Dynamic stiffness and damping of single piles in loess

Pluck tests were conducted to determine the lateral dynamic stiffness and damping of single piles embedded in loess. Two prestressed concrete, friction piles were tested just outside of Memphis, Tennessee. Each pluck test consisted of giving the pile head an initial deflection, then releasing it instantaneously. The dynamic stiffness and damping were determined from the free-vibration response of the pile. Each pile was tested at several initial deflections, which were progressively increased.

Dynamic stiffnesses ranged from 63 kips/inch at the smallest initial deflection to 13 kips/inch at the largest for one pile. For the other pile, the largest stiffness was 45 kips/inch and the smallest was 31 kips/inch. The smaller dynamic stiffness values determined for the first pile mentioned and the larger values determined for the second were believed to be artificially low due to saturated soil conditions. The damping ratios for the first pile ranged from 9% to 13% while those for the second pile ranged from 5% to 7%. Computer models of the test piles were developed to match the lateral stiffness determined from the tests. The models were then used to "transfer" the dynamic stiffness from the center of mass to the ground surface.