A numerical study on the effect of the hydraulic diameter of tunnels on the plug-holing phenomena in shallow underground tunnels

When the plug-holing phenomena occurs, the actual amount of smoke exhaust could not be able to satisfy the design criteria of the smoke ventilation. The plug-holing phenomenon is influenced by various factors, but has not yet suggested accurate criteria for the plug-holing. Especially, the effect of tunnel geometry was not considered, despite the smoke movement is influenced by hydraulic diameter of tunnel. In this study, we performed the numerical simulation to analyze the effect of the hydraulic diameter of a tunnel on plug-holing. According to the numerical result, it was identified that the total mass flux increased and the mass flux of smoke decreased as the hydraulic diameter of tunnel was increased. A new, modified Froude number is suggested for estimating the occurrence of plug-holing by including the effect of the hydraulic diameter for shallow tunnels. Additionally, the critical value of the modified Froude number is defined as 0.104 based on the results of the numerical simulation.