Concrete-faced rockfill dams (CFRDs) constructed in narrow and steep valleys are strongly influenced by the mechanical interaction between the dam body and abutment bedrock. Under long-term construction and reservoir impoundment, time-dependent frictional slip along the dam–abutment interface may alter deformation transfer within the dam system. This study investigated the Dashixia extra-high CFRD through large-scale contact rheological tests and three-dimensional finite element analysis. A contact rheological model was established from interface tests and incorporated into a full-scale numerical model considering valley topography, staged construction, and reservoir impoundment. The influence of contact rheology on dam deformation, face-slab response, and peripheral-joint behavior was evaluated. The results show that contact rheology has little effect on global dam settlement but significantly increases horizontal displacement and redistributes local deformation near the abutments. Under the normal reservoir water level, the maximum upstream displacement, downstream displacement, and settlement increase by 0.7, 3.4, and 1.5 cm, respectively. Meanwhile, the maximum peripheral-joint settlement increases from 43.7 to 69.8 mm, and the maximum tensile opening increases from 8.7 to 12.8 mm. For the Dashixia CFRD, inclusion of dam–abutment contact rheology increases the predicted maximum peripheral-joint settlement and tensile opening by 59.7% and 47.1%, respectively, highlighting the greater sensitivity of local joint deformation compared with global dam settlement.

As a key structural component of rockfill dams, the load-bearing capacity of large-sized concrete slabs under complex multi-axial stresses is directly related to the long-term safe operation of the dams. This study conducted uniaxial and biaxial lateral compression strength tests on C25 concrete slabs with dimensions of 1500 × 1500 × 150 mm using a large-scale bi-directional loading reaction frame test system, systematically revealing the mechanical properties and failure criteria of large-sized concrete slabs. The results indicate that the biaxial compressive strength of the concrete slabs is significantly greater than the uniaxial compressive strength. The stress–strain curves of the concrete slabs and standard specimens exhibit good consistency before failure. Based on uniaxial compressive strength data, the concrete size effect strength reduction formula proposed by Neville was modified, and a compressive strength prediction formula applicable to large-sized concrete members was established. Further integration with code-specified failure criteria led to the development of a biaxial failure envelope for large-sized concrete slabs, which was validated to agree well with measured data. The research findings can provide reliable experimental evidence and theoretical support for the strength reduction, load-bearing capacity assessment, and revisions of relevant design codes for large hydraulic components such as concrete face slabs in rockfill dams.