Abstract: The Shizishan tunnel of the Central Yunnan Water Diversion Project runs through the fracture structure clamping belt between F_Ⅲ-102 and F₁₆ where the D₁¹ calcareous shale and carbonaceous shale are loosely consolidated under high in-situ stress. Geological analysis shows that the residual high in-situ stress of geological compression structure is the major driving force of the large deformation of tunnel, and the low load-resistance and degraded broken structure is the main internal cause of the instability and deformation of tunnel. Site investigation reveals the damage of tunnel reflected as instability and collapse induced by excavation unloading, extrusion of working face, and severely deformed support. To tackle these problems, we propose a principle of “advance support, fast excavation, fast support, and fast closure” for tunnel construction. We summarize the construction measures applicable to the geological condition: 1) advance support with 15mΦ108 mm large pipe shed by using ST-20 pipe shed drilling rig, and grouting reinforcement of surrounding rock of working face; timely strong support with pressure-relieving anchor rods and long-lock-foot anchor pipes to restrain deformation; dynamic control and informatized construction by strengthening deformation monitoring of surrounding rock. Practice has proved that, for tunnels passing through crushed zone of high geostress, construction measures such as reinforcing the shallow surrounding rock and combining active support and passive support can effectively restrain the deep development and deformation of surrounding rock’s loose circle, hence guaranteeing safe and smooth construction.

Key words: diversion tunnel, high in-situ stress, loose consolidated structure, pressure relief instability, compressive deformation, Central Yunnan Water Diversion Project