Seismic fragility of a concrete gravity dam: integrating in-situ shear testing with displacement-dependent...

The concrete-rock interface plays a critical role in governing stability of concrete gravity dams. Existing design practices rely on mapped joint roughness coefficient (JRC) values that neglect both excavation-induced damage and displacement-dependent degradation resulting in unconservative overestimation of interface shear capacity. This study integrates 56 in-situ direct shear tests on Class I and II behavior rocks with displacement-dependent numerical modeling. Effective JRC values back-calculated from in-situ direct shear tests using the Barton-Bandis criterion revealed reductions of 40% (Class II) and 22% (Class I) relative to mapped JRC values that is attributed to excavation-induced damage. A two-stage model capturing hyperbolic pre-peak JRC mobilization and exponential post-peak degradation is presented and validated against in-situ measurements (R² = 0.92 and 0.89 respectively). The validated interface is implemented for a 140 m concrete gravity dam using 20 recorded seismic acceleration time histories representative of the regional seismic hazard. Under 1.0g peak ground acceleration interface shear stress reached 5.32 MPa with 968 mm permanent displacement. Seismic fragility analysis indicated a median peak ground acceleration of 0.213g for serious damage at 50% exceedance probability. The results indicate that conventional approaches are less conservative. The presented field-validated approach provides a reasonable method for performance-based seismic risk assessment of concrete gravity dams in tectonically active regions. Further work incorporating cyclic shear testing, CNS boundary conditions, three-dimensional modeling and long-term bond degradation effects is recommended to enhance the reliability of the presented framework.