Reaserch On Mechanical System Of The Start Of Butterfly Shape Coal And Gas Outburst In Excavation Roadway

Coal and gas outburst is one of the most serious disasters in coal mines around the world,because of its mechanism and related basic theory have not been mature,so thatscientific prediction,prevention and efficient management have not been effectively studied for this major disaster,and has becominginternationalcentury problems for mining,rock mechanics and other disciplines.The mechanics of coal and gas outburst is the sudden destruction of the coal body,resulting in the huge elasticity of its internal storage and the power failure that can occur simultaneously in a short period of time.If there is not failure,then there will not be outburst.Besides,if there is not gas in the broken coal body,then there will not be outburst either,which has long been known for people.The scholars at home and abroad have made a lot of fruitful results around the explanation of the prominent phenomenon and the summary of the prominent laws,discussing the causes,conditions,energy sources and their development process.It is mainly possible to answer the question of what kinds of factors that contribute to sudden destruction of coal and sudden eruption of gas and so on.It can basically carry out the mechanism of coal and gas outburst.The main reason is that the theory of gas-dominated hypothesis,the hypothesis of ground stress,the hypothesis of chemical essence and the hypothesis of synergy.In this paper,the mechanical mechanism conjecture of butterfly coal and gas outburst in excavation roadway has been put forwarded,and the mechanical nature of butterfly coal and gas outburst has been analyzed by the methods of theoretical analysis and numerical simulation.The change rules of the plastic zone have been studied systematically,and the start-up incentives of the coal and gas outburst cases in the Mazhuang mine have been analyzed by using FLAC3 D numerical simulation software.The main conclusions and innovations are as follows:(1)According to the theory of butterfly roadway rockburst,the analysis model of coal and gas outburst process has been constructed.The mechanical model of the former plastic zone is mainly used to explain the ground stress field and the distribution of the plastic zone in the coal rock before the occurrence of coal and gas outburst.The mechanical model of the butterfly plastic deformation is mainly used to explain the occurrence of coal and gas and the distribution of the plastic zone in the coal and rock mass.The conceptual model of the trigger event mainly includes the types of the sudden changes of the plastic deformation range of the rock and the abrupt change of the stress field of the surrounding rock caused by the disturbance stress or geological weakening eventsin production practice.The concept model of prominent dynamic phenomena is mainly used to explain the dynamic effect of coal and gas outburst.(2)The mechanics mechanism of butterfly coal and gas outburst in excavation roadway is put forward,and the physical mechanics process of butterfly coal and gas outburst in excavation roadway is clarified as well.That is to say,to meet certain prerequisites,due to the role of triggering events,coal seam in front of coal and rock body instantly formed butterfly-shaped plastic damage zone.Meanwhile,the inner gas energy and huge of elastic energy in the broken area,the huge energy will contribute to break through the prominent hole in the neck of the resistance,thus result in the rock and gas outburst,and released by the forms of vibration,sound and rapid gas emission.Among them,"the instantly formed butterfly-shaped plastic destruction zone in front of coal and rock body ahead of coal roadway" is the coal and gas outburst formation of the key prerequisite factor.(3)The general morphological characteristics of plastic zone and the main influencing factors of plastic zone distribution have been analyzed.When the excavation roadway is located in the uniform stress field of the three-way pressure,the plastic zone of the surrounding rock is evenly distributed,and the plastic zone is approximately circular in the roadway section,and the plastic zone in head position of the excavation roadwayis hemispherical.In the non-uniform stress field,the plastic zone of the surrounding rock in the section of the roadway will show oval and butterfly shape.However,it is worth noting that the plastic field of the butterfly leaf will appear in the top and bottom corner of the excavation head as well.The plastic zone of the head position will show a semi-ellipsoidal or butterfly-like semi-circular distribution as a wholeaccording to the difference of principal stress field.Besides,the size,direction and strength of the main stress field have a significant effect on the morphology and extent of the plastic zone of the surrounding roadway.When the direction of the principal stress field is constant,the magnitude of the principal stress field paralleled to the roadway section has the greatest influence on the plastic zone morphology,while the influence of the principal stress field on the axial shape of the roadway is relatively small.In a certain condition of stress and surrounding rock,the butterfly plastic zone will come into being around roadway and surrounding rock,the butterfly-shaped plastic zone will deflect or even disappear with the change of the principal stress field,but no matter how the direction of the principal stress field changes,the butterfly plastic zone which is always distributed in the vicinity of the bisector of the principal stress field at the intersection of the roadway.(4)The basic characteristics of the butterfly plastic zone in the roadway have been found.The plastic zone of the butterfly leaf in front of the working face of the roadway is consistent with the basic characteristics of the butterfly plastic zone at the time of the rock burst.That is to say the excavation roadway in front of the butterfly plastic area also has a high degree of sensitivity,unlimited scalability and direction of rotation.When the structure and strength of the coal and rock mass in front of the heading face change,the plastic zone of the butterfly will have morphological variability as well.The high sensitivity indicates that the plastic zone of the butterfly leaves will change drastically with the change of the stress field around the roadway area,and the non-butterfly plastic zone will not be changed drastically.The infinite extensibility indicates that there is a critical stress state in the plastic zone of the butterfly leaves,so that the plastic zone can be infinitely extended.Directional rotation shows that the morphology and range of the butterfly leaves plastic zone changes with the rotation of the regional principal stress field.The morphological variability shows the effect of the strength distribution of coal and rock mass on the alienation of the butterfly plastic zone.(5)The necessary conditions and sufficient conditions for the start-up of butterfly coal and gas outburst in excavation roadway have been put forwarded.The necessary conditions for the start-up of butterfly coal and gas outburst in roadway include: strength conditions,stress conditions,angle conditions and triggering events.Besides,it can be simply summed up as four points: “the maximum confining pressure is large enough,the minimum confining pressure is small enough,coal and butterfly directions are consistent,triggering incidents are in coincidence”.When to meet the four necessary conditions at the same time,it will constitute the sufficient condition for the start-up of coal and gas outburst.(6)Key factors affecting coal and gas outburst initiation have been obtained,including disturbance stress triggering events and geological weakening triggering events.Under certain conditions,for the disturbance stress trigger event,the small disturbance stress can sometimes make the excavation work area in front of the plastic zone range have an instantaneous rapid expansion,and at the moment to form a huge energy gathered in the open “gas package”.Meanwhile,for the geological weakening trigger event,sometimes a slight weakening of coal and rock mass will be occurred ahead of excavation roadway working face,thus will result in a closed plastic zone area in the weak areas of coal and rock mass.Besides,owing to the increasing amount of fissures caused by the failure of coal and rock mass appeared around the closed plastic zone area,the adsorbed gas will be gradually desorbed,thus to form a closed “gas package” which contains huge amount of energy.The closed “gas package” will be broken under the conditions of the continuous pushing of work face or appeared slight disturbance of external part,the work face will be excavated through,and the stored energy will be quickly released as well,thus coal and gas outburst will be formed.While in actual production,the geological weakening trigger event may be more.(7)Based on the engineering background of YanMazhuang mine and the actual cases of coal and gas outburst,the numerical simulation software has been used to analyze the start-up incentives of coal and gas outburst,and the physical process of gas outburst have been reproduced as well.The results of case analysis show that the disturbance stress and the change of coal and rock mass have a direct influence on the start-up of coal and gas outburst.Besides,whether the disturbance stress can directly result in the start of coal and gas outburst or not depends mainly on the regional stress states and the strength of the rock mass in the surrounding rock of the roadway before and after the disturbance stress.