Long, Curved Microtunnelling Drives for a New Force Main Through Environmentally Sensitive and Urbanized Community

Background: York Region, Ontario, Canada located north of the Greater Toronto Area (GTA) is one of the fastest growing Regional municipalities in Canada. Rapid growth along with aging infrastructure required the Region to develop a unique solution continue to service the population and provide redundancy in its sewage system. York Region retained GHD to develop a sustainable and innovative solution to provide the new force main through a highly urbanized city with environmentally sensitive areas. The new sewer force mains (FM's) would connect three Regional pump stations and would provide redundancy in the system which currently has only a single force main nearing 37 years of age. The system is at great risk since it has no standby FM and extremely limited maintenance access to significant portions located within the East Holland River floodplain. Rapid growth has also created significant utility congestion and interferences. Furthermore, additional hardships exist with the environmental restrictions placed on FM's construction alongside the Bailey Ecological Park. To address these challenges the design team undertook a detailed risk assessment and analyzed the impact of multiple construction methods. Multiple design scenarios were produced to evaluate costs, property impacts and limitations of trenchless technologies. The analysis demonstrated that the use of trenchless technologies would have the least social and environmental impact. After rigorous analysis, the design team concluded that micro tunneling 5.6 km (3.36miles) would be the most technically viable and cost-effective solution. Solution: York Region embarked on one of the longest sewage Micro tunnel (MT) projects in Canada's history. This project incorporates construction of a 1050mm (42') diameter sewage FM to be constructed within a precast concrete tunnel liner that traverses 5.6 kms (3.48 miles) across highly urbanized development and environmentally sensitive lands (refer to figure 1). Trenchless technology became imperative due to significant community fatigue associated with the recent construction of a new transit system and the need to construct along the sensitive Bailey Ecological Park and multiple crossing of the east Holland River. The designed tunneled portions included ten (10) shafts with several tunneled sections featuring compound curves with drive lengths up to 830 metres (2730 ft.). Following the tender process, the contractor Ward and Burke Microtunnelling Ltd. proposed even longer drive lengths up to 1132 meters (3715 ft.) in eight (8) shafts. With the project objective and challenges in mind, the Region's Capital Delivery Group requested a team of innovative thinkers from environmental planning, engineering infrastructure and the water construction industry to develop a sustainable and viable solution for the community. The preferred solution involved connecting three large sewage pump stations within the Town of Newmarket. Industry leading design and state of the art micro tunnelling methodology would need to be married together to maximize the spacing of work compounds and their respective tunnel shafts. During the design, the Region assembled a technical expert panel from the tunnelling industry to validate, enhance the design solution as well as provide confidence to proceed with long MT drive lengths under various geotechnical conditions. This collaborative effort successfully built confidence in the design of long curved tunnel drives, compact shaft compounds, need for an effective Geotechnical Baseline Report (GBR) and optimization of the Region's capital cost estimate. Each of the project tunnel drives exceeded the minimum 320 meters (1050 ft.) in length with the longest drive in excess of 830 metres (2730 ft.). The allowable tunnel size ranged in diameter between 2.25 metres (7.4 ft.) to 2.50 metres (8.2 ft.) internal diameter to construction of the new 1050 mm (42') FM and to allow for construction industry flexibility with machine availability. The geotechnical conditions would require new or newly refurbished microtunnel boring machines (MTBMs) with hardened cutting tools, hardened face, advanced survey technology and interjack segmental pipe thrusting system. Additionally, the tunneling machine was specified to include the ability to perform compressed air intervention at the face to change tooling. The project began construction in summer of 2019 and is expected to reach completion by 2021. The community participated in the design process and are satisfied with the reduced construction disruption. The local Conservation Authority guidelines and environmental protection requirements to construct within a critical river floodplain were satisfied. Conclusions: The project was tendered in spring 2019 resulting in the award to Ward and Burke Microtunnelling Ltd. in the amount of $110M CAD ($73.4M USD). The use of MTs for large diameter FMs with extended drive lengths is a feasible solution to minimize community and utility disruption while offering both schedule and cost advantages. Collaboration with the construction industry by conducting effective technical expert panel sessions during the design phase led to a more reliable design and more accurately assessed capital costs.