Crossing the Red: Managing Construction Changes on a Challenging 1200 mm Microtunnelling River Crossing in Winnipeg, Manitoba, Canada

The Northeast Interceptor services approximately 2,300 ha (5,685 acres) of land in the northeast quadrant of the City of Winnipeg (Manitoba, Canada). The 1,800 mm (72”) Interceptor crosses the Red River using a multiple-barrel inverted steel siphon configuration prior to it’s final leg before reaching the City’s largest wastewater treatment plant. The original siphon crossing, built in 1971, was at capacity during large wet weather events and required additional capacity to meet development plans and reduce upstream basement flooding risk. Thus, plans were developed to install an additional siphon. The size and construction methodology for the river crossing evolved through both the design and construction of the project. Originally starting life as a 600 mm (24”) horizontal directional drill crossing, system hydraulic requirements dictated the installation of a larger 900 to 1200 mm (36” to 48”) crossing to meet the desired design objectives. Due to the geotechnical, geometric, and surface use constraints, a 900 mm (36”) microtunnelled crossing was tendered. To facilitate microtunnelling through underlying limestone bedrock in an area where external access is extremely limited, the contractor (Ward and Burke Microtunnelling) opted to install a single pass 1200 mm (48”) siphon, increasing the capacity of the new crossing. Further changes to the project were proposed by the contractor and to an extent accepted by the project team. They included a vertically curved microtunnelling drive, eliminating the need to construct shafts within the underlying limestone aquifer, changes to the chamber designs to suit their preferred construction methods, and an alternative connection configuration to reduce tie-in risks. The challenge, as with any contractor driven design change, is to primarily ensure the original design intent and functionality of the project is not compromised. Secondarily, where construction risks have been shifted to other portions of the work, one must ensure they are of an acceptable magnitude and/or have been sufficiently mitigated. While this paper discusses the project as a whole, it primarily focuses on the evolution of the crossing design from inception to completion of construction. Managing design changes during construction is a critical component to any large capital project. This paper details AECOM’s efforts to manage proposed changes during construction, complete with a lessons learned and recommendations for future complex tunneling projects.

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