Building the Canso Causeway

July 8, 2009

Several sites were considered before the site from Cape Porcupine to Balache Point
on the Cape Breton side was decided on.

The Northern Construction Company chose to extract rock from the closest part of the mountain. Part way through the construction they considered changing their strategy but never did. Mining from the back they could have had a 40 foot face, but much longer running back into the wooded area out of sight. It would have been faster because they could have been drilling and blasting at one end of the long face and hauling stone from the other. As it was they had a 400 foot face. Coyote holes about 4 or 5 feet high by 4 or 5 feet wide and 100 feet deep were dug into the lower level face of the mountain. At the 100 foot mark they would branch off 50 feet each way all along the base of the face. These holes were then packed with dynamite and the entire thing blasted. The 400 foot rock lift would bring down 350 – 400 thousand tons of rock. There were disadvantages to using the front face of the mountain.

After the coyote holes were packed with dynamite all the equipment had to be moved out before blasting could take place. This slowed the process down somewhat. The rock was then loaded onto trucks driven to the water’s edge and dumped and bulldozed into the strait. After the 50th truck load of rock was dumped into the strait nothing much could be seen.
Many people felt it was an impossible task. They felt the rock would be washed away by strong currents and the large ice flow in the strait. Watching closely as large ice pans hit the causeway sometimes crashing over it they shook their heads. The closer the construction came to the Island the less they believed it could be finished. Large ice pans then washed up on the Cape Breton side and pile up on the causeway.

Engineers studying the site watched the tides and knew the tide stopped for a certain period of time each day. That is what they counted on for the completion of the job.

No one had ever built a rock fill in that depth before so no one knew what slope it would take. The engineer in charge figured it would be a ratio of 1 ½: 1, for every foot up it goes out a foot and an half. He was right.. As the job progressed it was necessary sometimes to place dynamite into a sack lower it and set it off to crumble the rock and have it settle.

Engineer Harry MacKenzie said he thought the strangest thing on that job was the change in the depth of the floor bed. Originally the depth charges showed the deepest spot was 187 feet. Soundings were taken every 50 feet to see what the slope was.
In the spring of 1954, 700 feet from the Cape Breton shore in the deepest part of the straight (187 feet) soundings were taken.
The instrument men reported a depth of 218 feet. That fact was verified.

When the opening became restricted between Balache Point and the end of the Causeway the rush of water through the700 feet opening was so strong it cleared 31 feet of overburden hardpan and boulders from the bottom down to the rock base. The last 700 feet of the Canso Causeway is sitting on solid rock.

To close the gap when there was a south tide the rock was dumped in on the north side and visa versa allowing the rock to settle in the middle. Huge slabs of rock were being washed away. It is thought that at the deepest part of the causeway it is probably 900 feet wide. Harry MacKenzie said “It was rolling boulders half the size of this room out of there then the tide would slacken and we’d give her the hell then.”

From Harry MacKenzie’s diary
December 11th noon Completion of Quarry run Rock fill, Coyotes 96. & 98 shot at 1:57 PM Dec. 15th, 1954. Amen.
Weds. Dec. 22, 1954; last load dumped in causeway at Noon today— Total in job 10,000,262.7 tons

Information taken from the “Cape Breton Magazine” and from Cape Breton History archives.


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