By Luke Dalinda, from his Palace Place Book.
Palace Place was setting records and gaining international attention before it had ever even soared into Toronto’s skyline with its spectacular 47-storeys.
While it has always been compared to Palace Pier, which had been completed in 1978, that similarity ends underground.
While Palace Pier stands on conventional strip footings on bedrock, Palace Place sits a top extra-deep shaft caissons designed in friction. Say what? Ok. Let’s back track.
Simply put, strip footings are strips of concrete placed into an area and reinforced with steel. A caisson foundation is a foundation system used to support tall buildings by anchoring them to solid bedrock. It consists of a prefabricated cylinder that is drilled into the ground to bedrock where steel beams and rebar are inserted through the tubes and into the rock to pin the caisson in place. The tube is then filled with concrete and capped to support the major columns of the building.
Caissons are typically used when shallower foundations, like strip footings, cannot be constructed safely. Caisson foundations offer help to resist the tipping effect of winds on the building and to control movement in an earthquake. The world’s largest skyscrapers employ caisson foundations.
To create the footprint of Palace Place, a bathtub was created of over 1,300 feet of sheet piling. Rather than keeping water in, the intention of this “bathtub” was to keep water out.
The geotechnical consultant for Palace Place was the renowned firm of Trow Ltd. Personally led by William Trow himself, it was discovered that within the 262 feet of space that separate the footprints of Palace Pier and Palace Place, the bedrock elevation had differed by as much as 82 feet.
Bramalea Limited had retained the services of internationally renowned Franki Canada Ltd. to take on the deep foundation job under water at a 1989 cost of $4-million led by the structural engineer for Palace Place, Alex Tobias.
Given the proximity of the lake, inspecting the caisson bases before concreting would prove impossible, as the caissons would be full of water. Therefore, Alex Tobias had opted not to go with end-bearing caissons, as there was no way that sufficient end-bearing could be guaranteed for the bottoms of the newly drilled sockets, so Alex had chosen shaft caissons designed in friction, with the load taken up between the concrete and rock in the 13 to 20 foot deep sockets. Any end bearing that was achieved had only increased the factor of safety.
Due to very fine, soft silt over the bedrock, permanent liners were used to help keep the freshly poured concrete as well as to help the rebar act compositely with the concrete. Shaft diameters were minimized to 760 mm and 860 mm.
Working up to 120 feet deep in a marine environment had caught the attention of engineering firms worldwide. During its 50-years of operation in Toronto, this was the deepest job ever taken on by Franki. The work was difficult. To add to the complications, the caissons had to be tremie-concreted, a process of delivering a fresh mix of concrete to an underwater floor, to their full depth.
Furthermore, socketing into the shale rock so deep gave rise to further complications due to the hard limestone in the shale. 55M bars that were rigidly tied together had formed the rebar cages that had gone into the caissons. Finally, dealing with the water build up in the caissons was yet another testament to this engineering feat.
In all, 145 tower caissons were drilled in for the tower, 26 caissons for the Palace Place pool, and a final 22 caissons for the Palace Place access ramp and pedestrian bridge to the west garage.
The result was a very sound, advanced foundation system found in the world’s best-known skyscrapers and in marine construction environments like New York and Hong Kong.