As part of Toronto Water’s proactive asset management program, numerous sewers are being rehabilitated using trenchless technology. In fall 2017, the City’s Engineering and Construction Division awarded the contract for rehabilitation of the North Mimico Sanitary Trunk Sewer (STS) and South Mimico STS sewer and maintenance holes to Capital Sewer Services for $11.3 million (CAD).
The objective of the rehabilitation project is to address structural and operational defects within the sewer and extend its useful life for a minimum of 50 years. Due to the high corrosion levels observed in the sewers, the recommended rehabilitation approach includes installation of a structural rehabilitation liner with corrosion protection barrier for the sewers, maintenance holes and replacement of appurtenances with non-corrosive products.
Built in 1961, North Mimico STS services a catchment area of 2,755 hectares, within the City of Toronto and extends into the Region of Peel. North Mimico STS and South Mimico STS meander through the Mimico Creek floodplain, passing through City Parks, Toronto Region Conservation Authority (TRCA) property and several private properties, all within a sensitive ravine environment. Access to the North Mimico STS and South Mimico STS required extensive stakeholder coordination, permitting and approvals efforts.
The scope of the rehabilitation work, designed by Jacobs, included CIPP of approximately 360 m of 1,200 mm to 1,350 mm diameter of the North Mimico STS, approximately 660 m of 525 mm to 750 mm diameter of the South Mimico STS and 25 m of the 600 mm diameter sewer connecting the South Mimico STS to the North Mimico STS, as well as CIPP lining of about 250 m of 375 mm to 600 mm diameter local sewers and structural lining of all associated maintenance holes (MH). The rehabilitation work begins in Echo Valley Park, crosses Kipling Avenue and continues through existing permanent easements within Islington Golf Course.
Design & Construction and Lessons Learned
To mitigate the impact to the golf course business operations, construction within the golf course was restricted to the period from Nov. 20 to April 5, with final restoration works completed by April 20. The work carried out during this 4.5-month period included construction and removal of 25,000 m2 of temporary access roads, staging areas and construction platforms, two temporary Bailey bridges over Mimico Creek, an 8-m deep temporary construction shaft, temporary above ground bypass of all sewer flows, CIPP lining of sewers, structural rehabilitation of MHs and restoration planting and seeding of all areas disturbed during construction.
The project team worked tirelessly to mitigate delays during construction to maintain the strict timeline necessary to complete all work during the golf course off-season. Jacobs’ design included keeping construction activities off fairways and greens to reduce impacts to grassed areas within the field of play and marking access routes for additional visibility during snow-cover to limit damage to other areas. Capital Sewer Services completed vigorous preplanning, fulfilling contract requirements for early submittals to allow sufficient time for review by Jacobs and City staff, and ordering 25,000 m2 of specialty sod in the fall to make sure it was available and ready for use in the spring to restore the golf course to its pre-construction original or better condition.
Due to the tight time lines and scheduling restrictions required to work within the golf course, there was no room within the schedule for delays or down time due to wet weather. Hence, Jacobs and the City stipulated that the contractor is to provide, install and maintain a temporary bypass capable of accommodating the full anticipated peak wet weather flow within the North Mimico STS of 1485 L/s. The existing maintenance holes were too small to house the seven, 12-in. suction pipes necessary to pump these flows, resulting in the need for construction of a large temporary suction shaft in Echo Valley Park. With only limited space available at this location, Capital Sewer Services came up with the innovative solution of using the CIPP liner itself as a sewer plug during curing to stop flows, avoiding the need for construction of a deep sump to prime the pumps and reducing the overall construction footprint.
The North Mimico STS, is located near the base of the ravine within the Mimico Creek floodplain. Due to the meandering nature the Creek, the bypass was required to cross the creek in two locations; the first crossing was submerged in Mimico Creek, under Kipling Avenue bridge and the second comprised a temporary pipe bridge to convey sewer flows over the river. Various alternatives to submerging the bypass within Mimico Creek were considered and reviewed, but these were ultimately rejected in favor of the submerged crossing due to permitting and space constraints.
For the submerged crossing, steel pipes were used instead of typical HDPE to protect them from possible damage by debris carried in the flood waters. The submerged crossing was secured in place with concrete ballasts to stop the pipe being washed away during storm events. To prevent erosion of the creek bed gabion rip rap was added upstream and downstream of the crossing. Fish passage was integrated with the submerged crossing. Additional mitigation measures included flushing and cleaning the bypass pipes while not in use to prevent sewage spills to the environment in the event of pipe damage during a flooding event. Although originally considered by some of the team to be over and above what was necessary, these robust submerged crossing mitigation measures proved to be worth the extra effort and expense as in March and April, during construction the area was hit by several large storms coupled with unusually high rates of snow-melt, causing water levels within the creek to rise very quickly. The mitigation measures performed well during this time and were able to withstand the high flows allowing the work to proceed without interruption or damage to the environment.
The second bypass crossing was within Islington Golf Course and used a temporary pipe bridge spanning 22 m over Mimico Creek. Immediately adjacent a second temporary bridge crossing was installed to facilitate construction vehicle crossing of the creek. The bridges were supported with abutments on either side of the river bank, installed outside of the 2-year flood line elevation.
The project team successfully completed the rehabilitation and major restoration work within the limits of the golf course on schedule (April 2018), despite challenging weather conditions. The remaining restoration and demobilization work within Echo Valley Park were completed over the following months.