Project Overview
Serving nearly 9 million ferry riders per year, the Colman Dock in Seattle, is Washington State Ferries’ busiest terminal and plays a critical role as a regional, multimodal transportation hub. After decades of wear and tear, the terminal’s trestle, which was originally supported by timber piles from the 1938 construction, was in desperate need of replacement. For this project, the Washington State Department of Transportation (WSDOT) and Washington State Ferries worked with project engineering firm WSP and Concrete Technology Corporation, choosing precast concrete as the best material for meeting both short- and long-term goals.
A Beacon for Seattle
Due to the massive nature of the project and restrictions for when in-water work could occur, construction at Colman Dock was spread out over the course of seven years. Rapid completion of the overall project was not a goal of the team. Instead, WSDOT officials aimed to accelerate individual phases of the work so that traffic patterns could be switched from soon-to-be demolished areas to newly completed ones, keeping the terminal open and functional throughout the process.
Mike Wray, senior vice president of ports and marine engineering for WSP, noted that, keeping Colman Dock open throughout construction was not the only priority. “Minimizing disruption to the surrounding areas and avoiding environmental impact were crucial,” he said. “And this needed to be accomplished while keeping the project on time and on budget.”
Colman Dock’s new trestle deck is supported by more than 750 precast and prestressed concrete panels sitting atop newly driven steel pipe piles. Trestle deck placement occurred over the course of five years and was performed in phases. Each of the panels was designed with a maximum thickness of 27-in. The precast concrete panel elements included 44 haunched, 32 prismatic, and 32 trapezoidal prismatic shapes. According to Wray, selecting precast for the panels brought a significant number of advantages to the project.
“The precise nature of precast concrete fabrication allowed for the advance coordination of complex utility attachments, eliminating the need for site drilling and installation under the deck and over the water,” said Jim Parkins, PE, vice president and director of marketing at Concrete Technology Corporation. “Additionally, the pretensioned elements of the precast concrete helped accommodate long spans of 40 feet, leading to lower costs for the substructure and mitigating environmental impact from the pile driving.”
The use of precast concrete also helped ease traffic in a highly congested area. Barges were used to deliver the precast concrete components, which represented 95% of the concrete installed on the project. If a cast-in-place alternative had been selected, more than 850 ready-mix trucks that would have been on the road.
Another advantage of the precast concrete design is the low permeability and high resilience of the precast and prestressed concrete panels. They boast a 75-year service life, meaning that everyone who uses Colman Dock can count on the transportation hub to continue servicing the region for years to come.
“Colman Dock is a beacon on the Seattle waterfront,” said Suanne Pelley, construction communications manager at Washington State Ferries. “The new facility is a vast improvement over the old one. Passengers benefit from a seismically sound concrete and steel trestle, main terminal passenger building, entry and welcome building, elevated pedestrian connections, retail spaces, and a passenger-only foot ferry.”
Mason Nichols is a Grand Rapids, Mich.-based writer and editor who has covered the precast concrete industry since 2013. |