Stability Analysis of the Front Slope of Small-Distance Highway Tunnel with Very Large Section Based on 3D Monitoring

Landslide due to highway tunneling is a threat to the site safety and the stability of tunnel lining, and leads to large economic losses. As to the tunnel front slope deformation behaviors of small-distance highway tunnel, it is more complex for the tunnel of large section than that of small section. Generally, the small-distance tunnel with large section will experience many complex construction steps and many support transformations, therefore, the excavation of workface will disturb the slope many times. Longtoushan tunnel, which is located in Guangdong Province of China, is a four-lane large section highway tunnel. With consideration of weathered rock and fracture zone existing in surrounding rock of the tunnel, the both side drift method has been adopted. 3D (three-dimensional) monitoring has been introduced to monitor the movement of the front slope. The basic characteristic of 3D deformation of the slope has been analyzed to estimate the stability of the slope. The results show that: (1) the deformation of the front slope is of obvious 3D features and with no significant correlation between horizontal displacement and vertical settlement; (2) the deformation of the front slope is mainly influenced by tunneling; (3) the front slope can be divided into three regions with different characteristics of displacements; and (4) the displacement characteristics of the front slope depends with great degree on the type of slope rock. Accordingly, based on monitoring results, the construction schedule and the construction program have been often adjusted to ensure safety.