RAPID: Pile Downdrag Behavior Based on Blast Liquefaction Testing

Brigham Young University, Provo UT

Investigators

Abstract

As a result of the extensive liquefaction damage in Christchurch, the New Zealand Earthquake Commission (EQC) is sponsoring a multi-million research study to determine the most cost-effective liquefaction mitigation strategies for repair work and subsequent reconstruction. Based on this test program, government approved approaches will be eligible for earthquake insurance reimbursements. Blast induced liquefaction trials will be performed to evaluate the ground improvement techniques under full-scale conditions. In collaboration with EQC we will install instrumented test pile foundations and measure negative skin friction and pile performance following liquefaction induced settlement. These full-scale measurements of liquefaction induced ground settlement, pile settlement and pile load distribution will provide fundamental understanding of the pile downdrag phenomenon. The absence of field measurements has led to contradictory design recommendations which may be either unsafe or uneconomical depending on the actual pile behavior. Because deep foundations are a common approach for supporting bridges and buildings in liquefiable sands, reliable design approaches are critical to preventing future earthquake damage. Liquefaction of loose saturated sands results in significant damage to civil infrastructure in nearly every earthquake event. Direct and indirect economic losses resulting from liquefaction are substantial costs to society. The tremendous damage to property and the indirect economic loss resulting from the Christchurch New Zealand earthquake sequence dramatically illustrate the broad social impacts of failure to properly design for liquefaction hazards. Large sections of the city are still uninhabited years after the earthquakes and the economy is struggling to recover. This collaborative research study in New Zealand will provide fundamental understanding of the performance of typical deep pile foundations for bridges and high-rise structures when subjected to liquefaction. 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. These results will help engineers to design safe, yet cost effective foundation solutions. Results from this research will benefit both New Zealand and US engineering communities and foster international collaborations. Leveraging funding from both countries expands the scope of work beyond what either country could accomplish independently. Considering the paucity of test data relative to liquefaction induced loads on foundations, these test results are likely to have a strong impact on design practice in seismically active areas around the world.