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Combined Sewage Storage Tunnel

Award of Excellence


One of the key objectives of the Combined Sewage Storage Tunnel (CSST), which comprises two 3-m diameter interconnected tunnels that provide more than 43,000 m3 of storage volume, is to reduce the volume and frequency of combined sewer overflows (CSOs) to the Ottawa River. The tunnel, which entered use in 2020, builds on the success of real-time control (RTC) for the operation of Ottawa's sewer system and acts as both a tunnel flow conveyance and a storage tunnel that drains by gravity, avoiding a costly dewatering pumping station.

The CSST comprises a 4.1-km long east-west tunnel (EWT) and a 1.9-km long north-south tunnel (NST), with an overflow frequency target of one to two CSOs per year. With its implementation, the city now meets and exceeds its CSO control objectives.

Beyond CSO control, the design maximizes resiliency to prevent basement flooding in the downtown core by providing a high-level relief outlet during extreme events. Twinning the collector system maximizes reliability by providing the ability to inspect and undertake the future rehabilitation of the main interceptor.

Underground issues The project’s challenges largely resulted from geologic subsurface conditions across the city’s downtown core and tie-ins to collector sewers, which entailed stakeholder engagement related to mitigation of construction impacts to existing facilities and utilities.

Construction required excavating more than 6 km of deep rock tunnel and eight shafts, including 25-m deep shafts for a single-pass tunnel boring machine (TBM) using a precast concrete tunnel lining (PCTL). The TBM would cross under a new light-rail transit (LRT) tunnel, the Rideau River and the Rideau Canal and above the existing interceptor outfall sewer (IOS).

Groundwater drawdown was a key risk, with potential for widespread underdrainage of the sensitive marine clay deposits overlying the rock and for subsequent building and structure impacts. To mitigate this risk, the project team specified the TBM’s operational control requirements, with segmental PCTL as excavation advanced and sealed shaft excavations in several locations.

Specific performance criteria were specified in areas of concern, such as high groundwater, low rock cover and fault zones, and the project involved an extensive geotechnical and hydrogeological conditions monitoring program.

A key feature of the CSST’s design is adaptability, as it can be extended all the way to the downstream wastewater treatment plant (WWTP) in the future, allowing the twinning of the entire length of the more than 60-year-old interceptor sewer, providing added resiliency, performance capability and capacity to address changing conditions.

After its first year of continuous operation, the CSST had already led to a reduction in the frequency of CSOs discharged to the Ottawa River. The innovative gravity-based tunnel design has allowed the city to meet its CSO control objectives within its funding limits. Further, it positions Ottawa as a leader in environmental protection among Canada’s municipalities.

Combined Sewage Storage Tunnel, Ottawa Award-winning firm (prime consultant): Stantec, Ottawa. (Adrien Comeau, P. Eng.; Gerald Bauer, P.Eng.; Pascal Pitre, P.Eng.; Steve Fradkin; Anil Dean, Eng.; Stephane D’Aoust, P.Eng.; Colin Goodwin, P.Eng.; Shawn Ireland, P.Eng.; Jordan Beukeboom, P.Eng.; Darren Vanderydt, P.Eng.). Owner: City of Ottawa. Other key players: Jacobs Engineering (subconsultant for design and construction inspection services), Golder Associates (subconsultant for geotechnical engineering), Dragados Tomlinson Joint Venture (general contractor), Power Precast Solutions (precast concrete tunnel liners), Herrenknecht (tunnel boring machine), ECS Environmental Solutions (odour control vessels), Floval (stainless steel gates), Auma (electrical gate actuators), Siemens (level instruments), Access Industrial (ladders, platforms and railings), MSU Mississauga (hatches).


CONSISTENT QUALITY: Proven strength, durability and dimensional accuracy of precast concrete CUSTOM FIT: Segment thickness, length and radius designed for specific tunnel requirements ASSURED SUPPLY: Pre-manufactured to eliminate construction delays COST-EFFECTIVE: Precast tunnel segments are more economical than other types of large diameter tunnel systems

Project: Combined Sewage Storage Tunnel, Ottawa, ON | Engineer: Stantec Consulting LTD Precast Supplier: Power Precast Solutions | Photo: Stantec



• Transit applications (subways, light rail) • New utility tunnels and enclosures • Relining of failing structures and large culverts • Sewer and water conduits • Tunnel lining • Vertical shafts

CPCI Congratulates Stantec Consulting LTD on their CCE Award of Excellence for the Ottawa Combined Sewage Storage Tunnel and also Congratulates our CPCI member Power Precast Solutions as the precast concrete product supplier on this award-winning project.


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