Combined Sewage Storage Tunnel Shoring Project

In 2016, the City of Ottawa launched construction of the Combined Sewage Storage Tunnel (CSST), one of the city’s largest environmental infrastructure investments designed to reduce combined sewer overflows into the Ottawa River. The project involved the construction of two major underground tunnels: an east-west tunnel beneath Ottawa’s downtown core and a north-south tunnel located near Canada’s Supreme Court building.

The CSST system required approximately six kilometres of tunnelling at depths ranging from 10 to 30 metres below ground level. To support tunnel excavation and underground access, multiple large-diameter shafts and complex shoring systems were required across a variety of challenging geological conditions in a dense urban environment.

Marathon Underground played a key role in delivering the shaft construction, shoring, and early tunnelling works for the project. One of the most significant components included the construction of a 15 metre diameter secant shaft. Secant shafts are formed using a series of overlapping concrete-filled drilled piles that create a continuous structural and groundwater control system. Primary piles are installed first, followed by secondary piles that overlap into the adjacent concrete to achieve the required structural integrity and ground retention performance. Pre-excavation grouting and ground consolidation were also completed to stabilize the surrounding soils prior to excavation.

The project also incorporated large-diameter drilled shafts designed by Marathon Underground’s in-house engineering team. To improve efficiency and accelerate production, Marathon designed and fabricated a custom 4 metre diameter auger specifically for the project. This innovative solution reduced construction timelines by approximately four months and provided significant schedule savings for the overall CSST program.

Additional shoring systems included traditional soldier pile and lagging walls, along with extensive shotcrete applications in overhead and restricted-access areas where conventional support systems were not feasible. Once the shaft and shoring works were completed, Marathon Underground’s trenchless division advanced the tail tunnels used to support the setup and launch of the larger tunnel excavation operations.

Following tunnel excavation, Marathon completed final underground stabilization measures including the installation of rock bolts and wire mesh reinforcement systems to secure exposed rock faces and maintain long-term tunnel stability.

The CSST project showcased Marathon Underground’s ability to deliver complex underground infrastructure solutions in highly constrained urban environments through integrated engineering, specialized construction expertise, and advanced trenchless and shoring capabilities.