Time-Dependent Capacity Increase of Piles Driven in the Puget Sound Lowlands

Abstract

Piles driven in the Puget Sound Lowlands frequently exhibit a high magnitude of soil setup or gain in capacity over time. In fact, the energy associated with production pile driving hammers is often insufficient to fully mobilize pile toe bearing resistance during dynamic restrike testing. Due to the potential for significant increases in capacity, practitioners often must consider the uncertain trade-off between the potential for setup and time to restrike. This study presents the results of 76 pile case histories from the Puget Sound Lowlands that were dynamically monitored during end of drive (EOD) and begin of restrike (BOR). The setup time (i.e., time between EOD and BOR) ranged from 5.5 to 312.5 hours. Signal matching using CAse Pile Wave Analysis Program (CAPWAP) indicated that the majority of the piles experienced significant increases in shaft resistance. To assess setup, piles were grouped into three categories: closed-end steel pipe piles, open-end steel pipe piles, and solid concrete piles. The accuracy and uncertainty of the well-known Skov and Denver setup prediction model was evaluated using the mean bias, defined as the ratio of measured and predicted shaft resistance, and the coefficient of variation (COV), defined as the ratio of the standard deviation of the point biases to the mean bias. The Skov and Denver model exhibited considerable uncertainty (COV = 45 to 59 percent) when evaluated using the local database. Recalibration efforts made using a hyperbolic function and least squares regression resulted in a reduction of the COV to 29 to 32%. Based on considerable scatter in the setup ratio (i.e., ratio of BOR to EOD shaft resistance), it was determined that the behavior of setup over time could not be adequately captured by considering total shaft resistance. Therefore, setup due to the overall increase in shaft resistance between EOD and BOR was characterized in an initial effort to provide guidance for incorporating setup into pile design. Closed-end concrete piles exhibited the largest average setup ratio (3.92), followed by closed-end steel piles (2.37), and open-ended steel piles (2.26).