Jackson's got vents in low places where the water runs under paved spaces, along East Main; sources hope it'll be OK

On either side of East Main Street in Jackson, between Interstate 55 and Old Orchard Road, at least three holes have opened.

That�s not good news, said city engineer Clint Brown, but that�s why city staff needs to bring in a specialized team of experts to survey the area and determine what the problem actually is.

Are they the product of subsurface erosion? That would be inconvenient but fairly typical, officials said.

Or are they sinkholes? That might indicate a much more serious situation, since sinkholes are typically part of a system, and would require much more extensive intervention.

The holes were discovered about a month ago, Brown said, by an alert city worker who noticed some exposed pavement where the shoulder meets soft ground.

A possible sinkhole, off of East Main Street between Interstate 55 and Old Orchard Road, is marked off Friday in Jackson.KASSI JACKSON

The holes are about 125 yards west of the red �Welcome to Jackson� sign, Brown said. Development plans are already in the works near the sign.

The first hole and one nearby, both on the north side of Main Street, were filled with flowable fill, a polymer mix similar to cement, Brown said.

�The hole was about 8 feet by 8 feet, and about 16 feet down,� Brown said, and one hole extends under the roadway.

The employee discovered the holes on a Friday afternoon about a month ago, Brown said, and two truckloads of fill later, the holes were stabilized.

�It�s a temporary fix,� Brown said, but for a more permanent solution, city staff will need more information about what exactly the problem is.

While a simple survey of the ground and surrounding area could have been handled in house, Brown said in this case, more is necessary.

Brown used to work in Perry County, Missouri, home of one of the biggest and most extensive karst systems in North America.

That means caves and sinkholes, and lots of them, he said.

The system�s presence complicates construction projects, he said, since companies must be sure not only to build on solid ground, but they must also control runoff so as to not damage the environment.

The Missouri Department of Transportation built the stretch of East Main Street in question, and construction was completed in 2007, said MoDOT official Andrew Meyer, construction and materials engineer for the southeast district. He worked on the project, and said the holes could indicate subsurface erosion.

�It�s not unusual to have ... erosion subsequent to construction, in particular when you have a lot of rain, as we have over the spring and winter,� Meyer said.

Water follows the path of least resistance, Brown said, and if there�s a shelf of rock under the soil surface, the water might trickle onto that, eroding soil and rock alike until the surface collapses.

That�s not a sinkhole, Brown said, but could look like one.

East Main was built in two sections, he said: one by the city, and the other by MoDOT. The portion MoDOT built was deeded back to the City of Jackson, Meyer said, and that�s why the city is looking at solutions.

�Typically, when we start a project, we send a drill crew to survey,� Meyer said. That crew will drill into the ground in a grid pattern within the limits of a project, at the centerline and the ditch line, then report what they�ve hit.

�That helps us determine what�s out there, and helps determine what quantities we will have,� Meyer said, adding that a contractor will bid differently if removing rock rather than dirt.

Meyer said the project did have a lot of limestone underneath, with veins of dirt and softer rock.

Survey crews did not run into any voids, such as a cave or sinkhole, when preparing for this project, Meyer said.

�We do pretty extensive work on the front end to make sure we don�t have problems during construction,� he added. �We follow appropriate steps out there, but 11 years later, it�s tough to go back and say what�s happened.�

That�s why the city is looking at this survey, he said. It�s not as disruptive as digging would be, and will tell city staff a lot about what steps to take next.

Brown said pending approval by the board of aldermen at the regular meeting today, the city would hire Geotechnology Inc., a St. Louis firm that would provide ground-penetrating radar equipment and the crews to operate it � and interpret the results.

The city would pay about $24,000 for equipment and services, according to city documents.

The survey area is about 1,400 feet long and covers all lanes of East Main, and the ground on either side to about where the hillsides are visible, Brown said.

According to a proposal from Geotechnology Inc., dated April 17, East Main would remain partially open during the survey, with traffic control directing drivers.

�We need the survey before we take the next step,� Brown said. �We don�t want to cut corners on this project � we need the best services for the best results.�

mniederkorn@semissourian.com

(573) 388-3630