In 2015, the Canadian government awarded a $4.3 Billion Design-Build contract to the Joint Venture (JV) team Signature on the Saint Lawrence (led by SNC-Lavalin) for the design and construction of one of the largest infrastructure projects in North America, Samuel de Champlain Bridge over Saint-Laurence River in Montreal. In addition to the new 3.5 km main bridge, the overall project involved the construction of tens of smaller bridges and over 20 retaining walls along the bridge approaches. The complexity of the project posed unique geotechnical challenges on many levels.
One significant challenge was the construction of part of the alignment over an old landfill where the subsurface investigation indicated up to 9 m of waste solid. To assess the foundation material properties, settlement monitoring was performed during the design phase on a 7-m high temporary embankment, constructed on the existing fill, using numerous settlement platforms. The “consolidation” and strength parameters were then back calculated and used in the roadway embankment design.
On the west approach, part of the new highway alignment was to be constructed over an existing 11x5 m COS collector with unknown structural condition, with the requirement not to apply additional load on the collector. Lightweight fill composed of expanded polystyrene was used for this purpose. Detailed numerical analysis of the stress-strain conditions was performed to optimize the design.
Moreover, the presence of a thick liquefiable silty soil deposit, covering a large area along the alignment, led the geotechnical team to design the foundation system to withstand seismic loading induced during a 2% in 50-year return period. Furthermore, the rock formation at the site was known to exhibit relaxation (decrease in resistance) phenomenon for piles driven to refusal on bedrock. A thorough dynamic testing program was developed to carefully assess the nominal pile resistance.
This paper addresses the issues encountered and concerns raised during the geotechnical design and how these were addressed and resolved. Design and construction procedures, challenges and solutions are discussed in detail.