Examining Setup Mechanisms of Driven Piles in Sand Using Laboratory Model Pile Tests

In this study, model pile tests were carried out in a custom-made pressurized chamber to explore the mechanisms of a driven pile setup in dry sand. A bender element system and tactile pressure sensors were used in parallel to monitor the stiffness and stress changes in the soil surrounding the pile during the test. The experimental results demonstrate that pile setup is not caused by the increase in at-rest radial stress sigma rp. Instead, it is mainly attributed to the increase in radial stress during pile loading sigma rp, as a result of soil aging (or creep). Pile installation pushes the surrounding soil to the side, thereby imposing additional loading on the soil inside the influence zone. This loading action initiates an associated aging (or creep) process during the setup period; the aging effects ultimately give rise to an increase in both sigma rp and pile shaft resistance. The measurements also reveal that the increase in aging-induced soil stiffness is due to contact normal forces among soil particles gradually becoming more homogenized during the setup period. This suggested setup mechanism could explain the absence of pile setup in bored (nondisplacement) pilesthe loading action induced by pile installation is too insignificant to trigger the required aging process. Using a similar logic, the mechanism could elucidate why the setup rate is higher in large-displacement piles than in small-displacement piles. (C) 2014 American Society of Civil Engineers.