Countermeasures against squeezing ground: A case study of Haidong Water Conveyance Tunnel

The Haidong Water Conveyance Tunnel (HWCT), a notable engineering feat located within Dali City, Yunnan Province, China, represents an ultra-long water conveyance tunnel situated in a region characterized by medium in-situ stress conditions. As part of the Central Yunnan Water Diversion Project, this tunnel was specifically engineered for soft-rock environments. The excavation of such tunnels presents significant challenges due to rock mass deformation, commonly referred to as squeezing ground behavior. These challenges are exacerbated when navigating through diverse geological and geomorphological units, particularly in areas with complex geological conditions. To address these issues, an innovative active support system utilizing prestressed anchor cables was developed for the HWCT. This study provides a comprehensive analysis and comparison of rock mass behavior between two support systems: a conventional passive system employing steel arches and the proposed active system using prestressed anchor cables. The numerical modeling was performed using FLAC(3)D software to simulate various scenarios, while an extensive monitoring program was implemented in several representative tunnel sections to measure key parameters including rock mass stresses, displacements, internal forces in steel arches, and axial forces in anchor cables. The results from both the numerical simulations and field observations were systematically compared. The analyses demonstrated the superior performance of the active support system using prestressed anchor cables in the HWCT, significantly enhancing overall rock mass stability and effectively mitigating large deformation issues throughout the tunnel.