Canadian Consulting Engineer
Toronto’s critical trunk sewer safe againWater & Wastewater Wastewater treatment systems
A complex construction project to bypass a 60-metre section of Toronto's most critical sewer pipe has now been completed and sewage at the rate of 400 million litres per day is now able to flow more safely.
A complex construction project to bypass a 60-metre section of Toronto’s most critical sewer pipe has now been completed and sewage at the rate of 400 million litres per day is now able to flow more safely.
The Coxwell Sanitary Trunk Sewer in East York serves 750,000 residents. Even in dry weather, the flows are six cubic metres a second, which is three times the flow of the Don River.
The pipe was hand dug during the 1950s. It runs from a ravine near O’Connor Drive and Coxwell Boulevard, south to the Ashbridge’s Bay Treatment plant near Lake Ontario.
Making things even more difficult, the 2.6 metre diameter concrete structure runs from a depth of 42 metres. Its maintenance holes are spaced only every 1.5 kilometres.
When a CCTV camera was sent down to examine the pipe in 2008, it was found that a 60-metre section at O-Connor and Coxwell was badly damaged and needed to be repaired and replaced.
Because of the tight residential site at one end, and the urgency of the problem, it was decided that the best solution was to construct a permanent bypass around the problem section of the pipe. Construction started in April 2010 and the bypass was in service by the end of this February.
Hatch Mott MacDonald were the engineers representing the city of Toronto on the project, and AECOM and McNally International jointly held the design-build contract.
A tunnel boring machine was used to dig the new bypass, entering in the ravine at Taylor Massey Creek Park in the ravine and exiting at the busy corner of O’Connor Drive and Coxwell Avenues.
The work involved not only excavating the exit shaft to 42 metres, but also removing the top portion of the existing sewer pipe and replacing it with a temporary flume to carry the flow while construction proceeded. The wastewater could not be redirected during the construction, making it critical to have a good connection between the bypass and original pipe. The connections had to be done under live flow conditions at both the entry and exit shaft. The project also involved decommissioning an old sewage pump house located underground and close to nearby homes. Then mid-way through the project the Ministry of Labour required the construction of a secondary escape route at the exit shaft.
A worker does a final walk-through check of the Coxwell Sanitary Trunk Sewer bypass days before it was put into service in February. Photo courtesy City of Toronto.