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Ottawa Combined Sewage Storage Tunnel (CSST)

TBM segmental lining design, rock mass grouting design and engineering support services during construction.
Estimated cost: Commencement:
February 01, 2016
Completion date:
110 Laurier Avenue West
Ottawa, ON K1P 1J1
Technical data:
Geology description:

The alignment of the tunnels mainly crosses limestone formations with laminated black shales of varying thickness. The rock is overlain by glacial till, glaciofluvial deposits and fill.

Geology types:
Bid cost: Final cost:
$232 million CAD
Service areas:
Construction methods:
Tunneling under:

The CSST project is part of the Ottawa River Action Plan and will greatly reduce the frequency of sewage overflows during storms entering the Ottawa River. The tunnels will hold up to 43,000 m3 of sewage overflow during major rainfalls which will be treated and returned to the river once the rainfall has subsided. Additional benefits include reducing the risk of basement flooding and increasing operational flexibility and redundancy of the sewage system. The project comprises design and construction of two TBM tunnels running east-west and north-south along 6.2 km in total, and multiple shafts and support buildings such as odour control facilities.

Overview plan of the CSST   (
East West Tunnel under construction (3m tunnel ID)
Excess wet weather flow is sent to the CSST (