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 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, resulting in a 3.4 m minimum inside diameter for the 765-m longstormwater conveyance pipe.
There are three distinct and contrasting soil conditions along the alignment. The first, approximately 550 m 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 where the ground changes from bedrock to cohesionless, coarse-grained channel fill soil deposits. The final, approximately 135 m, is through a full face of these channel fill deposits. The entire alignment is below the water table.
Microtunnel 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 tunnelling methodology and a value engineering process for contractors to present better ideas. It was acknowledged that local contractors have successfully completed a number of tunnelling projects in Calgary using slurry microtunnel boring machines.
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 & Burke Microtunnelling Ltd., proposed constructing the stormwater trunk using the microtunnelling method with an AVN2500 fitted with mixed face cutterhead and an up-skin kit to install the 4.0-m outside diameter pipe.
The tunnel lining consists of a 328-mm thick 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 long and weighing more than 32 metric tons. Langley Concrete, a state-of-the-art precast concrete production facility located ~100 km 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 tunnel boring machine data to monitor tunnel progress.
Tunnelling was completed within 12 weeks, between September and December 2022.
Fred Marquis, P.E., P.Eng, is with Delve Underground. This article originally appeared on the Delve Underground website – delveunderground.com – and is being reprinted with permission.
Northwest Inner City Upper Plateau Separation Microtunnel Project Specs
In December 2022, Ward and Burke Microtunnelling Ltd. successfully completed microtunnelling a 4.144 m diameter tunnel for the City of Calgary’s Northwest Inner City Upper Plateau Separation Project. To the best of the owner’s knowledge, this is the largest diameter microtunnelling project completed in North America.
Tunnel Length: 762 m long, S-curve with mixed subsurface conditions
Lead Engineer: Associated Engineering
Tunnel Design: McMillen Jacobs Associates (now Delve Underground)
Geotechnical Engineering: Thurber Engineering Ltd.
Prime Contractor: Whissell Contracting Ltd.
Trenchless Subcontractor: Ward & Burke Microtunnelling Ltd.
MTBM: Herrenknecht AVN2500 w/mixed face cutterhead and up-skin kit
Jacking Pipe Mold: Gollwitzer GmbH
Jacking Pipe Supplier: The Langley Concrete Group
- 4.0 m OD
- 3.4 m ID
Geotechnical Instrumentation: Englobe and Maidment Land Surveying