The largest diameter microtunnel in North America was successfully completed in December 2022 as part of the Northwest Inner City Upper Plateau Separation Project.
In response to major flooding events in the early 2010s, the City of Calgary identified the need for a new 3.4-m (11.2 ft) tunneled stormwater trunk. This system will intercept water runoff collected from higher elevation communities, referred to as the Upper Plateau, and separate it from the stormwater system of the community of Sunnyside, located on the north side of the Bow River immediately adjacent to downtown Calgary.
The design phase of the project was awarded to Associated Engineering in March 2018, with Delve Underground and Thurber Engineering as subconsultants, and was completed in July 2021. Delve led the design of the shafts and tunnel. The interface with the urban environment was identified as a major risk for the underground scope of the project. A feasibility study of alternative alignments and trenchless methodologies was performed, with the preferred methods recommended for detailed design.
The new stormwater trunk is constructed through the community of Sunnyside. The launch shaft is located near the intersection of 10 Street NW and the 8 Avenue NW right-of-way. A light rail transit (LRT) line operated by Calgary Transit runs through the project area, crossing the tunnel alignment just east of the launch shaft. The reception shaft is located immediately to the south of Memorial Drive at 7 Street NW and adjacent to the Bow River. Based on hydraulic analysis conducted by Associated Engineering, the trunk is designed to intercept runoff with flow rates of up to 20 m3/s (706 ft3/s), resulting in a 3.4 m (11.2 ft) minimum inside diameter for the 765-m long (2,510 ft) stormwater conveyance pipe.
There are three distinct and contrasting soil conditions along the alignment. The first, approximately 550 m (1,804 ft) in length, is in a full face of Paskapoo Formation bedrock consisting of a mix of mudstone, siltstone, and sandstone. This is followed by a transitional zone of approximately 80 m (262 ft) where the ground changes from bedrock to cohesionless, coarse-grained channel fill soil deposits. The final, approximately 135 m (443 ft), is through a full face of these channel fill deposits. The entire alignment is below the water table.
Tunnel Construction Overview
Procurement for the construction contract commenced in early 2021 and was awarded to Whissell Contracting Ltd. in September 2021. Site mobilization commenced in February 2022. It was essential for the City of Calgary to attract highly qualified contractors to bid on the project while leveraging the local market experience. To encourage this, a number of steps were taken in the development of the Contract Documents, including the selection of the tunneling methodology and a value engineering process for contractors to present better ideas. It was acknowledged that local contractors have successfully completed a number of tunneling projects in Calgary using slurry microtunnel boring machines (MTBMs).
Minimum performance requirements were included in the Contract Documents. Specifically, a closed faced tunnel boring machine was required to maintain full face support and to control groundwater inflow during construction, particularly within the middle and southern reaches. The tunnel subcontractor for the project, Ward and Burke Microtunnelling Ltd., proposed constructing the stormwater trunk using the microtunneling method with an AVN2500 fitted with mixed face cutterhead and an up-skin kit to install the 4.0-m (13 ft) outside diameter pipe.
The tunnel lining consists of a 328-mm thick (13 in.) precast reinforced concrete pipe designed to ASTM C76 and ASCE 27 specifications. A total of 230 pipe sections were required to complete the alignment, each 3.3 m (10.9 ft) long and weighing more than 32 metric tons (35.3 tn). Langley Concrete, a state-of-the-art precast concrete production facility located ~100 km (62 mi) east of Vancouver in Chilliwack, British Columbia, supplied the pipes for the project. During construction, Delve performed quality assurance reviews for the tunnel lining and shaft structures, as well as daily reviews of geotechnical instrumentation data for the protection of nearby infrastructure and TBM data to monitor tunnel progress. Tunneling was completed within 12 weeks, between September and December 2022.
This article originally appeared on the Delve Underground website – delveunderground.com – and is being used with permission.