Evaluation and Stabilization of a Slope Failure at Liard Highway No. 7, km 5.9 (Case Study)

Rita Kors-Olthof, Ed Hoeve, Bob Patrick

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Landslides and Geohazards

ABSTRACT: The Liard Highway (NWT No. 7) was constructed between 1969 and 1984. In summer 2007, the road was reconstructed at km 5.9, raising the vertical alignment. A fillslope failure occurred later that summer. The Government of the Northwest Territories, Department of Transportation, reconstructed the fillslope, and installed a French drain in the ditch to intercept groundwater flow. The repairs performed adequately until 2010, when the slope failed again. Tetra Tech EBA Inc. evaluated the site in 2010, finding an underlying swale and high groundwater inflow from upslope. Stabilization works included a longer, deeper French drain along the ditch, and two counterfort drains across the road. The gradient of the upper fillslope was reduced to mitigate surface ravelling of the fill. Follow-up monitoring suggests that slope stability has improved, but surface water erosion remains an issue. RÉSUMÉ La route Liard no.7 a été construite entre 1969 et 1984. À l'été 2007, la route a été reconstruite à son km 5,9 avec un dénivelé plus important. La pente du remblai s'est affaissée plus tard dans l'été. Le Département de Transport du Gouvernement des Territoires du Nord-Ouest a alors, reconstruit la pente du remblai et installé un drain français dans le fossé pour intercepter l'écoulement des eaux souterraines. Les travaux de réhabilitation ont bien tenu jusqu'en 2010 lorsque la pente s'est de nouveau affaissée. En évaluant le site en 2010, Tetra Tech EBA Inc. a trouvé une baissière sous-jacente et une infiltration importante d'eau souterraine venant de la pente ascendante. Les travaux de stabilisation ont inclus un drain français plus long et plus profond dans le fossé, ainsi que deux drains à contrefort traversant la route. La pente du remblai supérieure a été reconstruite avec une configuration moins pentue pour réduire le désenrobage en surface du remblai. Le suivi effectué montre que la stabilité de la pente s'est améliorée, mais que l'érosion due aux eaux de surface demeure préoccupante. 1 INTRODUCTION The original alignment of the Liard Highway (NWT No. 7) was constructed in between 1969 and 1984 by the Federal Government using local borrow materials, generally consisting of silt. In the summer of 2007, the Government of the Northwest Territories, Department of Transportation (DOT), reconstructed the area by raising the vertical alignment using crushed granular materials consisting of a layer of 50 mm minus-sized sub-base, and a surfacing layer of 20 mm minus-sized base course. A slope failure occurred at the subject site, km 5.9, in the late summer of 2007 (Figure 1). DOT repaired the slope in 2007 by removing the upper silty soil (fill and/or colluvium) and peat, and reconstructing the slope in consecutive blast-rock benches, and track-compacting the material. A French drain was installed upgradient in the north-bound lane ditch to intercept groundwater flow and direct it towards a culvert to the north. The repairs were understood to have performed fairly well until 2010, when renewed slope movements were observed at approximately the same location as in 2007 (Figure 2). In late October 2010, Tetra Tech EBA Inc. (Tetra Tech EBA, formerly EBA Engineering Consultants Ltd.), carried out a geotechnical site investigation, including the installation of standpipe piezometers and slope inclinometers in selected boreholes to assess the groundwater and slope stability conditions at the problem area. Figure 1. Slope failure in 2007 (photo courtesy of DOT) Based on Tetra Tech EBA's analyses, some recommendations for slope stabilization were made. Of primary importance was groundwater management, with drawdown to be achieved by installing a longer, deeper French drain along the ditch, and two counterfort drains across the road. After the groundwater control measures were installed, the upper fillslope of the road was restored to a slightly flatter configuration, by building up the fill from the first bench below the road.

RÉSUMÉ: ation and Stabilization of a Slope Failure at

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Rita Kors-Olthof; Ed Hoeve; Bob Patrick (2014) Evaluation and Stabilization of a Slope Failure at Liard Highway No. 7, km 5.9 (Case Study) in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper429,author = Rita Kors-Olthof; Ed Hoeve; Bob Patrick,title = Evaluation and Stabilization of a Slope Failure at Liard Highway No. 7, km 5.9 (Case Study) ,year = 2014}