Little Bras d’Or Cofferdam, Trestle and Bridge Replacement
Location: Little Bras D’Or, NS, Canada
Client: J&T Van Zutphen Construction, NS Department of Transportation and Infrastructure Renewal
Duration: 2010
Key Staff: Paul Burke, Robbie Fraser, Kyle Boudreau
Cofferdam and Trestle
Project Description
Local tourism in the Little Bras D’Or area is dependent on the access provided by the Little Bras D’Or bridge. It acts as the only non-load related link to the Newfoundland ferry in North Sydney. As the bridge was in a deteriorated state, it was slated for replacement in 2010.
To allow for the installation of the center pier of the new two-span steel box girder bridge in Little Bras D’Or, a large steel cofferdam and temporary trestle were required. The design of the cofferdam called for a complex construction sequence to allow for installation of the pier in both wet and dry conditions.
Harbourside’s Role
Harbourside completed the cofferdam design to retain over 13 m of water with large current loads (exceeding 5 knots). This was in addition to 5-meter of differential soil lateral pressures while maintaining an open annulus within the cofferdam. This was to allow for excavation of the soils and installation of the pier footing.
The entire rebar cage for the pier footing was constructed above water on a platform supported by the cofferdam and subsequently suspended from the cofferdam and lowered into position. This technique allowed the footing to be installed by tremie concrete method in the wet. Once the footing was installed and acting as a plug to resist uplift seepage pressures, interior bracing was placed within the cofferdam and the entire interior was dewatered to allow the remaining pier to be constructed in dry conditions.
To provide access for the installation of the center pier as well as access during the subsequent girder launch, a 68 m temporary trestle was required. HEC completed the design consisting of a timber crane mat deck spanning between steel stringers that were supported on rigid frame double pile steel bents driven to bedrock.
Unique Challenges
The unique aspect of working in a marine environment offers interesting challenges. The volume/depth of water required careful consideration for the design of the cofferdam.
Bridge Replacement
Project Description
Following the development of the Little Bras D’Or Cofferdam and Trestle, Harbourside was involved with the replacement of the existing bridge structure. Located on a critical stretch of highway, the new bridge replaces an old, deteriorated structure that represented the only non-load rated link to the Newfoundland Ferry in North Sydney.
Harbourside’s Role
Harbourside provided the detailed design of the new bridge as well as construction engineering services for the steel fabricator/erector (girder launch) and general contractor (deep water cofferdam and pier access trestle).
The Little Bras d’Or Bridge, located on Highway 105 in Cape Breton County, is a two span, 120 meter long (60m – 60 m), twin trapezoidal steel box girder bridge spanning over St. Andrew’s Channel. The bridge is supported on semi-integral piled abutments and a deep water reinforced concrete hammer-head pier supported on a rock anchored spread footing founded on bedrock.
Social Impact
Acting as a primary channel for visitors coming in on the Newfoundland Ferry, this stretch of highway is critical to the economy of North Syndey. Rapid development was necessary to ensure that the bridge replacement progressed as quickly and smoothly as possible. The Little Bras D’Or bridge is integral in keeping Newfoundland and Nova Scotia connected. Other than the ferry, the main link between Newfoundland and Nova Scotia (or the rest of Canada), is by air travel.