Christ Church Cathedral, in the heart of downtown Montreal, was pointed to, gaped at and discussed by thousands of Montrealers morning, noon and night. 
This was in 1987, and the focus of attention was the Cathedral-on-stilts, as it came to be called,the stilts bearing the caissons for the underground excavations and new foundations constructed as part of a multi-million dollar office and retail development.
The Cathedral was completed in 1859 based on the design of Frank Wills, who also designed Christ Church Cathedral, Fredericton. Architecturally it has always been regarded as a fine example of the English neo-gothic but its engineering design was not in the same league. From its completion the heavy central tower started to sink into the soft ground on which the foundations were built; by the 1920s the spire was leaning 4ft to the south. There was a landmark lawsuit as a result of early foundation problems (Wardle vs. Bethune) often quoted in connection with Article 1688 of the Quebec Civil Code. In 1927 the stone steeple, weighing 3.5 million pounds, had to be removed. It was not until 1940 that a replica steeple made entirely of aluminium was erected as an anonymous gift.
The development project comprised the building of a 34-storey office tower immediately to the north of the Cathedral which included a single parking level and two retail levels below the Cathedral, underground connections to Eaton's and The Bay department stores, and re-landscaped grounds. There is also a 10,000 sq. ft. mezzanine floor sandwiched between the Cathedral floor and the ceiling of the first retail level. This is occupied by the Canadian Bible Society, the Diocesan Bookroom and the Undercroft - home for the Cathedral's music, church school and out-reach programmes.
The Church Group - comprising the Cathedral, the Diocese of Montreal and the Canadian Bible Society - negotiated in 1985 and 1986 two agreements with the developers : Westcliff Development Corporation, First Quebec Corporation and Les Coopérants, a Mutual Insurance Company.
Underpinning of the Cathedral to make provision for the retail and parking levels was the engineering highpoint of the project. The work started at the end of February 1987, and was completed in November of the same year.
Thirty-three hollow cylindrical steel piles or 'caissons' were driven down to bedrock around the Cathedral walls and under the central tower. Twenty-three of these were just outside the Cathedral walls and were 36" in diameter. The ten driven from within the Cathedral crypt using a special machine were 26" in diameter. The caissons were driven to bedrock about 45-50 ft. below ground and a hole bored into the rock to ensure proper bearing.
The contractor lowered steel reinforcement cages into the caissons and filled them with concrete to create a column on which the Cathedral was supported.
On top of the caissons a grid of massive pre-stressed concrete beams was built, capable of carrying the weight of the Cathedral when spanning between the caissons after ground excavation was completed. The beams running across the Cathedral were generally 6' x 6' shaped in the form of a 'T'. The beams running from the back of the Cathedral towards the high altar were 4' deep x 3'9" wide.
Pre-stressing the beams means that they are able to carry much greater loads than ordinary reinforced concrete beams. High tensile steel wires were threaded through ducts placed in the beams and massive jacks used to stretch the wires and put the beams in compression before they took the Cathedral load (this is similar to carrying a small shelf-load of books from one location to another by pressing them together from each end).
Once all the beams had been tensioned the sleeves were filled with a concrete slurry to make sure the steel wires would not corrode and lose their strength.
The walls and columns of the Cathedral were held tightly in the tops of the beams using special steel gripping devices called Dywidags. A 8" concrete slab was poured just below the tops of the beams to create an effective sound and fire barrier between the wooden floor of the Cathedral and the retail level below.
The trickiest part of the operation was underpinning the tower. This meant carefully excavating by hand and chipping into the massive concrete foundations placed in 1939 to stabilize the original foundations and enable erection of a new aluminium spire which considerably reduced the weight on the Cathedral tower. Jacks were inserted during excavation and finally two massive concrete beams 14' wide 46' long and 5' deep were poured spanning across the pairs of caissons at each corner of the Cathedral crossing.
The Cathedral was carefully monitored for movement and excess vibrations throughout the whole operation. In addition the stained glass windows were surveyed and checked again once the main construction was completed. The main damage done during the underpinning was the smashing of the nave's stained glass window when a clam bucket, used to excavate inside one of the caissons, struck the edge and ricocheted through the window and into the Cathedral, narrowly missing the verger who had been standing there 30 seconds before.
There was some opening up of existing cracks as well as some new cracks and arching of some of the stained glass windows, but on the whole this remarkable operation went extremely well. The structural engineer for the project was Quinn Dressel Associates of Montreal and the Church Group retained Shector, Barbacki & Shemie Associates Ltd., also of Montreal, to check the design as an insurance measure. The general contractor was Magil Construction Ltd. and Petrifond Foundation Co. Ltd. was the specialist sub-contractor for the foundations.
Quinn Dressel Associates of Montreal was given the "Award of Merit" by the Association of Consulting Engineers of Canada for this project :
"The challenge faced by Quinn Dressel Associates was putting the church on new foundations without interrupting services during the construction period. The delicate and possibly strained condition of the building required an approach that would not allow any appreciable movement during or after the implementation stage ... (it was executed) successfully with movements never exceeding 3/16 inch ... "
At the end of 1987 the office tower itself was almost fully occupied. The retail development under the Cathedral, its grounds and two of the surrounding streets were opened in the fall of 1988.
The Project Development has not been without criticism: fears of our 128-year-old Cathedral being dwarfed by a 34-storey office tower came from people within and beyond the Cathedral Community, and it was suggested by uninformed people not connected to the Cathedral that with the underground development the Cathedral was being too commercial. And of course there were a few rumblings about the disruption of the surrounding green spaces and the inevitable breakdown of electricity, heat, water and telephones during the excavation process.
But few can deny that the office tower is a stunning backdrop to the Cathedral. Instead of dwarfing the Cathedral, the spire and steeple have emerged 'larger than life' throughout the reflection of the tower's warm copper-toned glass facade.
