Silty soil causes sinkholes

Published 12:00 am Wednesday, August 11, 1999

When Natchez’s famous loess soil gets wet, it has the consistency of soft ice cream. That’s the culprit behind sinkholes – such as one on U.S. 84 – that appear in Natchez, said City Engineer David Gardner.

&uot;When the soil gets wet, it loses its cohesive properties and is highly erodable,&uot;&160;Gardner said. &uot;When enough soil erodes, all of a sudden, a sidewalk falls in.&uot;

Still, holes as big as the 25-foot crater that fell through U.S. 84 near the river bridge after Monday night’s rains are rare. And the city is checking sewer and drainage lines to catch leaks before they erode enough soil to cause a sinkhole, Gardner said.

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Natchez’s soft, silty soil is one reason sinkholes are likely to form. And in a city more than 200 years old, aging sewer, water and drainage lines can crack, causing leaks that erode surrounding soil and cause sinkholes and mudslides.

Such was the case in January 1993, when a section of the 800 block of State Street, including one house, slid into a nearby bayou.

The neighborhood was built on a garbage dump built in the 1800s. When the garbage settled, it broke a water line, which leaked and caused the slide.

In recent years, slides have caused two deaths at Natchez-Under-the-Hill and caused the city to build retaining walls along Martin Luther King Jr. Street in 1994 and close Learned’s Mill Road several times.

Also, the city installed a new drainage system at Highland Boulevard and Pecan Way last year after erosion caused part of the intersection to sink.

But there are things the city is doing to check for leaks in sewer and drainage lines. To detect breaks, crews are now running video cameras into sewer lines and blowing smoke into sewers to see where the smoke escapes.

The city also videotapes its drainage lines on an ongoing basis, Gardner said. And crews have built more storm drains and ditches along roads to carry rainwater away.

Meanwhile, Mississippi Department of Transportation crews on Wednesday continued to work on repairing the U.S. 84 sinkhole.

Crews should fill the hole with water-resistant clay gravel and have the road repaired and open to traffic by the end of next week, said Transportation Commissioner Wayne Brown, who visited the site Wednesday. Crews will then drill a hole in the highway median down to the leaky drainage pipe to fix it.