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Geotechnical Challenges During the Construction of Denison Road East Railway Grade Separation


The Denison Road East (Denison) Railway Grade Separation, located in the Weston neighbourhood of Toronto, Ontario, is an integral component of Metrolinx’s Air Rail Link Improvements which provides express commuter service between Union Station and Pearson Airport in Toronto, Ontario. The project includes a new underpass comprising of two reinforced concrete railway bridges, tieback retaining walls, and a 4,000 m³ precast concrete storm water retention tank. Both the Metrolinx Bridge and CP Rail Bridge are three-span, 45-metre long structures with 1.5-metre thick reinforced concrete decks. The bridges carry four Metrolinx tracks and two CP Rail tracks over the realigned Denison Road.
R.V. Anderson Associates Limited (RVA) and Thurber Engineering Ltd. (Thurber) were retained by Metrolinx to provide the detailed design and construction engineering support services from 2009 to the project’s completion in 2015.
Due to the complexity of the project including its proximity to the residential community, the soil stratigraphy and high groundwater table, a sheetpile wall enclosure structure was selected to minimize potential adverse impacts to nearby structures that could result from bulk excavation and groundwater lowering. The enclosure structure also minimized the volume of groundwater being managed during and after construction. A total length of 465 metres of sheetpile wall was installed prior to commencing excavation below the groundwater level. During the construction stages, up to four rows of prestressed tieback anchors were installed along the maximum 15-metre high exposed front face of the sheetpile wall.
The focus of this paper is to show how the engineers overcame the challenges during design and construction. To achieve shoring wall stability and groundwater cut-off, the sheetpiles were penetrated into the underlying very stiff silty clay till/weathered shale. One of the challenges being faced during sheetpile installation was vibration while seating the sheetpiles within the very stiff to hard soils and weathered rock due to the close proximity of the sheetpile walls to adjacent houses and underground utilities. Vibration induced settlement and distress were concerns during construction and, therefore, pre-augured holes were advanced prior to sheetpile installation to mitigate vibration where necessary.
Another challenge was related to the installation of prestressed tiebacks near the bridges which support an active railway corridor. Due to the high groundwater table and the presence of loose soils based on borehole information, fine sands, silts and water were expected to flow out from the tieback holes which would have made the installation of the tiebacks practically impossible. A dewatering scheme was designed and implemented to lower the groundwater table and to depressurize the water-bearing soils in order to facilitate tieback installations.

Conference Paper Details

Session title:
Augustin, S.
Mehdawi, W.
Vierhuis, J.H.R.