Interpretation of Seismic Response Data from Full Scale Helical Piles Tests-Paper

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This study investigates the seismic response of helical piles using full-scale shaking-table tests conducted at the NEES/UCSD Large High Performance Outdoor Shake Table. Ten steel piles with different configurations were installed in dense, dry sand; the analysis presented focuses on six piles, including single- and double-helix piles, a driven pile, and a square-shaft helical pile. Piles were tested with different head masses under white-noise excitation and scaled records of the 1994 Northridge and 1995 Kobe (Takatori) earthquakes.<br /><br />Strain-gauge measurements were used to calculate bending moments and infer pile deflections. Because measurements were available only at discrete depths, the researchers compared global polynomial and spline methods for fitting moment profiles. Quintic interpolation splines performed better than global polynomials, which could develop large oscillations near the pile tip and produce substantial deflection errors after integration.<br /><br />White-noise responses were analyzed to estimate the natural periods of the soil–pile systems. Circular helical piles had periods of 0.43–0.53 seconds, while the driven and square-shaft piles had shorter periods of 0.24 and 0.29 seconds, respectively. The findings indicate that seismic response depends strongly on how closely a pile system’s natural period matches the dominant period of ground motion. Thus, circular helical piles responded more strongly to the longer-period Takatori motion, whereas the driven and square-shaft piles were more responsive to the shorter-period Northridge motion.<br /><br />Overall, the results show that spline-based moment fitting is suitable for interpreting the test measurements and that pile configuration and earthquake frequency content are important considerations in assessing helical-pile performance. The authors note that further research is needed to develop understanding of these foundations under seismic loading.

Keywords

helical piles

shaking-table tests

seismic response

soil-pile systems

bending moments

spline interpolation

white-noise excitation

Northridge earthquake

Kobe Takatori earthquake

pile natural period