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Innovative Structural and Geotechnical Strategies: Crossing Unstable River Valleys in Northern Alberta


Located north of the City of Grande Prairie and west of the Town of Spirit River in northwest Alberta, Highway 727 serves local farm residents, oilfield activities, and businesses. Generally speaking, the terrain along the 18 km long highway is flat with the exception of the Ksituan River and Howard Creek crossings, which have valley depths of 70 to 80 m. Geotechnically unstable, these valleys have experienced regular slide activities along the highway for over 30 years. Repair and maintenance measures have not been able to resolve the geotechnical instabilities, and slide movements were occurring at an increasing rate. In 2005 the Province of Alberta made the decision to realign Highway 727. The chosen alignment for the new highway provided improvements in a number of areas including roadway geometrics, environmental impact, and utility coordination issues. However, the new alignment still required crossing structures at the Ksituan River and Howard Creek. With large slide blocks in the valley slopes and weak compressible subsurface clay, bridges or traditional culvert crossings would not be technically or economically feasible at these locations. An innovative and creative engineering solution was required that would efficiently solve the technical problems at the sites, would be constructable, and would provide safety and value to the citizens of Alberta. Presented as a case study, the paper describes the matrix process that was used during preliminary design to evaluate various options for the crossing structures. For each potential highway gradeline, appropriate types and lengths of crossing structures, including their corresponding geotechnical issues, were evaluated. The selected option best optimized cost, safety, environmental impact, geotechnical loading, and constructability. It utilized an aggressive highway gradeline that crossed the valleys at right angles, with large cuts to offload the crests, and cast-in-place concrete culverts and embankment fills to buttress the toes of the slopes. The paper will also focus on the methodology applied to optimize the design of the culvert and mitigate the known geotechnical issues. Descriptions will be given on the use of driven steel shear piles, wick drains, and lightweight expanded polystyrene (EPS) as embankment fill material, to reduce culvert lengths and anticipated settlements. Construction techniques for the construction of the culverts, and placement of the EPS blocks are described. Practical recommendations and lessons learned during the design and construction stages are also outlined. 

Conference Paper Details

Session title:
Neil Robson
Gamal Ghoneim
Don Proudfoot
Donald Saunders
Darcy Taylor