Northwest Inner City Upper Plateau: North America's Largest Diameter Microtunneling Project

Abstract

In December 2022, the City of Calgary completed the largest diameter microtunnel in North America as part of the Northwest Inner City Upper Plateau Separation Project. This 765-meterlong tunnel, designed to manage stormwater runoff from the Upper Plateau, was developed in response to major flooding events in the early 2010s. The infrastructure intercepts stormwater from higher elevation communities to prevent flooding in the Sunnyside neighborhood, which is vulnerable to both overland runoff and Bow River surcharges. Designed to handle peak flows of up to 20 m³/s, the tunnel required a 3.4-meter internal diameter and 4.0-meter external diameter, unprecedented for North American microtunneling. The design phase began in 2018 and concluded in 2021, led by Associated Engineering with tunnel design and geotechnical input from Delve Underground and Thurber Engineering. A trenchless solution was selected to avoid disruptions in dense urban zones and traverse challenging ground conditions, including Paskapoo Formation bedrock and loose alluvial deposits. The alignment passed beneath a light rail transit line, residential roads, and public greenspaces, requiring precision planning and execution. Construction commenced in 2022, managed by Whissell Contracting Ltd., with Ward & Burke Microtunnelling Ltd. delivering trenchless execution. A Herrenknecht AVN2500 microtunnel boring machine (MTBM), equipped with a mixed-face cutterhead and up-skin kit, installed over 230 precast concrete pipes (supplied by Langley Concrete Group). Quality assurance protocols, including daily geotechnical monitoring and real-time performance tracking, ensured safe and efficient progress. The project marks a major milestone in trenchless innovation in Western Canada, extending microtunneling capabilities into the mega-diameter range. Its success illustrates the expanding role of trenchless methods for major urban infrastructure, demonstrating minimal surface disruption and strong flood mitigation impact. The NWIC project stands as a critical upgrade to Calgary’s stormwater network and a foundational element of the city's long-term flood resilience strategy.