Damping characteristics of full-scale grouped helical piles in dense sands subjected to small and large shaking events

Abstract

A full-scale pile testing program was implemented using the large outdoor shake table at the University of California – San Diego. Nine steel helical piles with varying geometry were embedded in dense sand and tested individually and in 2×2 groups, comparing fixed and pinned pile head connections. The test piles were subjected to shake motions ranging from pulses and white noise to replicated earthquakes. Strain gauges attached to the exterior pile walls and accelerometers placed on the pile caps and within the soil provided data for analyzing the behavior of these piles. Foundation damping (herein soil–pile system) is a substantial parameter in seismic design of the foundation–structure. Therefore, the damping characteristics of the soil bed along with the combined soil–pile system consisting of single and grouped helical piles are discussed based on the experimental pulse, white noise, and shake excitations. Several methods, including logarithmic decrement, half-power bandwidth, and energy (equivalent) methods, were implemented to estimate the damping ratio over a range of strains. Based on the experimental data gathered from this study, the suitability and accuracy of different computational methods to determine damping ratio as well as the effect of type and location of instrumentation on the calculated damping ratio were evaluated.