By Greg Christensen
The existing stormwater drainage system in the northwest sector of the City of Medicine Hat, Alberta was over capacity and could not meet the demands of continual commercial and industrial growth in the area. As such, the city imposed development restrictions in the sector until a solution could be found to relieve the capacity issues.
Furthermore, one high traffic intersection would flood frequently during rain events and would often require closure for public safety.
In 2016, the city decided to install a new stormwater trunk main to address these issues. The Northwest Storm Trunk will convey stormwater discharged from several existing and proposed stormwater management facilities to an outfall on the South Saskatchewan River.
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MPE Engineering Ltd. (MPE) was engaged by the city through a Request for Proposal process to provide stormwater modeling, preliminary design, detailed design, tender documents, contract administration, site inspections, and quality assurance testing for the project.
The Northwest Storm Trunk alignment closely followed a proposed alignment for a sanitary sewer trunk main that was also being planned for the same area. The design of both trunk mains was completed simultaneously by MPE and combined into one construction tender. The project included designing a stormwater and sanitary sewer trunk main, railway crossings, pipeline crossings, river outfall upgrades, energy dissipation structures, roadway design, and a stormwater dry pond. Included were 880 m of 1,350 mm diameter Class III reinforced concrete pipe (RCP) and 1,275 m of 1,350 mm diameter Class IV RCP.
Design was completed and the project was tendered for construction in March 2018. The contract was awarded to BYZ Enterprises Inc. of Medicine Hat and construction began in June 2018. Flexible pipe systems and rigid pipe systems were reviewed during the design process. Upon analysis of the project conditions, both MPE and the City of Medicine Hat concluded that flexible pipe systems were not suitable for the Northwest Storm Trunk. Therefore, only rigid RCP was specified.
The decision to use RCP was based on the following project challenges:
Depth of bury – The selected alignment had approximately 800 m of pipe buried between 6 m and 8 m deep and over 300 m of pipe buried between 8 m and 10 m deep.
Significant groundwater infiltration expected – Geotechnical investigations indicated that groundwater would be above the pipe depth for portions of the alignment and to expect varying native soils, including loose to very loose gravelly sandy wet soils.
The flexible pipe design for sections of this project would require specifying Class I material compacted to 95%. It is possible to design an embedment material and compaction effort to support a flexible pipe installation. However, it was MPE’s opinion that it is not practical to assume a contractor will consistently achieve >95% compaction within the pipe embedment zone, especially with large diameter pipe, at significant depths, while facing groundwater infiltration and loose soils.
The inherent strength of RCP compensates for any construction shortcomings from trench depths and trench conditions rather than relying on the pipe embedment for strength. Specifying RCP carried less risk in the installation for this project. It allowed for a more consistent installation as it is less structurally dependent on bedding and haunching. This would have been very challenging to install adequately to support other pipe materials in the wet trench conditions.
Also, the shorter pipe lengths of RCP allowed for easier install in deep, wet, loose native trench conditions, versus a six-metre-long section of flexible pipe.
Furthermore, at the depths and conditions expected, there were negligible expected cost savings by specifying flexible pipe systems over RCP. With at least three RCP manufacturers in Alberta alone, competitive pricing was seen in the tender.
There were other project challenges:
Steep grades – Long segments of the Northwest Storm Trunk have grade in excess of 6%. The potential for pipe abrasion was reviewed with RCP pipe manufacturers. Energy dissipation structures were designed to reduce the energy grade line of the Northwest Storm Trunk and manhole structures required reinforcement to prevent scouring.
Access to properties – The alignment is located within existing roads and installation required road closures. To maintain access to residences and businesses along the route, the project had to be phased around important intersections. Temporary roads were constructed parallel to the work space to maintain through traffic.
Affected property owners were engaged during the design phase to discuss access requirements and review options. Engaging them during design and having them provide input on the temporary access design reduced their frustration during construction.
Greg Christensen is with MPE Engineering Ltd. This article appears in ES&E Magazine’s December 2019 issue.
