A new analytical method for determination of discharge duration in tunnels subjected to groundwater inrush

Unexpected water inrush to tunnels is one of the most hazardous events, especially in karst terrains that could take place during mechanized shield tunneling mostly due to inappropriate site investigations. When water inrushes to tunnel front, actions like pumping out the water or grouting, for impeding or reducing the inrush and alleviating related damages, are often time-consuming, uneconomical, difficult, or sometimes even impossible. In most cases, it is sometimes necessary to wait for lowering groundwater within the tunnel to have again the possibility to excavate. Therefore, determination of the required time for the groundwater inrush to decrease to a certain level is extremely important for tunneling management plans and plays a significant role in decision-making for project managers. In this study, an assessment of the discharge time after water inrush to tunnel using the new proposed analytical approach is argued. This method is developed based on recession analysis and hydrodynamic of open channels. For this purpose, inrush rate to tunnel front and groundwater level should be measured on at least a daily basis, a feasible and practical action to be carried out on tunnel construction sites. The method is validated with available data from three tunnels with water inrush experiences. The results show that the analytically determined water inrush regime is in good accordance with the observed ones.