2021 #CCEawards Showcase: Mill Creek Ravine Pedestrian Bridge Rehabilitation

Photos courtesy ISL.

Category: Transportation

Award of Excellence Winner: ISL Engineering and Land Services

The Mill Creek Ravine is both a recreation destination and a commuter link to downtown Edmonton. Working with the municipal government, ISL rehabilitated two pedestrian and three historic railway-turned-pedestrian bridges within Mill Creek, improving their durability and safety.

Historic timbers were used where possible to maintain the original design, while structural engineering focused on reducing wear on bridge components interacting with the creek.

Preserving the past for the future

The City of Edmonton wanted to maintain the functionality of the Mill Creek Ravine trail system by extending the service life of its pedestrian bridges. Built in 1902, three of the bridges were originally part of the Edmonton, Yukon and Pacific Railway (EY&PR). The trestle bridges were converted for pedestrian use in the 1970s.

Community engagement sessions emphasized the importance of preserving the heritage value of the structures. Thus, retaining part of the original ‘building fabric’ in the final construction was a key objective of the design process. To achieve this goal, the bridges were completely dismantled and each piece of timber was carefully graded and catalogued. This step added a significant challenge to the construction process and required a high level of co-ordination between ISL and the project’s prime contractor, Alberco Construction.

A timber resistograph microdrill was used to determine which timbers could be salvaged. In the end, approximately 20% of the original timbers were salvaged and included in the final bridge construction.

Additionally, ISL had to consider safety improvements to meet current requirements. This included widening the bridge decks to meet current design standards for multi-use trails. New handrails were also designed to meet current safety standards for pedestrians and cyclists, but also to match the architectural style of the timber bridges. In these ways, ISL was able to maintain the original character of the bridges.

Reducing repairs

Timber rotting issues had been an ongoing maintenance challenge for the city. The existing timber pier columns were consistently rotted just below ground level and piers in the creek were in even worse condition.

ISL was determined to design a durable solution that would reduce repair demands. All foundations were converted to a concrete pile-and-cap design that elevated the timber just above grade. The team also removed the timber pier columns from the creek and replaced them with a timber truss span designed to maintain the authenticity of the historic bridge appearance.

Bridge decking had also been a maintenance concern for the city. The underlying problem was sandwiched layers of timber placed over the historic rail ties, which trapped moisture and promoted rotting. Addressing this concern, ISL designed a free-draining bridge decking solution, with large timbers connected from below the deck using galvanized hardware.

Approximately 20% of the original timbers were salvaged and included in the final bridge construction.

Minimizing disturbance

Trail closures were necessary to complete bridge construction. A pedestrian accommodation plan and construction staging were crucial to ensure logical connectivity of the trail system. Clear signage with multiple detour options was essential to accommodate all trail users. Much of the construction work was consolidated over a winter season with fewer pedestrians and park users, to reduce the impact on the public.

Construction access was also a challenge due to the sensitive environmental area and topography of Mill Creek Ravine Park. Minimizing disturbance to the existing park space was a key consideration during construction.

Construction roads were lined with rig mats to reduce damage and provide access to laydown areas closer to the bridge locations. Off-site fabrication of timber trusses was efficient and allowed erection of the bridge spans using cranes from the top of the ravine.

Slope stability had become an issue on the steep south bank of one of the trestle bridges, threatening its long-term structural integrity. Conventional methods for installing a concrete pile stability wall were not possible, due to restricted access for large equipment on the steep slope. The design team addressed this access problem by designing micropile foundations that could be installed with smaller and lighter equipment.

Site cleanup

Tree removal was required adjacent to the bridges for structural concerns, where trees had become entangled with the existing trestle piers. Modest tree clearing would also promote good air flow around the bridges, aiding in the drying process and improving the long-term health of the timber structures.

The project team worked with the city’s urban forestry group to document and provide compensation for the tree removals. The areas around the bridges were restored with low-height naturalization planting.

The original trestle bridge piers were removed from the waterways and replaced with timber truss spans. The new clear spans improved the structural durability and hydraulic performance of the bridges, which were previously susceptible to debris jams.

Many years of railway operation through the Mill Creek Ravine had contributed to accumulation of hydrocarbons in the soil adjacent to the trestle bridge. Soil remediation was included in this project to clean up the slopes.

ISL and Alberco worked through the winter months to minimize impact to the popular trails and ensure the public could enjoy the newly rehabilitated bridges throughout the following summer. They are now expected to last 50 years with regular maintenance before needing another upgrade.

Mill Creek Ravine Pedestrian Bridge Rehabilitation, Edmonton, Alta.

Award-winning firm (prime consultant): ISL Engineering and Land Services, Edmonton, Alta. (Troy Letwin, P.Eng.; James Chapman, P.Eng.; Scott Murray, P.Eng.).

Owner: City of Edmonton.

Other key players: Thurber Engineering (geotechnical), Golder Associates (hydrotechnical), Spencer Environmental Management Services (environmental), Twenty/20 Communications (public engagement), Alberco Construction (prime contractor).