Canada’s First Subway: Underground Downtown

Much of the downtown section of the subway was built directly underneath Yonge Street. Since the street could not possibly be closed during years of construction, the subway was built using the “cut and cover” method. First, vertical steel piles were driven into the ground every six feet on each side of the street to provide support for the sides of the tunnel excavation. Then, workers and machines began to dig downward from the street, making the “cut.” When the excavation was deep enough, horizontal steel beams were rested on the steel piles.

 

 

Huge timbers one foot square were then set across the beams, forming a temporary street-level surface, the “cover”. This was for traffic to use while work continued below. In places where the subway ran beneath buildings, the foundations of those buildings were deepened and reinforced while workers tunnelled from one cut to another. Once the concrete walls, floor, and roof of the subway had been poured, the piles were cut off, the remaining space was filled with earth, and Yonge Street was repaved.

 

 

 

 

 

 

 

 

Steel forms (used as molds to contain the wet concrete in shape until it dried) were used in most areas, though plywood was used in control areas, passageways, and entrances. The steel forms were built in twenty-foot sections mounted on a frame that rested on rails on the floor. The forms could then be easily moved to the next area of the excavation.

 

 

The subway tunnels were entirely reinforced concrete, while the stations had, in addition, steel columns and structural beams. All concrete was mixed at a central plant offsite and pumped from trucks at street level using tubes that the TTC nicknamed “elephant trunks.”

 

 

The arch of the roof in the stations conforms to structural design requirements, but was specially designed for visual effect as well. Original plans called for the centre supports (at the left of this image) to be concrete also, but they were instead made of steel columns to increase the feeling of spaciousness in the stations.