No abstract

In the process of top coal caving mining, the caving time and the tail beam swing angle have an important impact on the evolution of the coal gangue boundary, which determines the top coal recovery rate and gangue content rate. Therefore, it is necessary to study the influence of the caving time and the tail beam swing angle on the top coal caving effect. For this purpose, based on Hertz–Mindlin contact theory, this paper established a simulation model of top coal caving and carried out a coal caving simulation with different caving times and tail beam swing angles. In the process of coal caving, under a specific tail beam swing angle and different caving times, this work studied the evolution characteristics of the coal gangue boundary. A simulation model of coal caving with variable time is established, and the influence mechanism of time on the coal caving effect is analyzed. Then, the evolution characteristics of the boundary are studied under different tail beam swing angles and the same caving time. A simulation model of coal caving with variable angles is established, and the influence mechanism of the tail beam swing angle on the coal caving effect is analyzed. Finally, the effects of caving time and tail beam swing angle on coal gangue discharge are studied, and the effect of coal gangue release under different working conditions is analyzed. The boundary data and the discharge data are extracted through the simulation, and contrast curves are drawn. The results show a limit position of the coal gangue boundary under the fixed working condition, which is the middle line position of the coal caving outlet. When the coal gangue boundary does not reach the limit position, there are two characteristics. First, the greater the tail beam swing angle is, the faster the deformation speed of the coal gangue boundary is. At the same caving time, the position of the coal gangue boundary will be closer to the coal caving outlet, which leads to excessive mixing of coal and gangue. Second, the longer the drawing time, the greater the deformation of the coal gangue boundary, and the easier the coal gangue is mixed, resulting in an increase in the gangue content of the discharged coal. When the coal gangue boundary reaches the limit position, the boundary will be basically stable near the top of the coal caving outlet. The limit position and shape of the coal gangue boundary are mainly affected by the density difference of coal gangue particles but not by the coal caving time and the tail beam swing angle. After the end of coal caving, the coal gangue boundary finally presents a double arch structure. The research in this paper has important reference significance for the selection of caving time and tail beam swing angle and further research.

Top coal caving is an important way of thick coal seam mining. The current coal gangue identification technology is not mature, resulting in a low degree of automation of coal caving. The current numerical simulation pays little attention to the real shape of rocks. This paper aims to reveal the vibration response of the tail beam under the action of real shape coal gangue particles. First, the real shape rock and hydraulic support model are established, and the relationship between the Rock Mixed Ratio (RMR) and the vibration signal characteristics during the impact process are studied. The influence of falling velocity and coal strength on this relationship are analyzed. Finally, the influence of motion mode on this relationship is analyzed and discussed. The numerical results show that the increase of the RMR can significantly enhance the velocity and acceleration signals of the tail beam, and the signals caused by different RMR are still different under different velocities and strengths. The best recognition effect can be obtained when the coal gangue particles slip on the hydraulic support, and the effect is the the worst when the coal gangue particles impact upon and roll on the hydraulic support. The conclusion provides directions for further study of coal gangue identification based on vibration.

The water-rich red sandstone strata at the Lanzhou Metro site area have special engineering properties and vary greatly in their speed of disintegration when exposed to water. There is an urgent need for a comprehensive and systematic study of the engineering properties of red sandstone and their classification. From the disintegration speed of red sandstone encountered during the excavation of Lanzhou metro lines 1 and 2, the relationship between physical parameters such as particle size, composition, dry density, and permeability coefficient as well as mechanical parameters such as shear wave speed, dynamic penetration test (DPT), natural uniaxial compressive strength, and disintegration speed of red sandstone was analyzed through indoor and outdoor tests and geological exploration data statistics, and classification guidelines for red sandstone are given. The results show a significant correlation between dry density, permeability coefficient, natural uniaxial compressive strength, and disintegration speed. The red sandstone can be classified as I, II, and III according to the disintegration speed, dry density, permeability coefficient, and natural uniaxial compressive strength. The design of the foundations is differentiated according to the classification, and different support systems are used for the deep foundation pits of the metro stations. The category I red sandstone pit is supported by diaphragm walls with internal bracing, the category II pit by bite piles with internal bracing, and the category III pit by row piles with internal bracing. The study results can provide technical support and experience reference for the investigation, design, and construction of metro projects in similar red sandstone distribution areas.

In top coal caving mining, the coal rock collapse will cause an irregular impact on the tail beam jack of the caving control mechanism. The severe impact will lead to jack failure. The bidirectional fluid-structure coupling model is built on Fluent and Mechanical software to study the impact response of the tail beam jack. The dynamic flow velocity streamlines, hydraulic pressure distribution, stress field, and strain field of the jack under impact load are extracted. The response characteristics of the jack in the stationary state and motion state are analyzed. The conclusions are as follows: the stress and strain of the rodless cavity are much larger than those of the rod cavity, which is more likely to be damaged. The hydraulic pressure in the jack cavity is in vertical layered distribution. The flow velocity streamlines present spiral shapes. The response degree of the hydraulic pressure signal in the rodless cavity is stronger than that in the rod cavity, and the response degree of the flow velocity signal in the rod cavity is stronger than that in the rodless cavity. The impact response of the jack in the motion state is more sensitive and stronger than that in the stationary state. The coal rock collapse situation can be most effectively identified only by comprehensively analyzing the rodless cavity’s pressure signal and the rod cavity’s velocity signal. This paper innovatively visualizes the flow velocity streamlines and pressure distribution together. The bidirectional fluid-structure coupling method is innovatively applied to the tail beam jack. The findings of this study can help for better understanding of the tail beam jack’s structural design and failure prevention. This study provides a certain research basis for the intelligent coal rock identification technology in mining coal based on jack vibration signals.