Story URL: http://news.medill.northwestern.edu/chicago/news.aspx?id=220372
Story Retrieval Date: 10/30/2014 11:02:55 PM CST

Top Stories
Features

Breaking down the science behind sinkholes

by Theresa Chong
Apr 18, 2013


sinkhole

Photo by Harry L. Moore

Roadway collapses from natural sinkholes in the eastern U.S.

A 20-foot diameter sinkhole developed on Chicago’s South Side Thursday. By late afternoon, it had grown to 40 feet in diameter and swallowed three cars.

It was a dramatic element of the week’s storms, but it raised a question: What is a sinkhole?

To start -- there are two main types of sinkholes: naturally induced and human induced, according to Jim Kaufmann, a research physical scientist from the U.S. Geological Survey EROS Center.

Naturally induced sinkholes are formed when the bedrock – it’s usually limestone, dolomite and gypsum - below the soil softens from slight levels of acidic water. This process usually develops over a span of decades.

“The void develops as sediment, is carried away through crevices and conduits which have been enlarge by being dissolved by water,” Kaufmann said. “Eventually, the void enlarges and the soil is no longer able to bridge the void and a collapse occurs.”

Human-induced sinkholes are commonly formed from human-activity or aging infrastructure, such as old water mains and sewer systems.

“In the Chicago area, the soil is at least 100 feet thick. But another way you can get a sink hole is not related to bedrock, but more related to infrastructure, for example water mains,” said Samuel Panno, who works for the Illinois State Geological Survey’s Prairie Research Institute.

Geochemist Samuel Panno explained further: When water mains crack, sediment is sucked into the pipe and is carried away with the flow of water, which creates a void around the exterior of the pipe. It is often difficult to detect the presence of these voids due to rigid structures, such as roads and sidewalks, which act as a bridge over the void. Then, when a large load, such as a vehicle, is placed on top, the road collapses.

Panno also said that due to multiple cracks on road surfaces caused by weathering, it is difficult to detect with a naked eye if a sinkhole will occur. However, a handheld electromagnetic induction instrument is often used to detect voids below the surface.

According to Kaufmann, homeowners can help prevent excess amounts of water to buildup within their property limits.

“When building a structure, it is very important to preserve the original drainage as much as possible and to make sure that precipitation running off of roofs is piped away from buildings,” Kaufmann said.

“Downspouts which concentrate roof runoff are significant contributors to cover-collapse sinkholes. The best scenario is to capture as much of the runoff as possible, either in barrels or cisterns, and use it for water lawns or other non-potable uses.

According to Tom LaPorte, a spokesman for Chicago’s Department of Water Management, there will be a “methodical repair process due to the utilities” beneath the surface.