Laboratory evaluation of piles installed with vibratory drivers.

This report presents the results of a large-scale laboratory study on the basic behavior of displacement piles installed with vibratory drivers compared to impact hammers and the influence of various soil and driver parameters on the behavior of piles. In order to achieve the desired goals, a model testing system consisting of a long sand column capable of simulating deep sand deposits, instrumented 4-in.-diameter closed-ended pile, vibratory driver and impact hammer was designed and built. Among the driver parameters investigated are frequency, bias mass and dynamic force (eccentric moment) and sand parameters such as grain size, relative density (65 and 90%) and in-situ effective stress (10 and 20 psi). Two uniform sands with effective grain sizes of 0.2 and 1.2 mm were selected for this study, and a total of 22 large-scale model tests were performed. The optimum frequency for the test conditions, selected based on the maximum rate of penetration, was 20 Hz and was independent of bias mass and soil conditions. Among the variables investigated, the relative density of sand had the greatest effect on the rate of penetration during vibro-driving. Penetration rate also increased with increasing bias mass and decreasing in-situ horizontal stress. Grain size had a smaller, and variable, effect on rate of penetration and on bearing capacity. Impact-driven piles in sand with 65% relative density developed 25% higher shaft resistance and 15 to 20% higher toe resistance in compression than the vibro-driven piles, but this trend was completely reversed at 90% relative density, where the vibro-driven pile exhibited better static performance than the impact-driven pile. The uplift resistance that developed along the shaft of both vibro-driven piles and impact driven piles was 75% of the corresponding resistance developed in compression. Restriking of vibro-driven piles in sand with 65% relative density produced a very small increase in compression capacity, but there was no clear trend for the relative density of 90%. A design method has been proposed to predict the bearing capacity of a vibro-driven pile from rate of penetration, power delivered to the pile head and soil conditions.