Bridges built on saturated deposits of loose sand in seismically active areas are, regardless of their size, critical structures during seismic events. In fact, the location of these structures increases the risk of liquefaction-induced foundation failure. In addition, the consequence of their collapse is not limited to the direct human and economic losses but also creates a traffic impediment that severely affects postearthquake rescue of human lives and properties and imposes a long-term disruption of social life. Small to medium size bridges found in this environment are particularly vulnerable to the effects of liquefaction, as they are frequently built, especially in developing countries, on shallow foundations. Even though densification is frequently used as a liquefaction resistance measure for bridge foundations, design is based on poorly understood fundamentals. This paper presents the results of dynamic centrifuge modeling undertaken to investigate the behavior of a foundation-bridge system during an earthquake causing pore-pressure in the granular soil to rise. The results of a dynamic centrifuge test performed on a model of a bridge resting on liquefiable ground are compared with those obtained from similar tests where a zone beneath the bridge foundation was densified, using three different geometries. The tests demonstrate the dramatic consequences of ground liquefaction on bridge foundations and enlighten the different effects of the existence of a densified foundation soil zone on the behavior of the system. The influence of the geometry of the improved zone on the bridge performance under seismic loading is also assessed, providing important insight with respect to the optimum geometry that may be used in practice. The results shown in this paper suggest that the present understanding on the use of densification as a liquefaction resistance measure is still limited, the issue of post-earthquake pore pressure migration emerging as a key factor to consider. New concepts that should be more deeply explored in the future are disclosed. The conclusions presented suggest that the use of densification to mitigate liquefaction effects may be improved and are particularly relevant in view of the fact that full-scale observation of the performance of improved sites is scarce and often requires considerable subjective interpretation.
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