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Solution for Shoal Creek landslide coming soon

Friday, May 18, 2018 by Caleb Pritchard

City engineers are getting closer to figuring out how to fix a catastrophic landslide that knocked out a portion of the Shoal Creek Trail earlier this month.

“By the end of next week, we’re probably going to have an idea of what a solution is going to look like,” Mike Kelly, managing engineer with the Austin Watershed Department, told a gaggle of journalists outside One Texas Center on Thursday afternoon.

That reveal is pending the conclusion of ongoing geotechnical analysis of the conditions beneath the creek bed and the steep slope above it near the 2600 block of North Lamar Boulevard. That analysis includes drilling 30 to 40 feet below the surface to get a profile of the subterranean strata.

The city and at least three property owners in the Pemberton Heights neighborhood above the collapse are footing the bill for that drilling.

The slope collapsed earlier this month after a sudden storm swept through Central Austin. According to Kelly though, the amount of rain the storm brought wasn’t out of the ordinary.

“What was unusual about this rainfall was the intensity with which it fell,” he explained.

Three inches fell within just one hour, percolating through the cracked limestone surface. That created an underground slip-and-slide between that rocky layer and the clay it sat upon, sending the formation tumbling down into the creek.

Nearly two weeks later, the massive pile of rock, soil, broken trail components and vegetation is still unstable, according to Kelly, and any indelicate attempts to remove it could trigger further slope failures. For the time being, the city has detoured hikers and bikers along the trail to the sidewalk along North Lamar.

Other sections of the trail, however, will remain open. Kelly dismissed any concern of similar collapses happening along any other portions.

“We’ve done the geologic homework to know where these formations exist and where something like this potentially could happen. There’s no place right now that has the combination of the steep slope, the geology and the proximity to the trail,” he said. “This seems to be a localized phenomenon.”

In the meantime, the recent summer-like conditions are suiting the Watershed Department just fine.

“First thing we’re going to do is hope for continued dry weather because that decreases the chance that that soil is going to move on its own,” Kelly said.

On top of the threat of further washouts, the debris in the creekbed has partially dammed up the waterway. Kelly estimated that the flood plain in the surrounding area rose by two feet immediately after the collapse. Since then, crews have used chainsaws to remove large tree trunks and branches from the colossal pile. That alone has cut by half the impact to the flood plain, Kelly said.

As for the homeowners on the ridge above the landslide, Kelly said the city isn’t forcing any evacuations at the moment.

“Ultimately, it is their decision whether they live there on a day-to-day basis,” he said.

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