Seismic responses and damage mechanisms of the structure in the portal section of a hydraulic tunnel in rock

Abstract Tunnel portals, being shallow underground structures, are well known to be more vulnerable to damage during earthquakes because they are generally located in areas of poor geological conditions that likely lead to amplified ground motions; this is especially true for hydraulic tunnel portals, which are additionally subjected to considerable hydrodynamic pressure. In this work, a full three-dimensional (3D) seismic analysis of the portal structure of a hydraulic tunnel is presented. Additionally, the hydrodynamic pressure and the rock-structure interaction (RSI) are both considered in this analysis to approximate actual field conditions, and their impacts on the seismic response and the damage characteristics of the tunnel portal are studied in detail. The numerical results indicate that the hydrodynamic pressure and the RSI contribute to a greater seismic response of the tunnel structure, with the peak displacements likely increasing by 48% when the hydrodynamic pressure is considered. The tunnel sections as far as 40 m from the entrance may suffer severe damage, and the constraint effects of the surrounding rock can efficiently reduce the damage of the tunnel structure. Two typical damage patterns are deduced from the simulation results and can be reasonably interpreted by the corresponding damage mechanisms.