This report presents a state of practice and literature review study conducted as part of NCDOT Research Project RP 2018- 17 on foundations for coastal mast arm traffic signal structures. The original scope of this project involved experimental and computation research on alternative foundation systems for support of coastal mast arm traffic signal structures in areas with high wind loads, limited right-of-way, and poor geotechnical conditions. However, at the request of the Steering and Implementation Committee (SIC) of this project, the focus was changed to entail a state of practice (SOP) study to document the foundation systems used by coastal departments of transportation to support coastal mast arm traffic signal structures. A total of 12 DOTs participated in this SOP study. The main objective of this SOP study was to document the predominant foundation systems used and identify any special foundation design practices used for mast arm traffic signal structures in coastal environments with high wind loads, small right-of-way, and poor geotechnical conditions.
The SOP study revealed that the most commonly used foundation system to support coastal traffic signal mast arm structures was a single conventional drilled shaft. Occasional use of a drilled shaft with wing walls was reported by NCDOT, VDOT, and ALDOT for structures with high torsional loading demand on the foundation. However, both VDOT and ALDOT reported that in recent years their practice was moving towards eliminating wing walls in the foundation design due to construction and installation difficulties. The SOP study also revealed large differences in the procedure for selecting wind speed and the associated foundation loading demand. These differences are attributed to variations in timelines for transitioning from allowable stress design (ASD) to load and resistance factor design (LRFD) as well as significant changes in the load factors and wind speed maps used in the design of mast arm traffic signal structures. These differences make the comparison of design practices between coastal DOTs challenging.
This report also includes a literature review that summarizes research on drilled shafts under the complex, multi-directional loading present in traffic signal mast arm structures. Specifically, the combined eccentric lateral and gravity loads on mast arm traffic signal structures lead to axial, shear, flexural, and torsional loads transferred to the mast arm foundation. The few experimental studies to date that have investigated the behavior of drilled shafts under these combined loads have highlighted significant reductions in lateral load capacity of drilled shafts when they are simultaneously subjected to torsion. However, all SOP participants use decoupled approaches for the design of drilled shafts supporting traffic signal mast arm structures that do not account for these interaction effects. The literature review also presents a summary of alternative foundation systems that have been proposed for supporting coastal mast arm traffic signal structures including driven post-grouted concrete piles, large driven pipe piles driven open or closed ended, and finned pipe piles.
