On The Characteristics Of Ultrasonic Wave During The Progressive Failure Of Coal-Rock Masses Around The Boreholes

Drilling for gas extraction is one of the important techniques of mine gas control,and efficient sealing of boreholes is the key to ensuring efficient gas extraction.However,gas drainage boreholes are affected by complex stresses such as in-situ stress and mining stress,and factors such as low strength of the coal seam itself,the sealing section of the borehole is easily destabilized and damaged,causing gas leakage and other problems and greatly reducing the efficiency of gas drainage,which stands as a great hidden danger for the safe production of the min.Therefore,conducting quantitative researches on the mechanism of the crack extension of coal-rock mass around the borehole of gas drainage is of great importance to ensure the efficient drainage of gas and prevent major accidents in coal mines such as gas disasters.According to performing uniaxial compression tests on porous samples under different conditions by the DNS200 electronic universal testing machine,together with digital speckle related measurement technology and ultrasonic detection technology,this thesis quantitatively analyzed the principles of porous crack evolution during the destruction of coal and rock masses.This meanure combined methods of theoretical analysis,laboratory tests and field industrial test.The main contents of the research conducted in this thesis are as follows.(1)The progressive failure test of coal and rock bodies with different water content and different sealing materials has been carried out,and the digital speckle realted measurement technologies have been applied to obtain the evolution characteristics of the strain field during the crack propagation of the coal and rock body surface around the hole and to gain the information about the destruction of coal and rock masses.Subsequently the crack propagation principles of coal and rock masses with different water content and different sealing materials around the pores are discussed.(2)The RSM-SY7 ultrasonic system is applied to obtain ultrasonic characteristic data,analyze the ultrasonic characteristic parameters,such as transmission waveform,wave velocity,attenuation coefficient and others during the progressive destruction of the coal and rock body around the hole,and to acquire the law of influence of how the water and sealing material on the crack propagation around the hole and the ultrasonic characteristic parameters.(3)A theoretical model of wave velocity and attenuation coefficient with variable porosity parameters has established.By comparing the laboratory test results,this thesis find out that the moisture content of the sample at the early stage of loading is the main cause to the changes of the ultrasonic characteristic parameters.With the continuous loading,the occurrence and expansion of internal cracks in the rock mass become the main factors affecting the ultrasonic characteristic parameters.Therefore,in the failure stage,the experimental values are relatively consistent with the theoretical ones.(4)The parameter equivalent crack width is introduced.And based on the time difference calculation formula during the sound wave transmission,the equivalent crack width in the progressive destruction of the coal and rock mass around the hole has been calculated.The equivalent crack width model based on the ultrasonic characteristic parameters is established.(5)Given the ultrasonic propagation law during the progressive destruction of the coal and rock mass around the hole,an ultrasonic on-site industrial testing of the quality of the gas sealing hole in the Yuwu mining area,through the ultrasonic testing device around the drilling hole designed by the team.Combined with the analysis of the gas concentration,ultrasonic waveform and wave velocity in the borehole,the thesis comprehensively analyzed the sealing quality of the gas drainage borehole,and the main location of the gas leakage of the drainage borehole is found out to be around 2 to 6 meters in the sealing section.Then the new CF expansion cement material was applied,which can effectively avoid gas leakage in the sealing section of the borehole and ensure efficient gas drainage.