Bridges, Tunnels and other Transportation Structures
For river-crossing bridges, foundations are often constructed in water and embedded into soils. Saturated cohesionless soils may liquefy under earthquakes, leading to reductions of soil strength and stiffness, which probably causes damage to piles and associated failure of bridges. On the other hand, pile foundations constructed in water are often subjected to scour hazard that is reported to be one of the most severe hazards causing bridge failures in the United States. Phenomenally, scour renders caps and upper portions of piles exposed without surrounding soils. Technically, scour may cause degradations of foundation capacities and alter seismic behavior of bridges. Previous studies always considered the effect of liquefaction or scour separately, regardless of experimental or numerical studies. However, it is a common scenario in practice that bridges are located in flood-induced scour sites where saturated sands may liquefy under earthquakes. In other words, scour and earthquake-induced liquefaction hazards may occur simultaneously. However, to the best knowledge of the authors, studies on the seismic behavior of bridges under such combined effects of hazards have not been well documented, which may be partially because rare scour-monitoring sensors were installed for bridges that were reported damage due to earthquake-induced liquefaction.
Numerical predictions on the seismic performance of soil-bridge systems may be susceptible to varied model-input parameters. From a predictive point of view, it is desirable to characterize impacts of these parameters. To this end, this study aims to perform an in-depth sensitivity analysis to identify structural and geotechnical parameters as well as intensity measures of ground motions that have the most and least significant effects on the seismic performance of bridges under the combined effects of scour and liquefaction hazards. A coupled soil-foundation-bridge finite element model is established considering a typical three-span reinforced concrete bridge embedded into a multi-layered soil profile where loose sand overlies dense sand. This numerical modelling technique is validated using centrifuge tests in literature, where single and group piles were embedded into liquefiable loose/dense sands. Ground motion time-history analyses are performed to obtain the seismic performance of bridges that is represented by the column drift ratio, bearing deformation and section curvature ductility. Tornado Diagram and First-Order-Second-Moment methods are adopted to assess sensitivities of studied parameters including loose/dense sand properties (unit weight, friction angle, shear modulus, permeability and void ratio) and model parameters of steels and concretes as well as intensity measures of ground motions. Special attentions are paid to the impact of scour and liquefaction, separately and combined, on the sensitivity. Preliminary results indicate that the peak ground velocity and loose sand friction angle are the most sensitive parameters while the concrete strength shows the least significant effect. Other conclusions are under construction. This study can provide valuable insights into the efficient prediction of bridge performance under combined effects of scour and liquefaction hazards.