To reduce wet weather flow into Lake Ontario and the Don River watershed, the City of Toronto is implementing the Don River and Central Waterfront Wet Weather Flow Project. The project involves roughly 22km of tunnel, 27 connections and 12 shafts and has been split into five phases that are expected to be completed over the next 25 years. Black & Veatch in association with R.V. Anderson Associates Limited were contracted to perform the detailed design of the first phase of the project, called the Coxwell Bypass Tunnel, which includes 10.5km of 6.3 m inner diameter rock tunnel and five large diameter storage shafts within the Georgian Bay Shale rock formation.The five storage shafts range in diameter from 20-22 m and are up to 58 m deep extending through both soil and rock. The shafts feature a composite roof slab design consisting of precast concrete panels and a cast-in-place concrete topping to aid in constructability. Unique unbalanced soil loading conditions were evaluated at one site to accommodate a planned future development. Large hydrostatic water pressures at the base of the shaft led to the use of an arched base slab design to help resist the uplift forces applied over the long spanning shaft diameters. This paper will provide a comprehensive overview of the unique design conditions and challenges encountered during the detailed design of the shafts, a brief background on the analysis techniques utilized, and a summary of the resultant final shaft design.
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