September 30, 2011

CITY OF TORONTO

The Toronto Union Station concrete post project is probably the largest of its kind ever attempted and most challenging.

FEATURE | Concrete/Masonry

Construction crews re-engineer concrete posts at Toronto Union Station

PETER KENTER

correspondent

Engineers and construction crews are breathing new life into the City of Toronto’s Union Station as they build a second passenger concourse below the existing track.

The work is part of the $640-million Union Station Revitalization Project awarded to Carillion Canada. Costs of the project are being shared by the city and the provincial and federal governments. The project anticipates a doubling of GO Transit passenger volume by 2030.

Other goals of the project include restoring heritage aspects of the main building, constructing a new northwest pedestrian connection joining the northwest corner of Union Station to Wellington Street, enhancing pedestrian concourses and creating a new lower retail level below the station.

The operation currently underway: excavating approximately four metres below the existing floor level to create space for the lower-level pedestrian retail concourse and two new GO Transit concourses, one on Bay Street and the other on York Street.

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Crews are re-engineering the station’s 447 original concrete posts to the bedrock underneath the station so they support both the original track and the second floor level being constructed below. Of those posts, 185 currently support the train tracks above.

The concrete post project is probably the largest of its kind ever attempted and the most logistically challenging as train traffic continues to induce live loads above and passengers continue to use the facility.

The original concept was tested in October 2010 on a single column to determine how the concrete could best be cut and supported during the operation.

“We wanted to ensure that all of our design calculations were correct,” says Rick Tolkunow, the project’s principal engineer with the City of Toronto. “We learned quite a bit about different options, including the support structure required, standardized scaffolding and the construction of falsework to temporarily support the columns during excavation. We also wanted to learn how to break the existing concrete without disturbing the rest of the rebar, remove the lower part of the column and then form the new section of column down to the bedrock.”

Tolkunow says the concrete supports are in great shape at almost 100 years old, primarily because they’ve performed their function indoors, away from salt and other environmental corrosion.

The project team is tackling the columns in groups of six, distributed across the project. Excavators remove the soil around the posts one at a time and the posts are then supported by temporary steel falsework. As structural engineers sign off on each post, crews begin the next one.

Construction crews are currently removing between 600 and 900 tonnes of excavated material each night; almost all of it original fill placed on the site during construction of the station during the First World War.

The excavators and support equipment being used in the project are the same as those used in underground mining operations. The operation employs low headroom vehicles, which are primarily electrically powered. Necessary diesel equipment has been outfitted with scrubbers to facilitate indoor operation.

“Currently, we’re plugging away and the dig down is making good progress,” says Tolkunow. “Everything seems to be picking up speed. We have 30 columns at both York and Bay in various stages of production with some replaced, some supported and some cut.”

Excavation will be followed by further construction on the York concourse. Once complete, GO Transit will relocate to the new York concourse and construction on the Bay side will continue.

For now, the biggest project challenge is in minimizing the effects of construction on a fully operational train and subway station. That includes maintaining the mechanical and electrical systems, and controlling vibration and dust. “We have 250,000 passengers coming through the station each day and we want to maintain safety and comfort for them, for TTC workers, and for our own workers,” says Tolkunow.

The York GO concourses — York and Bay — are scheduled to open in 2013 and 2014, respectively.

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