In the fall 2020, the Regional Municipality of York initiated a project to rehabilitate a 127 m long concrete box culvert under Yonge Street. The Region retained Jacobs Engineering to provide design and construction support services.
The rehabilitation ultimately consisted of the installation of a fibre reinforced polymer (FRP) structural liner in the existing box culvert and microtunnelling a 1.5 m diameter overflow tunnel under Yonge Street. The Yonge Street Culvert rehabilitation and overflow pipe installation provides an excellent example of utilizing multiple trenchless technologies to satisfy complex site constraints, engineering design criteria and client demands.
Existing Site Conditions of the Yonge Street Culvert
The Yonge Street Culvert is located 100 m north of Eagle Street West in the Town of Newmarket, Ontario. The existing culvert was constructed in 1960 and extended to accommodate private development in the 1970’s. The structure is a cast-in-place open footing concrete box culvert with approximate dimensions of 3.05 m span, 1.55 m rise (from top of footing) and 126.6 m length. The original culvert is a straight section that passes under Yonge Street. The extension passes under a private parking lot adjacent to Yonge Street and consists of four monolithic sections with skew angles ranging from 14- to 55-degrees between adjacent sections. Yonge Street at the location of the culvert had recently been reconstructed, including the installation of bus rapid transit infrastructure.
The existing culvert showed extensive deterioration due to its age, warranting major rehabilitation or replacement. Jacobs worked closely with the Regional Municipality of York to develop an optimal solution that would reduce construction costs and traffic impact while satisfying the site constraints. Trenchless construction methods were preferred for several reasons, namely: minimized impacts to traffic on Yonge Street and adjacent private property and businesses, preservation of the recently reconstructed roadway and new bus rapid transit infrastructure, and lower capital cost than open cut excavation methodologies.
The culvert conveys flows from Western Creek, a tributary of the Holland River East Branch, in the jurisdiction of the Lake Simcoe Region Conservation Authority (LSRCA). Hydraulic modeling was performed to assess the hydraulic capabilities of the existing culvert. The driving design criteria for the site was that flood elevations up and downstream of the culvert could not be negatively affected. To achieve this, three hydraulically feasible alternatives were investigated: rehabilitation of the existing culvert with a FRP structural pipe liner and installation of a tunneled overflow pipe, rehabilitation with a corrugated steel pipe (CSP) liner with overflow pipe, and construction of a new full-size tunneled culvert with decommissioning of the existing culvert. The FRP liner with overflow pipe alternative was selected as it minimized the required size of the overflow tunnel and reduced risk of utility conflicts during tunneling operations.
Overflow Tunnel Installation
The overflow tunnel consisted of a 1.5-m inside diameter pre-cast concrete overflow pipe with HDPE liner and was constructed using micro-tunneling technology. Microtunnelling was proposed by the drilling contractor – Ward & Burke Construction Ltd. – as it has a high degree of alignment control and minimizes settlement/ground loss during installation with pressurized slurry at the tunnel excavation face. These factors were extremely important as the tunnelling operations were required to avoid several utilities and have no impact to traffic on Yonge Street above.
Settlement monitoring was carried out during tunneling operations. Surface settlement markers were installed every 10 m along the alignment of the overflow tunnel and along the north face of the commercial building south of the tunnel. One subsurface monitoring point was installed along the tunnel alignment and one orthogonal borehole geophone was installed over nearby utilities.
The launching pit was constructed on the east side of Yonge Street and tunnelling terminated breaking through an existing concrete retaining wall on the west. The alignment of the overflow pipe includes a bend 12 m west of the outlet and was chosen to allow the launching shaft to be constructed at the location of the bend, thereby reducing impacts to the creek and vegetation near the outlet. This also allowed the first 12.1 m of the overflow pipe to be installed in open cut.
Custom Fiber Reinforced Polymer Structural Liner
The FRP liner was designed to carry 100 per cent of the loading and does not rely on the strength of the existing culvert. Jacobs worked with an FRP liner manufacturer during design to customize a rectangular section with curved corners that would optimize the hydraulic conveyance capacity of the rehabilitated culvert. A LIDAR scan of the culvert barrel was undertaken during design to confirm optimal sizing of the liner. A mandrel walk-through of the existing culvert prior to fabrication was also undertaking to confirm that the FRP section would be able to pass the bends in the box culvert.
The final design utilized a section with 2.66 m internal span, 1.33 m internal rise, 48 mm wall thickness, and provided a reduction in waterway area of 70 per cent. The FRP liner sections were customized to include elbow pieces to be used at culvert bend locations and pieces with storm sewer outlet connections into the existing culvert. The sections were joined in-situ using bell and spigot joints and connected with an elastomeric sealant. The annular space between the existing culvert and the FRP liner was grouted using a low-density flowable cellular grout.
Utility and Site Challenges
Utility conflicts presented unique challenges during design and construction. Subsurface utility investigations identified the following utility conflicts with the proposed overflow pipe: a 600 mm watermain at the west side of Yonge Street, a 525 mm abandoned storm sewer and a 300 mm watermain at the east side of Yonge Street. The 600 mm watermain is located approximately 150 mm above the proposed tunnel and was protected during construction by wrapping the watermain in bond breaker, encasing it in a concrete cradle to the spring line, and placing insulation between the cradle and the tunnel. The abandoned storm sewer was removed prior to construction. The 300 mm watermain was relocated by diverting the pipe above the proposed tunnel using both horizontal and vertical bends.
The acquisition of private property was also required to facilitate the new overflow tunnel alignment and launching pit.
Construction and Lessons Learned
The overflow tunnel was installed first while the creek remained flowing through the existing culvert. Microtunnelling proceed well for most of the trenchless installation, however, near the end of the tunnel run, a previously un-identified obstruction was encountered. The obstruction turned out to be a soldier pile and lagging wall that had been used for temporary shoring in a previous construction project and had been left in place without documentation. Luckily, the obstruction was not under Yonge Street and was removed quickly through top down excavation.
Once the overflow tunnel was operational, the creek flows were diverted into the tunnel and the FRP liner installation was completed. The custom FRP liner was fabricated by Channeline in the United Arab Emirates. There were some schedule delays due to the impacts that the COVID-19 pandemic had on international shipping.
The tender price for construction was $3,883,000. Construction began in fall of 2021 and continued through to summer of 2022.