From roads and bridges to waterflow and drainage systems, or even animal crossings, structural culverts play a crucial role in maintaining connectivity, ensuring public safety and facilitating the flow of water beneath roadways, railways and other structures. Essentially, the structures have only two features : hold up the weight of the traffic and allow enough water to pass through. Made of timber, concrete, or steel, new culverts are designed for 75-year service life. Older culverts don’t always last that long, and when they start to deteriorate, the first step for the engineering team is to complete a site visit to review the existing culvert condition and site constraints.
Construction Staging Design
Once the culvert has been assessed, the engineering team will meet to review the site-specific staging needs. This is a crucial step, as replacing the culvert means affecting traffic, and the aim is to minimize the disruption to the community. A full closure with a temporary detour is always the preferred option, as it has the lowest overall cost and construction duration.
However, a full closure is not always possible, and if traffic needs to be maintained on-site, then the traffic must be shifted away from the culvert. This usually requires the culvert to be replaced in multiple stages, widening of the roadway, and the use of a shoring system to safely support traffic while excavation occurs next to the roadway. The size of the roadway shift depends on how many lanes of traffic need to be maintained and the width of the lanes needed. To protect both workers and vehicles, temporary construction barriers are typically installed within the work zone limits.
Where a shoring system or roadway widening isn’t possible, an innovative approach McIntosh Perry has successfully used to manage traffic is using temporary modular bridges to span the culvert excavation while work occurs below. The Traffic engineering team will then analyze the proposed staging to determine the expected traffic delays during construction.
Structure Type Selection
A culvert replacement involves engineers from different disciplines, working together. The Transportation Structures team will develop a list of feasible culvert replacement types. The most common culvert types used are Concrete Box culverts and corrugated steel pipe culverts (CSP).
The Drainage and Hydrology team will then review the existing hydrology to determine the size of the culvert needed. It is important that the size be optimized to ensure that it meets the design storm requirement and that it doesn’t drastically impact the water levels upstream or downstream of the culvert.
The Geotechnical team will perform soil investigations to determine if the existing soil below the culvert has enough geotechnical capacity to support the culvert without settling too much. This is important as settlement of the culvert can lead to premature deterioration of the culverts and/or impede the flow through the culvert.
The soil investigation also provides information necessary for the Contractor to design a dewatering system to permit installation of the new culvert in the dry. The dewatering system will often consist of a steel sheet pile cofferdam that will surround the culvert. Large pumps are then used to remove water from within the cofferdam.
The Environmental team will then review the impacts that each culvert type will have on the existing environment. The team will also highlight the constraints that will need to be followed during construction such as when can tree clearing be completed or during what periods is the Contractor allowed to enter the water.
Once a culvert type is selected and endorsed by the Client, the Design team will begin preparing the Contract Package that the Contractor will use to construct the new culvert.
The Contract Package typically includes:
- Tender Document that describes the Contract, the tender items, and specifications that need to be followed as part of the work.
- Contract Drawings that illustrate the work being done for each discipline.
- Quantity Sheets that provide a breakdown of the tender items and their location on the project.
Interim submissions of the package are typically completed at 30%, 60%, and 90% to provide an opportunity for the Client to review the status and provide feedback.
Once the Contract Package is complete, the client will advertise the project to Contractors for bidding. The bidding period will range from 4 weeks to 12 weeks depending on the value and complexity of the assignment.
CASE STUDY – LITTLE PASHKOKOGAN CULVERT REPLACEMENT
McIntosh Perry was recently tasked with the Detail Design for the replacement of the Ministry of Transportation (MTO) Little Pashkokogan structural culvert located on Highway 599, 92 km south of the remote community of Pickle Lake Ontario.
Client: Ministry of Transportation of Ontario Easter Region.
This culvert presented many unique challenges as the culvert was in a 12 m thick causeway built of rockfill. The presence of rockfill meant that conventional dewatering was not going to be practical as a cofferdam couldn’t be installed through the rockfill. This meant the Contractor would have had to install most of the culvert in the water, but the depth of water made installation of a box culvert or steel pipe culvert far too challenging as the Contractor would be working in 1.8 m of water.
To overcome this challenge, the Design team came up with a unique prefabricated concrete culvert design and carefully engineered construction sequence that could be used to complete installation of the culvert in the water.
The culvert would consist of a precast concrete slab connected on-site to precast concrete footings and walls.
Design and construction were completed under a traditional Design Bid Build (DBB) model. The Construction contract was awarded to LH North Ltd. in March 2021 for $2.77M.
The following provides a summary of the construction sequence including photos provided by the Contract Administrator (WSP) and the Ministry of Transportation Northwest Region (MTO NWR)
Step 1: Set up traffic control devices and stage 1 configuration.
Step 2: Install environmental protection measures (i.e. sandbags, turbidity curtains) upstream and downstream of the culvert. Subsequently, excavate for new footing installation and expose the existing culvert.
Step 3: Construct a 200 mm thick concrete working slab for the precast concrete footings without dewatering.
Step 4: Remove the top portion of the existing culvert and install a sandbag cofferdam around the concrete working slab.
Step 5: Install precast concrete footing and use the adjustable legs to achieve the top of footing design elevations.
Step 6: Remove water from within the sandbag cofferdam and fill the gap between the working slab and the bottom of the footing with non-shrink grout.
Step 7: Remove a remaining portion of the existing culvert and install new streambed material within the new culvert channel. Subsequently, install the precast prestressed top slab units complete with waterproofing.
Step 8: Install waterproofing across the top slab, backfill the structure, and reinstate the roadway.
The above-noted steps were then repeated to install the upstream end of the new culvert in stage 2.
The unique structural design and installation sequence proved to be very successful as the Contractor was able to complete the installation of the structure in a short window without issue.