Yoho Bridge Replacement
January 1, 2006
By Canadian Consulting Engineer
The replacement of the Yoho Bridge is part of a series of ongoing improvements being made to the Kicking Horse Canyon section of the Trans-Canada Highway east of Golden in British Columbia....
The replacement of the Yoho Bridge is part of a series of ongoing improvements being made to the Kicking Horse Canyon section of the Trans-Canada Highway east of Golden in British Columbia.
The road is an important east-west corridor between B.C. and Alberta, carrying tourist and commercial traffic. However, because of the rugged, mountainous terrain, the road is also one of the most challenging to engineer. No major improvements have been done in decades, so upgrading it to a modern four-lane highway is the “number one priority” of the B.C. Ministry of Transportation.
Due to the complexity of the undertaking, the upgrades along the 40-kilometre Kicking Horse Canyon stretch are being done in three phases, at a total estimated cost of $725 million.
The $23-million project to replace the 1950s Yoho Bridge is part of Phase 1, as is the upgrading of 1.9 kilometres of highway to the east of the bridge, which is still under way. Phase 2, involving the Park Bridge section, is going through a procurement process, and Phase 3 for long-term final upgrades to 17 kilometres is still in long-term planning.
Urban Systems of Vancouver was lead consultant and project manager for the Yoho Bridge replacement, with Sandwell Engineering, Golder Associates, Brybil Projects and the Ministry of Transportation’s Geotechnical Branch all playing important roles.
A difficult site
The canyon is steep and narrow, and has the Kicking Horse River and the Canadian Pacific railway line running along its course.
Making the site even more of a challenge is the instability of the terrain, with its avalanche and rockfall chutes.
The design team compared numerous possible alignments to replace the existing 1950s bridge. The original structure had an S-alignment across the river, while the new crossing was to be straight. First the team favoured a long, relatively high-profile structure that could clear the existing bridge and thus avoid closing the highway during construction. However, the higher profile structure would have needed high retaining walls, and extraordinary and expensive measures to stabilize the foundations. Eventually, the team selected a shorter, lower profile crossing as simpler and more economical. Road closures were kept to a minimum by building a temporary bridge connection after the existing span was demolished. Both steel and concrete bridge structures were designed, but the concrete option was chosen.
The $23-million construction contract was awarded in July 2002, and construction was completed in October 2004.
Following are four of the most important innovative approaches taken in the project.
The new bridge is 270 metres long and crosses the river at a skew angle of 60 degrees. Due to the skew and multiple constraints on where piers could be placed, it was initially thought that the main span would be at least 64 metres. A structural concrete span of that length would normally be a cast-in-place box girder, which involves methods such as extensive falsework, balanced cantilevering or incremental launching. Each of these solutions is expensive, so a new approach was needed.
The selected design uses steel delta frames on top of the pier columns. The frames branch outwards to support the superstructure at the quarter-span points. In this way, the effective superstructure span is reduced to a maximum of 32 metres, a distance that is manageable using Type V precast, prestressed concrete I-girders. The main delta frame legs are concrete-filled steel box columns. The Yoho Bridge is believed to be the first delta frame bridge to employ a four-legged, concrete-filled steel-box column design springing from a single-column pier.
Bridge deck drainage system
Because the bridge lies on a sag in the highway profile, there is a potential for water to pond on the deck. In response, the highway and structural engineers collaborated to develop a redundant drainage system that can accommodate extreme storms or system blockages. The system has primary, secondary and tertiary inlets. The primary inlets are typical deck drains adjacent to the parapet. The secondary inlets are deck drains at a 2-metre offset from the parapet, while the tertiary inlets are large scuppers in the parapet, set approximetely 150 mm above the road.
Piled slab support
The approaches on both sides of the bridge had a history of instability so the engineers concluded that any placement of high embankment material could cause settlement or rotational failures. Using piled or tied-back foundations or lightweight fills to stabilize the embankments would be expensive. Instead, the team developed a low-cost piled slab system that transfers loads directly to the bedrock below. The scheme consists of an elevated deck supported on a series of piles built out over the existing embankment sideslope. An articulated transition slab accommodates the potential differential settlement of the slopes relative to the pile-supported slab.
Rock catch wall
To combat a perpetual problem with rock falls along the west approach, a massive retaining/rock catch wall was designed. It is 485 metres long, reaches a maximum height of 11.6 metres above the highway, and can withstand the impact of rocks up to 5,000 kilograms travelling at velocities up to 35 m/s.
A unique three-layer energy absorption system applies controlled forces to the upper wall. It consists of a gabion wall with a reinforced concrete core wall and an expanded polystyrene crushable layer. The gabion wall is wrapped in heavy gauge chain link fence to ensure its integrity and longevity. Rebound ties are detailed to ensure the stability of the freestanding core wall following an impact.
The project won an award of excellence from Consulting Engineers of B.C. in 2005.
Client/owner: B.C. Ministry of Transportation
Prime consultant, highway and drainage design, project management: Urban Systems, Vancouver (Tim Blackburn, P.Eng., Tom Lowe, P.Eng., Trevor Towers, EIT)
Structural subconsultants: Brybil Projects, Burnaby (David Harvey, P.Eng.) and Sandwell Engineering, Vancouver (Dick Reynolds, P.Eng.)
Geotechnical, bridge foundations: Golder Associates, Burnaby (Mark Goldbach, P.Eng.)
Geotechnical, retaining walls and slope stabilization: B.C. Ministry of Transportation Geotechnical Branch (Mike Walsh, P.Eng., Ian Pilkington, P.Eng.)
Hydraulics and scour protection: Northwest Hydraulics, Vancouver (Bruce Walsh, P.Eng.)