Halton Region is undergoing a series of infrastructure initiatives to service population and employment growth to the year 2021. One of these initiatives is a strategic watermain that conveys Lake Ontario water from the Burloak and Burlington...
Halton Region is undergoing a series of infrastructure initiatives to service population and employment growth to the year 2021. One of these initiatives is a strategic watermain that conveys Lake Ontario water from the Burloak and Burlington water distribution systems to service increased water demand in North Oakville and Milton.
A challenging part of the project was crossing Bronte Creek. The creek valley is approximately 30 metres deep and 200 metres wide, and with its diverse habitats for aquatic and terrestrial species it is designated an Environmentally Sensitive Area and an Earth Science Area of Natural and Scientific Interest.
Halton Region retained R.V. Anderson Associates to carry out the Class Environmental Assessment (EA) Study, detailed design, and construction contract administration of the watermain crossing.
The Class EA study considered three alternatives: (a) supporting the watermain on a new pipebridge founded on four old piers that remain from a former highway bridge spanning Bronte Creek; (b) tunnelling the watermain under the Bronte Creek valley; (c) supporting the watermain from the existing highway bridge.
Ultimately concept (a) using a pipebridge on the historic piers was determined to be the preferred solution. A pipebridge is very similar in design to a traditional roadway bridge, but contains a narrower deck slab and a lower load-carrying capacity since it is not expected to support heavy traffic. Since entirely new piers were not required in this concept, it offered the least impact on Bronte Creek’s environmental features. It could be constructed at the lowest capital cost, and it could be implemented more quickly than the other alternatives.
The four free-standing piers were once the major support system for the Tansley Bridge, a highway bridge that was constructed between 1917 and 1919. Most of this bridge’s superstructure was demolished in 1948 after an adjacent highway bridge was erected.
The piers reflect the engineering and architectural achievements of the early 20th century. They are cast-in-place concrete structures, each measuring 30 metres in height, 5.5 metres in length, and 2 metres in width. Each pier includes a large arch shape opening, intricate cross-beam design with recessed rectangular facings, and a gradual three-directional tapering of thickness from the pier’s base to its top. Due to their history, the piers were identified as having value as Built Heritage Resources.
A specialist in built heritage assessments, Unterman McPhail Associates, was retained to ensure that the old bridge piers were properly managed. The reclamation and refurbishment of the concrete piers preserves the history of these distinctive structures.
A steep climb
Although the piers survived as firm, rigid structures for almost a century, the integrity of the concrete and steel had to be verified before detailed design could be undertaken.
Visual inspections found areas of spalling and cracks consistent with the piers’ age and use, as well as exposed rebar and rust stains. Given their locations within the valley, access was challenging and limited to non-traditional methods. The Region engaged a climbing specialist, Remote Access Technology, to ascend the piers and report on the surface concrete. Their skilled rope access technicians took core samples for compressive strength tests (to assist in determining its load bearing capacity), and they performed cover meter surveys (to determine the rebar locations and depth of concrete). The results showed that the piers were suitable for the intended purpose.
Following extensive coordination meetings with the approval agencies and local utilities, aerial wires invading the air space over the future bridge were decommissioned and removed. The contractor, Varcon Construction, then commenced with the pier refurbishment.
“Pier patching” was conducted by chipping down to sound concrete and installing anchors to tie reinforcing steel. Once the concrete was placed and cured, the forms were removed, and the complete surface area of each pier was covered in a cementitious coating to protect it from water damage and salt spray from the adjacent highway bridge.
New east and west abutments were constructed as the existing abutments needed replacing. The demolition and construction were challenging due to the close proximity of the existing roadway bridge and steep terrain. The abutments comprise concrete grade beams supported by drilled concrete caissons anchored into bedrock.
Cranes in tandem, a wagon, and other innovations
With the piers complete and the abutments in place, Varcon mobilized two large cranes to erect the girders. The precast concrete girders support a cast-in-place deck. Two girders span between two piers. Each girder is approximately 30 metres in length and weighs about 43 tonnes.
The environmental sensitivities and concerns about the load-bearing capacity added challenges to the construction. While a strategically placed crane could complete most of the lifts, using two cranes working in tandem allowed the work to be completed mostly outside of the valley and expedited the construction.
The 900-m diameter watermain was installed on the bridge using an innovative concept developed by Varcon that incorporated a gantry crane, in conjunction with a wagon propelled by manpower. Each pipe segment is 6.8 metres in length, elongated from the typical 6.1 metre lengths to better facilitate construction. The main was constructed using concrete pressure pipe with flexible joints to manage normal bridge movements. Insulation and heat tracing were installed around the exposed portions of the pipe, with the entire assembly later encased in galvanized steel.
Another unique feature used was a custom-built trolley, complete with a crane and winch system, to install the concrete parapet walls onto the pipebridge. The walls were designed to match the former 1919 bridge structure, incorporating patterns reflecting the original style and exposed aggregate finish. The aesthetics are an improvement over a utilitarian pipebridge and heighten the built heritage significance.
Long-life and overall project sustainability were key considerations throughout the design and execution. The precast parapet walls, as well as the concrete deck, incorporate stainless steel reinforcements to mitigate corrosion. The parapet walls’ aggregate finish is covered with a brush-applied clear-coat sealant to further enhance their durability against the environment and the deicing salt overspray from the adjacent existing roadway bridge.
New functional infrastructure
Varcon finished construction in October 2013 at a cost of $9.5 million and the watermain is now augmenting the water supply to the northern reaches of Halton Region.
The Bronte Creek valley area has been rejuvenated with fresh vegetation and enhanced with erosion control measures. The architectural features of the two adjacent bridge structures — existing and new — now complement each other in both style and functionality, and the abandoned piers from the original 1919 bridge have been repurposed into a new functional piece of water infrastructure that will last for decades to come. cce
David Simpson, P.Eng., is manager of water planning services, and Joe Proietti is project manager II, water services, both with Halton Region, Ont. Nathaniel Andres, P.Eng. is associate, project manager, with R.V. Anderson Associates of Toronto.
Owner: Halton Region
Prime consultant: R.V. Anderson Associates, Toronto (Nathaniel Andres, P.Eng., Vireak Hinh, P.Eng., Hans Vierhuis, P.Eng.,
Ken Collicott, P.Eng., Reg Andres, P.Eng., Don Hull)
Other key consultants: Unterman McPhail Associates
assessment); Remote Access Technology
(pier concrete assessment); Coffey Geotechnics (geotechnical
investigation). LGL (natural environment)
Contractor: Varcon Construction Corporation