RAPID: Downdrag Behavior of Piles & Drilled Shafts After Liquefaction

Brigham Young University, Provo UT

Investigators

Abstract

Earthquake shaking can cause loose sands below the water table to liquefy and behave like heavy liquid, causing buildings, bridges and ports supported on shallow foundations to settle, slide or tip over. Direct and indirect economic losses resulting from liquefaction are substantial costs to society. One solution to this problem is to use deep foundations which penetrate through the liquefiable sand into a dense layer that doesn't liquefy. However, settlement of the liquefied sand can still cause downward friction on the foundation wall, increasing the load and leading to excessive settlement. In the absence of field measurements, engineers are uncertain about the amount of downward friction that will develop and how much settlement will occur. Because of this uncertainty contradictory design procedures are used. To remedy this problem, small explosive charges will be used to produce liquefaction under full-scale conditions and measurements will be made of the downward friction and foundation settlement. Tests will be performed on six different foundations at a site in Arkansas. Primary funding will be provided by the Arkansas State Highway and Transportation Department; however, supplemental funding from NSF will significantly expand the scope of the project. 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 safe, yet cost effective foundation solutions which will remain stable during earthquakes. To improve our understanding of liquefaction-induced downdrag on deep foundations, blast induced liquefaction tests will be performed on three full-scale drilled shafts and three driven piles (steel pipe, H sections, and pre-stressed concrete) at a site in Arkansas. NSF funding will be used to provide the static load that will be applied to each test foundation to simulate dead load while soil settlement and pile settlement will be measured after liquefaction. The objective is to measure negative skin friction and downdrag induced settlement. With the inclusion of a static load, 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.