Permanent fix to Peaceful Valley landslide threat likely months away, but city continuing South Gorge Trail work

The city may be months away from shoring up a hillside threatening a small section of Peaceful Valley with a landslide, but development of the adjacent South Gorge Trail will continue .

“We’ve coordinated some changes to that project so we can keep that moving forward even though this slide area is right in the heart of a lot of the needed infrastructure for that project,” Kyle Twohig, the city’s director of engineering services, explained to the Spokane City Council’s Public Infrastructure and Environmental Sustainability Committee on Monday.

The city has cut out a 350-foot section of the South Gorge Trail project where it passes beneath the slide, and the contractor will circle back to complete it when the hillside is secure – either late this fall or in 2022.

“If the stars align, we’ll get our solution built, then they could build the rest of the project this fall,” Twohig said.

The intersection of Clarke Avenue and Elm Street has been closed for weeks after city officials determined the hill to its south poses a landslide risk.

Last week, the city was able to install a geotechnical drill on the hillside that will allow engineers to understand at what depth the land is slipping, and the makeup of the soil. The city has also installed subsurface monitoring equipment.

“We need to basically watch to see what happens for a little bit and understand the nature of what’s happening beneath the surface, and then we’ll start evaluating the potential solutions,” Twohig said, adding that the fix is months away.

But that work won’t hold up the next phase of construction of the South Gorge Trail, a 10-foot-wide pedestrian and bike trail, which is set to run through that very intersection this year.

Avista has already taken action to protect its infrastructure. The utility company has begun to relocate between eight and 10 utility poles that were threatened by the landslide, Twohig said. In January, its equipment was struck by a tree, causing a temporary power outage to the surrounding neighborhood.

The slide is not believed to pose a risk to surrounding residences at this point.

With help from Avista crews, the city has already gone to the hillside and removed trees that were at risk of falling.

“We literally had trees sliding down the hill and rotating,” Twohig said.

Removing vegetation from the unstable hillside may seem counterintuitive, but the landslide is actually occurring several feet below the surface.

The city has brought in a geotechnical engineer to help.

Thus far, 2,000 yards of dirt have been placed at the bottom of the hill to secure a city-owned retaining wall and resist the pressure of land above.

“That has started to slow the slide considerably. It’s doing its job … we’ve bought ourselves some time to implement a fix,” Twohig said.

Councilwoman Candace Mumm asked Twohig if there was any concern the instability would expand from east to west.

That isn’t a concern, Twohig said, but the city is continuing to monitor signs of movement that would place homes and apartment buildings on Riverside Avenue at the top of the hillside in danger. However, he said “there’s still a long ways between the top of the slide and where you would start to undermine” structures above it.