Christ Church Cathedral,
Montreal, Quebec

Key to the success of the underground shopping mall, which was part of the "Cathedral Place" high-rise office development in the heart of downtown Montreal, was the feasibility of building a two-storey shopping mall and one level of parking under Christ Church Cathedral.

The church foundations, which had a history of settlement, were masonry spread footings supported on a layer of clay with glacial till and bedrock about 45 feet to 50 feet below grade. Putting the church on new foundations without interrupting services during the construction period was a considerable challenge.

The technical challenge was to minimize the building movements during or after the implemenation stage. The solution was a post-tensioned system which balanced the church loading and prevented any vertical movements of the already strained building. The wooden floor was left in place and all work was confined to the basement area, thus ensuring continuation of religious services throughout construction.

Caissons, socketed into bedrock at a depth of 18 m, were installed outside the church. Concrete beams - 1.8 m deep, designed to carry the building in section - were threaded through the existing foundations. A secondary system of 1.2 m deep girders was designed to enclose all masonry foundation elements and transfer the load to the main concrete beams. The soil was then excavated from under the building to leave it dramatically perched on stilts.

Throughout the entire operation, the cathedral remained open for regular services and special events.