Field Method for Back-calculation of Dynamic p-y Multipliers and Damping

Johns Hopkins University, Baltimore MD

Investigators

Abstract

The design of pile foundations subjected to lateral dynamic cyclic loading such as caused by earthquakes, ship collision, etc., is still in need of a rational analysis method. In recent years, the applicability of the Beam on Nonlinear Winkler Foundation (BNWF) method for these problems has been studied. The BNWF method has been proven to work well for problems involving static, monotonic loading. A number of difficulties occur when trying to extend the static BNWF to dynamic problems. Among these, the difficulties of selecting a suitable damping model (series or parallel) and the corresponding damping values, a suitable method for accounting for the cyclic pore pressure build up, and a rational method for accounting for the pile-soil gapping stand out the most. One method of addressing these difficulties is to conduct a suitable field experiment, monitor the behavior and back-calculate the needed parameters from the field data. Recent studies indicate that the Statnamic testing device might be used for conducting lateral pile load tests. The Statnamic device produces a single pulse loading. We have analytically determined that, instead of applying the loading directly on the pile head, if it is applied through a single degree-of-freedom oscillator involving a mass and a spring, the motion of the pile head becomes cyclic. The objective of the proposed study is to develop a method of back-calculating the p-y multipliers and damping parameters directly from a set of field data derived from a dynamic cyclic lateral load test on single piles. In achieving the objectives, a series of field tests will be conducted at the test site of Turner-Fairbank Highway Research Center (TFHRC) of FHWA. Some of the tests will be conducted in a 18-foot deep test pit on wet and saturated sands and silty sands of different densities, and the others outside the pit in the natural soil which is clay. The pile and the surrounding soil will be adequately instrumented for measuring the needed quantities. Existing p-y curves based analysis methods will be modified to account for pore pressure effects and pile-soil gapping. Also, experiments will be conducted on a nine-pile group, and methods of applying the single pile data to analyze pile groups will be explored. The study will be conducted by a doctoral student and supervised by the PI. The doctoral student has already spent over an year on the analysis portion of the study and developed computer codes based on existing p-y curves based methods. The proposed modifications to the analysis method and experimental work will be completed in a period of two years.