ABSTRACT
We present data obtained from field testing of vertical steel tube piles, some installed by driving and others as screw piles. The 8 m long piles were 220 mm in diameter with a wall thickness of 8 mm. The soil profile, a stiff cohesive material, had CPT qc values of about 2 MPa and a shear wave velocity of about 180 m/sec. A total of 12 piles were installed, 6 driven and 6 screw piles. The piles were arranged in 4 rows with 3 piles in line. Using the outer piles as reaction the central pile was pulled in alternate directions and the dynamic response recorded after sudden load release. This loading–snap release pattern was repeated under gradually increasing pullback forces for two piles; for another two piles the first phase of loading was at a large pullback force followed by series of tests at gradually increasing loads from an initial value of 5 kN. The results show that the driven piles had greater lateral stiffness than the screw piles. The cycle by cycle damping during the snap-back response depends on the amplitude of the vibration. Immediately after release the equivalent viscous damping ratio was 10 to 30 %. With each cycle the vibration amplitude decreased as did the damping ratio. It is suggested that the large damping value immediately after snap release is appropriate for design calculations, rather than the smaller values observed when the amplitude of the motion has decreased to 1% or less than the pile shaft diameter. After each snap-back response the vibration of the pile under hammer blow excitation was recorded, from which the elastic response of the system was observed and the natural frequency obtained. It was noted that the natural frequency decreases as the pullback load increases. This is interpreted as a consequence of gap formation between the pile shaft and the surrounding soil. From the decrease in natural frequency the depth of the gap, up to about 2 pile diameters, was inferred.
