RESILIENT BRIDGESBridges Under Pressure

Can a bridge withstand an earthquake? One of the big unknowns is how far a bridge might settle from seismic shaking, especially if the shaking triggers a quicksand-like soil response called liquefaction.

Scientists at the University of California (UC), Davis have compiled the most detailed experimental data yet seen on how liquefaction-induced downdrag can add to the structural load applied to a pile foundation during earthquake shaking.

Their dataset was awarded a 2022 DesignSafe Dataset Award, which recognized the dataset’s diverse contributions to natural hazards research.

The dataset was also made publicly available on the NHERI DesignSafe cyberinfrastructure.

“This research will help in predicting how much settlement pile foundations will have if piles go through liquefiable deposits,” said dataset co-author Bruce Kutter, a professor emeritus at UC Davis.

“Our research also helps our understanding of this phenomenon move more strongly in a direction where we can account for the soil and the structure as a system from the foundation, all the way to the superstructure,” said dataset co-author Katerina Ziotopoulou, an associate professor at UC Davis.

Modern bridges are supported by large columns called piles that are driven deep into the ground. Liquefaction, which is the loss of soil strength due to the development of excess pore water pressures during earthquake shaking, can make the normally stable sand layer under the bridge pile lose their shear strength.

Additionally, when the excess pore water pressures dissipate after shaking has ceased and the soil regains its strength while settling, that can also affect the piles which can be then dragged downwards.

The data report has a number of experiments where scientists varied the thickness and stratigraphy of the liquefiable layers; the intensity of shaking; and the piles. They monitored how the downdrag forces on the bridge foundation pile build up.

“Our dataset refined the sequence of all those interdependent factors—the shaking, the increase in water pressure, the dissipation, the settlement, and the subsequent increase in axial load. Those things happen in sequence,” Kutter said.

Dr. Sumeet Kumar Sinha, the lead graduate student researcher, under the supervision of professors Kutter and Ziotopoulou, designed a series of model tests at UC Davis that included layered soil profiles of coarse sand, clay, loose sand, and dense sand. Instrumented model piles were inserted into the soil layers.