AWARD OF EXCELLENCE: Leslie Station, Sheppard Subway, Toronto

Category: Transportation

YOLLES & THURBER ENGINEERING

The Toronto Transit Commission’s Leslie Station is one of five stations along the 6.4 kilometre long east-west Sheppard subway line under construction in north Toronto.

Located at the corner of Sheppard Avenue and Leslie Street, in difficult soil conditions and close to existing buildings and natural features, the Leslie Street Station proved to be a very technically challenging engineering project. It consists of an underground public subway train station, train running track tunnel, below-grade electrical substation, two-storey bus terminal and an entrance building. The station is designed to have future connections to an adjacent college building, a GO transit station, North York General Hospital to the east, and a Canadian Tire store site to the west.

The base of the station penetrates a confined aquifer with a head nearing the original ground surface. The resulting large buoyancy forces affected the gravity design of the station, as well as the sliding and passive soil resistance necessary to counter the large lateral forces imposed on the structure.

Measures also had to be taken to control differential settlements since the station and running track structures are founded on three different soil deposits, including sensitive clay deposits supporting an adjacent college building. That building is less than 600 millimetres away from the new structure.

The station vertical alignment was restricted by the existing grades and roadways over the station at the west end, and by the elevation of Leslie Street and the flood level of the Don River at the east end. In some areas there is no cover over the station roof and the roof profile matches the grade. The structure also had to be constructed very close to, but beneath, an active roadway. These restrictions complicated the engineering solutions to the buoyancy and founding conditions. Horizontal alignment was complicated by the super-elevated track curve at the east end.

Structural details

The train station is a 138-metre long, two-level underground structure consisting of the concourse and platform. The structure is a reinforced concrete box, cut approximately 15 metres deep into an earth embankment. It supports lateral loads from the adjacent four-storey masonry college building located immediately to the south, and from earth and water pressures above and to the south. The large mass required to resist the high buoyancy forces meant the station could be designed with an 18.5 metre clear span at essentially no cost premium. The roof supports a road link between Old Leslie Street and Sheppard Avenue.

The train running track is a 142-metre long, three-celled reinforced concrete box structure. The centre cell provides public access between the station platform and entrance building. It has two large light well openings in the roof, which added to the complexity of the design.

The bus terminal roof is a 54 x 44 metre reinforced concrete space frame structure supported partially on the train station roof and partially on caissons. Architectural considerations led to the roof being designed without expansion joints. Special low-shrinkage concrete mix specifications were developed, and special column base details were designed to relieve thermal and shrinkage stresses in the roof structure. Perimeter overhangs are created by 8-metre long cantilever beams.

The existing college building is founded well above the excavation base, over highly sensitive clay material. Calculations indicated that de-watering the aquifer at the base of the excavation would dry out the sensitive clay layer that supported the college. The engineers developed a groundwater replenishment system using existing well technology to maintain the water pressures in the sensitive upper clay during the de-watering of the aquifer. Monitoring showed that settlement and movement of the existing building were minimal throughout the course of the work.

The Sheppard subway will provide area residents with a convenient transport link to downtown, and is located close to numerous parcels of land suitable for future development. Close cooperation between the transit commission, the consultants and the contractor ensured the budget and schedules were met, and the project was completed late in 2000 at a cost of $60 million. The Leslie Station was the first station design completed along the line, and many of its design standards and innovations were subsequently used in the other stations. The subway service will begin operating next August.CCE

Project name: Toronto Transit Commission, Sheppard Subway, Leslie Street Station

Owner: Toronto Transit Commission

Award-winning firms: Yolles, Toronto (structural engineer of record); Thurber Engineering, Toronto (geotechnical sub-consultant for excavation support)

Project team leaders: Michael Meschino, P.Eng., David Stevenson, P.Eng. (Yolles); Paulo Branco, P.Eng. (Thurber)

Prime consultant: Moriyama and Teshima Architects

Other key players: Delcan-Hatch Joint Venture and Golder Associates (program managers for TTC)