Major Stormwater Retention Tank Installation Method
By Adam Polski
Constructing massive precast concrete structures with CON/SPAN® provides choices for design engineers and assembly options for contractors.
The Denison Road Storm Water Retention Tank in Toronto (Georgetown South Project) by Metrolinx (GO Transit) is one of many applications of CON/SPAN. It is not a new application, but the size and installation method used make the application noteworthy.
Design and construction of the retention tank was needed for the treatment and disposal of stormwater run-off from a total catchment area of 4.86 hectares. The structure was also required to provide enhanced water quality protection and discharge via an existing storm sewer network for all storm events up to and including a 100 year return storm.
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Design of the system resulted in a 4,000 m³ underground stormwater retention tank comprised of 62 precast concrete units measuring 9,755 mm wide by 2,740 mm high. Some pieces were specially designed skewed end units for bends, while other pieces were narrower at the beginning and end of the structure. All were designed to S6.1S1-10 – Supplement #1 to S6.1-06, Commentary on CAN/CSA-S6-06, Canadian Highway Bridge Design Code and CSA A23.4-09 (R2014) – Precast Concrete – Materials and Construction.
Pre-project planning started in 2011 with the concept for the CON/SPAN option, which would require a cast-in-place foundation and channel. The units were produced and shipped from the Guelph, Ontario facility of Con Cast Pipe.
Another reason for selecting a precast concrete retention system was the complexity of the tank location. It sits directly underneath the Denison Road GO Transit grade separation and a metre below Denison Road, where it passes under the railway overpass. The structure had to be constructed in a short period within an established neighbourhood and roadway right-of-way where it was difficult to make use of heavy equipment. Cambridge Rigging’s 300-tonne mobile boom crane was used to install CON/SPAN units to the grade separation. However, a Goldhofer crawler was needed to install them under the rail overpass. Using precast concrete units saved time, limited the impact of construction on the neighbourhood and commuters, and helped the contractor meet the construction schedule.
Upon completion of the foundation, Dufferin Construction, a division of Holcim (Canada) Inc., constructed cast-in-place bulkheads at each end, connected catch basins into the CON/SPAN structure, and constructed a cast-in-place maintenance-hole base on top. This allowed the precast riser sections to be installed to the finished grade of Denison Road. Stormwater would be pumped from the tank to an oil/sediment control structure for treatment, and then discharged into the nearby storm sewer system.
Cambridge Rigging and Engineering in Motion were responsible for planning and executing the installation of all of the precast sections.
Construction of the tank began with excavation for a poured-in-place channel that would also serve as the base of the structure. Footings for the CON/SPAN units were poured-in-place. The first unit was set on June 24, 2014, using the mobile crane.
As installation approached the grade separation, Cambridge Rigging switched from the boom crane to the crawler to install the precast units. The process involved a precast unit being carefully centred on the crawler by the crane and tieback retaining wall system, and the new precast stormwater retention tank.
When completed, the public will see the Denison Road grade separation that accommodates a rail system for commuters and travellers to and from Lester B. Pearson International Airport, a pedestrian underpass, and improved road pavement. What won’t be visible is a modern underground precast concrete stormwater retention and treatment system designed to last for generations.
Adam Polski, C.E.T. is with Con Cast Pipe. This article appeared in ES&E’s November/December 2014 issue.