Numerical Investigation on Influence of Two Combined Faults and Its Structure Features on Rock Burst Mechanism

With the increase in coal mining depth, engineering geological conditions and the stress environment become more complex. Many rock bursts triggered by two combined faults have been observed in China, but the mechanism is not understood clearly. The focus of this research aims at investigating the influence of two combined faults on rock burst mechanisms. The six types of two combined faults were first introduced, and two cases were utilized to show the effects of two combined faults types on coal mining. The mechanical response of the numerical model with or without combined faults was compared, and a conceptual model was set up to explain the rock burst mechanism triggered by two combined faults. The influence of fault throw, dip, fault pillar width, and mining height on rock burst potential was analyzed. The main control factors of rock burst in six models that combined two faults were identified by an orthogonal experiment. Results show that six combinations of two faults can be identified, including stair-stepping fault, imbricate fault, graben fault, horst fault, back thrust fault, and ramp fault. The particular roof structure near the two combined faults mining preventing longwall face lateral abutment pressure from transferring to deep rock mass leads to stress concentration near the fault areas. Otherwise, a special roof structure causing the lower system stiffness of mining gives rise to the easier gathering of elastic energy in the coal pillars, which makes it easier to trigger a rock burst. There is a nonlinear relationship between fault parameters and static or dynamic load for graben faults mining. The longwall face has the highest rock burst risk when the fault throw is between 6 and 8 m, the fault dip is larger than 65°, the mining height is greater than 6 m, and the coal pillar width is less than 50 m. The stair-stepping, imbricate, horst, and ramp fault compared to the other fault types will produce higher dynamic load stress during longwall retreat. Fault pillar width is the most significant factor for different two combined faults, leading to the rise of static load stress and dynamic proneness.

Study on Web Pillar Stability in Open-Pit Highwall Mining

When highwall mining technology is applied to recover large amounts of residual coal left under the highwall of a big open-pit mine, reasonable coal pillar width is the premise for maintaining the stability of web pillars. By adopting the numerical simulation method, the characteristics of the abutment stress distributions in the web pillars under different slope angles and mining depths are studied, and the function of the stress distribution in the web pillar is established. The relationship between the abutment stress and the ultimate strength of the web pillar under different widths is also analyzed and used in combination with the failure characteristics of the pillar yield zone to explore the instability mechanism of the web pillar. The retaining widths of the web pillars are determined. Based on the modeling results, a mechanical bearing model of the web pillar is established, a cusp catastrophe model of pillar-overburden is constructed, and the formula for the web pillar instability criterion is obtained. By analyzing and calculating the ultimate strength of the web pillar, the formula for calculating the yield zone width at both sides of the pillar is achieved. Using the instability criterion of web pillars in highwall mining, a reasonable pillar width can be deduced theoretically, which provides significant guidance on the application of highwall mining technology.

Substantiation of the parameters of open stoping mining method with abandonment of unrecoverable pillars

Relevance. Developing inclined ore bodies of low and uneven thickness using mining systems with caving results in great losses and dilution, deteriorating the conditions for ore gravitation. As a consequence, it becomes necessary to slice the barren rock in the lying side to form slopes with the desired angle. There are also characteristic ore losses at the dead end of the drawing drift independent of the ore body thickness, therefore acquiring a significant role under a lesser thickness. The height of the sublevels, limited by the ore body’s inclination, leads to a considerable amount of preparatory and development operations. The absence of actual in-process monitoring of useful component content in the ore drawing doses often leads to increased dilution rates due to the drawing of barren caved rocks of the previously worked overlying sublevel. Thus, the technological solutions search and their design parameters substantiation for the conditions of inclined ore bodies of low thickness is an urgent scientific and technical problem. Research aim is to substantiate the parameters of the open stoping mining which provides for leaving unrecoverable pillars which ensure the drawing of the maximum volume of pure ore under the cantilever of the hanging side rocks. Research methods. The work used a comprehensive research method based on search and design of technically rational options for geotechnology, their technical and economic assessment and mathematical modeling, and determination of stable parameters of mining system structural elements – the chamber span and the pillar width. Results analysis. The optimal variant of the open stoping mining system has been determined. In comparison with the basic technology of sublevel caving, the specific consumption of preparatory and development operations per 1000 tons of mined ore has been reduced by 34%, the cost of mined ore – by 12%, and ore losses and dilution – by 2 and 2.9 times, respectively. The stable parameters of the chamber span and pillar width have been established. Conclusions. The developed technology of sublevel open stoping with double chambers with frontal ore drawing using remote-controlled loading and hauling machines and subsequent caving of unrecoverable pillars of minimum width allows to significantly increase the efficiency of mining.

Study on Dynamic Behavior Law and Microseismic Monitoring in Stoping Process of Roadway with High Gas and Wide Coal Pillar

In order to study the dynamic characteristics and microseismic distribution in the mining process of roadway with high gas and wide coal pillar, combined with the two dynamic events of N2105 working face in Yuwu Coal Industry, theoretical analysis and field measurement research were carried out. According to the theory of structural mechanics and geomechanics, the causes of dynamic appearance are analyzed. Combined with the specific situation, the influence of mining depth, coal pillar width, gas pressure, and content on the dynamic performance is analyzed. Stress monitoring and microseismic monitoring are carried out on one side of coal seam. The results show that, with the increase of the mining distance, the backside roof of the goaf is prone to unbalanced fracture due to the lack of lateral stress, and the impact pressure generated is used for the reserved protective coal pillar behind the goaf, causing the floor heave of coal seam. The combined stress generated by the anticlinal structure below the working face interacts with the abutment pressure of the working face to produce superposition effect, which promotes the occurrence of dynamic appearance. The critical depth of rock burst in Yuwu Coal Industry is about 600m. The increase of coal elastic energy caused by roof subsidence is more uniform with the increase of coal pillar width. The decrease of gas pressure in coal seam promotes the rock burst disaster. The vertical stress of coal seam at one side of the working face shows different evolution characteristics along the trend and strike. The vertical stress of coal seam in the lateral range of 53 m is adjusted to different degrees and tends to be stable until 300 m behind the working face. The active microseismic area in the middle of the working face was located 50 m in front of the working face, and the microseismic activity continued to 30–50 m behind the working face. The active microseismic area at the side of the roadway was located 30 m in front of the working face, and the microseismic activity continued to 100–180 m behind the working face. The inflection point, where the stress in the elastic area of coal pillar increases sharply, corresponds to the active microseismic area, which indicates that the dynamic characteristics in the mining process of roadway with high gas and wide coal pillar are related to the distribution law of microseismic. This study has a certain guiding significance for optimizing the width of reserved coal pillar, monitoring the coal seam stress/microseismic, and understanding the dynamic disaster of coal and rock under complex conditions.

Research on the Reasonable Coal Pillar Width and Surrounding Rock Supporting Optimization of Gob-Side Entry under Inclined Seam Condition

In order to study the optimal coal pillar width and surrounding rock control mechanism of gob-side entry under inclined seam condition, the 130205 return air entry adjacent to 130203 gob in Yangchangwan No. 1 well is taken as a typical engineering background. By means of engineering background analysis, theoretical analysis based on inside and outside stress field, numerical simulation by FLAC3D software, and in situ industrial test and relevant monitoring methods, the optimal coal pillar width and surrounding rock control technology are obtained. The results show that the influence range of inside stress field is about 12.2∼12.8 m based on theoretical calculation result; under the influence of 10 m coal column, the overall deformation of the roadway is relatively small and within the reasonable range of engineering construction, so the width of the coal pillar along the return air roadway is set to 10 m which is more reasonable; the cross-section characteristics of special-shaped roadway lead to asymmetric stress distribution and fragmentation of surrounding rock, and then the asymmetric surrounding rock control technology under the coupling effect of roof prestressed anchor + high-strength single anchor cable + truss anchor cable support is proposed. The monitoring results of this support method are effective for the maintenance of gob-side entry, and the study conclusions provide new guidance for the surrounding rock control mechanism of gob-side entry under inclined seam conditions.

Dynamic Simulation Study on Protective Coal Pillar Width on Floor Roadway

Determination of reasonable protective coal pillar width has a great significance for the safety and stability of the coal seam working face and the surrounding structures and facilities. For studying the reasonable width of protective coal pillar, based on the platform of ABAQUS, program with the PYTHON language to make dynamic model which can simulate the dynamic changes of different width. Then the deformation law of floor roadway under different protection coal pillar width is analyzed. The finite element optimization analysis program is compiled based on MATLAB, and the minimum deformation of roadway and protective coal pillar is used as the optimization goal to make calculation for the dynamic model, and the reasonable width of protective coal pillar is proposed. Through the comparison between the simulation result and the field monitoring data, validity of the calculation result and rationality of the method are verified. The research results provide the reference for the setting of protective coal pillar for similar mining conditions.