Opportunity
In the course of upgrading the Keswick Water Pollution Control Plant, the Region of York became aware that the existing outfall would not provide enough capacity for the future expansion of the plant.
The existing outfall was about 1,800 meters (5,905 feet) long, of which 900 meters was onshore and 900 meters offshore. The project posed a number of significant challenges:
- The onshore alignment crossed a complex “five corner” intersection and heavily traveled roadways that provide primary access to emergency and fire stations. Only minor construction related lane closures would be allowed.
- The onshore alignment also traversed a city park and a residential neighborhood with narrow roadways. Although the new outfall would sometimes pass within about three feet of property lines, a preapproved environmental assessment prevented the acquisition of more property for permanent easements.
- In order to avoid abandoned solid-sheet shoring extending more than 36 feet (11 meters) below grade, the new outfall had to pass beneath the existing sewer with a separation of less than 18 inches (45 centimeters).
- Geotechnical conditions included soft, loose soil, clay, and sand, and areas of glacial till containing cobbles and boulders. The groundwater table was at or near the surface for the whole alignment. These subsurface conditions require sophisticated pipeline support considerations.
- Obtaining permits for work that might disturb the shoreline of Lake Simcoe would be difficult.
- The use of trenchless technology for the onshore-offshore transition was selected to address the public and environmental concerns.
Solution
In 2011, HMM, a joint venture of Mott MacDonald and Hatch, was retained as the prime consultant for the project. Our role encompassed preliminary and detailed design of a new outfall with a diameter of 47 inches (1.2 meters), including the following:
- Coordinating onshore and offshore geotechnical, geophysical, and hydrogeological investigations to determine the soil’s characteristics, strength, and other properties
- Assessing the route’s archaeological significance by onsite investigations
- Developing and selecting the cost-effective outfall route and vertical profile
- Selecting the appropriate excavation and tunneling methods to satisfy the primary concerns
- Developing and selecting tunnel shaft and staging areas, prequalifying contractors, and coordinating permits and approvals
- Project management, QA/QC, scheduling, constructability and risk assessment, cost estimating, tender phase services, and construction administration
Based on our evaluation of various construction methods, a combination of open-cut trenching, microtunneling, and marine dredging was selected. To prevent the microtunnel boring machine from sinking or going off course in the very soft, loose soil, the contractor was required to use the machine’s lightest configuration and to install a minimum length of trailing cans to help distribute the machine’s weight.
According to Trenchless Technology, “Through collaboration between the owner, design consultant and contractor team, several notable microtunneling firsts were achieved on the project.” These included the following:
- First offshore/underwater reception of a microtunnel drive completed in Canada
- First curved microtunnel drive completed in Canada
- First compound curve microtunnel drive completed in North America
Outcome
“The project has proven to be a tremendous success,” reported Trenchless Technology, which named it the New Install Runner Up in the competition for 2013 Project of the Year.
“The microtunneling work was completed on time, under budget and with no third-party damage claims. All four project drives were completed on line and on grade. In completing this project, the limits of what can be done using microtunneling have been pushed forward significantly, both in Ontario and across North America.”
The project was also named a 2014 Public Works Project of the Year by the American Public Works Association (APWA).