We report on snap-back testing of vertical 220 mm diameter steel tube piles driven into a stiff clay soil profile at a site to the north of Auckland in New Zealand. The site where the piles were installed was investigated with CPT and dilatometer measurements and shear wave velocity profiles were obtained through geophysical testing. The piles were subject to gradually increasing lateral loads prior to snap-back release which provided information about the static lateral load behaviour of the piles. We present three damping curves: (ⅰ) as a function of the lateral displacement at snap-back, (ⅱ) when the cyclic displacement amplitude is 1% or a little less than the pile shaft diameter, and (ⅲ) when the cyclic displacement amplitude is 0.2% or less than the pile shaft diameter. The fact that these curves are different indicates that damping under lateral pile head vibration cannot be charactised by a single value for the equivalent viscous damping ratio. However, we suggest that if working within in a displacement based design framework how it might be possible to decide on a representative value for the damping parameter. We modelled the inertial dynamic response of the piles with the finite element software OpenSeesPL which gives 3D dynamic modelling of pile-soil interaction, allows for nonlinear stress-strain behaviour of the soil, and prevents tensile stresses in the soil near the pile shaft. The nonlinear finite element analyses indicate that after many snap-back releases the stiffness of the soil adjacent to pile shaft is much degraded. Reasonable modelling of snap releases from small pull-back forces was achieved but for large pull-back forces, after many snap releases from smaller forces, the frequency of the response was matched only with a large reduction in the soil stiffness, but the damping in the computed response was considerably less than for the measured values.
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