Incorporating Setup Effects into the Reliability Analysis of Driven Piles

Driven-pile setup is referred to a phenomenon in which the bearing capacity of driven piles increases with time after the end of driving (EOD). The setup effect can significantly improve the bearing capacity (ultimate resistance) of driven piles after initial installation, especially the ultimate shaft resistance. Based on the reliability theory and considering the setup effects of driven piles, this article presents an increase factor (Msetup) for the ultimate resistance of driven piles to modify the reliability index calculation formula. At the same time, the correlation between R0 and Rsetup is comprehensively considered in the reliability index calculation. Next, the uncertainty analysis of load and resistance is conducted to determine the ranges of relevant parameters. Meanwhile, the influence of four critical parameters (factor of safety FOS, the ratio of dead load to live load ρ = QD/QL, Msetup, the correlation coefficient between R0 and Rsetup, and ρR0,Rsetup) on reliability index are analyzed. This parametric study indicates that ρ has a slight influence on the reliability index. However, the reliability index is significantly influenced by FOS, Msetup, and ρR0,Rsetup. Finally, by comparisons with the existing results, it is concluded that the formula proposed in this study is reasonable, and more uncertainties are considered to make the calculated reliability index closer to a practical engineering application. The presented formula clearly expresses the incorporation of the pile setup effect into reliability index calculation, and it is conducive to improving the prediction accuracy of the design capacity of driven piles. Therefore, the reliability analysis of driven piles considering setup effects will present a theoretical basis for the application of driven piles in engineering practice.

Time-lapsed shear-wave velocity (Vs) tomographic images and stress in sand surrounding displacement pile during setup

This study reports model pile tests designed to characterize the underlying mechanisms of driven pile setup in dry sand by means of stress measurement with the tactile pressure sensors and spatio-temporal, shear-wave velocity (Vs) distributions using an automated high-speed tomographic imaging system. The pile-load test results demonstrate a distinct increase in the pile shaft resistance after pile setup. The measured stress and Vs in the soil surrounding the pile suggest the increase in the radial effective stress during pile loading [Formula: see text], as a result of the aging-induced stiffness increase, is the dominant mechanism of pile setup. The spatio-temporal evolution of Vs distributions reveals that during initial aging (before pile installation), Vs is similar at any distance to the pile shaft and exhibits a similar aging rate S0 in terms of stiffness increases. After pile installation, the soil exhibits a higher aging rate S1. In addition, the ratio S1/S0 decreases with increasing distance from the pile shaft. A lower initial Vs and a higher aging rate S1 are also observed for the measurements at the three sensing layers of different depths, suggesting that more soils disturbed by pile installation tend to recover at a relative higher aging rate.