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Precast concrete box sections specified for storm water detention structure

The detention tank is placed below the P1 level of the parking structure, to accommodate the large volume of storm water.
The detention tank is placed below the P1 level of the parking structure, to accommodate the large volume of storm water.

Precast concrete box sections were specified for an underground storm water detention tank to enable the construction of the Humber Mews Townhomes project on Evans Avenue in Toronto. The system was selected because it could reduce the total peak runoff being discharged from the site during all storm events. Using precast units would reduce construction costs and time. Other pre-manufactured solutions were considered, nevertheless, a precast system was selected as the preferred option. The structure met the service life requirements through the proven durability of precast concrete.

The precast system was customized to fit site-specific conditions, layout and structural requirements. While a detention tank would normally be placed on the P1 level of an underground parking structure, the design team decided that the storage system could be placed below the P1 level to accommodate the large volume of storm water.

Designed to withstand loads

Shallow municipal storm sewers on Evans Avenue, along with a level grade, suggested a tank design on P1 with a reduced internal height. This design limitation required a tank footprint covering greater area, thereby reducing required parking spaces. The consulting engineering company, R.V. Anderson Associates Limited provided the volume and discharge parameters. If the tank were to be installed below the P1 level and the connecting storm sewer, a pumping system was required to discharge storm water from the tank. The storage system had to be structurally designed to withstand loads from the parking structure.

Con Cast Pipe was contracted by CDC Contracting to supply the precast concrete boxes and other precast products to construct the system. Con Cast Pipe retained the services of GM Blueplan Engineering Consultants Limited to provide the structural design of the box sections. The system was composed of a combination of horizontal box sections and a vertical precast concrete structure with plugs and caps to contain the storm water on site. The horizontal portion of the system was composed of seven 4.8 m (span) x 3.8 m (rise) box sections with a 4.8 m x 3.8 m precast cap and plug at each end of the box system. The vertical portion was comprised of two 4.8 m long x 4.8 m wide box sections, with a 4.8 m x 4.8 m precast base slab and transition slab. On top of the transition slab, a standard 1,200 mm manhole taper top was provided to allow maintenance access from P1. The heaviest section of the 4.8 m x 3.8 m box unit weighed 41,587 kg. The horizontal box sections (combined) measured 12.1 metres. The heaviest element of the 4.8 m x 4.8 m vertical chamber was 38,755 kg with a height of 6.8 metres.

Structural water-tightness was achieved through the installation of butyl rubber jointing material to fill the annular space within the joints. A secondary water-tightness safeguard was achieved by wrapping the external portion of the joint with Mel-Rol joint sealer and filter fabric to help mitigate the potential infiltration of ground water and sedimentary fines.

The installation, from the initial ground breaking, excavation for the placement of the units to backfilling took two days. Ease of product installation reduced the possibility of performance issues resulting from improper installation. 

Structural water-tightness was achieved through the installation of butyl rubber jointing material.

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