Ottawa CSST

Shoring Excavation Project: Ottawa Combined Sewage Storage Tunnel


The Challenge

In 2016 the City of Ottawa started work on their new Combined Sewage Storage Tunnel (CSST). This was a significant infrastructure investment for the city that would require the construction of two major tunnels: an east-west tunnel through the downtown core and a north-south tunnel just behind Canada's Supreme Court building.

The tunnels would need to be six kilometres in length, approximately three metres in diameter and 10 to 30 metres below ground level. These tunnels required significant access shafts in multiple different geological conditions.

The Solution

A 15-metre wide secant shaft was the largest of the shafts. Secant shafts are formed by constructing a series of overlapping “primary” and “secondary” concrete-filled drill holes. The primary piles are constructed first, followed by secondary piles, which are cut into the previously placed primary pile concrete. The amount of overlap required between the adjacent piles is a function of the structural design requirements. The area was grouted in preparation for excavation of the shafts as well.

The drilled shafts were also designed by Marathon and utilized on the project. This included a custom fabricated 4-metre auger that was designed specifically for the drilled shafts, that saved four months of project schedule.

The project had traditional soldier pile and lagging shoring systems and shotcrete was required for overhead or challenging locations.

Once the shoring was complete, Marathon’s trenchless team went to work boring the tail tunnel, which is used to aid in the set up for the larger tunnel excavation to come. Once the tunnels were complete the final piece was for Marathon to stabilize and support the tunnel walls with rock bolts and wire mesh.

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