Rock Doc: Weak bedrock can make your house a hole

MONDAY, APRIL 15, 2013

Sometimes “solid rock” turns out to be anything but sturdy stuff.

Limestone and a couple other related sedimentary rocks are common in some parts of the country, including in Florida. The chemistry of limestone and groundwater can combine to make for sinkholes, or vertical holes in bedrock that can open up quickly.

Acidic groundwater percolating down from the land surface causes sinkholes. And acids eat away at limestone, dissolving it. Over time, limestone bedrock can start to resemble Swiss cheese, with caverns and holes within it. If a hole grows large enough, it undermines the ground at the surface. The surface layer then falls into the hole created in the limestone bedrock.

Earlier this year a Florida man in a Tampa suburb fell into a sinkhole that opened beneath the bedroom of his home. He called to his brother for help.

Jeremy Bush tried to aid his brother, scrambling down into the hole and digging with a shovel. But Jeff Bush wasn’t to be found. When police arrived, they pulled Jeremy Bush out of the hole, saying it was unsafe because it was still spreading and potentially would undermine the whole house.

John and Tina Furlow, another Florida couple, face a more slowly expanding sinkhole that threatens their home. For more than a year they’ve watched a sinkhole on their property expand, undermining a room in their house. It’s an ongoing tale that may make fresh headlines at any time.

Sinkholes are one feature of what geologists call karst topography. Around the world, some karst regions have thousands of caves and sinkholes. The voids form as groundwater seeps through cracks or bedding planes in the bedrock. Slowly the bedrock dissolves and the voids grow. As they do, they increase the rate of groundwater percolation so that water drains from landscapes via the subsurface instead of streams above. In some karst areas, streams simply sink into the ground, disappearing from view at the surface. A karst “fenster” occurs where an underground stream emerges from a spring at the surface for a few feet, but then disappears back underground, often cascading down into a sinkhole.

The acid in the groundwater is, by and large, completely natural. There’s a little bit of carbon dioxide in the air, produced by the respiration of ecosystems and augmented since the Industrial Revolution by the burning of fossil fuels. Rainwater with dissolved carbon dioxide in it seeps through soil where more carbon dioxide is added to the water by plant root systems. The resulting carbonic-acid solution can dissolve limestone and related rocks.

Chemistry and the water cycle create karst topography. Unfortunately, from time to time, voids open quite suddenly at the surface of the earth, as was the case under the place where Jeff Bush was sleeping. The Furlows face a more gradual change, but it’s plenty dangerous. Sometimes underground changes set the stage for results none of us would choose.

E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard universities. This column is a service of the College of Agricultural, Human and Natural Resource Sciences at Washington State University.

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