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below the existing subway line due to concerns regarding vibration and settlement risk that could cause a disruption in service. Instead, a TBM exit shaft was constructed west of the existing subway line and a TBM launch shaft was constructed east of the subway line, with the future station to be built in between. These shafts formed part of the advance tunnelling contract awarded to Crosstown Transit Constructors (Kenaidan Contracting, Obayashi Canada, Technicore Underground and Kenny Construction Company). These shafts were constructed by Deep Foundations Contractors (now Green Infrastructure Group [GIP]) based out of Toronto and were designed by Isherwood Geostructural Engineers.

To advance the schedule of the tunnelling operations, it was decided by the tunnelling project team to hoist the still assembled TBM out of the exit shaft via a large gantry crane provided by Mammoet Canada. Due to limited space available to support the gantry crane, the designer and crane provider worked together on a solution to support one side of the gantry crane on the exit and launch shaft secant pile walls, adding loads in excess of 2,000 kN (460 kips) to the shoring walls that travel along the wall at the gantry leg location. The TBM was then placed on a specialized hydraulic trailer used to balance ground pressure, to traverse 100 metres (330 feet) along Eglinton Avenue, above ground and the existing subway line, before being lowered via a second gantry assembly into the launch shaft, again supported on the launch shaft secant pile walls.

The existing subway box, located in the middle of the proposed traverse, was constructed in the 1970s and showed signs of corrosion and concrete spalling in the section under Eglinton Avenue. The existing subway structure was also designed under an earlier design code and as a result did not have the spare capacity to support the additional load from the TBM carrier. To solve this concern, a temporary bridge was constructed to allow the TBM to cross over the subway line, supported on micropile foundations on either side of the existing subway structure. This allowed the TBM carrier to traverse Eglinton Avenue without putting additional stress on the existing structure. The entire lift operation took place overnight outside of the existing subway operating hours, over just two nights. It finished ahead of schedule, allowing Eglinton Avenue to be reopened earlier than planned. The overall performance of the support of excavation and TBM bridge during the TBM lift was excellent, with negligible movement detected during real time monitoring of the shaft walls during lifting and lowering of the TBMs, and negligible settlement of the bridge foundations after TBM traverse.

Main station excavation

In July 2015, the Eglinton LRT Station contract was awarded to Crosslinx Transit Solutions (EllisDon, Aecon, ACD Dragados Infrastructure Canada and SNC Lavalin). Construction of Cedarvale Station required support of excavation between the exit and launch shafts for the three new station entrances, station platform, concourses, and air shafts, all with varying and sometimes sloping excavation depths ranging from five to 20 metres (16 to 65 feet) depth. To meet the staging needs, various design and construction strategies were employed including temporary staging walls, buried tension struts between adjacent excavations, partial top-down construction to support live traffic and other methods of support around the 28 internal corners were utilized. Support of excavation and traffic deck installation for the main station was constructed by GIP and designed by Isherwood.

The new station is located at an extremely congested location within the city, the intersection of Eglinton Avenue and the junction of the separated highway, William R Allen Road. The new station overlaps and ties into the existing Eglinton West Subway and Bus Station, which is serviced by four main bus routes by way of a loop whose entrance and exit are between William R Allen Road South and William R Allen Road North, all of which needed to be maintained throughout construction. The existing Eglinton West Station also services the Yonge University Spadina subway line, which had to be underpinned to allow for an excavation of 8.5 metres (28 feet) below the existing subway line for the future ECLRT tracks. The combination of bus and pedestrian traffic from the Eglinton West Station and the vehicular traffic from William R Allen Road made the sequencing of installation of the shoring a significant challenge.

During the construction of Cedarvale Station, traffic flow had to be maintained on Eglinton Avenue, William R Allen Road, and the Eglinton West bus loop as well as all intersections. Approximately 45,000 square feet (4,200 square metres) of steel frame decking with timber mats were constructed and provided a staging area for the construction activities below. The traffic deck was installed in stages. The initial stages were planned to maintain traffic on one side of the road while the shoring, shallow excavation and decking were installed on the other side of the road. These stages had to be further divided when working on the north side of the road due to all the

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Ph. +1 905 607 8468 www.p1international.ca | sales@p1international.ca intersecting streets that had to remain active (i.e., William R Allen Road North and South and the Eglinton West Station bus loop entrance and exit). This staged installation method necessitated the design and construction of several small temporary supports for the decking and shoring that were later demolished during the excavation.

Another challenge was the location and depth of the existing subway box. The support of excavation had to be designed around it with a series of perched shoring piles bolted to the top of the TTC box to retain this small height of soil. Challenges arose when determining how to both install these perched piles a top the existing subway box and how to move the heavy construction equipment from one side of the box to the other because the box dissects the middle of the future station and required excavation. A concrete pad was designed and constructed by removing the top one metre (three feet) of soil and replacing it with a lightweight concrete slab that would remove some of the dead load on the existing TTC box and disperse the load of the equipment that would be travelling across the structure and operating on top of the structure without causing damage or undue stress. Once the shoring piles had been installed, the lightweight concrete slab was removed, and the traffic deck was installed spanning over the box to avoid adding any load greater than the existing condition on the structure.

Excavation below the existing subway

To support the existing subway line within the excavation so the new ECLRT tracks could pass below, Entuitive, the project’s structural engineer, designed two eight-foot-deep (2.4-metre) plate girders bolted to the side of the existing subway line and spanning across the excavation. Crosslinx brought in Dr. Sauer and Partners, a tunnelling consulting group based out of Salzburg, Austria, to provide the temporary mining support design underneath the subway box that provided structural underpinning support installation. The initial excavation below the subway line took place in six galleries perpendicular to the existing subway boxes. The galleries were designed with shotcrete facing and temporarily supported the subway box above with vertical props. When a gallery was completed, needle beams were installed through the galleries and connected to the plate girders on either side of the subway line. The props were then removed. Mining gallery construction was completed by HC Matcon.

During the design phase, it was decided to use the shoring system to support the suspended subway box during the underpinning phase to reduce the number of support caissons required, which would impact the future excavation works and future train testing inside Cedarvale Station. This design change resulted in significant additional load 2,050 kN (460 kips) being carried through the secant pile wall on each side of the existing subway box. The structural plate girders were supported by a recessed connection in the secant pile wall on the south side and structural columns on the north. Jacks were set up on each needle beam and the plate girders to allow adjustments to accommodate movement of the support system during excavation.

The support of excavation under the subway adjacent to the underpinning mining galleries was particularly challenging because of very limited space for equipment due to the 2.4-metre (eight-foot) mining gallery height and four-metre (13-foot) gallery width, further congested by the underpinning beams supporting the now undermined subway boxes. Any support of excavation in this area also had to account for small allowable movements of the soil supporting the adjacent subway line. The innovative solution to this tricky challenge was to use two stacked 12-metre-long (40-foot) wide-flange waler beam sections installed between the webs of the shoring piles on either side of the subway. The north and south double walers were preloaded toward each other using a series of tie rods, like a bow and arrow, before being encased in reinforced concrete. Once the concrete had reached a minimum design compressive strength, the tensile preload on the tie rods was then released transferring the preload to the reinforced concrete waler beams and soil behind prior to further excavation. This system was designed using the observational method and eliminated the need for cross site bracing in the very tight access and allowed excavation to proceed with enough headroom to install the lower shotcrete wall and four rows of soil anchors. The preloaded waler was monitored throughout preloading, unloading and excavation to ensure it met the design requirements. If movements were to exceed allowable limits, a strut would be installed midspan to limit deflections, optimistically at a late enough construction stage to not impede construction.

The overall deflection of the preloaded waler system after final excavation was 12 millimetres (0.5 inches) on the north wall and 0 mm on the south wall, within the 1/1,000 (12 millimetre/0.5 inch) limit imposed by the TTC, and much less than the theoretical non-preloaded deflection of 30 millimetres (1.2 inches). The overall deflection of the anchored shotcrete shoring system was nine millimetres (0.3 inches), and the existing subway boxes adjacent to the excavation saw less than two millimetres (0.08 inches) differential settlement at the joint locations, within the acceptable range.

Acknowledgments

The overall construction of this station spans over a decade and two contracts and involved many companies to

Temporary support make it a success. Both project teams are responsible for accomplishing this challenging construction saga over and under the existing TTC subway line.

Shawna Munn, P.Eng., is a senior engineer with Isherwood Geostructural Engineers in Mississauga, Ontario. She has more than 12 years of experience in the shoring and specialty foundations field with a focus on complex excavation retention systems and subway infrastructure.

Sarah Speir, P.Eng., is the youngest –and one of the only female – project managers working on the Eglinton LRT project for Crosslinx Transit Solutions. Before starting on the project in 2015, she got her bachelor’s degree in civil engineering from Queen’s University. l