Abstract: To determine the law of gas migration in the process of tunnel excavation in a highgas area of complex coal strata, a thermalhydrologicalmechanical (THM) coupling model of gas permeability based on the mechanism of multiple physical fields is derived. The characteristics of gas migration over time are discussed. The results show the following. (1) The gas pressure decreasing speed decreases with increasing distance from the tunnel face, and the gas decreasing speed is highest at the tunnel face. The pressure field reaches a new balance after 20 hours, and the greater the distance from the tunnel face, the more time is needed for gas pressure to reach a new balance. (2) The porosity of the rock mass shows a decreasing trend overall. The rock mass porosity clearly decreases in 10 hours, and the decreasing amplitude decreases and time for balance increases with increasing distance from the tunnel face. (3) When the coal seam has not been excavated, the gas pressure ahead of the tunnel face is relatively low, and the gas pressure decreases rapidly in 15 hours; the gas pressure varies at different distances from the tunnel face, and it reaches a new balance after 40 hours.

Key words: highway tunnel, coal strata, fractured rock mass, gas migration, thermalhydrologicalmechanical (THM) coupling