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 UC Davis Center for Geotechnical Modeling 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, recognizing the dataset's diverse contributions to natural hazards research, and 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, professor emeritus of civil and environmental engineering at UC Davis.
DesignSafe is is part of the NSF-funded Natural Hazard Engineering Research Infrastructure (NHERI), of which the UC Davis Center is also a part. It provides cloud-based tools to manage, analyze, understand, and publish critical data for research on the impacts of natural hazards. The cyberinfrastructure and software development team is located at the Texas Advanced Computing Center (TACC) at The University of Texas at Austin.
“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, associate professor of civil and environmental engineering at UC Davis.
Liquefaction and bridge piles
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.
Lead graduate student researcher Sumeet Kumar Sinha 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.
Accelerometers, pore pressure transducers, linear potentiometers and settlement markers measured the pile acceleration and settlement of soil and pile. The model was then tested on the nine-meter radius geotechnical centrifuge and shake table at UC Davis.
The authors describe their experiment further in a study published in April 2022 in the Journal of Geotechnical and Geoenvironmental Engineering.
“We developed new contactless displacement tracking procedures with cameras and laser lines that were placed above the model," Ziotopoulou said. "And we developed image processing techniques so that we could combine the two and get a very clear idea of how the piles move the soil."
“We found something unexpected, but also substantiated by the data. Several mechanisms that lead to pile settlement occur during shaking,” Ziotopoulou said.
The scientists had expected that most of the damage would occur after the shaking had finished.
“We also learned that it wouldn't be likely for the pile to settle more than the ground around it,” Kutter said.
Good news for bridge design
This is good news for bridge design, as it was previously thought that if the downdrag forces on the pile were large, it might lead to significant plunging of the pile.
“We only observed significant plunging of the pile when the strength of the soil around the pile tip was significantly reduced by pore pressure migration from the liquefied layer to the load bearing stratum around the pile tip. The importance of the migration of pore pressures was surprising," Kutter said.
The authors intend for the data to be used by as broad a range of researchers as possible from experimentalists to numerical modelers.
“We wanted to make sure that the data is versatile enough so that other experimentalists could see how we went about our protocols, get inspired by it and learn from it. For numerical modelers, we provided both raw data recorded at the centrifuge, but also processed data to save them time in processing,” Ziotopoulou said.
Data and modeling helps us understand bridges, piles and the soil beneath them as a single system, Ziotopoulou said. “They're not islands. The soil, the piles, the building — they work as one. And as we move forward, we should be addressing them as such and evaluate their interactions.”
Sinha is now a postdoctoral researcher at UC Berkeley. This work was funded by the California Department of Transportation under Agreement 65A0688. The authors would like to acknowledge Caltrans engineers and staff for their initiation of the project and guidance along the way. The geotechnical centrifuge facility at UC Davis is part of the NHERI program under award CMMI 2037883.
NHERI is supported by multiple grants from the National Science Foundation, including the DesignSafe Cyberinfrastructure, award #2022469.