There’s no way around it – the South Saskatchewan River poses challenges for many of the key players involved in the Saskatoon freeway project, including geotechnical engineers, designers and road builders.
Yes, river crossings are inevitable for this project. The previously approved corridor shows the freeway running around the perimeter of Saskatoon and because the river essentially halves the city and flows southwest to northeast, the only option is to cross it.
That’s easier said than done. There are a limited number of viable locations to build a bridge on the river. The location needs to balance engineering, environmental, heritage and cost considerations. If the freeway is moved further north, costs increase and fewer vehicles from the city and the surrounding area will use it.
The structural design of a river crossing, in addition to the environmental and geotechnical considerations, are also extensive, according to Kimberly Doran, manager of geoscience and materials at SNC-Lavalin in Saskatchewan.
For most of her career, Doran’s focus has landed on transportation geotechnical engineering. With the Saskatoon freeway project, Doran is leading the geotechnical group and executing the foundation design and the evaluation of stability for not only bridges, but also other structures like interchanges.
Years before heavy machinery arrives on site, geotechnical engineers like Doran and her team evaluate the land and its stability, analyze soil conditions and assess any risks posed by the site.
For example, when it comes to bridges, engineers will consider the stability of the South Saskatchewan River riverbank. The design will be very dependent on how the bridge structure will impact the slope, and if it does, engineers will need to determine how best to stabilize it to ensure safety.
But it is not only the soil underneath a potential bridge that Doran is interested in learning about. A geographic information system (GIS) database with borehole information has been built so Doran and her colleagues could locate gaps in information throughout the entire proposed route and determine what additional info is needed to complete the geotechnical design.
The perceived notion is that the work conducted by geotechnical engineers occurs before the design of the project takes shape. However, Doran said with this project, geotechnical engineers are working very closely with a structure and design team because the soil samples collected by engineers are immediately inputted into the study.
“With this project, looking at alternatives for a river-bridge crossing and potential locations for interchanges and alignment affects the proposed locations for drilling to define the soil conditions at those locations,” Doran said. “This is very much an interactive and back-and-forth process.
So, why do geotechnical engineers drill so many holes?
“Sometimes people don’t realize the difference of soil conditions from an interchange location here to one 200 metres away,” Doran said. “Maybe people think we overkill drilling holes. It’s very important to understand what the soil is doing at that particular site in order to provide the best design.
“A lot of different options are being presented to get the optimal solution for the Ministry of Highways and the general public. Both are considered – it’s not just the capital cost. There are things like aesthetics, environmental impact, operations and maintenance cost … so many variables.”