Study on Establishment of Prediction Model and Identification System for Outburst Risk of Coal Roadway Working Face
Author:Wang Chao Jie
Supervisor:yang sheng qiang jiang cheng lin
The working face of coal roadway is the most frequent location for underground accidents.It is extremely important for coal and gas outburst(hereafter,referred as ?outburst‘)control to carry out the work of outburst occurrence mechanism and outburst prediction system.When carrying out outburst prediction for an actual coal roadway working face,the accuracy and credibility of the prediction results depend on the rationality of the prediction index selection and the reliability of the prediction index critical value,the scientificalness of the prediction boreholes layout,and the accuracy of hazard identification of dynamic phenomena in prediction boreholes.Based on the theory of rock fracture mechanics and gas seepage theory,in the paper the theoretical analysis,numerical simulation,laboratory test and engineering application test are used to analyze the causes of the influence factors of outburst risk in a coal roadway working face;then,based on the analysis of the critical conditions of stress required for coal masses failure under different stress loading modes,a theoretical model of outburst prediction can be presented to characterize the maximum risk level of coal roadway working face.Meanwhile,the platform of prediction index measurement of simulated coal roadway working face is set up;based on the platform combined with the theoretical analysis of the influencing factors weight of the drilling cutting weight measurement,the laboratory test measuring the drilling cutting weight is conducted.The reliability of the platform is tested,which shows the superiority of the outburst prediction model.On the basis of the test,the linkage system between borehole dynamic phenomena(e.g.gas and coal being ejected from the borehole(GCEB)and drill pipe being stuck by coal mass in the borehole(DPSC))and their direct connection with the outburst risks are revealed.Based on the prediction index called ―initial gas emission from borehole‖,the critical value delimitation system for initial gas emission from borehole is established.Additionally,the layout pattern of outburst prediction boreholes in coal roadway is quantified.Finally,with a measurement system of initial gas emission from borehole used in the field test the engineering applications is successfully implemented.The main conclusions can be drawn as follows:(1)The instability failure mode and critical stress conditions of coal-rock mass in coal roadway working face change with different stress loading modes.Simplifying the state of stress during the sudden exposure of coal and rock mass in the coal roadway as confining stress with the hypothesis of gas leak-free in the exposed coal rock mass,the coal-rock mass has the highest level of outstanding risk.Based on the condition,the predictive index measurement platform is developed,with which the obtained prediction index critical value discriminating the outburst risk can achieve the discriminating non-outburst risk accuracy rate of 100%.(2)Based on the laboratory-constructed outburst prediction index measurement platform simulating coal roadway working face,the drilling cutting weight measurement experiment is carried out,and the critical values to determine outburst risks of the drilling cutting weight are determined according to that of the initial expansion energy for releasing gas.The prediction accuracy of the outburst risk is 93.75%,which is far greater than the accuracy judged by the critical value provided in the ―Provisions on Prevention of Coal and Gas Outburst‖.It shows the superiority of the developed outburst prediction platform for the working face of the coal roadway.(3)The higher the outburst risk level of the coal roadway working face is,the greater the occurrence frequency of prediction borehole dynamic phenomena.The occurrence of GCEB phenomenon indicates that the coal mass in the predicted area has outburst risk,and the drill bit being prevented from advancing(DBPA)phenomenon can be regarded as a precursor to the occurrence of GCEB.The reasons for the occurrence of DPSC phenomenon are diverse,and the phenomenon of DPSC which is characterized by the outburst risk of coal mass is generally accompanied by the phenomenon of GCEB,which are triggered successively constituting a linkage system.(4)The ?initial gas emission from borehole‘ is a comprehensive outburst risk prediction index of the reactive in situ stress,gas pressure and coal properties.When in the laboratory to obtain the critical value applied in the field guidance,the most influencing factors are the gas pressure of the simulated coal seam and the drilling speed of the borehole.(5)The opening location distance of prediction borehole from roadway wall is directly proportional to the control range of roadway and the predicted lead distance,and inversely proportional to the predicted depth.When the prediction borehole distance is 0.5 m,it can be ensured that the working face continuing to advance can enter the effective prediction band formed by a new round of forecasting,after a new round of outburst prediction under different prediction conditions,when the driving distance reaches the predicted lead distance left by the previous round of prediction.(6)As for a working face of coal roadway without obvious soft coal stratification having tectonic soft coal,the prediction borehole should be evenly arranged within the control coal mass range,and the width of the borehole maintained in the final borehole section should be 4~6m;meanwhile,the height should be based on the general scope of geological structure which has been explored.The prediction borehole arrangement can be further adjusted based on the predicted outburst amount of coal that may be ejected from the coal roadway working face after the prediction borehole arrangement.The paper has 93 figures,29 tables,and 222 references.