PROJECT REPORT | TRENCHLESS


Left: Noise wall at launch shaft.


prior to excavation being fully completed. Following removal of the MTBM, the Inner City trunk was exposed. Grouting and sealing of the chamber was completed as were construction of concrete walls and a diversion channel to allow flows of the Inner City trunk to eventually flow through the newly constructed IST. The chamber was constructed with a slide gate to ensure flows from the trunk to the new IST, and stop logs were also included for operational flexibility and future maintenance. There were three live sewer connections in Phase Two,


including connection to the two Memorial Drive trunks (1200mm and 1950mm diameters) and to Phase One at chamber 1-1. Workspace for the Memorial Drive trunks was constrained as both pipes were located in the driving lanes of an 80 km/hr major expressway. The 1950mm- diameter connection was completed first utilising an expandable steel flume, which was installed once the pipe was supported and the top of the pipe was cut and removed. Once the flume was installed the contractor was able to complete the chamber construction in the dry. The flume designed for flows of 2.4 m3/s before any upstream surcharging would occur. Peak dry weather flows in the 1200mm trunk was 35 l/s and could be pumped with a 150mm-diameter self priming pump with an additional pump supplied for 100% redundancy. The final connection in Phase Two was to chamber 1-1, which was constructed with removable roof slabs to provide full access for MTBM retrieval and bypass operations. The bypass installed in Phase One was activated again for construction operations, including permanent benching installation. Once retrieval was completed, permanent benching was installed and the bypass filled with concrete. This design greatly reduced the length of construction activities in the residential neighbourhood.


LESSONS LEARNED


Mitigation of construction risk The Bow River is an environmentally sensitive area, known for its pristine clear turquoise water, a source of pride for the locals and city. It is heavily environmentally regulated and protected. Any negative impacts from construction would be met with public outcry and possible fines. A rescue shaft would not be possible in the river section.


The tunnel alignment crossing the Bow River was


mainly in a coarse granular section with a small section in bedrock. The coarse granular soil unit contains cobbles and boulders. Nested cobbles and boulders have been reported in similar soils and were expected. To mitigate construction risk, the contractor proposed


upsizing the tunnel from 1500mm to 1800mm concrete pipe to allow the use of a MTBM with a hyperbaric chamber. This would allow disc changes and access to the face for repairs in case of emergency. To meet minimum velocity requirements for flows in operational life, these pipes were lined inside with a 1500mm fibreglass reinforced pipe and its annulus was grouted to the structural pipe. The combination of pipes lowered risk, while satisfying project flow and minimum cleansing velocity requirements.


Liner failure All concrete pipe and manholes were designed to be HDPE-lined for corrosion protection from H2S. The Phase One contractor proposed an alternative spray liner to speed construction schedule and for ease of application to the large octagon-shaped shaft and manhole chamber. During Phase Two construction, as the contactor was


prepping for the retrieval of the MTBM in the octagonal- shaped shaft the roof slabs were removed and the chamber was opened up. Liner defects found included bubbling and delamination. The defects were repaired by the original coating installer; however, the repairs were unsuccessful. The project team elected to bring in a different coating company, removed entire liner, repair any concrete defects, profile the concrete and apply a new liner. This work took approximately eight weeks and cost Can$1.1 million (US$0.8 million). Control of ambient conditions in the field can be


difficult, especially in a chamber with live sewage flowing and temperatures below freezing. Initial headaches with installing an HDPE liner, welding seams and fastening holes do however provide advantages in the end in hard- to-control environments.


Trunk sewer location IST’s Phase Two needed to tie into the existing 1950mm- diameter trunk located in Memorial Drive, a major


June 2026 | 23


Below: Vibration unit installed in residence.


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