Study On Temporal And Spatial Evolution Law Of Electric Potential Response Of Damage And Failure Of Gas-bearing Coal

Coal resource has been still the main energy for a long period in the future,and it is aslo the guarantee of energy security in China.With the contunous reduction of shallow resources,the mining depth and intensity are gradually increaseing,meanwhile the mining stress,gas pressure and gas content increase.It results in more severe and complex dynamic disasters such as coal and gas outburst.The prection and forecast of dynamic disaster are more and more difficult and laborious,which seriously threatens the safety and efficient production of coal mining.The study on the precursory characteristics and accurate prediction methods of coal and gas outburst is an urgent scientific problem to be solved.Previous research achievements indicate that the electric potential(EP)signal has the advantages of strong sensitivity to stress and fracture process of coal and rock,and good anti-interference ability.According to the application demand of monitoring and farecasting of coal and gas dynamic disasters in the mining field,this paper closely focuses on the scientific problems of spatiotemporal evolution law of EP response during the process of damage and destruction on gas-bearing coal.The methods of experimental research,theoretical analysis,field verification are adopted to analyze the spatio-temporal response law of EP,response mechanism of EP,and EP-damage evolution constitutive model.Further,the law of EP distribution and evolution in driging face and the fine identification method by EP mearuring are studied.The main results are as follows.The experimental system of loading and EP measuring of gas-bearing coal is established,and the temaporal response characteristics and spatial distribution law of EP under the coupling effect of loading stress and gas impact are studied.The results indicate that:(1)There is the positive correlation between the EP response and the load and the damage status of gas-bearing coal specimen;when the local damage of the specimen is severe,a"step"mutation occurs in the EP intensity,which is most significant when the specimen failures.With the increase of gas pressure,the peak value and coefficient of variation of EP intensity increase,and the EP response is promoted.(2)The temoral series of EP data present multi-fractal characteristics and critical slowing characteristics.The mul-tifractal parameters??,?f_m and variance V can be utilized as the temoral precursor information to monitor the instability and failure of gas-bearing coal.Before the main fracture,the mul-tifractal spectrum parameter??suddenly increases to the peak value,while?f_m decreases to the negative value rapidly,and the variance also increases to the peak value.(3)In the area with severe damage on coal specimen,the cloud map of EP spatial distribution shows local high-value center and isoline dense features,which is corresponding to the inversion results of acoustic emission(AE)moment tensor(fracture type,spatial distribution and motion vector)and physical photograph of crack growth as verification results.The connection line of the high value center is the penetration zone of the fracture zone that induces coal fracture.The anomalous evolution law of the spatial distribution of EP reflects the localized characteristics of coal damage,and it can monitor and identify the harzard areas of dynamic failure gas-containing coal.The study reveals the EP response mechanism of during the damage and destruction process of gas-bearing coal,menahwile,the EP-damage evolution constitutive model is established.The results show that:(1)The charge separation is the material basis of EP response,mainly derived from the stress concentration,crack propagation,friction and other processes at the crack tip of the coal mass under load;the migration of primary charge defects and the new charges in the non-uniform deformation of coal-rock media induce changes in the charge distribution;the adsorbed gas erodes the coal matrix,and the free gas changes the stress distribution of the fracture surface of coal media,which promotes the coal deformation and fracture,and enhances the EP response.(2)The electrostatic charge electric field can induce the coal medium to form a polarized electric field.The combined action of both induces the potential signal on the surface of the gas-bearing coal medium;the equivalent static electric field,the polarization effect and the response characteristics during the charge diffusion process are analyzed theoretically;the mathematical model of EP calculation is constructed with separation variable method.(3)Based on the dual pore model,the effect of crack distribution and expansion on coal damage characteristics is studied,and the effective stress equation of gas-bearing coal is revised;the EP-damage evolution constitutive model is constructed.Meanwhile,the characterization method of values of the effective stress and damage of gas-containing is obtained.The results show that the calculation results of the model are in good agreement with the measured results in terms of value and change trend,whihch can reflect the loading state and damage degree of gas-bearing coal.The multi-channel EP measuring device for mining is developed.The EP distribution and evolution law of the coal body in the driving face of coal seam with outburst risk are tested and analyzed in the field.The EP inversion results are utilized to propose a new fine discrimination method for risk of coal and gas outburst.The results indicate that:(1)With the advance of the driving face,the EP signal of the coal body in front of the driving face increases firstly and decreases subsequently,which is consistent with the change of mining stress;the EP signal of coal mass behind driving face is with high value and fluctuates violently when the shock bump occurs or the gas concentration increases suddenly in the roadway.The temporal response of EP is generally consistent with the change of mining stress and electromagnetic radiation(EMR)signal produced by deformation and fracture of coal mass in the corresponding area,which can reflect the loading state and damage evolution of coal mass.(2)Based on the multi-point EP measuring results of driving face,the EP inversion imaging results are carried out to study the relationship between the EP distribution and the abnormal damage area of coal mass.The results show that the high value areas in EP inversion map of unilateral behind driving face basically coincides with the high value areas of EMR index and the distribution areas of the side faults and floor heave,which can reflect the local area of coal mass with severe damage;the high value areas in EP inversion cloud map of double side in front of driving face have good consistency in spatial distribution with the areas where drilling cutting or initial speed of gas emission is higher than the warning value,or areas with hole sprarying or dril withstanding.(3)The results of bilateral EP inversion are utilized to identify the outburst risky areas in front of driving face.The critical value of EP inversion is determined based on fuzzy mathematics method,which verified in combination with conventional indexes.The results show that the EP identification results have high accuracy and strong pertinence without omissions,whicn can accurately estimate and decide the risky areas of coal and gas outburst.The research reveals the spatiotemporal evolution law of EP response during damage and destruction process of coal mass under the coupling of stress and gas pressure,meanwhile the fine identification method is put forward for coal and gas outburst risk by EP measuring.It is of significantce on engineering application for the in-depth understanding of the evolution and development process of coal-gas dynamic disasters,and to ensure coal mine safety further.There are 100 figures,7 tables and 199 references in this paper.