Deformation Mechanism and Influence of Surface Pumping Control in Shallow-Buried Tunnel in Water-Rich Fully Weathered Granite

Abstract

Fully weathered granite exhibits high water sensitivity and poor water stability, making tunnel construction in water-rich fully weathered granite formations challenging. This study analyzes the water sensitivity of fully weathered granite and the impact of surface pumping on the deformation of shallow-buried, water-rich fully weathered granite tunnels, focusing on the Beitouling Tunnel in China. Geotechnical tests and numerical simulations are used to investigate these effects. The key findings are as follows: 1) The fully weathered granite in the project shows significant water sensitivity, with mechanical strength parameters decreasing as moisture content increases. Despite this, the stress-strain characteristics at various moisture levels exhibit strain-hardening behavior, and the failure mode is consistently “waisted drum-shaped.” The granite’s particle and compositional characteristics also contribute to surge disasters; 2) As the water level drops, groundwater moves toward the bottom of the “pumping funnel” along the pumping curve. Below this curve, intense seepage is observed around the tunnel lining, causing sidewall convergence and invert heaving. A “preventive layer” forms beneath the tunnel, reducing groundwater seepage toward the unsupported face; and 3) When pumping reaches the first excavation stage’s bottom, crown settlement decreases by more than 50% compared with the initial water level, providing the best control. However, sidewall convergence increases by 9% to 13%, and invert uplift rises by 23% to 36% when pumping reaches the tunnel invert.