Effect of plastic-softening and dilatancy of coal on sealing property of borehole

Sealability of borehole is one of the key foundations of efficient gas drainage. The mechanical properties of coal play important roles in the selection of sealing parameters and the results of borehole sealing. Therefore,this paper aims at understanding the environment of surrounding coal and rock of borehole and analyzing the impacts of coal plas- tic softening and expansion characteristics on borehole sealing and providing a reliable foundation for the selection of borehole sealing parameters. A linear elastic-plastic stress softening model of “three-fold line” was proposed based on the full stress-strain curve and the mechanical parameters of coal. According to the Linear Mohr-Coulomb Yield Crite- rion,the radius and displacement of elastic zone,plastic softening zone and plastic flow zone and displacement of the model were calculated. Borehole imaging and data monitoring of gas drainage were conducted. The results show that ra- dial displacement and theoretical sealing radius of surrounding of boreholes was increased by 12. 93 and 2. 60 times compared with the calculated results of Kastner solution. Compared with the adjacent borehole which is unbroken and the sealing parameters are the same,the initial average extraction gas concentration of the borehole decreased by 59% , the average extraction gas concentration decreased by 64. 92% over the same period of 29 d and gas concentration de- cay rate increased by 3 times. Based on the results,the influence of plastic-softening and dilatancy is one of the main reasons inducing the decrease of initial sealing performance and the decline of gas extraction in late drainage stage. The theoretical permeability radius and grouting pressure for sealing boreholes obtained from Kastner equation was not reasonable. The sealing parameters of boreholes should be selected according to the characteristics of borehole sur- rounding rock as well.