Calgary, Alberta
Construction: 2012
Owner: H&R Real Estate Investment
Client and Shoring Contractor: HC Matcon Inc.
Architect: Foster + Partners
Structural Engineer: Read Jones Christoffersen Consulting Engineers
At 236 metres The Bow is Calgary’s second tallest tower and was designed by Norman Foster. Encompassing two city blocks on a major axis through the downtown core, The Bow has become the most recognizable building in the city’s skyline.
An impressive 24-metre cut into the weak Calgary shale shear zone provided a geostructural challenge. Past excavations in the area experienced large amounts of rock movement, the mechanics of which were poorly understood. In close coordination with the shoring contractor, Isherwood designed an innovative anchored earth retention system comprised of two components. Secant piles were drilled through the overburden soils and toed into the soft shales below. The rock face was subsequently shotcreted to the base of the excavation. This system was able to successfully control groundwater movement, rock degradation, and soil loss in the extensive cobble layers and the weak bedrock.
The design constructability allowed for 6th Avenue, which straddles the site, to be demolished, excavated completely, and rebuilt in less than eight months – just in time for the 99th annual Calgary Stampede Parade to pass over it.
An in-depth FLAC analysis combined with an extensive monitoring program proved successful during the design and construction of the project. Results from the study indicated that past large horizontal movements were caused by a phenomenon known locally as the ‘shear band effect’ which describes the movement of a rock mass over a thin, extensive layer of extremely weak rock. The analysis also provided a range of expected movements of the shoring wall, to ensure the design was working as required. The information was used to further refine the FLAC analysis for more accurate predictions during excavation and construction, as well as allowing Isherwood to quickly address any unexpected shoring wall behaviour.