Diagnostic method for underwater damage zones of concrete face slabs in concrete-faced rockfill dams based on seepage and deformation observations

Cracking and compression damage of concrete face slabs directly influence the leakage condition and operational safety of concrete-faced rockfill dams (CFRDs). During operation, however, underwater face-slab damage is difficult to perceive and localize in a timely manner. Rockfill deformation is a dominant factor affecting the damage state of concrete face slabs. Accordingly, this paper proposes a diagnostic method for underwater damage zones in concrete face slabs of CFRDs by capturing the intrinsic relationship between seepage and deformation observations. First, Random Forest (RF) is combined with SHapley Additive exPlanations (SHAP) to reduce the dimensionality of the influencing factor set for seepage discharge, which is constructed from monitoring data on rockfill settlement, face-slab joint deformation, and environmental loads. A RIME-optimized Light Gradient Boosting Machine model is then developed to establish the seepage discharge–deformation mapping relationship. The model-calculated seepage discharge is subsequently compared with a seepage-discharge control threshold determined by a bootstrap-assisted empirical quantile method to identify abnormal deformation periods. Based on SHAP analysis of the seepage–deformation coupling relationship during these abnormal periods, underwater damage zones in concrete face slabs are diagnosed by identifying key abnormal deformation locations that contribute significantly to abrupt increases in seepage discharge. A case study of an in-service CFRD confirms the effectiveness and feasibility of the proposed method, with the identified underwater damage zones in concrete face slabs showing good agreement with inspection results. These findings highlight the practical value of the proposed method for rapid underwater damage diagnosis and structural health monitoring of CFRDs.