Structural Aspects of the Stabilization of Ten Mile Slide-Paper

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Ten Mile Slide on British Columbia’s Highway 99 had been moving since the 1980s, retrogressing more than 250 m upslope and threatening a vital route between the coast and the province’s interior. Before stabilization, the roughly one-million-cubic-metre slide moved about 10 mm daily, accelerating after rainfall. Highway repairs, load restrictions and single-lane traffic had become costly and recurring.<br /><br />The Ministry of Transportation and Infrastructure selected a structural solution designed to raise the slope’s factor of safety to at least 1.2. It consists of 276 post-tensioned soil anchors in five rows above the highway and a tied-back tangent pile wall below it, alongside reconstruction of the road and its drainage. The wall includes 148 large-diameter piles and 125 tie-back anchors. A top-down construction sequence and time limits for each stage were specified so the works could withstand ongoing movement as stabilization progressed. Composite steel-shell piles provided early strength while concrete piles were installed and cured.<br /><br />Extensive investigations, test anchors, laser scanning and monitoring informed the design. Limit-equilibrium analyses and two- and three-dimensional numerical models were used to assess slope stability, structural loads, construction stages, seismic effects and load sharing among wall components. An observational approach used anchor load cells, inclinometers and surface surveys to compare actual performance with predictions and guide adjustments.<br /><br />Construction took place from 2017 to 2021. Monitoring showed that the slide responded as predicted; movement above the wall is below detection limits, and anchor loads are stable or declining slowly. No additional anchors were needed. The highway has returned to full two-lane service, reducing maintenance demands and restoring reliable connectivity for nearby communities, including Xaxli’p.

Keywords

Ten Mile Slide

Highway 99

British Columbia

landslide stabilization

soil anchors

tied-back pile wall

slope stability

geotechnical engineering

road reconstruction

infrastructure monitoring