Degraded cast-in-place structures eventually need to be replaced but the costs associated with clear-span structures may not be affordable to all clients. Innovation in the pre-cast industry may be at a new level for ecological consideration with the new Barefoot Box Culvert™ designed by RJ Burnside & Associates Limited (Burnside). The Barefoot Box Culvert™ was designed to promote groundwater upwelling while maintaining thermal conditions – an acceptable pre-cast replacement for watercourse crossings.
The Township of Melancthon (Township) is responsible for numerous concrete structures that convey headwaters, which support sensitive cold-water species such as Brook trout that are protected and regulated under the Fisheries Act, governed by the Department of Fisheries and Oceans (DFO).
A structure replacement was required on a headwater stream of the Pine River that supports all life stages of Brook Trout, with numerous areas of groundwater input (critical habitat). The existing degraded structure was a “cast-in-place open-bottom culvert”. The old structure was experiencing severe deterioration and had a 10 tonne load limit posting which prevented local agricultural equipment and machinery from travelling over the structure. The Township requested Burnside to design a replacement for Structure 2027, located on Sideroad 15, approximately 0.2 km west of CR124, north of Shelburne.
Burnside worked with Con Cast Pipe to ensure that the Barefoot Box Culvert™ design could be precast, integrating all of the required specifications relating to structural integrity of the precast concrete box units and cut-off walls. The box units were designed according to OPSS 1821 and CSA S6 (CHBDC). The five pieces of 3658mm x 1829mm box units and two precast concrete cut-off walls were manufactured to CSA A23.4 – each box unit was 2.0m in length. The installation contractor, Drexler Construction Limited, was responsible for the removal of the existing structure, the installation of the new precast concrete culvert and the construction of the cast-in-place distribution slab that was required due the very low cover over the culvert.
Based on the sensitivity of the site and headwater system that supports Brook Trout, it was important to replicate the form and function of the watercourse as well as the existing conditions. Burnside proposed the 3658mm x 1829mm precast concrete box culvert that was designed to promote groundwater input through strategically placed perforations in the bottom slab of the box units. The Barefoot Box Culvert™ incorporates unique design characteristics such as the preformed holes in the bottom slab of the box units for groundwater discharge as well as strategically placed river stone simulating natural substrate, accommodating a meandering low flow channel within the structure. The plan view below illustrates the strategic placement of preformed holes to promote groundwater upwelling.
The profile below illustrates the comparison between the Barefoot Box Culvert™ and a typical open bottom culvert with footings. Based on the design comparison we note the following observations:
• No footing that forms a barrier blocking lateral ground water input
• Cut-off wall promotes ground water upwelling into perforations
• Cut-off wall creates the required hydro-static pressure to promote groundwater upwelling
• Speed of construction between a precast box culvert and a clear span requiring cast-in-place footing (substantial cost savings and ease of construction)
Supply and Construction
A budget estimate of $300,000 (plus HST) was allocated to complete the engineering and construction of the new structure. Burnside was required to identify a suitable design option that addressed the safety and environmental concerns at the site and fit within the Township’s budgetary constraints. The contract included demolition and removal of the existing 3.0 m span concrete open bottom structure, which was determined to be in poor condition and recommended for replacement.
Burnside successfully managed the culvert replacement by providing engineering services and contract administration for the Township. Con Cast Pipe delivered the client a precast box unit structure that addressed the environmental, safety and budgetary conditions associated with the project.
The 3658mm x 1829mm box units were manufactured at Con Cast Pipe’s Guelph, Ontario manufacturing facility. The units were cast in advance of the project installation date and were delivered to site in a “just-in-time” delivery scenario to eliminate wait times on site and expedite the installation of the precast box units. The openings cast in to the bottom slab of the box units were placed in accordance with the details provided by Burnside to ensure that the Barefoot Box Culvert™ units would perform as designed and would function properly to improve the man-made environment within the invert of the box units.
In order to monitor the success of the Barefoot Box Culvert™ and this new approach, a number of metrics needed to be determined prior to actual construction and placement of the structure. To determine if the design (base slab perforations, cut-off walls, bedding) worked, Burnside installed shallow stream bed piezometers at various depths to determine groundwater upwelling and hydraulic gradient. Each piezometer was outfitted with an Automatic Water Level Recorder to determine water levels within the piezometer compared to the natural stream conditions. Based on the results of the surface and groundwater data collected, all piezometers located within the Barefoot Box Culvert™ show an upward hydraulic gradient (increased water levels) that promote groundwater discharge and upwelling within the culvert (See Graph below). Piezometers revealed a 2-4cm increase in groundwater level above existing base flow conditions which could significantly increase groundwater contribution.
Additional monitoring is still being conducted with regards to groundwater levels, Brook trout use and underwater footage of spawning downstream of the culvert (<5m downstream) has been captured by Burnside.
As-built conditions have also been reviewed by DFO and the Conservation Authority to ensure all mitigation measures and approved detailed design was completed as proposed.
The new structure brought the site to current standards in all respects, including safety, geometry, road grades and load capacity. It is Burnside’s understanding that the Barefoot Box Culvert™ is the first of its kind approved by the Nottawasaga Valley Conservation Authority and DFO in Ontario. Collaboration between Burnside’s aquatic ecologists, hydrogeologists, structural engineers and Con Cast Pipe’s precision manufacturing has taken pre-cast innovation to a new level in watercourse crossing design with the Barefoot Box Culvert™.
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