Failure Mechanism of Primary Support for a Shallow and Asymmetrically Loaded Tunnel Portal and Treatment Measures

Abstract

Shallow and asymmetrically loaded tunnels are very common in mountain tunnel portals, and localized excessive deformation, cracking, or even collapse often occur during the construction process. In this study, a shallow and asymmetrically loaded tunnel entrance is used as an example to discuss the primary support failure mechanism by means of field observations and monitoring and three-dimensional (3D) numerical simulation. Asymmetrical loading, rainfall, and mismanagement were the three main triggers of failure events. The continuous rainfall infiltration significantly reduced the quality of the surrounding rock, resulting in an increase in the initially high asymmetrical loading. The retaining wall and surrounding rock failed to provide sufficient counterpressure on the shallowly buried side, leading to large deformation of the primary support at the beginning of excavation. The deformation increased continuously and quickly exceeded the allowable value; however, the project was not stopped in time, and further excavation occurred due to a communication failure between the monitoring and construction crews. Thus, severe damage to the primary support occurred when the tunnel face reached a distance of 24 m from the entrance. To avoid more hazardous damage, a temporary support mainly composed of 20b I-steel was installed immediately to prevent deformation; 30 days later, a cutting slope was created on the deeply buried side to reduce the asymmetrical loading, and the excavated rock mass was stacked at the shallow buried side to increase the counterpressure. Then, the temporary support was removed, and a secondary lining was applied in a timely fashion. The monitoring data showed that good results had been achieved. This study provides useful recommendations for the construction safety of shallow and asymmetrically loaded tunnel portals.