The seismic performance of bridge structures and appurtenant components (i.e., approach spans, abutments and foundations) has been well documented following recent earthquakes worldwide. This experience demonstrates that
bridges are highly vulnerable to earthquake-induced damages and loss of serviceability. These damages are commonly due to soil liquefaction and the associated impact of ground failures on abutments and pile foundations.
Current design methods for evaluating permanent, seismically-induced deformations of earth structures are based on rigid body, limit equilibrium and “sliding-block” procedures that are poorly suited for modeling soil liquefaction and establishing the pattern of embankment-abutment-foundation deformations. Recent advances in the seismic design of bridges have addressed some of the limitations of the current design procedures; however practice-oriented methods for estimating permanent deformations at sites that contain liquefiable soils and/or where soil improvement strategies have been employed to mitigate liquefaction hazards are still at an early stage of development. In Oregon, the evaluation of soil liquefaction and abutment performance are complicated by the rather unique seismo-tectonic setting and the prevalence of silty soils along the primary transportation corridors in the Portland/Willamette Valley region and along the Columbia River.
This study has focused on numerical dynamic, effective stress modeling to determine the seismic performance of sloping abutments and the effectiveness of soil improvement for reducing permanent ground deformations. Recommendations are provided for evaluating the dynamic behavior of regional silty soils, the application of soil improvement at bridge sites, and comparisons have been made between the deformations computed using the advanced numerical model and the rigid-block methods used in practice. The results have been presented in the form of design charts, where possible, that can be readily used by design engineers in preliminary design and incorporated into the ODOT Liquefaction Mitigation Policy. This study has demonstrated the utility, and limitations, of soil improvement solely by densification techniques. In some cases soil densification techniques for mitigating seismic hazards may not be adequate in limiting deformations to allowable limits, indicating that other methods of soil improvement (e.g., cementation, stone columns, drainage) or structural improvements may also be required.