Evolution Law Of Plastic Zone And Burst Failure Mechanism Of Gateway In Yima Coalfield

With the increasing of China’s coal mining depth and intensity year by year,the frequency and intensity of burst dynamic disaster as one type of mutational disaster increases dramatically,and 85%of burst dynamic disasters occur in the roadway.The roadway burst failure mechanism and its prevention and control caused by burst dynamics pressure has become a major problem needs to be solved immediately,so that the coal mine can realize safe and efficient mining.In recent years,more than hundreds of roadway burst failure events occurred in the five coal mines(Qianqiu coal mine,Yuejin coal mine,Changcun coal mine,Gengcun coal mine and Yangcun coal mine)in the central Yima coalfield,resulted in thousands of meters roadway were damaged to varying degrees,.The roadway burst failure event occurred in 21141 working face headway in Qianqiu coal mine is most.Although many scholars have conducted a lot of studies on the mechanism of roadway burst failure occurrence and its prevention and control technology,there is still no unified understanding of the occurrence mechanism,so that the forecast and prevention and control technology of roadway burst failure is progressing slowly.This doctoral dissertation takes Qianqiu coal mine in Yima coalfield of Henan Province as the engineering background,the energy characteristics of coal under different loading conditions are analyzed,and takes the morphological characteristics of the plastic zone of roadway surrounding rock as the main line,the occurrence mechanism of burst failure occurred in gateway is revealed accordingly and the key influencing factors are obtained,based on the study of stress field characteristic of roadway surrounding rock,the evolution law of gateway plastic zone,and the impact of boundary load on the morphological characteristics of plastic zone,by way of site investigation,laboratory test and numerical simulation.The gained main conclusions and innovations are as follows.1.The burst failure characteristics of roadway and their distribution laws of Yima coalfield are obtained.(1)Due to the overburden layer with large thickness and the influence of thrust fault,the roadway in Yima coalfield is in a complex high-stress environment.Under the action of dynamic load factors such as mining disturbance,roadway repair,blasting within the roadway and so on,the roadway burst failure events are frequent.The main features of roadway burst failure are severe floor heave,sharp contraction of two sides,severe damage of supporting body,and even closure of roadway.The burst failure events of roadway is mainly occurred during mining of working face,and they occur at a large buried depth and are within the influence range of mining dynamic stress.According to the statistical results,108 burst failure events of roadway that occurred in Yima coalfield from 2006 to 2015,the total number of events with a location depth greater than 600m is 90,accounting for 83.3%of the total.The total number of events during mining period is 55 times,accounted for 50.9%of the total.The number of burst failure event occurs in Qianqiu coal mine is 41,which is biggest number among the five coal mines,63.4%of the events occurred during face mining and account for 58.5%of the accident total number of Qianqiu coal mine occurred in headway of 21141 working face.Through the analysis of precursory features of microseismic monitoring of the burst failure event occurred in headway of 21141 working face of Qianqiu coal mine,it is found that the maximum energy fluctuation of microseismic monitoring is not obvious before burst failure of roadway,and each sharp increase in energy is accompanied by the occurrence of burst failure event.(2)The acoustic emission signal energy characteristics of coal samples under different loading conditions are analyzed.Under different load-bearing states,the acoustic emission signal of specimens in the triaxial compression process has gone through three stages with time,namely the silent period,the explosion period and the post-peak release period.In the silent period,the original crack in the specimen is closed and elastic deformation occurs,and the overall acoustic emission ring count and energy are less.Most of the energy input by the press is converted into the elastic energy of the specimen.During the explosion period,the original crack in the specimen expanded and run through,gradually forming macroscopic crack.The acoustic emission ringing count and energy release showed an explosive growth.When the specimen reached the peak stress,the acoustic emission ringing count and energy release also reached the maximum.In the post-peak release period,the acoustic emission signal decreases or even disappears as the stress drops.Both loading rate and confining pressure have significant influence on the burst failure of specimens.When the confining pressure remains unchanged,as the loading rate increases,the number of acoustic emission events of the specimen gradually decreases,and the peak energy gradually increases,and the specimen damage is more serious.With the same loading rate,as the confining pressure decreases,the number of acoustic emission events of specimens increases gradually,and the peak energy also tends to increase gradually.The damage degree of specimens is more serious,and the acoustic emission events on the upper part of specimens are significantly higher than those on the lower part.Under certain conditions,both the loading rate and the confining pressure can induce the occurrence of high-energy events and lead to the burst failure of specimens.2.The characteristics of the mining stress field and the evolution law of the plastic zone of gateway in Yima coalfield are obtained.(1)Because of the influence of working face mining,the magnitude and direction of regional principal stress field of gateway will change.Along the axial direction of gateway,the maximum principal stress shows a trend of sharp increase and gradual decrease,and the decrease amplitude is smaller and smaller,and the distance from the peak value of the maximum principal stress to the working face is 15m.The angle between the maximum principal stress and x-axis increases with the increase of the distance to the working face and is close to the vertical direction.After the minimum principal stress reaches its maximum at about 25m away from the working face,it slowly decreases with the increase of the distance to the working face.Under the action of mining stress,the maximum size and direction of the plastic area of the gateway change obviously.Working face advancing to a certain position,the morphological characteristics of plastic zone at different distance from working face are different.With the decrease of the distance to the working face,gateway shoulders angle of the plastic zone extended to the deep,its shape by irregular gradually evolved into butterfly-shape,and influenced by the maximum principal stress,the butterfly leaf direction of plastic zone will deflect.The shape of the plastic area at a certain position gradually evolves into a butterfly-shape from the irregular shape along with the advance of the working face,and the direction of the butterfly leaves will also be deflected.In the process of 21141 working face advancing,the stress concentration "triangular area"is formed at the comer connecting the goaf of 21121 working face with the front of 21141 working face.When the advance distance of working face is 270m,400m and 700m respectively,contour lines of the maximum principal stress in front of the working face are approximately in the shape of "L","L+U" and "U" respectively.The contour line density of the minimum principal stress within 20m range before the working face is relatively large and tends to be stable after reaching the peak value.(2)When the load ratio is 1 and 1.5,as the vertical load increases,the plastic zone of gateway surrounding rock gradually approaches to the circular and elliptical shape from irregular shape.When the load ratio is 3,the plastic zone of gateway surrounding rock is distributed in butterfly-shape(or incomplete butterfly-shape).With the increase of vertical load,butterfly leaf of butterfly-shape plastic zone gradually expands to the deep part.When the vertical load reaches a certain limit value,the coal seam will be destroyed.3.Transient extension characteristics of butterfly-shaped plastic zone of gateway surrounding rock are found.Based on the relation curve(RPP curve)between the maximum size Rmax of plastic zone and the boundary load P1 and P3 of gateway surrounding rock,it is clarified that the plastic zone of gateway surrounding rock has two response characteristics:slow increase and sharp increase under diffrent stress conditions.There is a linear relation between the maximum size and vertical load in the non-butterfly plastic zone of roadway surrounding rock,while the maximum size of butterfly-shaped plastic zone has a positive exponential relationship with the vertical load.The RPP curve shows that the butterfly plastic area is extremely sensitive to the increase of vertical load.Under certain stress and surrounding rock conditions,the slight increase of vertical load will lead to the instantaneous expansion of the butterfly-shaped plastic zone.Only when the butterfly-shaped plastic zone appears in gateway surrounding rock,the instantaneous expansion of the plastic zone is possible,that is,the roadway burst failure.Furthermore,from the perspective of energy,the change characteristics of the elastic energy in the surrounding rock when the roadway burst failure occurs under the influence of external disturbance are analyzed.4.The burst failure mechanism of gateway in Yima coalfield is revealed.Under the influence of factors such as mining stress,the plastic zone distribution of gateway is uneven.Because of the induction of triggering events,such as influence of square,gateway repair,blasting and so on,the regional stress field of surrounding rock of the gateway will be suddenly changed,bi-directional load of gateway surrounding rock is obviously change accordingly which lead to butterfly-shaped(or incomplete butterfly-shape)plastic zone of surrounding rock in instantaneous expansion,and a plenty of elastic energy which is gathered in the body of coal will be suddenly released in the form of vibration,sound and coal and rock mass throw that stands for a dynamic phenomenon of explosive damage.5.The key factors influencing the burst failure of gateway in Yima coalfield are summarized.Both the magnitude of the principal stress and the strength of surrounding rock have significant influence on the morphology characteristics of gateway plastic zone,and the magnitude of the principal stress has more influence on the morphology characteristics of the plastic zone than the strength of surrounding rock.Under certain stress and surrounding rock conditions,when there is butterfly-shaped plastic zone in gateway surrounding rock,the increase of maximum principal stress and the decrease of surrounding rock strength will lead to the expansion of butterfly leaf of plastic zone and energy release.Under certain conditions,when there is no butterfly-shaped plastic zone in the gateway surrounding rock,the transient plastic zone of gateway surrounding rock will also be butterfly shaped when subjected to external disturbance.If the butterfly-shaped plastic zone expansion is instantaneous,it will induce roadway burst failure.When the strength of the surrounding rock decreases,the irregular shape of the non-butterfly plastic zone of the surrounding rock doesn’t change,and the roadway burst failure will not be induced.On this basis,the important role of roadway layout and large diameter drilling in the prevention and control of roadway burst failure are introduced.