An overhanging slab of limestone weighing an estimated 300,000 pounds will need to be removed from the St. Paul river bluff before Wabasha Street can safely reopen south of downtown, city officials said Friday.

Kathy Lantry, the city's director of public works, said that under the most optimistic scenario, the street would open again to traffic 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.

On April 28, an estimated 400,000 pounds of rock and soil slid down a slope that abuts Wabasha south of Plato Boulevard. 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 blocked all traffic from Plato to Cesar Chavez Street and began an immediate study of what caused the slide and what needs to be done to stabilize the bluff.

Brent Christensen, an engineer with public works' bridges division, said analysis by the city and Itasca Consulting Group 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 also are using drones and lasers to scan and survey the site to determine the next steps.

Debris covers Wabasha St. after breaking off from the bluff above. ] JEFF WHEELER ï jeff.wheeler@startribune.com A portion of the bluff broke off above Wabasha St. on St. Paul’s West Side over the weekend, closing the street until city engineers decide whether to preemptively remove some more of the bluff to prevent another landslide. The aftermath was photographed Tuesday afternoon, May 1, 2018 in St. Paul.

No final decisions have been made, but the recommendation is to remove a sizable chunk of the bluff before reopening Wabasha, Christensen 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. But none of the costs to reopen Wabasha or further stabilize the bluff will be borne by the property owner atop the bluff, she said.

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

Christensen showed several slides of the rock 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 within those joints works to loosen slabs of stone from the bluff. Now engineers must determine which of those slabs, and their adjoining material, are in danger of sliding down the bluff.

The city will initially stabilize the bluff before allowing Wabasha to reopen. The second, long-term phase will assess the risk, costs and landowner impact of future action.

James Walsh • 612-673-7428