Study on control mechanism and method of gas concentration in gas drainage borehole of underground mine

In order to solve a problem of low gas drainage concentration at the mine site, a method to determine a sealing depth of the borehole was provided, a |o cation of the air intake at the main control cross section was revealed, a gas flow control equation was provided in a consideration of the gas adsorption effect influence and a method to control the gas drainage concentration with a multi stage adjusted drainage negative pressure was provided.The results showed that, a minimum seali ng depth of the borehole should be over the location of the stress peak. The main control section of the air intake could be divided into a modification zone of the seale d borehole, cracking zone, plastic zone, elastic failure zone and in-situ rock stress zone. There was an obvious stress concentrated phenomenon around the coal and r ock failures, because the effective stress was higher than the nominal stress and with the gas pressure increased, the effective stress would be reduced. Under the con dition of the same porosity, with the nominal stress increased, the stress concentrated coefficient around the pores would be increased. The air intake volume and the e ffective radius of the sealed borehole would be in a negative correlation and the air intake volume and the drainage negative pressure would be in a positive correlation. The engineering practices showed that a method to optimize the borehole sealing depth and the rational multi stage adjusted negative pressure could be applied to effe ctively control the gas drainage concentration in the borehole.