Experimental Study On The Law Of Magnetic Field Effect In The Process Of Coal And Rock Failure Under Load

The situation of coal and rock dynamic disasters such as rockburst,coal and gas outburst in China is still severe,an accurate and effective monitoring and early warning of which is of great significance.Magnetic field changes can occur in the coal rock deformation and fracture process,but what is the underlying mechanism of such changes?Is it possible to monitor the deformation and failure of coal rock by the changes in the magnetic field?These are fundamental questions that worth investigating.Therefore,this thesis combines laboratory experiment,theoretical analysis,and field verification to systematically study the magnetic field characteristics and laws of the failure process of coal rock under load.The magnetic field generation mechanism of the failure process of coal rock under load is revealed from the perspective of microscopic electricity and magnetism.Based on the difference in magnetic induction intensity at different measurment points in the coal rock failure space,a magnetic field spatial sensing imaging method is established for the coal rock stress state and the damage failure area,which is verified in the real tunnel environment.The main research results are as follows:(1)A fluxgate magnetic field monitoring method is studied and preferentially selected,which is suitable for the coal rock failure under load.Besides,a magnetic field effect monitoring non-magnetized experimental system for the coal rock failure under load is designed and established.The reliability of the experimental system is tested using titanium alloy samples and coal samples.The results show that the magnetic field signals produced during coal failure are higher than the ambient level of the magnetic field noise,and the magnetic induction intensity has a good correlation with load and acoustic emission.In the early stage of coal loading,the magnetic induction intensity does not change obviously;in the middle and late stage of loading,the magnetic induction intensity increases significantly;when the main failure occurs,the magnetic induction intensity rapidly increases to the maximum value.The average correlation coefficient between the magnetic induction intensity and the cumulative acoustic emission counts reaches up to 0.9.This strong correlation indicates that the generation of the magnetic field is closely related to the deformation and failure of coal rock.(2)The magnetic field characteristics and laws of coal rock failure process with different types,different loading conditions and different prefabricated crack inclination angles are systematically studied.The results show that the magnetic induction intensity of low-strength coal and coarse sandstone increases slowly with the increasing load,and increases suddenly when the load drop occurs.The magnetic induction intensity of high-strength fine sandstone,limestone and granite increases steadily with the increasing load,and rapidly increases to the maximum at the moment of main failure.The higher the loading rate,the greater the peak value of magnetic induction intensity of coal rock failure.In the early and middle stages of multi-stage loading,the magnetic induction intensity increases gradually in the load-increasing stage,and decreases slightly in the load-holding stage.At the late stage of loading,the magnetic induction intensities increase in both the loadholding and-increasing stages.In the process of cyclic loading and unloading,the magnetic induction intensity increases gradually in the loading stage and decreases gradually in the unloading stage.The peak value of magnetic induction intensity of 30°,60°and 90° pre-cracked samples first decreases and then increases.(3)The micro-composition,structure,electrical and magnetic characteristics of coal and rock are studied and analyzed.The magnetic field generation mechanism of the coal rock failure under load is analyzed from the perspective of electricity and magnetism.The source of the current generated by the magnetic field in the process of coal rock failure is obtained,and the generation model of coal rock failure magnetic field based on current element is established.The dominant mechanism of the magnetic field generation of coal and rock in different loading stages is revealed.Using the crack propagation speed and the amount of charge generated in the coal rock,the magnetic induction intensity in the process of coal rock failure is estimated to be～102 nT,which is on the same order of magnitude as that measured in the experiment.As a result,the theoretical model and experimental results can be mutually verified.(4)Based on the difference of magnetic induction intensity at different measuring points in the coal rock failure space,a spatial perception imaging method of coal rock failure magnetic field is established.The results show that the spatial perception imaging results of coal rock magnetic field have a good correlation with the crack evolution process and acoustic emission localization area of coal rock,showing the stress state and damage failure area of coal rock.The magnetic field perception imaging cloud image carries certain precursor information,which can reflect the damage position of the sample.Using the spatial monitoring results of the magnetic field,the accurate perception and localization imaging of the stress distribution and damage state of coal rock can be achieved.(5)The magnetic field response characteristics of rock mass failure process and the spatial distribution rule of magnetic field in tunnel face area are studied by field test.The results show that the blasting of rock mass in tunnel excavation can produce significant magnetic field signals.The magnetic field signal appears at the moment of blasting,in the form of a long-term pulse.The magnetic induction intensity of different test distances is distributed in the range of 102～103 nT,whose duration is ten to dozens of seconds.The magnetic field distribution at the tunnel face is uneven,displaying high intensity on both sides of the tunnel face while low intensity in the middle of the tunnel face.The stress concentration area of the numerical simulation corresponds to the high-value magnetic field area,which verifies the feasibility of spatial sensing imaging method of coal rock failure magnetic field.In summary,the research results provide a new non-destructive and noncontact magnetic field monitoring method for evaluating the instability and failure of coal rock,which is expected to be applied to underground engineering fields,such as mines,tunnels,and urban underground spaces.There are 102 figures,11 tables and 216 references in this paper.