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 is generally 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. 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, but the new alignment still required crossing structures at the Ksituan River and Howard Creek. Bridges or traditional culvert crossings would not be technically or economically feasible at these locations. 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 paper will also focus on the methodology applied to optimize the design of the culverts 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. For the covering abstract of this conference see record control number 201111RT334E.