The Research On Failure Characteristics And Disaster-causing Mechanism Of The Loaded Coal In Vibration

The dynamic disasters, such as coal and gas outburst and rock burst, occur suddenly, develop rapidly, have a large destructiveness, sweep a wide range and cause secondary accidents easily, which have undoubtedly became a great threat to the mine safety production. Doing research on the mechanism of these disasters is an important way to improve the prediction accuracy and reduce the accident rate and the degree of risk. The instability of coal and rock is the essential premise of the dynamic disasters. However in current academic circles, there are some different views about how to destroy the instability of coal and rock. This paper, based on a great deal of experimental data, proposes that static loads and vibration loads cause the instability of coal and lead to the dynamic disasters of coal and rock eventually.The occurrence of the instability in coal and rock can be divided into two categories. In the process of mining, because of the influence of the original stress field and the mining operation, the original mechanical balance is broken, the load transfers to a certain area, and the local stress is concentrated. When the mining stress field is superimposed on the original stress field and the stress is over the ultimate strength, it may directly cause the large range of destruction of coal and rock, and lead to the occurrence of structural instability under certain conditions. In some cases, although internal damage has occurred in coal and rock, it does not appear the large area of damage, stubbornly maintains a critical state and saves some energy. If it is disturbed by external dynamic disturbance, it will break this critical state and cause the instability of structure rapidly. Meanwhile, the internal energy will be released in short time and may lead to the occurrence of the dynamic disaster of coal and rock directly or with other factors(gas, etc.) coupled. These two types of instability are called spontaneous instability and disturbance instability. Statistics shows that, comparing with the spontaneous instability, disturbance instability has a large proportion and it is much more dangerous, so the research and discussion on disturbance instability is quite meaningful.The coal mine has a large amount of disturbance instability, such as blasting excavation, blasting down coal, drilling operation, mechanical vibration, roof fracture and adjacent working surface disturbance. It is obvious that the larger disturbance such as blasting operation and blasting down coal is easy to cause the instability of coal, which has been widely studied and identified. In fact, some dynamic disturbance seems little affected, but it can induce the occurrence of the dynamic disaster of coal and rock, e.g. drilling operation, mechanical vibration, picking coal with pneumatic, picking coal with hands, and even the disturbance from adjacent working surface. When the large disturbance occurs such as blasting operation, workers tend to be evacuated to the distant region, at which time, it takes a series of measures to protect workers. While the small disturbance "or even the" non disturbance "(" adjacent disturbance ") occurs, there are a large number of people in front of working face, and it will lead to very serious consequences if the disasters happen. More seriously, the smaller the dynamic disturbances was, the harder it would be to cause the attention of workers, and the more death after the disaster it would cause.In view of this, combining expriments of static load and cyclic dynamic load, this paper focuses on the analysis of "small disturbance", especially on how to cause the failure of coal and rock failure, and finally to induce the coal and rock dynamic disasters. The research results would help to replenish and perfect the occurrence mechanism of coal and rock dynamic disasters, improve the prediction accuracy of the prevention and control of coal and rock dynamic disaster, and provide the theoretical guidance and technical support for the applications of corresponding engineering prevention measures. In detail, the main contents and conclusions of this paper are as follows:(1) "Small disturbance" is an important inducing factor of coal rock failure instability. This kind of disturbance can be simulated by the vibration failure experiment of coal bearing.Theoretical analysis shows that there are four stages of the gestation, occurrence, development and termination of coal and rock dynamic disasters. In most of the cases, the coal and rock mass before the failure induced by dynamic disturbance has been in a critical state. Therefore, "small disturbance inducing damage of coal and the structural instability needs two premises: coal itself has been in a state of critical limit by the original force field and mining operations, the second is failure to lose stability is caused by multiple small disturbances. It needs a relatively long slow mechanical loading process prior to coal in the limit stress state, and the static load in the relatively slow breeding process often plays a leading role in the mechanics, therefore static load in the process of gestation can be analyzed by using uniaxial or triaxial compression, while lab simulations can be made on multiple effects of "small disturbance" with the aid of vibration. In view of this, this paper builds a static and dynamic combination(loaded coal mass vibration) damage coal experiment and simulation system, which focuses on the breakdown characteristics of briquette specimens under different loading conditions, and states the effect of the load on coal. With the evolution of vision algorithm, analysis of macroscopic cracks, by using high speed camera, summarized the distribution of coal surface crack. Combined with the characteristics of microseism and electromagnetic radiation signal in the destruction of coal crack evolution process, the whole failure process of briquette specimens was explored. Eventually integrated coal body surface and the overall damage state, it is put forward that combining static load and dynamic load of the loading process, the static and dynamic loads prompted the coal body to destruction and failure criterion of stability conditions.(2) The incident wave and the reflection wave are in phase when the vibration of loading coal is destroyed, and the interaction between the two leads to the destruction of the coal body.When loaded coal vibrates, test pieces left end surface is the leading role area of the pressure force, and right end is for tensile force. The two show similar symmetric distribution, and due to overlying static load it has produced a certain degree of offset. As a special kind of solid material, the tensile strength of coal is far less than the compressive strength, so in the test, the main failure form of coal is the tensile destruction, namely tensile stress plays a dominant role in the destruction of coal.In failure test of vibration, taking coal as the research object, because the acoustic impedance of the fixture(steel plate) is much larger than that of the specimen(coal), most of the incident wave energy will act on the coal in the form of reflected wave, the reflected wave and the incident wave phase have the same phase, energy is larger, and the interface between them occurs at the back of specimen. Eventually, the tensile stress incuded results in coal body tension failure. In addition, other than the incident wave with sustained energy source(vibrator), reflection wave in the propagation process decay faster, so the more far from the right end is, the smaller energy of reflection wave is, and the superposition of incident wave and a reflected wave energy is smaller, tensile force produced also decreases, while coal often produces crack in the maximum point of tensile stress, and then along the interface evolution, which is the main reasons that fracture of coal body distribute at the middle-rear part of the test piece.(3) When loaded coal mass is in vibration and damage, the larger sample size is, the samller density and molding pressure is, and the stronger the cracks of pixel values and acoustic signal energy is, which shows the destruction of coal is more serious, and is more likely to cause damageand instability.This paper builts the load vibration test platform of coal body, and simultaneous analysis of failure process of crack evolution and acoustic signal characteristic is made. The results show that the larger the granularity of coal briquette is, the larger seismic and electromagnetic radiation signal energy under the same disturbance conditions is, and the more pixels are, indicating that at this time the destruction of coal is more serious. When the size is the same, the grater density of coal preparation and forming pressure are, the smaller specimens fracture in the same disturbance condition is. According to the above description, size and molding pressure not only affects the physical and mechanical properties of briquette samples, but also reflects the ability in resisting damage of coal body in the same disturbance conditions. In the field practice, the smaller the particle size of the coal body is, the lower the degree of loose is, the less the fracture caused by the disturbance is, the less likely it is to be failure and instability. Specific to the characteristics of MS signals for single coal sample, it is found that the looser the coal body is, the smaller the molding pressure is, the larger number and smaller amplitude of MS signals are. The corresponding is when the molding pressure is increased, the emergence of microseismic signal time is more concentrated, and the amplitude is relatively large.In addition, compared with the statistical results of MS, EMR signals and crack pixels under same granularity, forming pressure and disturbance conditions, found that the energy and crack point of No. 10 Mine of Pingdingshan are higher than those of Tashan Mine, which indicates that soft coal is more prone to crack evolution under the same disturbance conditions, and easier to lead to the occurrence of failure and instability.(4) Large seismic signals appear consecutively and repeatedly, which is an important criterion for static critical damage of coal.Coal and gas outburst always occurs in soft coal or coal soft layer, while the rock burst often concentrates in relatively strong area of coal, resulting that the physical and mechanical parameters of the coal require an intensive analysis when researching coal and rock dynamic disaster. Therefore, this paper designed test on the consistence coefficient and the ultrasonic. The specific performance is when three or more large microseismic signals form "earthquake swarm mode, the cracks appear in coal surface. Thereafter, using hydraulic pump to load slowly, pressure gauge reading is no longer increasing, while the crack evolution speed is accelerating apparently. 10 seconds later, the macro cracks are throughout the coal surface, Jack pressure gauge reading decreases rapidly, and specimen has been destructed and loss bearing capacity.(5) Under different working procedures, the MS signals and EMR signals, tested local signals, are basically synchronous, and the energy of these signals are influenced by blasting explosive volume and the propagation distance. The RC oscillation circuit caused by P wave is the primary cause of the cluster tufted electromagnetic signal.At the practical section, in view of the homology between the micro-seismic and electromagnetic radiation signals, regarding tunneling faces and working face in No. 10 Mine of Pingdingshan as the research object, micro-seismic and electromagnetic radiation signal characteristics is revealed in the process of the tunneling blasting, mechanical cutting and drilling, and different denoising processes between micro-seismic signal and the electromagnetic radiation signal also is clarified, meanwhile it is found that sound signals show strong temporal synchronicity with electric signals of coal damage under different operation processes. In addition, treating the acoustic signals from explosive operations as the main research object, P-wave and S-wave were separated from the micro-tremor signals, and electromagnetic signals were divided into three types, i.e., single electromagnetic pulse section(Ⅰ), cluster section(Ⅱ) and low frequency oscillation(Ⅲ). The results show that all the energy of signals above have a positive correlation with blasting explosive volume and a negative correlation with the propagation distance. Based on the principle of homology, mechanism of acoustic signal correlation in coal and rock failure was discussed, indicating that the RC oscillator circuit may be the main cause of cluster section(Ⅱ) electromagnetic signals.