Roles of pile-group and cap-rotation effects on seismic failure mechanisms of partially-embedded bridge foundations: Quasi-static tests

Pile foundations, especially partially embedded in soils, are vulnerable parts of bridges under seismic loadings. In this study, to investigate seismic failure mechanisms of partially-embedded reinforced concrete (RC) pile-group foundations, a series of quasi-static cyclic loading tests were conducted on 1 x 1, 2 x 2 and 2 x 3 pile-foundation specimens in sand. Observed hysteretic behavior and pile damage were recorded, along with pile curvature distributions. Then, numerical models based on the Beam-on-Nonlinear-Winkler-Foundation (BNWF) approach were developed and validated by test data. The failure process of each specimen was reached, characterized by the sequence and locations of plastic hinges on piles. Combining the test and numerical results, seismic failure mechanisms of all partially-embedded pile specimens with different layouts are revealed: side piles yield before center piles, each pile forms two plastic hinges, and pile heads yield before underground portions. Special attentions are paid to the roles of cap rotation and pile-group effect (PGE) on the failure mechanisms. The results reveal that the existence of PGE leads to lower lateral loads and greater displacements at different limit states such as the first yielding and ultimate states etc., while the impact of cap rotation only increases corresponding lateral displacements at these limit states.