Designing Deep Underground Stations That Work
Integrating Transit & Existing Buildings Expertise to Successfully Introduce Underground Structures
The introduction of deep underground transit stations within urban centres affects many different stakeholders throughout the full design and construction process. Building owners and transit agencies often find themselves interfacing with each other where the vertical build meets the linear network. These projects are structurally complex and require an expert understanding of not just the two types of structures separately, but also how to integrate them seamlessly.
Municipal officials, transit agencies, developers, and business owners have all found success with Entuitive’s integrated approach to deep underground stations, which sees our transportation and building structural teams closely collaborating to deliver these projects. Drawing upon their knowledge of both vertical and linear infrastructure, our team of experts solves the challenges encountered when it comes to adding a new vertical building above transit or adding a new transit line within built communities or below busy roadways.
Often the integration necessitates a shared common concourse, foundations, and structural systems. Solutioning the physical integration of the various pieces of infrastructure may be the first step but addressing the asset owner’s perceived risks is the most important. These risks are often perceived to take place during construction, including the often-lengthy disruptions to operations and the concern that the infrastructure asset will be damaged as result of integration.
The additional integration of our construction engineering expertise in deep underground stations results in all stakeholders being engaged, concerns being heard and addressed throughout the process, continuous live operations during construction, and consultation and mitigation of impacts to adjacent businesses.
Keep reading to learn how our integrated approach benefits our clients from design through construction, resulting in the successful introduction of complex structures underground.
Protecting Your Assets & Ensuring Ongoing Daily Operations
At the design phase of a project, our team comes together to answer key questions regarding the engineering of deep underground stations. How do we protect the adjacent buildings? How do we protect utilities? What needs to be accommodated for existing transit facilities and the rail corridor? How do we mitigate impacts to the operating businesses? Infrastructure and businesses can’t be shut down or continually disrupted over the many years it takes to complete the construction of a new transit line.
We approach each deep underground project with this attitude of protection from the start. The aim is always to ensure a smooth process for all stakeholders, which is built into our designs.
Few things are more frustrating than endless construction on roads and in/around buildings and businesses frequented every day. Indeed, businesses can see drops in sales and revenue due to prolonged construction in their vicinity. Integrating our transportation and building engineering expertise, as well as construction engineering, enables our team to develop construction mitigation solutions that minimize closed doors on businesses and maintain their accessibility.
The outcome of properly integrating vertical buildings with linear transit is a successful deep underground station that not only serves the community but whose construction considered the needs of all stakeholders at all stages. This includes successfully minimizing the construction impact on existing buildings and businesses, minimally disrupting above-ground traffic, and maintaining existing underground train operations without delays to the end user.
Our work in this area illustrates the benefits of our integrated transit and building expertise.
Projects We’ve Delivered
Eglinton Crosstown LRT, Toronto
The Eglinton Crosstown Light Rail Transit project in Toronto is a new 19km light rail line that will run across Eglinton Avenue between Mount Dennis (Weston Road) and Kennedy Station. The line will provide 15 underground stations, including two subway interchanges and two terminal stations, 10 at-grade stops, a vehicle maintenance and storage facility, an operations control centre, and other supporting infrastructure. It will also include a 10km underground portion between Keele Street and Laird Drive.
Entuitive provided structural engineering for Cedarvale station, Oakwood station, and Fairbank station, and construction engineering for Yonge Eglinton station.
Cedarvale station is currently under construction in Toronto. It will serve both as an LRT station and as an interchange that connects to the TTC Yonge-University subway line. Three new entrances are provided: two on the north side of Eglinton Avenue immediately west and east of Allen Road, and one on the south side of Eglinton Avenue.
This three-level, below-grade station also includes a 95m LRT platform, 55m of back-of-house space at the platform ends, a 93m crossover structure east of the station, a lower concourse level that extends north under the existing TTC heritage bus terminal structure, an upper concourse level that connects the LRT station to the new entrances through a below-grade tunnel, as well as various ventilation shafts servicing all station levels.
Entuitive provided all structural engineering services for this station, including the structural design of temporary and permanent structures used to underpin the existing TTC subway structure and heritage bus terminal.
The most challenging aspects of this station structure related to its construction. The interchange station required the new LRT platform to be built beneath the existing subway line, requiring passenger connections to the existing subway platform that extend under the existing bus terminal structure. Project performance requirements dictated that the new LRT construction be completed without impacting the existing subway operations, and without causing more than 3mm differential movement between adjacent segments of the existing subway and bus terminal structure.
A complex underpinning of the existing TTC subway and bus terminal structures was developed through close collaboration between Entuitive, the geotechnical consultant, and the construction team. Sophisticated 3D modelling was used to assess the staged underpinning construction sequencing, demonstrating that the movements of the existing structures always remained within the limits specified.
Our team’s understanding of the importance of maintaining a safe and uninterrupted TTC subway service drove us to think creatively. Informed by a detailed analysis of the existing infrastructure, our team developed underpinning concepts that took advantage of construction engineering practices and that allowed for efficient and staged construction. This context and unique design solution gave our client confidence, addressing concerns of damage to the existing infrastructure and of interruption to critical transit service. Most importantly it maintained the safety of transit users.
With the client’s approval, construction commenced and a monitoring program that recorded actual movements to fractions of a millimeter was implemented to check the actual movements of the existing subway and bus terminal structures in real time throughout construction. Actual movements were measured to be slightly lower than those predicted from our 3D analyses; an achievement that we are proud of, as it demonstrates that what might seem too risky can be mitigated and practically solved.
Yonge Eglinton Station
Similar to Cedarvale, Yonge station will serve as both an LRT station and as an interchange that connects to the TTC Yonge-University subway line.
Entuitive provided construction engineering services for this station, which also required a careful underpinning to allow for excavation below. Our work included the design of the sequential excavation to install the needle beams and underpinning in order to construct the vertical circulation between the new LRT platform and the existing subway platform. As with Cedarvale, the complex underpinning of the existing TTC structure was carefully modelled and developed through the close collaboration of all stakeholders.
Oakwood station consists of two new entrances constructed using cut and cover techniques. The main entrance is located at the east end of the station on the north side of Eglinton Avenue immediately north of Oakwood Avenue. The secondary entrance is located at the west end of the station on the south side of Eglinton Avenue. It is one of the few Crosstown stations where the station cavern was constructed using the sequential excavation method (SEM) technique in order to avoid disruption to an old, deep utility over top of the station and to prevent surface traffic disruption.
The station consists of a 95m platform, a concourse level, and back-of-house areas at either end. The main and secondary entrances each include stairs and escalators for vertical circulation, as well as tunnel ventilation shafts. The main entrance also includes an elevator and has a large, double-height, glass-enclosed public area adjacent to a two-storey service building. Entuitive was the Structural Engineer of Record for the cut and cover entrance structures.
The two cut and cover entrances provided a means of access for the SEM construction, allowing the movement of materials in and out of the excavation. The entrance support of excavation was installed and the entrances fully excavated to provide access for the SEM equipment. Once the SEM cavern was complete, the entrance structures were built.
The cut and cover entrances have large openings for the SEM cross-cut sections (adits). These large openings created unbalanced loading on the entrance box structures, which thus relied on the adits for support of their lateral forces. This interaction of the cut and cover and SEM structures required careful coordination with respect to force transfer, waterproofing detailing, and contraction joint locations.
The main entrance roof was also higher than the adjacent existing buildings. This was found to cause potential increased snow build-up on the adjacent structures beyond what they were originally designed to carry. A special snow drift study was undertaken to establish parapet heights on the new entrance in order to mitigate this issue.
A major part of any underground rail line project is determining the method of construction. The method of excavation selected is based on numerous factors, including ground conditions, adjacent structures, requirements to maintain traffic overhead, utilities, schedules, and more.
Our understanding and experience with all methods of construction allows us to approach the design of any deep underground station project with confidence and the ability to respond to challenges as they arise in the field.
Fairbank station is located at the intersection of Dufferin Street and Eglinton Avenue, with the main entrance located at the southeast corner, and a secondary entrance at the northwest corner of the intersection. This is a two-level, below-grade station that includes a 95m LRT platform, 10m of back-of-house space at each end of the platform, a concourse level with a large central opening, and large ventilation shaft structures. The two-storey main entrance building includes a dramatic double-height, glass-enclosed public space immediately adjacent to a separated Traction Power Substation (TPSS) and service building area.
Entuitive provided all structural engineering services for the new LRT station. Our team was also part of a select focus group that worked in collaboration with the construction team to develop a design procedure that enabled the construction of four Crosstown stations, including Fairbank, using top-down construction.
There were a number of interesting challenges in the design of Fairbank station. One of the most challenging aspects related to its top-down construction approach. In order to expedite reinstatement of the road surface to minimize traffic disruption, the station was designed so that the roof was constructed first and supported on the excavation support system. While the roof was backfilled and the road reinstated, excavation and construction continued below for the remainder of the structure.
The excavation support system therefore became part of the permanent structure, sharing vertical and horizontal loads with the permanent station walls. This process required complex staged analysis to model the various structural conditions of the partially constructed structure. It also required extensive coordination, with the excavation support design being completed by a construction engineering firm. Special rocker bearings were designed to allow the ends of the roof slab to rotate during backfilling operations prior to construction of the permanent structural walls.
Sensitivity analyses were also performed to assess potential variation in relative stiffnesses between the station invert subgrade support and excavation support system caissons supporting the roof.
Additionally, the TPSS located at this station resulted in a significant amount of electrical conduit being embedded within the structure, routing between the TPSS and the station platform level. Extensive coordination between Entuitive, the electrical and architectural disciplines, and the construction group was performed using a joint 3D Revit model to ensure that the conduit could be embedded within specific zones so as not to compromise structural integrity.
Fairbank station is nearing completion. The top-down construction sequencing was completed as planned, and the structure is performing as designed.
We see ourselves as part of a larger team to deliver on the complexities of underground stations. We are collaborators providing expertise to help provide structural solutions that are responsive to the needs of our industry colleagues who are experts in their own fields.
Projects We’re Working On
College Station, Toronto
College station is an existing subway station in downtown Toronto, located at the intersection of Yonge Street and College Street. This project involves making College Subway Station more accessible by adding two elevators to its station platforms. The work is part of the TTC’s Easier Access Program to make its stations more accessible to all.
The Yonge and College area in Toronto is very dense, with limited space to build new infrastructure. To accommodate the two new elevators, a new concourse has to be built below the street on top of the existing TTC subway station. The new concourse will be adjacent to bustling commercial and retail buildings, such as College Park, a nearly 100-year-old building. In fact, access to the new concourse is being provided through a new opening in the existing College Park foundation wall. To achieve this, our team specified temporary shoring posts with jacking forces to transfer the floor loads away from the wall being demolished. We then specified a wide flanged steel beam within the opening with its own jacking forces to ensure proper load distribution.
Further below grade, access to the existing platforms is being provided by introducing several large new openings in the existing subway station walls, which were constructed in the 1960s. It is very important that the existing structure retains its original structural behavior in order to mitigate the impacts from the new additions and modifications taking place. Our design achieves this with reinforced concrete and post-installed dowels constructed on top and all around the opening being considered, all happening prior to its demolition. The benefit of this approach is that it eliminates the need for temporary shoring and greatly simplifies and accelerates the construction schedule.
Our team also developed construction mitigation plans to ensure adjacent businesses can remain open during construction. The staging process we developed for the excavation and construction of the concourse will allow store fronts to remain open without blocking access to them. We also specified temporary decking to act as a sidewalk during the construction, and any necessary works were completed overnight so that stores can remain open during the day.
There are many factors to consider when introducing new infrastructure within a live operating environment. We understand clients and asset owners are concerned about disruption to their patrons as a result of construction, damage to their existing infrastructure, the future safe operability and maintenance of their renewed infrastructure, and the safety of the public and their assets.
The complexity of introducing new infrastructure is always context specific. Our team of structural and construction engineering specialists have spent their careers delivering projects of various complexities, learning techniques from all sectors and using those solutions in new and innovative ways to simplify the complex. Our expertise in deep underground stations stems from our understanding of how to seamlessly merge infrastructure, protect assets, and find solutions that do not compromise the client’s needs.
Yonge North Subway Extension, Toronto
Another exciting project we’re working on is the Yonge North TTC subway extension. Entuitive is part of a consortium providing technical advisory services for this project, which will extend the TTC’s Line 1 service north from Finch Station to Markham and Richmond Hill. The latest plans propose four stations along an extension of roughly 8km.
Entuitive is providing structural engineering and construction technical consulting services for this project.