Numerical analysis of gas-liquid two-phase flow after water inrush from the working face during tunnel excavation in a karst region

Increasingly, deep tunnels are under construction, which greatly promotes the development of tunnelling technology. Complex geological conditions and frequent geological disasters have become great challenges during tunnel excavation. Among them, casualties and economic losses caused by water inrush are on the top levels in all types of tunnel geological disasters. In the present study, numerical simulation of the gas-liquid two-phase flow is carried out by using the FLUENT software to probe water flow characteristics after water inrush. Three common cases of water inrush from the working face during double-line tunnel excavation are researched. The variation rules of velocity, pressure and water volume fraction in the tunnels after water inrush are analyzed. The water movement laws under the conditions of different excavation situations and different water inrush positions are further discussed by comparing the three case studies. When water inrush occurs on the left tunnel working face, the following conclusions are drawn: (1) The velocity is smaller at the lower location of the left and right tunnels. In the width direction of the cross passage, the velocity is smaller on the left and right sides of the section, and in the height direction, the velocity is smaller on the upper end and lower end of the cross passage. (2) The pressure decreases gradually with the increase of height in the left tunnel and the cross passage. In the vicinity of the cross passage, the pressure on one side of the left and right tunnels close to the cross passage is the minimum, and then it gradually increases towards the other side. (3) The water volume fraction decreases gradually with the increase of height. (4) The velocity, pressure and water volume fraction changes greatly at the intersection of the cross passage and tunnels. In addition, the velocity nearby the working face without water inrush is very small.