The City of Trail, B.C. was faced with both a problem and a one-time opportunity.
The Old Trail Bridge across the Columbia River was cresting 100 years of age and had been closed to traffic — vehicles or pedestrians — since 2010.
Temporary rehab of the bridge would have cost an estimated $10 million.
But the secondary purpose of the structure, supporting a large sewage pipe across the Columbia, provided leverage for another approach. If the Regional District of Kootenay Boundary agreed to replace the sewer inceptor, a shared-cost pedestrian/pipe bridge could be affordably built across the Columbia.
"That’s really what the genesis of the project was about," says Aidan Connell, project manager, Graham Infrastructure LP, the company chosen to build the new bridge. "How can we take the old sewer pipe from the old bridge and find a new home for it?"
The solution was a 1,000-foot pedestrian suspension bridge that could support a sewer pipe, water pipe and additional future infrastructure, including a fibre optic line.
"In our 90-year history, we have built hundreds of bridges," says Connell. "But we had yet to build a suspension bridge and our first one turned out to be one of the longest pedestrian suspension bridges in North America."
The total cost of the project was $15.5 million, with costs shared between the city and the Regional Sewer Service. The bridge represents the single largest capital project in the city’s history.
The project was designed by COWI North America Ltd. Project management was provided by TRUE Consulting. The original construction contract with Graham Infrastructure totalled $12.2 million.
The bridge is supported by two concrete and steel towers, anchored by steel piles, one on each side of the river.
The four main cables that form the suspension bridge were gradually extended across the chasm using a winch. Four additional backstay cables on either side of the river were anchored to the ground to keep the towers erect under tension.
"There was a lot of engineering that went into balancing the tension between the two bridge towers," says Connell. "We had to re-tension the backstay cables periodically as we added weight to the bridge, in order to keep the towers plum."
The contractor built a crane system by suspending steel cables on temporary steel structures fixed to the tops of the towers. A platform attached to the cables moved personnel and materials across the length of the bridge.
The deck of the bridge was made of steel, topped by a lightweight fibre-reinforced polymer decking. The weight of the material allowed for cost savings by reducing the size of the suspension cables and allowing the bridge to be built with single-leg support towers.
The deck was suspended anywhere from 10 to 15 metres above the Columbia River depending on the depth of the river at the time as determined by BC Hydro, which operates dams nearby.
"The last elements to be installed were the 16-inch wastewater and 12-inch water lines," says Connell. "These were built from fusible HDPE, which was fused in 50-foot sections and then gradually pulled across the underside of the bridge using a plastic roller system."
The 12-foot-wide bridge can also accommodate a quad-track emergency vehicle and trailer to be used if an emergency requires a patient to be transported across the river.
The bridge, christened the Columbia River Skywalk, now forms part of the Trans Canada Trail network. It officially opened on Dec. 15, 2016, 13 months after construction began.
As much as Connell enjoyed the ribbon cutting ceremony, he says that his most enduring memory of the project was hauling the first man basket between the two towers.
"The design and engineering process was a lengthy endeavour," he says. "Knowing that our workers were travelling safely across that space represented the success of a strong collaboration between designer, engineer and contractor."