Bridges, Tunnels and other Transportation Structures
For bridges across rivers, pile foundations constructed in water may often suffer scour hazard, which is reported to be one of the most severe hazards causing bridge failures in the United States.
For the seismic design of bridges, the pile-group foundation is generally one of the most vulnerable parts of bridges. This becomes particularly challenging when bridges are subject to the scour hazard, because scour can cause degradations of foundation capacities, pile-soil interface friction and alter seismic behavior of bridges. In this regard, full-embedded pile-group foundations that are originally designed based on the principle of capacity-protection are probably subject to inelastic behavior under the effect of scour hazard. An alternative solution to this issue is to treat scoured pile-group foundations as potential energy-dissipation components with limited ductility. To this end, the seismic behavior of scoured pile foundations should be well documented first. Relatively rare experimental and numerical studies had been done. Among them, hardly any research paid attention to the impact of pile uplifting. In this regard, this study aims to reveal the impact of pile uplifting on the seismic behavior of scoured pile-group-foundations using quasi-static cyclic loading tests on a pair of 2×2 reinforced concrete square-pile foundation specimens embedded in layered soils of sand and gravel. The aboveground height of piles in each specimen is 0.80 m, corresponding to a global scour depth of 5.33D (D is the width of square-pile section, D=0.15 m). The embedded depth of piles is 24.67D. The pile-uplifting effect is achieved through additional bending moments applied on caps that facilitate the rotation of pile-group foundations. More specifically, for Specimen A, the reversed lateral loading was applied on the cap by a horizontal hydraulic actuator, while for its counterpart, Specimen B, additional moment on caps was achieved by introducing a rigid RC pier, fixed at the cap. The reversed lateral loading was then applied at the pier head. Meanwhile, axial compressive loads that represent dead loads of bridges were applied on both specimens through vertical hydraulic actuators. The hysteretic behavior of specimens, cross-section curvature distributions along piles, cap rotations and aboveground pile damages were recorded during the test. After tests, underground pile damages were also inspected. The test results reveal that the pile-foundation specimens with and without additional moment on caps behaved totally different, in terms of seismic failure modes, hysteretic behaviors, pile damages and so on. More specifically, a spindle-shaped hysteretic curve was recorded for the case considering the additional moment (Specimen B), which means a distinct pile-uplifting failure mode that was rarely reported in pile-foundation tests before, as compared to the typical ductile failure mode detected in Specimen A. Both the hysteretic behavior and observed pile damages of Specimen B indicate that the seismic energy was mainly dissipated by the pile-soil interface friction, rather than the inelastic pile damages as observed in Specimen A. The test results generally suggest the effect of pile uplifting should be considered carefully when evaluating the seismic behavior of laterally loaded pile foundations under the effect of scour.