About the Project
The Broadway Subway Project is a 5.7 km extension of the Millennium Line, that will replace part of the 99 B-Line route in Vancouver. The project includes the construction of six stations, five of which are on Broadway: Mount Pleasant, Broadway-City Hall, Oak-VGH, South Granville, and Arbutus Station. The tunnel for the line is being constructed using tunnel boring machines (TBMs), and the stations and underground structure are being built in open excavations. Given the SkyTrain’s rail alignment, the construction of the stations requires excavation beneath active lanes of traffic. The primary challenge of this project was to minimize traffic interruption on one of the busiest transit routes in North America. This required the installation of temporary traffic decks that act as bridges for traffic to pass over the worksites. The design of the traffic decks had two key objectives. First, they must support both the weight of traffic above, as well as act as struts between excavation faces, supporting earth pressure. Second, they needed to be modular to follow the phased construction sequence. This enabled the installation of the decks in pieces which offered flexibility in routing traffic to minimize construction activity impact. The two-phase installation of traffic decks has been completed on all the stations except Arbutus, which is currently in phase two. The excavation has reached more than 10m at four stations, with it being fully completed at Mount Pleasant.
Each traffic bay consisted of multiple prefabricated deck panels supported on a beam-girder system which were then supported on steel soldier piles. The aim was to achieve unification of structural elements that work in harmony with the construction sequence without creating impediments to installation. In designing our two-phase construction sequencing, we accounted for excavation of half of Broadway’s width and erected the first half of the transverse beams and deck panels. Once complete, our unique temporary street deck could accommodate traffic, allowing to excavate the other half of the road. Next, the other half of the transverse beam was erected, allowing to excavate for a full station width of 22m. The structural elements for modular construction of this deck enabled an efficient three step installation process:
- Drilling soldier piles (spaced 3m apart) on north and south sides of Broadway every 12m.
- Placing two prefabricated W1100x499 boxed-by-brace girders simply supported on a girt beam welded between two soldier piles. These girders not only support the vertical traffic load, but also act as a strut member to accommodate horizontal soil load.
- Placing 12mx3m deck panels on the girder system which are sufficiently wide to accommodate one lane of traffic. Each panel is completely prefabricated and consists of 6 stringer beams with a deck plate on top. This is then prefinished with an asphalt epoxy bonded gravel layer, ready to be loaded with traffic.
The novel construction sequence offered seamless construction process allowing phased regularization of traffic and leading to the following specialty solutions being applied to the structure:
– Bolted splices on the main beam allowing for modularization and easy transportation
– Non-bolted connections between the girder and deck
– Adjustable non-bolted connections for quick girder installation on steel driven piles
– Supporting Broadway’s massive utility infrastructure underside the decks
Allnorth Consultants Limited (Allnorth) is the Structural Engineer of Record for the list of following items and has developed these concepts in conjunction with the General Contractor.
– Temporary bridges for five stations on Broadway
– Support of excavation structure (for Broadway-City Hall, Oak-VGH and South Granville Stations)
– Utility supports under the bridge for all stations
– Pedestrian sidewalks adjacent to and on the bridge
– Conducting onsite engineering and inspections to confirm structure release to traffic
Broadway Subway Constructors General Partnership