Study on failure characteristics and control technology of roadway surrounding rock under repeated mining in close-distance coal seam

In this study, taking the Sheng’an coal mine as an engineering background, the failure characteristics of the surrounding rock of a roadway under repeated mining in a close-distance coal seam is comprehensively illustrated through field measurements (e.g., drilling imaging), theory analysis and numerical simulation (finite difference method (FDM)). The results show that although the return airway 10905 remains intact, the apparent failure of the roadway’s roof and the coal pillar can be observed. In addition, the expression of floor failure depth caused by upper coal seam mining is obtained through elastic-plastic theory. Meanwhile, the deformation of the surrounding rock of the roadway increases with the increase of repeated mining times, especially for the horizontal displacement of the roadway on the coal pillar side. Moreover, the cracks’ evolution of surrounding rock in the roadway can be observed as asymmetric characteristics. Finally, the stability control technology of “asymmetric anchor net cable + I-steel” is proposed to prevent potential mining disasters, and the feasibility of this support scheme is verified by numerical simulation and field practices. It can meet the requirement of safe mining and provide guidelines to effectively solve the failure of a roadway in close-distance coal seam mining

The breaking characteristics of thick-hard roof and determination of support capacity in fully mechanized caving face

It is difficult to determine the first roof weighting interval of hard roof on fully mechanized caving face.Based on the working face 8301 of Dongzhouyao coal mine, a model of two edges fixed in thick and hard roof was established by using energy method.According to the model, the horizontal stress distribution in the main roof and the analytic formula for the first roof weighting interval of the main roof were obtained.The characteristics of roof breaking and working resistance of the support were analyzed by using numerical simulation.And the working resistance of support in different positions of the working face under different advance distance was attained based on field observation.The results showed that mine pressure behaviors were more intense under the condition of thick and hard roof, with larger the first roof weighting interval and higher working resistance of support as compared to general conditions.To be specific, the first roof weighting interval was about 58 m in working face 8301, while the periodic roof weighting length ranged from 15 to 20 m.The rated working resistance of support was 13, 000 kN, but the observed maximum working resistance was 11 000 kN, only 78.7% of the rated value.It is believed that safety and efficiency of coal mining under the condition of thick and hard roof can be achieved

Research on Vibration Propagation Law and Dynamic Effect of Bench Blasting

To address the problem of damage to adjacent buildings (structures) caused by bench blasting construction, blasting in a sand and gravel mine in Guizhou Province was used as the background. Through on-site monitoring and numerical simulation, the blasting vibration propagation law and dynamic effect characteristics under the joint action of different bench heights and horizontal distances were studied. The regression model was established. The results show that: the peak vibration speed in all three directions with the increase in the horizontal distance of the burst center is a decaying trend, and the field measurements are basically consistent with the safe vibration speed and do not exceed 1.5 cm/s, so the house is in a safe state; shear stress with the increase in the horizontal distance of the burst center strictly decays, so the source of the shear stress and vibration speed decay faster in the near zone, with the slow decay in the far zone; analysis found that the shear stress and vibration speed are quadratic and exponential. Through the analysis of the regression model, it is obtained that there is no co-linearity among the influencing factors, which has a significant effect on the regression equation and regression coefficient, and so the multiple linear regression equation fits well. The model can predict the blast vibration intensity, which can be used as a safety criterion for buildings under the action of blasting, and provides a reference for blast vibration control, hole network parameters, and the design index

Optimization of drilling parameters for coal seam gas extraction considering fluid–solid coupling and field application

Abstract Affect gas extraction efficiency to find out the optimum well location parameters, based on the gray correlation analysis of borehole diameter, borehole spacing, and extraction pressure three boreholes parameters that influence efficiency of gas extraction from the design of orthogonal experiment, using COMSOL simulate the effective radius of extraction under different experiments, and the effective radius of extraction for optimizing index quantitative gas extraction efficiency. The results showed that: factors affecting the gas extraction efficiency of the correlation between high and low aperture drilling, borehole spacing, and extraction from extraction negative pressure, it is concluded that the influence parameters of optimal layout parameter values, respectively, borehole diameter 8 cm and 3 m borehole spacing, extraction pressure 50 kPa, combined with the numerical simulation analysis using the optimal layout of gas drilling extraction from about 30 days. The decrease in gas pressure is about 45%, which basically meets the extraction standard, and the decreasing trend of gas pressure is obviously slowed down when the extraction time is longer than 30 days. The engineering experiment is carried out in the 9306 transport lane of a mine in Guizhou Province, and the results are basically consistent with the simulation results, which verify the rationality of the gray correlation analysis method to optimize the drilling parameters, and provide a theoretical basis for the optimization of drilling parameters

Failure analysis and prediction of roof instability in end face under repeated mining using early warning system

Abstract The overlying strata of the lower coal seam is easy to be collapsed causing the roof caving accident at the end face of the mining working face under repeated mining in close-distance coal seams. In order to predict the roof instability of the end face, the mechanical model of the granular arch structure is established in this study to further analyze its main influencing factors. The results show that the mining height of the working face, the advancing speed, the distance of coal seams, the tip-to-face distance, the strength of the surrounding rock and the support setting the load of the support are the main influencing factors on the roof caving of the end face. Subsequently, the prediction model of roof instability in the end face under repeated mining is constructed through the radial basis function neural network (RBFNN) and the above main influencing factors are regarded as input layer indexes. Meanwhile, the roof subsidence, coal wall deformation and support load are determined as the output layer indexes. The predicted results are closer to the results of sample tests. Finally, the early warning system, including monitoring and early warning, data query, emergency management, user management, and system settings, is designed to monitor roof conditions of the end face and timely warn the roof accidents. The field application proves that the system has good practical value, which is of great significance to intelligent prediction of coal mine stope disaster and prevent the end face roof disaster under repeated mining and. This will promote the safe and efficient construction of coal mine production

Comprehensive Identification of Surface Subsidence Evaluation Grades of Mines in Southwest China

Due to their complex geological structure, it is difficult to systematically analyze the surface subsidence of coal mines in southwest China, and the factors that cause surface subsidence are also different from other coal mines. Focusing on the problem of surface subsidence caused by mining in southwest China’s mines, a grade evaluation system for surface subsidence of southwest mines is constructed based on the analytic hierarchy process, and ten evaluation indicators are established from the perspectives of mining disturbance and geological structure. A matter–element model of surface subsidence based on matter–element extension theory and a cloud model of surface subsidence based on cloud theory are then constructed. A coal mine in Anshun, Guizhou, is taken as an example to calculate the evaluation level of surface subsidence and thus verify the scientificity of extension theory and cloud theory. The results show that the main factors that affect the surface subsidence of southwest mines are the number of coal seam layers, mining height and comprehensive Platt hardness of rock, similar to that of northern plain coal mines. Surface slope and subsidence area are also very important. The comprehensive correlation degree of each grade of the coal mine is −0.29836, 0.192232, −0.1093 and −0.46531, and the coal mine is concluded to be in grade 2. The calculated similarity of the overall index evaluation cloud map of the coal mine and each grade is 0, 0.3453, 0.7872 and 0, respectively. The coal mine is in grade 2, which is a relatively safe state. Consistent with the calculation results of the extension model and in line with the field situation, the extension matter–element model and cloud model built in this paper can verify each other and have a certain scientificity

Instability Control of Roadway Surrounding Rock in Close-Distance Coal Seam Groups under Repeated Mining

In order to solve the problems of roadway stability and easy instability under repeated mining of close-distance coal seam groups, the mechanism and control technology of surrounding rock instability under repeated mining were studied via indoor testing, field testing, physical similarity simulation experiment, and numerical simulation. The results show that the surrounding rock of roadway has low strength, low bearing capacity, and poor self-stabilization ability, and it is vulnerable to engineering disturbances and fragmentation. Affected by the disturbance under repeated mining, the roadway surrounding rock cracks are developed and the sensitivity is strong, and it is prone to large-scale loose and destroyed. The location of the roadway is unreasonable, and the maximum principal stress of the roadway is 3.1 times of the minimum principal stress, which is quite different. Thus, under a large horizontal stress, the surrounding rock undergoes long-range expansion deformation. On the basis of this research, the direction and emphasis of stability control of roadway surrounding rock under repeated mining of coal seam groups in close-distance are shown. A repair scheme (i.e., long bolt + high-strength anchor cable + U-shaped steel + grouting) is proposed, and reduces the risk of roadway instability

The influence of job satisfaction, resilience and work engagement on turnover intention among village doctors in China: a cross-sectional study

BACKGROUND: As the gatekeepers of rural residents’ health, teams of village doctors play a vital role in improving rural residents’ health. However, the high turnover of village doctors, both individually and collectively, threaten the stability of village medical teams. This research evaluated the influence of job satisfaction, resilience, and work engagement on the village doctors’ turnover intention, and explored the mediating role of work engagement and resilience between job satisfaction and the turnover intention of village doctors in China. METHODS: A quantitative study using a self-administered questionnaire containing mostly structured items was conducted among village doctors with a sample size of 2693 from 1345 rural clinics in Shandong province, China, during May and June 2019. All variables including demographic characteristics, job satisfaction, resilience, work engagement and turnover intention were based on available literature, and measured on a 5- or 6-point Likert scale. Such statistical methods as one-way ANOVA, bivariate correlation, exploratory factor analysis (EFA), and Structural Equation Modelling (SEM) were used. RESULTS: Up to 46.9% of the subjects had a higher turnover intention and more than 26.3% of them had a medium turnover intention. The job satisfaction of village doctors could not only have a direct negative effect on turnover intention (β = − 0.37, p < 0.001), but also have an indirect effect through work engagement (β = − 0.04,=< 0.001). Meanwhile, work engagement also had a direct negative impact on turnover intention (β = − 0.13, p < 0.001), and resilience had an indirect negative impact on turnover intention through work engagement (β = − 0.09, p < 0.001). The above results of this study strongly confirmed that job satisfaction, resilience, and work engagement were early, powerful predicators of village doctors’ turnover intention. CONCLUSIONS: According to the results, the following should be taken seriously to improve job satisfaction: reasonable and fair income, effective promotion mechanism, fair social old-age security, reasonable workload, and strong psychological coping mechanisms for work stress. The turnover intention of village doctors could be reduced through improving job satisfaction, resilience and work engagement

Comprehensive Identification of Surface Subsidence Evaluation Grades of Mines in Southwest China

Due to their complex geological structure, it is difficult to systematically analyze the surface subsidence of coal mines in southwest China, and the factors that cause surface subsidence are also different from other coal mines. Focusing on the problem of surface subsidence caused by mining in southwest China’s mines, a grade evaluation system for surface subsidence of southwest mines is constructed based on the analytic hierarchy process, and ten evaluation indicators are established from the perspectives of mining disturbance and geological structure. A matter–element model of surface subsidence based on matter–element extension theory and a cloud model of surface subsidence based on cloud theory are then constructed. A coal mine in Anshun, Guizhou, is taken as an example to calculate the evaluation level of surface subsidence and thus verify the scientificity of extension theory and cloud theory. The results show that the main factors that affect the surface subsidence of southwest mines are the number of coal seam layers, mining height and comprehensive Platt hardness of rock, similar to that of northern plain coal mines. Surface slope and subsidence area are also very important. The comprehensive correlation degree of each grade of the coal mine is −0.29836, 0.192232, −0.1093 and −0.46531, and the coal mine is concluded to be in grade 2. The calculated similarity of the overall index evaluation cloud map of the coal mine and each grade is 0, 0.3453, 0.7872 and 0, respectively. The coal mine is in grade 2, which is a relatively safe state. Consistent with the calculation results of the extension model and in line with the field situation, the extension matter–element model and cloud model built in this paper can verify each other and have a certain scientificity