Vibratory Loading of Pile Foundations

Abstract

This paper describes a unique investigation of the response of piles subjected to both dynamic and static loading, and presents findings relative to performance of pile foundations under these conditions. Preliminary load tests were conducted at NASA's Mississippi Test Facility prior to final design of massive static test firing stands for Saturn V rocket boosters. Friction-type, steel H-piles and point-bearing, concrete-filled pipe piles primarily supported by a dense sand layer overlain by fat clay and silty sand were loaded (a) statically up to 200 tons and (b) dynamically with combinations of 50, 100, and 200 tons dead load and a vertically directed, steady-state loading of dynamic force amplitudes ranging from 1 to 10 tons and frequencies ranging from 4 to 16 Hz. Also, a cluster of four H-piles was tested under combined loading. Typical data in the form of steady-state displacement and phase angle versus frequency plots indicated that resonance occurred at frequencies ranging from 5 to 10 Hz depending on the static load. The experimental data were compared with a single-degree-of-freedom analytical model with linear springs and viscous damping. At resonance, dynamic stiffness was greater than static stiffness for comparable piles, and damping was slight with computed damping ratios of 0.04 to 0.05 for single piles and about 0.10 for the four-pile cluster. Resonant frequencies could be predicted on the basis of static load-settlement curves, but resonant displacement could not be reliably extrapolated from test data at nonresonant frequencies. Permanent settlements occurred at resonance at higher dynamic force levels. Skin friction distribution along the pile length, as determined by strain gages, appeared similar in dynamic and static loading.