Impact pressure control

I. Mechanism of impact ground pressure Impact ground pressure, also known as rock burst, refers to the dynamic phenomenon of sudden and severe damage due to the instantaneous release of elastic deformation energy, often accompanied by coal rock mass throwing. Out, loud noise and air waves. It is very destructive and is one of the major disasters in coal mines. Before 1992, more than 50 coal mines in China had impact pressure. More prominent are the Beijing Mining Bureau Mentougou Coal Mine, Fushun Mining Bureau Longfeng Coal Mine, Zaozhuang Mining Bureau Taozhuang Coal Mine, Datong Mining Bureau Zhangzhou Kiln Coal Mine, Sichuan Tianchi Coal Mine and Xinwen Mining Bureau Huafeng Coal Mine.

(1) The impact of China's coal mine rockburst has the following characteristics: 1. Sudden. There is generally no obvious precursor before the occurrence, and the impact process is short, and the duration is from several seconds to several tens of seconds. 2, the general performance is coal explosion (coal wall burst, small block ejection). Shallow impact (occurring in the range of 2m ~ 6m in the coal wall, large destructive) and deep impact (occurs in the depth of the coal body, the sound is like thunder, the degree of damage is different). The most common are coal seam impacts, as well as roof impacts and floor impacts, and rock explosions in a few mines. In the coal seam impact, most of the performance is the coal block thrown, a few dozens of square meters of coal body movement, accompanied by huge sound, rock vibration and shock waves. 3. Destructive. It often causes the coal wall to help, the roof to sink, the bottom drum, the bracket to break, the roadway blockage, and casualties. 4. It has complexity. In natural geological conditions, except for lignite, the depth of mining is from 200m to 1000m, the geological structure is from simple to complex, the thickness of coal seam is from thin layer to extra thick layer, the dip angle is from horizontal to steep, and the roof includes sandstone and ash. Rocks, oil shale, etc., all have impact rock pressure; in terms of coal mining methods and coal mining technology, regardless of water mining, blast mining, general mining or comprehensive mining, the goaf treatment uses all fall The method or the hydraulic filling method is long-wall, short-wall, room-column mining or column mining, and all of them have impact pressure. However, the long-wall mining method without coal pillars has fewer impacts.

(2) Classification of impact ground pressure The impact ground pressure can be classified according to the stress state, the apparent strength and the different locations and locations where it occurs. 1. Classification according to the stress state of the original rock (coal) body (1) Gravity stress type impact ground pressure. Impact ground pressure caused mainly by gravity, without or with minimal structural stress. Such as the impact of ground pressure in Zaozhuang, Fushun, Kailuan and other mining areas. (2) Structural stress type impact ground pressure. It is mainly affected by the impact pressure caused by tectonic stress (structural stress far exceeds the self-heavy stress of the rock layer), such as the impact pressure generated by Beipiao Mining Bureau and Tianchi Coal Mine. (3) Intermediate type or gravity to structural type impact ground pressure. It is mainly caused by the combined impact of gravity and tectonic stress. 2. Classification according to the apparent intensity of the impact (1) ejection. Some individual fragments are shot from coal or rock mass under high stress conditions, accompanied by strong sound, which is a micro-impact phenomenon. (2) Mine shock. It is the impact ground pressure inside the coal and rock, that is, the deep coal or rock mass is destroyed. The coal and rock are not thrown into the occupied space, only the strip or collapse phenomenon, but the coal or rock mass produces obvious vibration. With a loud sound, sometimes coal dust. The weaker mines are called microseisms, also known as coal cannons. (3) Weak shock. Coal or rock is thrown into the occupied space, but the damage is not great, and there is basically no damage to the bracket, machine and equipment; the surrounding rock generates vibration, and the general magnitude is below 2.2, accompanied by a loud sound; coal dust is generated. There may be a large amount of gas escaping in the gas coal seam. (4) Strong impact. Some coal or rock is broken sharply, and a large amount of it is thrown out to the occupied space. There are bracket damage, equipment movement and surrounding rock vibration. The magnitude is above 2.3, accompanied by huge sound, forming a large amount of coal dust and generating shock waves. 3. Classification according to the magnitude of the earthquake and the amount of coal thrown (1) Slight impact: The impact ground pressure of the coal level below 10t and the magnitude below the level 1 is thrown. (2) Medium impact: The impact ground pressure of the first to second grades of the coal is between 10t and 50t. (3) Strong impact: The impact ground pressure of the coal level above 50t and the magnitude above 2nd level. When the surface wave magnitude Ms=1, some residents in the vicinity of the mining area have a sense of earthquake; when Ms=2, there are different degrees of damage to the upper and lower sides of the well; when Ms>2, the ground buildings will have obvious crack damage. 4. Classification according to the location and location (1) Coal body impact. Occurs in the coal body and is divided into surface, shallow and deep impacts according to the depth and strength of the impact. (2) Surrounding rock impact. Bottom slate occurs in the top layer, the bottom plate and top plate shock impact depending on the location.

(III) Mechanism of the cause of impact ground pressure The interpretation of the causes and mechanisms of impact ground pressure mainly includes strength theory, energy theory, impact tendency theory and instability theory. 1. Strength theory The theory holds that the condition of impact ground pressure is that the mine pressure is greater than the comprehensive strength of the coal body-surrounding rock mechanics system. The mechanism is that the harder top and bottom plates can clamp the coal body and hinder the deformation of the deep coal body itself or the coal body-surrounding rock junction (see Figure 1). The frictional resistance and lateral resistance parallel to the layer hinder the movement of the coal along the layer, making the coal body more compact, withstand higher pressure and accumulate more elastic energy. In the sense of limit equilibrium and elastic energy release, the clamping acts as a latch. In the coal body clamping belt, the pressure is high and a relatively high elastic energy is stored, and the high pressure belt and the elastic energy accumulation area may be located near the coal wall. Once the high stress suddenly increases or the system resistance suddenly decreases, the coal body can suddenly break and move, throwing it into the occupied space and forming the impact ground pressure. 2. Energy theory The theory holds that when the energy released by the mechanical equilibrium state of the ore body and the surrounding rock system is greater than the energy consumed by the damage, the impact pressure will occur. Rigidity theory is also an energy theory. It considers that the condition of impact ground pressure is: the rigidity of the mine structure (mine body) is greater than the stiffness of the mine load system (surrounding rock), that is, the energy stored in the system is greater than the damage and movement consumed. When the energy is applied, the impact pressure will occur. However, this theory has not been fully confirmed, that is, the impact ground pressure occurs under the condition that the surrounding rock stiffness is greater than the stiffness of the coal body. 3. Impact tendency theory The theory holds that the condition of impact ground pressure is that the impact tendency of the coal body is greater than the limit value determined by the experiment. Some test or measured indicators can be used to estimate or predict the extent of the impact of the mine pressure. The measure of this index is called the impact tendency. The condition is that the actual impact tendency of the medium is greater than a prescribed limit value. These indicators mainly include: elastic deformation index, effective impact energy index, ultimate stiffness ratio, and failure rate index. The above three theories propose three criteria for the occurrence of impact ground pressure, namely the strength criterion, the energy criterion and the impact tendency criterion. The strength criterion is the coal body failure criterion, and the energy criterion and the impact tendency criterion are sudden failure criteria. The establishment of the three criteria at the same time is the necessary and sufficient condition for the impact of ground pressure. 4. Instability theory In recent years, some scholars in China believe that according to the full stress-strain curve of rock, the ability of coal and rock to resist deformation (including cracks and cracks) is increased in the upper convex hardening stage, and the medium is stable. In the softening stage of the depression, the crack propagates rapidly and spreads due to the fact that the external load exceeds its peak strength, and the phenomenon that the microcracks are dense and connected causes the deformation resistance to decrease and the medium to be unstable. In an unsteady equilibrium state, once there is a small external disturbance, it may be unstable, so that a large amount of energy is released in an instant, and a sharp and violent destruction, that is, an impact pressure, occurs. Thus, the strength and stability of the medium is one of the important conditions for the occurrence of impact. Although the external load does not reach the peak intensity, due to the creep property of the coal rock, its deformation will increase with time and enter the softening stage. This static fatigue phenomenon can make the medium unstable. The energy released by the system during the instability process can change the coal rock from static to dynamic, that is, sharp and violent damage.

(IV) Influencing factors of impact ground pressure 1. Geological factors mainly include mining depth, geological structure, coal rock structure and mechanical properties. The increase in the depth of mining increases the value of the earth's stress. Generally, the impact ground pressure begins to occur after a certain mining depth is reached. This depth is called the critical depth of the impact ground pressure. The critical depth value varies with conditions, generally greater than 200 m. The general trend is that as the depth of mining increases, the risk of impact increases. This is mainly due to the increase in the original rock stress as the depth of the mining increases. Geological structures such as folds, faults, coal seam dip angles and sudden changes in thickness also affect the occurrence of impact ground pressure. The slanting axis is easy to form the impact pressure; if the fracture is a mining boundary, if the mining direction is toward the fault plane, the impact risk increases; the coal seam dip angle and the thickness of the sudden change zone are actually the local geological tectonic stress accumulation zone, so the pole It is prone to impact ground pressure. Coal rock structure and performance are also the main factors affecting the impact of ground pressure. Hard, thick layer, strong overall roof (old roof), easy to form impact ground pressure; direct top thickness is moderate, combination with old top is good, not easy to fall, impact risk is large; coal strength, elastic modulus Large, low water content, high degree of metamorphism, large proportion of dark coal, and generally strong tendency to impact. 2. Mining technology factors When mining multiple coal seams, any factors that cause stress concentration, such as unreasonable mining procedures, unclean mining in this layer, and unsuitable misalignment of adjacent two layers, are all unfavorable for the prevention and control of impact ground pressure. From the point of view of prevention and control of ground pressure, ç’§ mining is superior to column mining, dry mining is better than water mining, linear working surface is better than curved working surface, and the caving method is better than filling method. Coal pillars and mining boundaries are the most important stress concentration factors, and the harmful effects of these factors should be avoided and reduced as much as possible. A large number of practices at home and abroad have shown that impact ground pressure is often accompanied by certain processes in the downhole production process (such as blasting, roofing, coal mining, etc.). These factors are called induced factors. The energy of the inducing factor itself may be small, but the energy it emits by inducing the impact pressure is very destructive. Therefore, the inducing factor is also a non-negligible factor in the occurrence of impact ground pressure.

II. Prediction of impact ground pressure (I) WET method This method was proposed by the Polish Mining Research Institute to determine the coal seam impact tendency. WET is the ratio of elastic energy to permanent deformation energy consumption. The Polish Mining Research Institute stipulates that WET>5 is a strong impact tendency; 2

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