BC’s Historic Landslides – What They Are and How We Manage Them

BC’s historic landslides are large-scale tracts of land that move over time, impacting communities, roads and bridges.

They may “creep” as slowly as a few millimetres a day or year. Then, they can “reawaken” and move downward more rapidly and dramatically, like areas in the Cariboo – burying or washing out roads, at times cutting off communities.

What can’t be seen below the ground, is what’s at play. Geologic materials such as clay, silt, sand, gravel and even rock can collapse due to changes in groundwater.

The movement over time of these slides is what distinguishes them from more “typical” short-term, instantaneous slide events like debris flows, or rock falls, which typically occur due to rain events, freeze-thaw cycles or warming temperatures. (Though both water and temperature can re-awaken a historic slide.)

Historic slides can be massive, complex and extremely challenging and expensive to manage and stabilize. For example, Ten-Mile Slide northeast of Lillooet is 200 metres wide at Highway 99 and 300 metres long. About 750,000 cubic metres in size, it’s part of a large inactive “tunnel earthflow.”

What to Do with Historic Slides?

Historic slides can have a major impact on the people who live and travel an area. So, what can be done to manage these situations?

When it comes to solutions for highways, roads and bridges that have been affected, we look at numerous factors like engineering, construction, environment, community and Indigenous considerations to ensure that our transportation infrastructure is safe and reliable for motorists.

Generally, we have three approaches:

• Monitor and make improvements, while inspecting regularly to assess changes, patterns and hazard levels, and conducting maintenance as required.
• Local realignment, or simple stabilization methods such as removing water from the slide, taking away some materials (e.g. rock, sand, clay) from the slide’s “crest” (highest point) to reduce the slide’s driving force, or placing reinforcing materials at the slide’s “toe” (bottom area) to resist the slide’s force.
• Provide permanent alternative access, or stabilize the slide with a structural solution which may include retaining walls and/or soil anchors.

Each slide is different, each scenario is different. West Fraser Road, south of Quesnel, which washed out in five places in 2018, is one location where moving the road to a more stable area was the best option. Construction work began in 2021, on a new bridge and a 5.6-kilometre stretch of road. In 2023, work was completed on the two-lane bridge over Narcosli Creek, and two-lane section of road that bypasses the active slide areas.

How decisions are reached is complex, and multiple aspects are examined for each situation, including economic activity, travel time, community connectivity, environmental impacts and costs (to name only a few). We have identified some below:

­­Recognizing the impacts of changing weather patterns, we have continued to evolve our strategy. Our design engineers and consultants are considering how future climate events will affect infrastructure and what can be done to make our roads and bridges more resilient, so they remain reliable and open.

Some works include upsizing culverts, building bridges where culverts are no longer suitable, redesigning drainage channels for future flow and armouring of water channels. This approach considers climate adaptation is over the life of our infrastructure. Additionally, we’re incorporating design stabilization approaches that are more resilient to the impacts of climate change.

Following are a few case studies of how we’ve handled historic slides.

Cariboo Road Recovery – Works, Assessment in Progress

Changing weather patterns have reactivated historic slides with dramatic results, from Williams Lake to just north of Quesnel. Recent higher-precipitation spring and summer seasons have combined with spring snow melt. Major wildfires in the area have damaged vegetation, making it less effective at absorbing moisture and holding soil together. The resulting additional water in the ground contributed to unprecedented slides and road washouts in 2020 and 2021.

Recovery projects are being undertaken to address major transportation impacts at 10 locations, some of which have more than one slide area. Preliminary road options are being considered for all 10 sites. Community engagement is ongoing with information available about each site’s characteristics, preliminary road options, plans and processes, and a presentation captured on video. Resident feedback is being sought to identify local considerations, as we evaluate various possibilities. Further investigations, feedback and other considerations will all play a role as we move toward a long-term solution.

Work underway in these areas involves conceptual design for repairs and alternate alignments, geotechnical and hydrotechnical investigations, and climate-resilience and environmental reviews. Geotechnical monitoring includes electronic remote sensing and evaluation of the slide area, monthly aerial LiDAR (laser scan) surveys, slope monitoring and subsurface investigation.

Among the 10 Cariboo sites, is the historic Highway 97 at Cottonwood Hill slide which reawakened in 2020. Although the slide remains active with ongoing monitoring, travel through the area is safe. Work to stabilize the area is underway.

Another area, Quesnel-Hydraulic Road, was closed after multiple historic landslides reactivated there during the 2020 spring freshet. Unstable slopes, and continuing erosion at the Fraser River, make the affected segment unsafe for public use. French Road serves as an alternate route.

When Kersley Dale Landing Road was found to be unsafe due to ongoing slide movement, five households with no other way to access their properties, had to be evacuated. A temporary access road was built, and residents returned home in late 2021. Options, including road relocation, are being considered.

Ten Mile Slide – Soil Anchor Stabilization

Ten Mile Slide, located within the Xaxli’p First Nation community, northeast of Lillooet on Highway 99, is a highly dynamic slide. It has been described as ancient, and since 1988, the slide’s average movement has ranged from 10 millimetres a day to up to 50 millimetres a day.

For a few years, slide activity required the stretch to be limited to 24/7 single-lane alternating traffic, commercial vehicle load restrictions were required, and considerable road maintenance was needed.

This portion of Highway 99 is the primary connector between Lillooet, Xaxli’p and Kamloops, and vital to local communities and the regional economy. The Ministry of Transportation and Infrastructure and Xaxli’p worked collaboratively over time to reach a long-term solution to the slide’s ongoing movement, and engineering consultants and construction contactors performed extensive analysis and extraordinary works.

In October 2021, structural stabilization concluded. The final solution was 276 soil anchors installed through individual 2.5 metre square concrete blocks above Highway 99. Below the highway, there is a tied back concrete pile retaining wall composed of 148 large-diameter piles with 125 tie-back anchors (closeup photos here). Geotechnical Engineer Tom Kneale describes the lower structure, “Like a whole bunch of football players standing at the bottom of the hill, holding the slide back.”

This section of highway was also re-aligned and restored to two lanes. In the short-term, the road remains a gravel surface until the site settles, which is anticipated to be in late 2023.

Seeking Solutions in Shifting Circumstances

Managing historic slides takes a great deal of analysis to assess the slide’s behaviour and then, with expert information and community input, to choose the best option. Like the slide and the earth itself, things shift and change, just as our climate is changing. Our commitment is to keep you moving safely through it all.

We hope you’ve found this explanation of historic slides, their impacts and how we manage them, to be informative and interesting. For other geotechnical topics see these blogs:

• Dig into the Work of a Geotechnical Engineer
• What Happens After a Rock Hits a BC Highway
• Our Geoscientists Dig Deep to Keep BC Highways Moving Safely
• What Happens After a Washout Hits a Highway
• Three Ways We are Working to Protect BC Highways from Climate Change

Thanks to BCG Engineering, whose project summary we drew on, for the section on Ten-Mile Slide.