Alternatives study
At the outset of preliminary design, an alternatives study was completed to determine the preferred method for replacement. The possible methods investigated included open cut replacement, installation of a new transmission watermain in tunnel and structural rehabilitation of the existing pipes using the Cured in Place Pipe (CIPP) Watermain Lining. This study compared the three alternatives based on various environmental, economic, and technical considerations. Using preliminary costs based on initial conceptual designs, CIPP was identified as the most cost-effective and efficient method of renewal.
Using CIPP allowed the city to avoid unnecessary financial and scheduling burdens.
See how WSP has helped the City of Toronto rehabilitate over 500 km of watermain
In addition to addressing an existing infrastructure issue using an innovative method, the rehabilitation of the watermains extended the lifespan of existing materials and limited disruption of critical services. With increasing demands for responsible resource use and service reliability, it is anticipated that this type of rehabilitation method will be increasingly adopted to upkeep infrastructure at a pace matching future trends and service demands.
Project Challenges and Resolutions
Watermain Isolation
To complete both the CCTV during the condition assessment and the actual lining of the pipelines, isolation of the watermains was critical. However, when isolation of the transmission watermains was attempted during construction, it was discovered that the perimeter valves were inoperable and could not provide adequate isolation. To isolate the 750 mm diameter transmission main without disrupting the adjacent distribution system, it was necessary for two new valves to be installed: an insertion valve and a line stop.
The installation procedure for the insertion valve is unique given that it can be installed on live mains without disrupting watermain networks. This solution provides a fully functional valve without requiring any valve shuts, disinfection procedures or large equipment on site. The installation of an insertion valve on a main of 750 mm diameter is rare and was a significant accomplishment on this project. The installation of the line stop included a flanged fitting and temporary valve, which allowed for the pipe to be hot-tapped under pressure. A stopping head was then inserted to stop flow and allow the isolation of the transmission main.
Condition Assessment
As required by the terms of reference for the project, a condition assessment of the existing transmission mains was completed at the outset of design. This assessment included a thorough review of as built records, a detailed inspection of all valve chambers, and a CCTV investigation to visually assess the condition of each watermain and determine appropriate renewal method.
GAME Consultants were contracted to complete the CCTV investigation and determined the most appropriate tool to successfully inspect the pipes. It was determined that a track mounted Remotely Operated Vehicle (ROV) would be used to complete the inspection. The completion of the CCTV inspection was a challenge due to difficulties in gaining access into the existing watermains. The ROV was inserted into the 900 mm main through the removal of a 900 mm blind flange. For the 750 mm diameter main, a new 300 mm diameter tapping sleeve and valve had to be installed inside an existing chamber.
The CCTV investigation was successfully completed and no significant pipe deficiencies were identified that would preclude the rehabilitation using CIPP.
Pipe Transitions
On the transmission watermains, the existing cast iron pipes in all liner access pits and new valve chambers were replaced with new stainless steel pipes.
At one of the transition locations, an unexpected condition was found, which required reengineering the connection detail. The as built drawings relied on during design were not up to date and instead of a straight pipe, a 750 mm diameter sweeping bend encased in concrete was found. The concrete encasement had to be carefully removed and alternate coupling options had to be investigated. A Hymax coupling was selected for use at this location, which required modifications to the new stainless steel pipe already manufactured.
Quality Assurance and Quality Control
In addition to detailed CCTV video review of the interior of the lined watermains, this project included a full liner material property testing regime as part of the QA/QC process. Liner coupon plate samples were first taken from the liner at access pit locations. One sample was taken from the 150mm distribution main, four samples were taken from the 750 mm transmission main, and six samples were taken from the 900 mm transmission main. All samples were submitted to the lab at Paragon Systems Testing for analysis. Paragon tested each coupon plate according to the ASTM D790 (flexural properties), ASTM D638 (tensile properties), and ASTM D5813 (wall thickness) tests. The results of these tests were then utilized to validate the liner designs. to ensure that the liners installed exceeded the external and internal pressure requirements specified by the contract documents.
