The Broadway Subway Project is a 5.7 km extension of the Millennium Line, a route on Metro Vancouver’s SkyTrain rapid transit rail system. The extension will run from the VCC-Clark Station to Broadway and Arbutus Street and will replace part of the 99 B-Line bus route. The project includes the construction of a total of six stations, five of which are on Broadway. The tunnel for the line will be constructed using a Tunnel Boring Machine, while the stations and underground structures are being built in open excavations.
The Broadway Subway Project Corporation (BSCGP), an Acciona-Ghella joint venture, has been awarded the contract for the design and construction of the project. A key constraint for BSCGP has been to mitigate the project’s impact on the city of Vancouver during construction, and to include strict times when traffic could be closed on Broadway.
Given the project’s need for rail alignment, construction of the stations requires excavation beneath active lanes of traffic. This necessitates the installation of temporary traffic decks as bridges for traffic over the worksites.
Allnorth has been engaged to be the Structural Engineer of Record (EOR) for these required traffic decks and has developed the concepts in conjunction with BSCGP.
Allnorth developed a novel design which allows for installation without halting traffic, while supporting both vertical traffic loads and horizontal soil pressures on station excavation.
The design of the traffic decks has two key objectives. First, the decks must support both the weight of traffic above, as well as act as struts between excavation faces, supporting earth pressure. Second, a construction sequence, installing the decks in pieces, must be developed to minimize the impact on traffic.
This construction sequence has led to the following specialty solutions to be applied to the structure:
- Bolted splices on the main beam to allow for modularization
- Precast foundations for easy installation
- Adjustable connections to allow for quick installation of the deck on steel driven piles
Considering the loading and the construction sequence, the design has resulted in a total of 6,500 tonnes of steel to be used for temporary purposes. Each traffic deck constitutes multiple bays of a beam-girder system with deck panels consisting of maximum 12m long x 3m wide x 11 mm thick plates, supported by six W460 beams, which are in turn supported by W1100 transverse girders underpinned by W610 piles.
This project is exceptionally complicated as it takes place in an extremely busy area of Vancouver. As a result, there were continuous challenges that necessitated ongoing problem solving and the application of out of the box solutions. Several sophisticated innovative approaches were developed such as a novel and unique construction sequence solution that resolved the continuous traffic flow problem which drastically reduced the construction schedule. Another major challenge was supporting the Capilano #5 watermain, a 1,800mm diameter pipe with wall thickness of 12.7mm spanning across 20m over the excavation, ensuring contingency and non-disruptive utility service throughout construction.
The complete project is estimated to be completed in 2025.
- Structural design
- Structural engineering
- Project management
- Construction engineering
- Traffic management
- Working space
- Public safety
- Construction noise
- Construction tolerance
- Installation time
- Placement of installation equipment
- Surcharge loads
- Massive infrastructure of utilities beneath