Leaders in Environmental and Transportation Infrastructure Services

Combined Sewage Storage Tunnel (CSST)

Project Details

Project Name: Combined Sewage Storage Tunnel (CSST)
Location: 10 project sites throughout Ottawa
Timeframe: 2016-2020
Partners: Dragados Tomlinson Joint Venture
Tomlinson Divisions: All Tomlinson Construction Divisions


  • Reduce the risk of basement flooding for several low-lying lands in the Glebe/O’Connor area
  • Increase operational flexibility and redundancy to major collector sewers
  • Construct 15 major access shafts, five drop structures, six flow diversion chambers, four odour control facilities
  • Install two inter-connected storage tunnels, 6.2km long
  • Divert 43,000m3 of sewer overflow during major rainfall

Project Description

The City of Ottawa’s Combined Sewage Storage Tunnel (CSST) project is part of the Ottawa River Action Plan and includes the construction of two tunnels, an east-west tunnel and a north-south tunnel. Tunnels spanned 6km in length and 3m diameter tunnel under urban Ottawa. The project also includes construction of all supporting infrastructure including access shafts, utility relocations, odour control buildings, and sewage and storm system tie-ins.


A tunnel boring machine excavated the tunnel. Conveyers transferred excavated material back to the shaft entrance. As the machine moved forward, precast concrete liners were placed and grouted behind.

Combined sewage storage tunnel
Combined sewage storage tunnel