Soil-structure failure mechanisms at two bridges in the 2016, muisne Ecuador earthquake

This paper presents observations from a series of failures of earth backfills behind three singlespan bridge abutments in the province of Manabí, Ecuador during the 2016 Mw7.8 Muisne earthquake. The failures had a similar combined pattern of dynamic instability and soil softening due to liquefaction or high seismic strains. Subjected to recorded peak ground accelerations ranging from 0.3 to 1g, the bridge structures seemed to limit soil failure by partially yielding, with failures observed at the abutment wall and/or the bridge deck. This pattern is different from typical failures of retaining walls and embankments that appear primarily in the soil backfill. In the presented cases, the bridge deck acted essentially as a strut, transferring active earth pressures to the opposite riverbank, thus generating a passive earth pressure, relieving soil displacements and preventing geotechnical failure. Documentation of drawings and design details of the bridges, including seismic force-reduction factors assumptions, along with available geotechnical and structural data and observations with actual strong ground motion records are presented. The information is then used to create dynamic models to demonstrate the observed "interrupted soil failure" behavior pattern, with seismic load transferred to the structural elements according to their relative stiffness, with the weaker element suffering the most damage, even reaching full collapse. A resilience-based retrofit approach is proposed, with bridge strengthening and enhancement details to make the bridges able to resist the observed transfer of loads in the 2016 earthquake and anticipate a better behavior in future earthquake.