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After Rockslide Blocks Road, St. Paul, Minn., Uses Drones and Lasers to Survey Situation

Mattress-sized slabs of limestone fell onto a road in St. Paul, Minn., recently.

by James Walsh, Star Tribune / June 1, 2018

(TNS) — An overhanging slab of limestone, estimated to weigh 300,000 pounds or more, will need to be removed from the St. Paul, Minn., river bluff before Wabasha Street could be safely reopened south of downtown, city officials said Friday.

Kathy Lantry, the city's director of Public Works, said that the most optimistic projection would have the street reopened to traffic sometime in the third week of June.

"We don't know what we're going to find once we start peeling back that soil," Lantry said of the more than 20 feet of weathered and solid limestone and shale that remains atop the bluff.

An estimated 400,000 pounds of rock and soil slid down a slope that abuts Wabasha south of Plato Boulevard on April 28. No one was hurt as slabs of limestone, some as large as a mattress, slid down the bluff and shattered on the street below.

The city closed the street to all traffic from Plato to Cesar Chavez and began an immediate study of what caused the slide and what needs to be done to make the bluff safe to reopen the road.

Brent Christensen, an engineer with Public Works bridges division, said analysis by the city and Itasca Consulting shows the potential for several additional large slabs of limestone to fracture and fall. The owners of a home and garage at the top of the bluff have given the city access to study the area from above and engineers are using drones and lasers to scan and survey the site to determine the next steps.

No final decisions have been made, but it has been recommended that a sizable chunk of the bluff should be removed before it is safe to reopen Wabasha below, he and Lantry said.

Lantry said the city has not yet determined the total cost of making the bluff safe, saying she has almost $300,000 "in bills on my desk." St. Paul and Ramsey County officials have declared a local state of emergency in the hopes of qualifying for disaster relief funds to help pay for the work. But none of the costs to reopen Wabasha or further stabilize the bluff will be borne by the property owner above, she said.

"This is not something that a private property owner could withstand," Lantry said.

Christensen showed several slides of the slide Friday, illustrating how layers of limestone several feet thick are segmented by fissures and vertical "joints" that over the years let water flow down through the rock. The annual "freeze/thaw" process then forms joints in the rock, creating slabs of stone that can detach from the bluff. The work now is to determine which of those slabs, and their adjoining material, are stable and which are in danger of sliding down the bluff in the future.

The work to make the bluff stable will be in two phases: Immediate work to allow Wabasha to be reopened and long-term work that will include assessing the risk, costs and landowner impact of future action.

"The bluff is always sloughing off," he said of a continual geologic process that can take many years.

©2018 The Star Tribune (Minneapolis), Distributed by Tribune Content Agency, LLC.

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