Risk Identification and Reinforcement Strategy for Tunnel Excavation in Clay Altered Rock

The excavation of tunnels in clay-altered rocks, formed through long-term geological and hydrothermal interactions, often results in disasters. It is challenging to identify and prevent these disasters during the excavation in clay-altered rock. To address these challenges, this study first analyzes 12 typical tunnel cases in clay-altered rock. The results reveal that clay-altered rocks can be classified into three types: vein-type, full-section-type, and pocket-type. Collapse incidents are prone in vein-type altered rock. Excavation in full-section-type rock exhibits substantial deformation of soft rock, easily leading to tunnel blockage caused by catastrophic collapse. The pocket-type is prone to large collapses and may encounter water and mud/rock inrush disasters when groundwater is abundant. Subsequently, methods for risk identification of the three types of altered rocks are proposed. Risk identification for vein-type rocks is based on the width, density, and angle with the tunnel axis of the alteration zone. The Global Altered Index (GAI) is proposed from the thorough hydration products of surrounding rocks and is used for risk identification of full-section-type rock. Risk identification for pocket-type rocks relate the filling material in pocket-like rock cavities. Clay-altered rock zones serve as excellent water transportation channels and water storage zones. Risks are exacerbated by the hydro-mechanical coupling mechanism in altered rock. Advanced geophysical exploration combined with borehole exploration can effectively prevent excavation risks. Proactive drainage, strong support, and closely following the second lining are essential for the smooth excavation process. The research results provide a scientific basis and technical support for the identification and prevention of risks associated with tunnel excavation in clay-altered and weathered rock formations.