock roadway support in deep mine, the support mode of "truss anchor beam" is proposed, and the support mode technology of truss anchor beam in deep soft rock roadway driving is studied by FLAC simulation software. Combined with the measured displacement of coal and rock around underground roadway under this support mode, it is concluded that the truss anchor beam support mode can introduce the high stress of shallow coal and rock around roadway into deep area, thus effectively controlling the deformation of coal and rock around roadway and ensuring safe and efficient production of mine.

Study on Reinforcing Anchor Cable Support in Overreach Section of Deep Roadway

With the increasing depth of coal mining, the conditions of deep working face are becoming more and more complex, the stability of surrounding rock of roadway is getting worse and worse, and the problem of deep roadway support is becoming more and more prominent. In this paper, the 24100 working face of Pingdingshan No.11 Coal Mine is taken as the engineering background. In view of the problems of roof crushing and deformation and more net pockets in the return airway affected by the mining of the working face, the grouting anchor cable reinforcement and support technology in the advance section of the deep mining roadway is proposed, and the feasibility of the support effect is verified by numerical simulation. The results show that the influence range of the advance abutment pressure of the air roadway is about 60 m, and the reinforcement support strength required for the roof of the roadway is 50.6 kN/m2. The grouting anchor cable reinforcement support technology system of the return air roadway can control the deformation of the surrounding rock of the roadway and achieve safe and efficient mining.

Optimization of Support and Relief Parameters for Deep-Buried Metal Mine Roadways

The management of rock mass deformation in high-stress roadways is a pivotal aspect of deep geotechnical engineering. Given the fruitful outcomes of research in rock mechanics regarding traditional confining pressure control methods, scholars have increasingly turned their attention to exploring pressure-relieving techniques, including borehole pressure relief and blasting pressure relief. However, there is limited research on pressure relief methods for deep-buried hard rock tunnels. This article commences with an overview of pressure relief in the roadway and conducts a detailed study on the parameters and methods of pressure relief in the roadway. To address safety and mining efficiency challenges, such as severe deformation leading to support failures, this study conducted a parameter analysis using the Sanshandao Gold Mine as a case study. Based on existing support methods, a strategy for arranging pressure relief roadways at varying distances from the main roadway is proposed, significantly enhancing the stress environment there. Numerical simulation software was employed to model two scenarios: (1) excavating the pressure relief roadway, main roadway, and maintenance roadway simultaneously and (2) first excavating the pressure relief roadway, followed by the main roadway and the maintenance roadway simultaneously. Simulation results indicated that the first pressure relief approach outperforms the second. The optimal position for both pressure relief roadways is 15 m from the main roadway, resulting in maximum deformation of the main roadway within 100 mm. These findings align with on-site stress monitoring data and satisfy safety production criteria. The research offers a theoretical foundation for similar pressure relief techniques in deeply buried, high-stress roadways.

Study on Pretightening Loss Effect of Bolt Support in Deep Soft Rock Roadway

The magnitude of pretension force serves as an important indicator of the effectiveness of anchor bolt active support and is one of the significant factors influencing the stability of tunnel surrounding rock. Therefore, in practical engineering, it is crucial to establish the relationship between pretension torque and pretension force. However, current research in this field has some limitations. With a specific mine in western China as the engineering background, this paper first analyzed the influence factors of surrounding rock deformation and failure, established a mechanical model for pretension structure of anchor bolt support, and derived the quantitative relationship between pretension torque and pretension force based on hypothetical conditions. Finally, it proposed the loss effect of anchor bolt support and modified the pretightening loss coefficient for engineering application. The research results show that the actual pretightening force of anchor support is lower due to the influence of deep soft coal seam, which leads to failure of coal pillar under mining stress. The loss effect of surrounding rock on pretightening force was represented by a coefficient k, and the modified value of the coefficient was calculated as 0.19∼0.43. By applying the modified relationship between pretightening torque and pretightening force, it was found that the actual pretightening torque of 300 Nm applied to 11307 return air roadway effectively controlled surrounding rock deformation and failure, with coal pillar displacement less than 120 mm.

Differential roof cutting for roadway support in dual gob-side entry retention on a single working face − Multilevel continuous anchor-grouting control technology: A case study

The coal pillar-free gob-side entry retention technology (GER), which enhances resource recovery rates and reduces the amount of tunnel excavation, has been applied in multiple mining areas in China. However, cases of implementing gob-side entry retention on both sides of a single working face are rare. The asynchronous extraction of long-distance working faces leads to significant differences in the stress environment of roadways on both sides, and uniform technical means are poorly adapted to the control of surrounding rock on both sides, affecting the safe and efficient production. This paper, taking the dual gob-side entry retention on both sides of the 61001 W working face with thick mudstone roofs in Zhao Guan Coal Mine, Shandong Province, as the engineering background, utilizes a combination of theoretical calculations, numerical simulations, and on-site detailed monitoring to analyze the uncoordinated deformation characteristics of the upper part bulging and the lower part concaving of the two roadways. It clarifies the differentiated spatiotemporal evolution law of the number of fractures and the development turning points at different depths of the surrounding rock of the two entries, revealing the differential deformation mechanism of the surrounding rock of the two entries caused by stress state and fracture development differences. A “cutting roof to relieve pressure-multilevel continuous anchor-grouting” combined control technology system is proposed. By cutting off the stress transmission of the roof through drilling and blasting, and then reconstructing the integrity of the surrounding rock and improving the rock strength with the help of anchor-grouting. With the application of this new technology, the deformation of the surrounding rocks of the two entries was reduced by more than 60 % and 29 %, respectively, and the maximum crack depth of the roof was controlled within 3 m, significantly enhancing the stability of the roadways. The research results of this paper provide a reference for the control of surrounding rock in gob-side entry retention under similar conditions.

Anchorage performance of a new rebar bolt under different surrounding rock strength and borehole depth

The theory and technology of rock bolting are fundamental research topics for strata control in civil and mining engineering. Rebar bolts are commonly used for roadway primary support in underground coal mine. To adapt to deep resource mining, a new left threaded rebar bolt has been developed. Compared to conventional rebar bolts, the result of installation test showed that the new bolt reduced of 41.5% and 57.9% in stirring resistance force and torque, respectively. In laboratory pullout tests, PVC and aluminum sleeves were used to simulate weak and medium strength surrounding rocks. The average peak pullout force, displacement at the peak load and energy absorption increased by 27%, 107% and 108%, respectively, using PVC sleeve; and increased by 113%, 109% and 212%, respectively, using aluminum sleeve. Field tests were conducted under soft coal, hard coal and medium strength rock geo-conditions. Different borehole depths were selected to precisely calculate the average anchorage performance of the new bolt. Results showed that the average peak pullout force of the new bolt increased by 37%, 38% and 28%, respectively, under different surrounding rock conditions. Moreover, based on on-site test results, the pullout curves in field-testing were summarised and classified into 6 different patterns, which were discussed from a viewpoint of causality mechanism. The research findings validate that the newly developed bolt has better anchorage performance compared to conventional rebar bolts, making it a new anchorage material for deep resource mining.

Main factors that control roadway damage when a steeply dipping coal seam crosses a fault

When the working face of a steeply dipping coal seam crosses a fault, the dynamic disturbance caused by the mining load induces fault activation, causing large deformation and failure of the roadway. In this study, a physical similarity simulation and numerical calculation method were used to determine the main factors that the deformation and failure of fault zone roadway and reveal the underlying mechanisms. The results show that when crosses a fault, the high static load formed by the coupling of mining stress and fault pillar stress induces mining stress-type fault activation, which leads to large deformation and damage to the roadway in the fault zone. The main controlling factors are the fault location and mining load intensity. In terms of fault location, the vertical stress concentration away from the roadway side of the fault surface was high, the principal stress deflection angle was large, and the surface deformation of the fault on the roadway left side was the greatest. In terms of mining load intensity, the deflection angle of the principal stress of the surrounding rock initially increased and then decreased, and tensile failure easily occurred near the fault surface. These results provide a reference for roadway support.

Coupling support technique for coal roadway under double gobs in close coal seams

AbstractIn the process of lower coal roadway support under double gobs in close coal seams, there are problems such as the whole destruction of the lower coal seam and the intermediate strata, the thin roof of the lower coal roadway with the smaller support space, the upper rock of the lower coal roadway without anchor bearing layer, the roof falling, the anchor cable offing, and so on. On the basis of the engineering background of No. 11103 haulage roadway in Fenxi coal mine, the research methods of theoretical modeling analysis, numerical simulation analysis and field engineering test are comprehensively adopted. In this paper, the layout of the roadway with floor insertion in the lower coal seam, the distribution characteristics of surrounding rock deviatoric stress, the control principle of broken surrounding rock and the coupling support method of shed–cable–prop are systematically studied. The study shows that many measures are needed to ensure the stability of the surrounding rock in the strong mining roadway of the lower coal seam. The lower coal seam roadway section was modified from the original rectangular section to adopt an arched section to counteract the broken roof. The staggered position of the lower coal seam roadway layout was modified from the original coal roadway position to a coal–rock roadway, so as to avoid the area of high deviatoric stress in the coal pillar and the area of crushed coal body, and to leave an effective support space for the roof. The support scheme of the lower coal seam roadway was improved to use anchor cable and single prop to support the weak section of U‐type steel shed with local coupling, and realize unequal force coupling by adjusting the size of the force at different coupling points. In the field monitoring the deformation of the roadway is small, and the support effect is good.

Influence of advance direction on surrounding rock stability of main ramp under deep high ground stress

Due to the widespread use of trackless equipment in metal mines, the main ramp has become a major development roadway in large mines. Under deep high ground stress condition, the stability of the surrounding rock of the main ramp is subject to significant variability due to the change in the advance direction, leading to structural failure in different areas of the roadway. Because the support design scheme of surrounding rock of main ramp will not pay attention to the advance direction of roadway, leading to the key support area of the surrounding rock is not clear, resulting failure of the local supporting form occur frequently. Therefore, this paper adopts the combination of on-site investigation and numerical simulation to carry out the stability analysis of the surrounding rock under different advance directions of the roadway. Through on-site investigation, various failure modes of surrounding rock induced by the change of the advance direction of the roadway are summarized, and the key supporting area of surrounding rock of main ramp is determined according to the angle change between the roadway advance direction and the intermediate principal stress. The extent of the risk zone of the roadway surrounding rock is defined based on the Drucker-Prager criterion, which can be used as the new basis for the design of roadway support parameters. The results show that: As the angle between the advance direction and the intermediate principal stress increases, the degree of deformation of the surrounding rock decreases linearly, and the volume of the plastic and risk zones of the surrounding rock decreases nonlinearly. Based on the distribution of the strain energy density of the roadway surrounding rock, when the angle between the advance direction and the intermediate principal stress is small, the roadway surrounding rock is more prone to the floor heave phenomenon. When the angle between the advance direction and the intermediate principal stress is larger, the roadway surrounding rock is prone to the full-section failure phenomenon. Based on the block theory, when the advance direction is changed, the location and production of key blocks have significant variability, leading to the need for reasonable adjustment of the support parameters for the surrounding rock. The research results of this paper are of guiding significance for the optimization of the surrounding rock support scheme of the main ramp.

Mechanism of bolt breakage in deep mining roadway under dynamic load and advanced strengthening support technology

To reveal the problem of local instability caused by bolt breakage and failure in deep mining roadway under impact dynamic loads, and explore strengthening support technologies that can reduce bolt breakage and improve the stability of roadway support bodies. This article adopts theoretical analysis, numerical simulation, and on-site industrial experiments to conduct research, investigate and obtain the fracture failure characteristics of bolt in deep mining roadway, quantitatively characterize the deformation pressure of roadway surrounding rock and dynamic load of overburden fracture, establish the mechanical criterion of bolt fracture, and analyze the influence law of different anchoring factors on bolt force under static load and dynamic and static load combination. The results show that when the anchor angle is different, the range under dynamic load increases by 169.3 % compared with static load, when the anchor length is different, the range increases by 863.3 % relatively, and the influence is significant; while the anchor support resistance increases by 11.6 % at different times, and the range decreases by 41.5 % at different pre-tightening torque. Then, based on this criterion, the influence law of bolt failure on the stability of surrounding rock is analyzed, and the optimal support scheme is obtained by simulation research, that is, bolt cable +U steel support scheme. After adopting this scheme for support, the deformation of surrounding rock is reduced by about 60 %, and the maximum concentrated stress of surrounding rock of roadway is reduced by 15.0 %. Based on this, the support parameters of the on-site roadway were optimized and a strengthening support scheme was determined, which includes “roof bolt surface support reinforcement + anchor cable combined with U-shaped steel deep reinforcement, and two strong bolts advanced reinforcement”. The on-site strengthening support practice showed that the bolt fracture rate was reduced by 87.3 %, the deformation of the surrounding rock was effectively controlled, and the stability was significantly improved.

The Distribution Law of Ground Stress Field in Yingcheng Coal Mine Based on Rhino Surface Modeling

The distribution law of the ground stress field is of great significance in guiding the design of coal mine roadway alignment, determining the parameters of roadway support, and preventing and controlling the impact of ground pressure in coal mines. A geostress inversion method combining Rhino surface modeling and FLAC3D 6.0 numerical simulation software is proposed. Based on the geological data of the coal mine and the results of on-site measurements, a three-dimensional geological model of Yingcheng Coal Mine is established for the geostress inversion, and the distribution law of the geostress field in Yingcheng Coal Mine is obtained. Research shows the following: (1) The horizontal maximum principal stress values of the Yingcheng Mine are between 33.9 and 35.3 MPa, the horizontal minimum principal stress values are between 23.6 and 25.4 MPa, and the direction of the horizontal maximum principal stress is roughly in the southwest to west direction; (2) the three-way principal stress magnitude relationship is σH > σv > σh, indicating that the horizontal stress dominates in the study area, which belongs to the slip-type stress state; (3) The maximum principal stress of No. 3 coal seam is 33.1–34.8 MPa, the middle principal stress is 27.5–29.2 MPa, and the minimum principal stress is 17.3–22.9 MPa. Due to the influence of topography and burial depth, there is a phenomenon of stress concentration in some areas. By comparing the inversion values with the measured values, the accuracy of the geostress inversion is high, and the initial geostress inversion method based on Rhino surface modeling accurately inverts the geostress distribution pattern of the Yingcheng coal mine.