Toronto's historic Union Station covers an entire downtown city block and is one of the busiest commuter rail terminals in North America. An ambitious renovation and expansion of the facility began in 2010 with completion scheduled for 2015. A key element of the project is the construction of a new retail concourse below the existing railway viaduct structure. The viaduct is to remain in full service during the project.
The viaduct structure is constructed of reinforced concrete and is supported on concrete columns set on unreinforced concrete piers founded on shale bedrock at an average depth of 5 m below the existing floor slab. Removal of this slab and 4 m of overburden excavation is required to enable construction of the retail concourse. The existing piers are being removed and their respective concrete columns extended to new foundations on bedrock. Temporary support of the railway viaduct above is a key element of this operation. To prepare for this technically challenging work, a prototype column replacement scheme was constructed in order to refine the temporary support procedure and design prior to starting full scale production. Quantum Murray LP was selected as the trade contractor for the prototype work and in turn selected Geo-Foundations to design and construct micropile foundations to carry the loading from the temporary column support system.
For the prototype, one 1200 kN (service load) cased, rock-socketed micropile was installed at each of the 4 corners of the existing pier. Work was carried out from the existing floor slab level in 4.5 m of headroom. Tolerances were extremely tight for two crucial reasons: 1) the micropiles had to line up with pre-installed jacking plates fastened to the underside of the railway viaduct slab above, and 2) the micropiles would eventually have their uppermost 4 m exhumed in order to enable excavation and removal of the existing pier. Following micropile installation, each 345 mm diameter micropile casing was trimmed and extended upwards to mate up with the jacking plates, thus serving as temporary steel shoring supports. A preload was then jacked into the viaduct slab with real-time instrument monitoring in place to record slab deflections. Observed movements were well within specified levels; pier removal and downwards extension of the concrete column proceeded without any setbacks.
 |
 |
 |
 |
 |
 |
 |
 |
 |