Abstract Manager

Saskatchewan’s river valleys contain numerous unstable slopes subject to landslides, across which roads, railways, pipelines and other infrastructure are aligned. The Site is on provincial Highway 914 which serves as the only road access to the McArthur River and Key Lake Uranium Mines. An erosion failure at a river bend and upslope landslide on the Haultain River valley slope was posing a hazard to the highway integrity. This was caused by a combination of high groundwater table (GWT) along with failure of an existing drainage system installed in highly permeable, loose, silty sand local subgrade material. The soil conditions were allowing seepage, which in turn resulted in noticeable erosion and progressive displacement along the riverbank.

Saskatchewan Ministry of Highways (ministry) conducted a comprehensive risk assessment as part of its Geohazard Risk Management Program (GRMP). The landslide was  rated with the landslide risk at a probability factor (PF) of 11 (active landslide with moderate, increasing rate of movement) and an erosion risk at a PF of 13 (active landslide with high rate of movement at steady or increasing rate). Mitigation of the site instability included technical design, construction supervision, and quality assurance and quality control. Through the investigation and analyses, the recommended mitigation option included a drainage system comprising: (i) construction of a 600 m long perforated subsurface drainage pipe, adjacent to the east side of the highway; (ii) replacement of a solid drainage pipe; and (iii) a manhole to connect the two pipes. The GWT was lowered by a well point dewatering system. The perforated pipe was protected with a gravel and geotextile wrap and graded to drain by gravity. Trenchless, horizontal drilling was conducted to convey the outlet pipe from the manhole location through the highway embankment. Thereafter, backfilling of the excavation was completed using the in-situ material. Next, the east ditch of Highway 914-03 was reconstructed. Replacement of an existing damaged culvert with a 600 mm diameter corrugated steel culvert was also carried out. The outlet of the culvert was tied into the west side ditch and riprap was strategically placed to prevent erosion and scour. Finally, recommendations for maintaining optimal operational conditions of the drainage system and the highway were made. This case study demonstrates an example of a strategic drainage system reconfiguration resulting in an effective, low-cost mitigation that reduced landslide risk and restored the geotechnical integrity of this segment of Highway 914.