Wave velocity evolution law and quantitative index of damage degree of soft coal

In order to reasonably evaluate the failure state of soft coal,taking the 11518 coalface of Panyi Coal Mine in Huainan mining area as the background,coal briquette is selected to replace raw coal,the laboratory test of multipath wave velocity monitoring of coal body under uniaxial compression is carried out,and the variation characteristics of wave velocity are analyzed. The quantitative index of damage degree is defined based on the propagation law of acoustic waves. The state of coal medium in different failure stages is explored,and an engineering application of the index is carried out. Results show that:① With the increase of axial strain of coal,the wave velocity presents the basic law of stable,slow decrease,sharp decrease,and then stable. At the initial stage of loading,the wave velocity of the coal briquette is stable,which is different from the acoustic characteristics of rock materials.② The quantitative index of damage degree K is the ratio of the time of longitudinal wave passing through a crack in a certain failure stage to the time of longitudinal wave passing through a crack after the complete failure in coal body. ③ With the increase of axial strain of coal,the index experienced four change stages:stable near 0,slow increase,sharp increase,and then stable at about 1. The index evolution is highly consistent with the change in coal surface morphology. Compared with the traditional damage variable,the index highlights the influence of crack on wave velocity,has stronger sensitivity to failure,and can more accurately reflect the failure state of coal. ④ The coal medium state in front of the working face is zoned according to the index change laws:the coal body 26.5 m away from the coal wall is in the continuous medium area,the coal body 13.5-26.5 m away from the coal wall is in the quasi continuous medium area,the coal body 5.75-13.50 m away from the coal wall is in the discontinuous medium area,the coal body less than 5.75 m away from the coal wall is in the bulk medium area. The research results can provide a new way to quantify the damage degree of soft coal and prevent the surrounding rock disaster.