2013 Project of the Year - New Install Runner Up
Keswick WPCP Effluent Outfall Project, Keswick, Ontario, Canada
Oct 28, 2013
The community of Keswick lies on the shore of Lake Simcoe and is a rapidly growing part of the Town of Georgina on the outskirts of the Greater Toronto Area (GTA) in Ontario. Keswick was once a rural community, but is now a bustling commuter hub due to growth in the GTA and an extension of the 404 Highway.
The Regional Municipality of York (York Region) has being expanding the existing Keswick Water Pollution Control Plant (WPCP) to meet the demands of the growing community, the existing outfall expansion is part of the overall plant expansion to provide sufficient conveyance capacity to accommodate future flows from the plant expansion. Hatch Mott MacDonald (HMM) was retained by York Region to undertake detailed design and construction administration for a new, 750- to 1,200-mm outfall. McNally Construction Inc. was awarded the construction contract as the general and marine contractor, with Ward and Burke Microtunnelling as the microtunneling subcontractor. The new outfall is approximately 1,800 m long, and runs from the WPCP to a point 900 m offshore in Lake Simcoe.
Along its 900-m onshore alignment, the outfall traverses regulated wetlands, arterial roadways containing both ambulance and fire stations, a busy “five corner” intersection containing numerous existing utilities, a quiet residential subdivision with mature trees and narrow roadways, a city park and a regulated shoreline/floodplain. Once offshore, Lake Simcoe is subject to stringent environmental regulations intended to maintain the lake as a valuable ecological and recreational resource.
Geotechnical conditions along the alignment consist of very soft/very loose silts, clays and sands, along with local areas of glacial tills with cobbles and boulders. The groundwater table is at or near the ground surface along the entire alignment. The combination of very soft/very loose soils and a high groundwater table indicated that unstable (i.e., flowing) ground behavior would be predominant. Key project challenges include the following:
1. To preserve “pre-approved” environmental assessment status, no permanent property acquisitions could be made. This posed significant issues as separation between the new outfall and property lines would locally be less than 1 m.
2. Significant impacts to traffic would not be tolerated. Full closures and detours would not be considered. Only minor lane closures could be permitted.
3. The community’s primary trunk sanitary sewer crossed the outfall alignment, and had been constructed using solid-sheet shoring that extended over 11 m below grade and was abandoned in place. Only one crossing location could be identified where abandoned shoring was not present. At that location, to maintain hydraulics the new outfall needed to cross underneath the existing sewer with a separation less than 0.45 m.
4. Unstable ground conditions, along with the need to traverse a residential subdivision would result in significant risk for third-party damage claims. Many homes were of older construction, and considered highly susceptible to damage.
5. Permitting of works that would result in disturbance to the natural shoreline of Lake Simcoe would be difficult to obtain, and likely contain onerous conditions of approval. The use of trenchless methods to complete the onshore-offshore transition was considered favorable.
In order to address the identified project challenges, several alignment alternatives and construction methods were evaluated with regard to cost, schedule, community and environmental impacts. Evaluated construction methods included open-cut trenching with continuous solid-sheet shoring, horizontal directional drilling and microtunneling.
Microtunneling was considered to provide the lowest risk for schedule, community and environmental impacts. Microtunneling was subsequently recommended as the primary onshore construction method despite having the highest anticipated construction costs. This recommendation was significant as until this point, the history of microtunneling in Ontario had been limited to approximately only 10 projects. Most of those projects had occurred in the 1990s and had met with limited success.
There were also several constructability issues that needed to be addressed. The presence of very soft/very loose soils was considered problematic for microtunnel stability due to the risk of machine sinking and/or inability to make steering corrections. This was mitigated by requiring the contractor to configure the microtunnel boring machine (MTBM) to its lightest configuration and to install a minimum length of trailing cans to aid in the distribution of MTBM weight. Provisional measures for intermittent ground improvement were included in the contract to provide an additional safeguard.
For the trunk sewer undercrossing, the limits of existing sheet pile shoring were verified by geophysical surveys, as well as by direct probing using horizontal directional drilling. The risk of settlement and/or disruption of the trunk sewer was mitigated by a combination of advance jet-grout support, bypass pumping and having a cured-in-place liner stocked and available should damage occur.
The project has proven to be a tremendous success. Through a pre-qualification process, York Region was assured of having qualified, experienced contractors to do the work. Through collaboration between the owner, design consultant and contactor team, several notable microtunneling firsts were achieved on the project including: the first offshore/underwater reception of a microtunnel drive completed in Canada; the first curved microtunnel drive completed in Canada; and the first compound curve microtunnel drive completed in North America.
Of particular note is the collaborative effort that led to the incorporation of curves into the microtunnel design. The contractor proposed the curved drives as a means of eliminating two shafts, which resulted in more than $1 million cost-savings for York Region. However, in order to accommodate this change, the diameter of the outfall had to be upsized from 750 to 1,200 mm and modifications to existing permits had to be obtained. The design and contractor teams, along with York Region, worked together to evaluate the technical risks and benefits of these changes. Once it was decided to adopt the changes, the teams further collaborated to make the modifications necessary to implement them.
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.
Project Owner: The Regional Municipality of York
Engineer: Hatch Mott MacDonald
Contractor: McNally Construction (General), Ward and Burke (Microtunneling Subcontractor)
Manufacturers/Suppliers: Herrenknecht, Munro, VMT