Experimental Study of Stress Release Using a Pilot Heading for a Highly Stressed Soft-Rock Tunnel

Geological disasters causing large deformations often occur when a tunnel passes through highly stressed soft rock. Such a disaster is greatly harmful to a project in terms of high risk treatments, expensive maintenance costs, and a delayed construction time. Therefore, preventing and treating the large deformation of a highly stressed soft-rock tunnel is a key and complex issue in tunnel engineering. Based on tests in the Muzhailing and Maoyushan tunnels on the Lanzhou-Chongqing railway, it was found that stress release by pilot heading can control large deformations. The stress is released within 3~7 days if the pilot tunnel is about 3m away from the tunnel crown and sidewalls. Additionally, the deformation of the main tunnel can be reduced by nearly 30%~40% compared to projects without pilot tunnels during the enlarging excavation.