Diagnosis and causal-chain-based treatment of water-leakage in in-service mountainous highway tunnels: an engineering application

Water leakage in in-service mountainous highway tunnels is difficult to diagnose due to concealed geological conditions, coupled seepage processes, and frequent recurrence after local repair. This study proposes a causal-diagnosis and causal-chain-based treatment framework for tunnel water leakage. The water-leakage mechanism is interpreted as a progressive process involving groundwater recharge, fracture-controlled seepage transport, drainage obstruction, water-pressure accumulation behind the lining, and failure at weak points in the waterproofing system. A diagnostic method integrating leakage characteristics, groundwater conditions, geological anomalies behind the lining, drainage performance, and waterproofing defects is established, followed by a three-stage treatment strategy. The framework was applied to the Dashiling Tunnel on the Shenzhen–Cenxi Expressway (G2518), China, using leakage surveys, seismic reflection imaging, and visual inspections. A total of 312 leakage defects were identified, with 48.1% occurring at secondary-lining construction joints and 37.8% occurring in wall-foot-to-sidewall zones. Fracture-related anomalies behind the lining and severe blockage in 50.0% of the inspected side drains revealed a compound leakage mechanism. After targeted treatment, two years of monitoring showed that the total area of pavement leakage was reduced to approximately 3.9% of its pre-treatment level. The number of tunnel water-leakage defects exhibited an approximately linear decreasing trend, with an overall reduction of more than 70%. The densities of tunnel water-leakage defects in all statistical sections were less than 0.3 points per linear meter, demonstrating good treatment effectiveness. The proposed framework provides a practical approach for the systematic diagnosis and treatment of water leakage in comparable in-service mountainous highway tunnels with similar hydrogeological and drainage conditions.