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Three days after chunks of rock the size of mattresses crashed down onto busy Wabasha Street, the St. Paul Department of Public Works is considering blasting away a portion of the bluff to prevent future rock slides.

The city has hired Itasca Consulting Group Inc. for engineering work, and city staff said they'll have a plan in place by the end of the week. The city may remove part of the bluff proactively, said Public Works Director Kathy Lantry, but so far nothing has been decided.

"We're getting started on this very complicated thing," she said. "There will be options on how to remove it."

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A hardhat-wearing engineering crew worked along the bluff between Plato Boulevard and Humboldt Avenue Tuesday morning, surrounded by caution tape and plastic barriers and flanked on one end by police cars. Massive pieces of rock still littered the ground.

A retaining wall catches the small amounts of rock that regularly fall from the bluff, Lantry said, but it was no match for the rock slide Saturday afternoon.

"The volume of material is just extraordinary," she said.

No injuries or damage to private property were reported from the slide, which occurred around 3:30.

Among the river bluffs of the Twin Cities and southeastern Minnesota, rock slides are a natural and sporadic occurrence. A rock slide in 2011 along Wabasha Street smashed in the back wall of a bakery.

The risk is especially high in spring, when ice inside the bluff thaws and destabilizes the rock, said Tony Runkel, chief geologist at the Minnesota Geological Survey.

"When you go through that multiple times in the spring, that just makes any already unstable rock just even more prone to fail," he said.

A big snow melt — which the Twin Cities experienced in the last two weeks — can soak the ground and put additional pressure on the bluff, Runkel said.

Preventing rock slides along the Wabasha Street bluff will depend on how much money the city wants to spend, Runkel said. Jason Richter, chief engineering geologist at the Minnesota Department of Transportation, said the rights of property owners above the bluff and aesthetic considerations could also come into play.

According to the city, Wabasha Street will stay closed between Plato Boulevard and Cesar Chavez Street "until a full assessment can be made of the bluff, and crews can safely work in the area."

It's not clear how long the road closure will last. Council Member Rebecca Noecker, who represents the area, said West Side residents are already feeling the pinch from closures at the High Bridge and on Water Street, and she will advocate to get Wabasha Street open as soon as possible.

"For my constituents on the West Side, there's an urgency to know what the future will be for that particular street," she said, "and then just to regain that freedom of movement within and from and to the West Side."

Emma Nelson • 612-673-4509