RAPID: Group Effects on Pile Downdrag Following Liquefaction

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Several of the earthquakes that occurred during the 1811-1812 New Madrid earthquake sequence were identified by the United States Geological Survey as being the largest earthquakes East of the Rocky Mountains and within the top 20 of the largest domestic earthquakes United States history. The seismic waves from these earthquakes propagated for a great distance because of the competency of the surrounding bedrock, and because the earthquakes occurred in the middle of the continental plate. Soil liquefaction, where soil below the water table temporarily becomes a viscous fluid following shaking, was observed to occur during the New Madrid earthquakes. Liquefaction results in a loss of strength in the soil that serves as the bearing material for most infrastructure. If the same size earthquake as the 1811-1812 sequence were to occur today, the United States Federal Emergency Management Agency warns that the earthquake could result in the highest economic losses due to a natural disaster in the United States. Because the areas near the earthquake are now densely populated (St. Louis, Missouri, and Memphis, Tennessee) additional understanding of behavior of civil infrastructure when exposed earthquake induced liquefaction is needed to prevent loses. To remedy this lack of understanding, small explosive charges will be used to produce liquefaction under full-scale conditions and measurements will be made of the foundation settlement and the downward friction force on the foundations. Tests will be performed on two pile groups a site in Arkansas. Primary support will be provided by Nucor Yamata Steel Corperation; however, supplemental funding from NSF will significantly expand the scope of the project. This Rapid Response Research (RAPID) grant will provide that supplemental funding. The results from these tests will allow engineers to evaluate the accuracy of existing design procedures and develop modifications to produce better agreement with measured behavior. Improved design procedures will allow engineers to design safer yet cost-effective foundation solutions that will remain stable during earthquakes. Limitations of the previous study at the Turrell Arkansas Test Site will be overcome by 1) testing groups of piles instead of individual piles, 2) leaving the deadload on the foundations for a longer period of time prior to blasting to allow the neutral plane to develop, and 3) leaving the deadload on the foundations for a longer period of time after blasting to allow the neutral plane to redevelop.To improve our understanding of liquefaction-induced downdrag/dragload on deep foundations, blast induced liquefaction tests will be performed on two different pile groups (steel pipe group, H-pile group) at a site in Arkansas. The objective is to measure negative skin friction and downdrag induced settlement. With the inclusion of a static load placed on top of the pile groups, the results from these tests will allow researchers to evaluate the accuracy of existing design recommendations and develop modifications to produce better agreement with measured response. Because deep foundations are a common approach for supporting structures in liquefiable sands, reliable design approaches are critical to provide cost-effective foundations that are safe in future earthquakes. The results obtained from this project will be used to mitigate liquefaction induced damage at other earthquake prone sites with a similar geologic profile.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.