Overcoming Environmental and Community Constraints by Executing Record Length Curved Microtunnelling Drives

The Regional Municipality of York (Region), located north of Toronto, is one of the fastest-growing municipalities in Ontario. Rapid growth along with ageing infrastructure requires the Region to develop unique solutions to service the growing community, all while improving operational flexibility and durability of the sewage collection system. As part of a Forcemain Twinning project, the Region required the installation of a new 1,050 mm 42” forcemain with similar performance characteristics as the existing forcemain linking three pump stations at Newmarket, Bogart and Aurora. The new forcemain traverses the Town of Newmarket and includes crossing of 4 regional roads, 11 crossings of the Holland River and associated environmentally protected wetlands, and makes 3 crossings of the CN Rail/GO commuter rail corridor. Due to significant constraints on the availability of land for worksites, the Region developed a state-of-the-art microtunnelling solution utilizing long curved microtunnelling drives. On completion of microtunnelling operations, the new forcemain was installed within the precast microtunnelling pipe. In the execution, the project team evaluated various alignments and different trenchless solutions, developed an environmentally sensitive and low impact solution, and managed all work associated with the approvals, design, tendering and construction of the new forcemain system. The complexity of the project was a major challenge for the owner and experienced trenchless designers and required extensive coordination with approval agencies, and local stakeholders. The design and eventual construction of the 5,145 meters (16880 feet) of forcemain resulted in a solution that accomplished the aim of low community and environmental impacts, achieved through execution of long curved microtunnelling drives that broke North American records for such an approach. The microtunnelling solution included ten shafts and nine tunnelled sections. A favourable outcome was achieved for all key performance indicators that included overcoming significant land constraints while protecting the environmentally sensitive lands (Bailey Ecological Park), and highly urbanized areas that included industrial, commercial, and residential areas of Newmarket. Project success required comprehensive data collection, critical analysis, various design assessments, including a collaborative process between the main design parties and highly experienced microtunnelling contractors. In studying the requirements for the new forcemain, it became imperative for the project outcome to manage all risks associated with the approvals, design, tendering and construction of the new system. The planning of a successful microtunnel project would challenge experienced designers, approval agencies, the owner and tunnellers alike. During the investigation period of the project, geotechnical experts recommended a phased approach in collecting borehole and hydrogeological data. The phased approach included initially drilling boreholes to various depths along the planned three alignments. The finding from the initial boreholes’ investigation included which of the three alignments would prove to be best for tunnelling. A second round of boreholes was drilled along the preferred alignment. This approach provided an opportunity to collect critical data along the alignment that would satisfy the needs for both design and construction as well as mitigate project risk. During the preliminary design, the team engaged an expert panel with representation from the owners, geotechnical experts, tunnel designers, contractors and tunnel equipment manufacturers to validate and enhance the design assumptions. The main goals of the expert panel session were to firstly transfer project information to the contractors improving their knowledge of the upcoming project, secondly gain insight from the tunnelling community and thirdly determine the best means to address tunnelling risks. The workshop provided confidence that the tunnelling industry in Ontario was not only capable, but up to the task of constructing the long microtunnel drives. During the detailed design, each of the project tunnel drives exceeded 362 metres (m) (1187 ft.) in length, with the longest drive-in excess of 821 m (2693 ft.). The tunnel team provided a range in tunnel diameters between 2.50 m (8.2 ft.) and 2.25 m (7.4 ft.) internal tunnel diameter. This range in tunnel diameters allowed for flexibility during construction while providing sufficient working room to place and grout the new 1,050 mm (42 in.) FM. The proposed geometry of the alignment allowed for contractors to consider going beyond the proposed designed alignment by omitting intermediate shafts if their equipment and execution strategy was determined to be effective during the early stages of the contract. The first tunnelled section length would include the second-longest drive of the project between S1 and R1, a total of 818 m (2,683 ft.). The findings and outcomes of the first drive would allow the team to consider drives of equal or greater lengths.