Too much moisture in the soil caused Whitehorse landslide

Too much moisture in the soil caused Saturday's landslide in Whitehorse, that saw about 3,000 to 4,000 cubic metres of sand, silt and clay fall across a main thoroughfare and into the Yukon River.

Mayor Laura Cabott said a landslide specialist engaged by the city made the determination based on information provided by city engineers.

"Essentially, [the escarpment] just couldn't hold any more and so it let loose," said Cabott.

The city has closed Robert Service Way, one of two thoroughfares into the city's downtown, as well as nearby trails.

Cabott said the city still doesn't have a timeline as to when it will reopen the road.

Taylor Eshpeter, manager of engineering services with the City of Whitehorse, told city council Monday evening that city engineering crews discovered "tension cracks" where the landslide took place.

"[It indicates] the site is still active and still a high risk," said Eshpeter.

The mayor added there is a possibility there could be further slides.

"The city will not start clearing the debris until it is safe for workers to do so. Likewise, the city will not be reopening Robert Service Way or Millennium Trail between Yukon Energy and the SS Klondike roundabout until it is safe," Eshpeter told city councillors Monday evening.

Usually stable

Panya Lipovsky, a geologist with the Yukon Geological Survey, said the slide began about a quarter of the way down a section of the escarpment's steep 60-metre bluff.

"It dropped about 50 metres down to the road and trail below. And it deposited the equivalent of about one Olympic-sized swimming pool of material, and a thin sheet about half a metre thick that extended across the road and the trail and the river ice below," she said.

The escarpment bounds the west side of downtown Whitehorse and is composed primarily of sand, silt and clay.

Lipovsky said the escarpment is usually stable but is susceptible to landslides in the spring when the snow melts or following heavy rainfalls.

She added there have been periodic landslides on the escarpment since at least the 1940s.

"I believe most of the time they're only quite small, shallow failures, but occasionally larger failures do occur like the one we saw on Saturday," she said.

She said it's difficult to say whether landslides will become more common in the future.

"I think if we have a wetter climate or higher snowpacks occurring, I think we can expect to see more events like this happening in the future, for sure," she said.

She added the city manages an escarpment control zone and has identified areas where it's hazardous.

Eshpeter told city council the city's engineering department will be assessing the entire escarpment over the course of the summer, something it had already planned to do before the landslide took place.

On Tuesday morning, traffic was backed up and moving slowly in all directions at the city's other main access into downtown — where the Alaska Highway intersects with Hamilton Blvd. on the west and Two Mile Hill on the east. 

"Make sure that you're giving yourself extra time to get where you need to go right now, especially if you're coming in from south of town," said Whitehorse RCMP Constable Carlie McCann.