SOIL SCIENCE: What causes landslides?

SOIL SCIENCE: What causes landslides?

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Photo courtesy of the St. Paul Fire Department Photo courtesy of the St. Paul Fire Department

As the investigation continues into the fatal field trip at Lilydale Regional Park in St. Paul, many are wondering how often the ground just gives way in the city and FOX 9 News looked into the science behind landslides.

Landslides are a pretty regular occurrence in the Minnesota River Valley, but a researcher at the University of Minnesota told FOX 9 News it's uncommon for one to take place in a St. Paul park.

So why did the landslide happen at Lilydale? Experts say it's important to take the park's landscape into consideration.

Photos sent in by Tina Hoikka of a different group of fourth-grade students from Susan Lindgren Elementary on a similar fossil hunting expedition in the area show the scene.

"We climbed down a few hills and up a few hills," Hoikka recalled. "You know, it just started to rain a tiny bit at the very end, but the trail we were walking on felt very safe."

Hoikka admitted it's hard to believe her kids walked through the same spot just one day before the Peter Hobart Elementary School field trip turned fatal.

"I felt terrible that we had so much fun but it ended up as a nightmare for someone else," she said.

While investigators work to pinpoint the cause of the accident, Satish Gupta, with the U of M's Soil, Water and Climate Department, said he believes the rainy weather played a role -- but he said it's unlikely anyone could have known it was coming.

"You cannot forecast these things because every hill slope does not slump," he explained.

Gupta said the park's landscape includes buried layers of rocks. That means when it rains, the water can't get through the rocks and moves toward the hill slopes instead of seeping down into groundwater. Eventually, that weakens the soil and causes a landslide.

He also demonstrated the consistency of a till, a hard soil type found in many hill slopes across Minnesota that can change drastically after being soaked in water for a few minutes.

"The strength of the soil goes down," Gupta showed. "It's more like taking a piece of bread and putting it in water. You lift it up, it's going to disintegrate."

Researchers say it's hard to predict a landslide, and the best time to search for fossils usually follows a heavy rainfall because the ground loosens up and makes it easier to dig -- but the amateurs are now being advised to stay away until conditions are dry.

St. Paul officials say the fossil portion of Lilydale Regional Park will remain closed indefinitely.

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