Influence of mining on floor water inrush in the h

2022-08-08
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The influence of coal mining on floor water inrush

floor water problem is very prominent and common in the production process of coal mines in China, involving most coal fields in China. It includes most late Paleozoic coalfields in North China and South China and individual Mesozoic and Cenozoic coalfields. It is rare in the world for its wide range, large amount of water and heavy harm. There are many factors that affect the water inrush from the coal seam floor, and the high-pressure water head is an important condition that causes the water inrush. The aquifer located in the coal seam floor can not be directly exposed in the mining face, in which there is impervious or weakly permeable rock stratum confined water at intervals. To enter the mining space, there must be a certain force to break through the impermeable layer or flush the cracks of the weakly permeable layer; Geological structures, such as faults and collapse columns, will reduce the strength of rock mass, weaken the water blocking capacity, and develop water inflow channels; Rock structure and lithological characteristics, such as the strength, integrity, water isolation and water resistance of rock mass; Mechanical action, such as the size and direction of the original rock stress, the redistribution of surrounding rock stress and the re expansion of structural plane caused by mining activities; Physical, mechanical and chemical effects of confined water on rock mass

1 main forces controlling water inrush from coal seam floor

there are many main forces affecting water inrush from coal seam floor. From the point of view of mechanics, they mainly include: ① the head pressure of the aquifer under the coal seam floor, including dynamic pressure and static pressure; ② Dynamic and static pressure of mine; ③ In situ stress; ④ The gravity of the relative water resisting layer between the coal seam floor and the underlying aquifer; ⑤ Rock mass strength relative to the water resisting layer

dynamic pressure includes the impact force of rock blocks on the floor when the roof falls, the vibration force of coal mining machinery or blasting, etc., which induce water inrush from the floor. Water head pressure and mine pressure play a role in destroying the bottom plate of the water resisting layer and promoting water inrush from the bottom plate; The strategy and strength of the waterproof bottom plate is to maintain the integrity of the bottom plate and restrain the water inrush of the bottom plate. When the binding force is enough to balance the force, the bottom plate will remain stable without floor heave and water inrush; Otherwise, disasters will not happen

2 the damage of coal seam mining to the floor and the formation conditions of water inrush

2.1 the damage of mining to the floor

(1) the change of stress state. When the working face starts mining from the open cut, the surrounding rock stress of the working face will change. With the advance of the working face, the front of the coal seam floor is compressed under the action of supporting pressure. After the working face is pushed, the stress is released and the bottom plate is in an expanded state. With the falling of the roof and the compaction of the falling gangue in the goaf, the floor at a certain distance behind the working face is restored to the original rock stress state. As the working face is in the process of continuous advancement, the bottom plate is in the state of compression expansion recompression. In the transition zone between compression and expansion deformation, the bottom plate is most likely to be damaged. Near the open cut hole, the vertical stress concentration is the highest, and the bottom plate is prone to deformation and cracks. The floor of the coal seam near the upper and lower exits is not only affected by the bearing pressure, but also by the static pressure. Therefore, the mining fissures along the floor near the upper and lower exits are relatively developed, which is easy to form water inrush points. While in the upper and lower exits and cut holes, the probability of water inrush is greater, and the investment of talent development funds of Chinese enterprises is greater. At the stoping line of the working face, because there is no influence of dynamic pressure, the deformation and damage of the bottom plate are small. Water inrush is unlikely

(2) re expansion of structural plane. Engineering Mining can cause the expansion of the original structural plane again, and comprehensive and multi-level energy-saving technological transformation should be carried out. Due to the differences in the physical properties of the structural plane itself, its hydraulic conductivity after expansion is different. According to the change of water inflow, the degree of mining induced fracture can be divided into five basic types: ① increasing type, the fracture is subject to the rock pressure stretched in the short axis direction and compressed in the long axis direction, and there is less filling; After being loosely stressed, the cracks gradually open and the water inflow gradually increases; ② Descending type, the crack is compressed in the short axis direction and stretched in the long axis direction to pull the ground pressure, so as to avoid large losses caused by the loosening of fasteners, and there are many fillings; After the stress, the crack is compressed, and the water inflow gradually decreases; ③ The fracture trend forms a certain angle with the direction of rock pressure action, the stress is uneven, there are many fillings, and the water inflow is sometimes large and sometimes small, but the overall trend is downward; ④ The fracture trend forms a certain angle with the direction of rock pressure action, the stress is uneven, the filling is less, and the water inflow tends to be stable behind the working face; ⑤ The fissure is subject to bi-directional compression of rock pressure, with less filling, basically no deformation after stress, and the water inflow is stable

(3) formation of mining fissure zone. Due to the action of mining ground pressure and water, within the mining failure depth of the floor, there are generally three kinds of cracks in the floor rock: ① vertical tension cracks, which are distributed in the uppermost part close to the coal seam floor, are tension cracks formed by the laminar tension failure when the floor expands; ② Layer wise cracks, the main rock layer appears in the form of separation, which is generally developed in the shallow part of the floor. It is caused by the floor being subjected to ground pressure during the advancement of the coal mining face, and the reverse displacement is weak along the layer. In recent years, China's plastic machinery enterprises have increased the opening of structural plane separation in the emerging market of extruders; ③ Shear fractures are generally divided into two groups, with reverse cross distribution at about 60 °. This is because the goaf, coal wall and goaf roof fall in the pressure area, and the rock stratum is formed by reverse stress shear

the above three kinds of fractures seriously damage the integrity of the floor rock mass. When they communicate with the aquifer (or confined water uplift zone or water diversion fault), water inrush from the floor will occur. The failure depth and degree of floor rock mass are mainly related to the size of stope ground pressure, the intensity of ground pressure appearance and the characteristics of floor. The former is affected by mining depth, direct roof thickness and mining height, coal seam inclination, working face size, roof lithology and structure, etc; The influencing factors of the latter include floor lithology, rock mass structure and structural characteristics

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