Assessment of the Unified Design Method for Piles Subjected to Negative Skin Friction Using Field Studies and Numerical Modeling

This study presents field measurements and numerical simulations to evaluate drag force and downdrag as a result of negative skin friction on an H-pile driven into clay till stratigraphy subjected to embankment loading. The validated numerical model results with field measurement data as well as pile dynamic test results are compared with predictions from the unified design method. The comparison is made for two pile positions in the cap, the most and the least critical, based on axial force along the shaft. Findings indicated that both methods demonstrated the neutral plane at nearly the same depth. However, the unified design method overestimated the maximum axial force along the shaft, as well as drag force and downdrag, in both pile scenarios. This method also indicated a smaller difference in maximum axial force between the most and least critical pile locations compared to the numerical modeling. Finally, the unified design method may neglect the shaft setup effect in calculating the target resistance of piles in soils with this potential. Considering the setup effect in evaluating the pile resistance subjected to negative skin friction can lower the neutral plane, reduce downdrag estimates, and increase drag force capacity, leading to a more efficient design.