28 October 2022
The Duffey Lake landslide: old logging roads and slope failures
On 15 November 2021 a series of landslides occurred during a period of heavy rainfall in British Columbia, Canada. I wrote about those landslides at the time. Substantial damage was caused to both Highway 1 and Highway 99. In the most deadly failure, now known as the Duffey Lake landslide, five people were killed and substantial damage was caused to Highway 99.
The Canadian media organisation CBC has a piece that explores the causes of these landslides, highlighting the role of old resource (logging) roads on the hillside above Highway 99. There is a very interesting video, which last about 10 minutes, in the article. It includes this diagram about the two landslides triggered in the Duffey lake area:-
The landslides at Duffey Lake appear to have originated from the sites of an old logging road high up on the hillsides. This road was constructed in the 1960s, and used through to the mid-1990s but then abandoned. The location is at about 50.473, -122.226. I have indicated the approximate location of the crown of the landslide and the impact point on Highway 99 on the Google Earth image below:-
The site was investigated by geologist Pierre Friele. His findings are described in an article on the Maple Ridge-Pitt Meadows News website:-
The problem, he said, was in the area of a switchback. Two older, small landslides had left debris resting on the road below the switchback, plugging a ditch. The water then ran down the road instead of crossing over the road and going down a stable slope.
It was the diversion and concentration of water from the road that triggered the initial failures high above Highway 99. Given the steepness of the terrain, they were able to turn into a major landslides that impacted the main road at the foot of the slope.
Interestingly, even the 2019 Google Earth imagery appears to show a couple of small failures on the resource road in the area of the landslide source:-
I have written extensively about poor quality road construction, and its impact on slope instability. In the case of these resource (logging or mining) roads in Canada, it is poor maintenance, or a failure to properly deactivate the roads when they are no longer needed, that is causing the problems.
The CBC article highlights that these roads pose a substantial long term hazard in Canada. It is estimated that there are over 1.5 million kilometres of resource roads – British Columbia alone has 600,000 km of these roads.