Canadian Consulting Engineer

City of Ottawa tunneling for combined sewage storage tunnels complete

September 24, 2019

The tunnels are 4.4km and 1.6km long respectively and 3m in diameter each, adding 43,600 cubic-metres of storage.

The tunnel-boring machine has completed its 4km east-west journey, marking a milestone for the City of Ottawa’s Combined Sewage Storage Tunnel project.

With the 2km north-south tunnel already completed in 2018, the end of east-west tunneling means all 6km of tunnel needed for the project have now been excavated.



East-west tunneling started in November 2018, and the machine continued for more than 4km to its destination, digging up to nine storeys below street level. The 250m-long tunnel-boring machine is now being dismantled and removed from the end of the tunnel.

Construction on the project began in 2016, and the City is continuing work to have the water works project operational in 2020.

Like many North American cities, Ottawa’s downtown has a historic combined sewer system, which uses the same pipes to convey both sanitary and surface water to its treatment facilities. During heavy rains or snowmelts, the system can become overwhelmed, resulting in untreated water flowing into the Ottawa River.

Once completed, the new system will reduce overflows when wastewater combines with surface water and debris washed off the streets.

The tunnels are 4.4km and 1.6km long respectively and 3m in diameter each, adding 43,600 cubic-metres of storage.

The tunnels range from 20m to 35m deep with six major drop shafts 2m to 3m in diameter.

With a capacity of approximately 18 Olympic-sized swimming pools, the Combined Sewage Storage Tunnel will help protect the Ottawa River and will also reduce the risk of basement flooding in some low-lying areas in the city’s core and help future-proof the existing downtown sewer infrastructure by adding capacity and redundancy.

The Government of Canada and the Province of Ontario are each providing $62.09 million, with the City committing an additional $108 million towards the $232.3-million component of the Ottawa River Action Plan, a collection of 17 projects to improve the overall health of the river.

The project design team consisted of Stantec Consulting Ltd. in conjunction with CH2M. Subconsultants to Stantec were Golder Associates (geotechnical consultant for the design team) and Jacobs Associates (part of the team’s Quality and Risk Review Panel).

Contracter: Dragados Tomlinson Joint Venture


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