Geological Evaluation of Undersea Tunnel Excavation in Fault Fracture Zones Based on On-Site Drilling Tests

Geological risk assessment is crucial for ensuring construction safety in undersea tunnel projects, particularly when traversing structurally complex fault zones. This study quantifies the hydrogeological and geomechanical risks associated with the Qingdao Jiaozhou Bay Second Undersea Tunnel crossing of the Cangkou Fault Zone and develops targeted mitigation strategies based on empirical data. Comprehensive field investigations—including water pressure tests (maximum: 48.38 Lu), pumping tests (permeability coefficient K = 6.74–14.56 m/d), and hydraulic fracturing stress measurements (σHmax = 4.57 MPa, N67°E orientation)—revealed two primary challenges: high-permeability fracture networks and anisotropic in situ stress conditions (lateral stress coefficients λ = 1.2–1.8). The results indicate that conventional grouting techniques are inadequate under these conditions, prompting the implementation of an 8-m full-section grouting design. This method effectively penetrates fracture networks and reinforces the surrounding rock mass upon solidification. Key limitations include the localized nature of the drill core data (limited to five core samples) and the assumption of homogeneous fracture distribution beyond the sampled zones. These findings provide an actionable framework for fault-zone tunneling in similar coastal geological settings.