Comprehensive Study of Induced Drag Force on Integral Abutment Piles

Large structures such as tall buildings, towers, and bridges transfer their loads to competent soil layers through pile skin friction and/or end bearing. When a pile is installed in a compressible soil layer, it may experience additional skin frictional force called “drag force” due to excessive soil settlement relative to the pile resulting from external loadings such as embankment construction. The location of maximum drag force, where skin friction is equal to zero or soil settlement and pile settlement become equal, is called the “neutral plane”. This thesis discusses the results of a comprehensive long-term field-monitoring program of three instrumented abutment piles and adjacent soil in order to evaluate the developed drag forces along the shafts of the three piles. The data collected from the field-monitoring program are presented and discussed in terms of measured responses with time, and load distribution along the pile shafts. In addition, the results were used to examine the unified design method and assess the design codes with respect to drag force. The findings indicated that the piles most likely behaved independently, and each experienced different magnitude of drag forces that extended to different elevations (i.e., neutral plane locations). A reasonable prediction of drag force can be achieved using the unified design method by considering the soil in the pile vicinity to be normally consolidated using both the total stress method (a) and effective stress method (b). The design of piles subject to drag force can be costly when following the AASHTO (2014), while the design following CHBDC (CSA 2006) and FHWA (2016) would result in safe and efficient design. Furthermore, the data were utilized to validate three-dimensional (3D) finite element models that were then employed to conduct a comprehensive parametric study to assess different aspects related to negative skin friction on single piles as well as group effects on drag force distribution among piles installed in small and large groups. The results revealed that the soil along the pile shaft moved with the pile and no soil plug was observed at the pile toe. Moreover, the analysis showed that the group effect might be neglected for piles installed in one row; however, it cannot be neglected for piles installed in large groups. Finally, design charts to calculate the toe resistance and group factor to calculate the drag force for a group of piles are presented in this study.