Sinkhole safety

by Michael Mosher on February 6, 2014

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Sometimes, the low spot in the lawn is just a depression from a rotting tree stump, or from disturbed soil that wasn’t fully compacted. Sometimes, a hole could just be a small critter’s front door.

However, there are situations that are much, much more involved. They need more attention than a quick fill-in with a bag of topsoil.

A sinkhole can appear to have opened overnight. But a true sinkhole evolves over time, developing long before any surface evidence is detected, according to Randy Gibble, specialty services director at B.R. Kreider and Sons, 63 Kreider Lane, Manheim.

“The first and foremost thing is that if you have a situation you’re not sure about, tape it off and keep people away from it until it’s evaluated,” Gibble says. “We remind people not to put weight around the edges and definitely don’t go into any holes that have developed.”

Areas in which there is a lot of underground rock and clay soil are ideal for sinkholes. “Clay soil is made up of really small, wet particles,” says Gibble. “Where there are voids deep in the rock below, the clay passes into the cracks and leaves a hole just below the surface. Probably 75 percent of those situations will open up,” he adds. “Usually, you’ll see a depression first.”

A common misunderstanding is to think that a sinkhole is the actual hole in the rock, according the Pennsylvania Department of Environmental Protection. But, really, the sinkhole is what we see on the ground surface because of the hole in the rock below. The void in the rock takes hundreds or thousands of years to form.

“Typically, the sinkholes I deal with around here are 3 or 4 feet deep,” Gibble says, “but I’ve put a tape measure into a hole and found that they can be 15 or 20 feet deep sometimes.

“Just because it’s a small hole, you really can’t tell how big the sinkhole below really is,” he says.

Gibble first probes around with a rod that penetrates the ground. He will work his way out from what appears to be the center of the situation to see if he feels a hollow area below the surface. After determining the diameter of the sinkhole, the depth is checked.

“If we have any doubt, we suggest that we uncover and expose the sinkhole to evaluate its extent,” Gibble says.

Developments that lead to sinkholes include a decline in water level through drought or pumping; soil disturbance; the addition of water to a specific point, perhaps from a leaking pipe; a concentration of water flow; water impoundments such as basins, ponds or dams; a heavy load on the surface from buildings or dense equipment; and vibration.

Gibble adds that ideal conditions for a sinkhole can be created by human influence, when there’s a lot of blasting of rock to prepare areas for building. The bedrock is disturbed and repositioned and leaves voids.

And, according to Gibble, groundhog holes can start the sinkhole process as well.

“If a lot of surface water can get to that area, the water will go toward the low spot and, over time, things can settle.”

What to do

Mark Harmon, an engineer with ARRO Engineering, 108 W. Airport Road, Lititz, says an excavator usually is called first, followed by an engineer if there is significant evidence for concern.

Based on location-specific characteristics, he says, “I will work with an excavator to open the area to reevaluate.

“Evidence of surface displacement, such as cracks in sidewalks, curbs or the house foundation, (or) displacements in the property surface are evidence of subsurface movement.”

Harmon, a licensed geologist with the state, explains that water can act as a filler in a subsurface cavern. When the water table goes down, it might take a long time, but the walls of the cavern can fall in on itself.

“The surface depression you see is not necessarily the center of a sinkhole,” Harmon warns.

Contacts

  • Notify your insurance company of the situation before any real digging goes on, There’s a chance coverage could be denied if proof of previous evidence can’t be documented.
  • Professional excavators are also aware of other potential concerns, such as underground utilities. Dial 811.
  • Keep the municipality in the loop. That helps local government keep a record of similar issues in the area and track potential causes and concerns.

The fix

The second part of fixing a situation is often overlooked. According to Harmon, there is always some component that can reduce the probability of this situation happening again.

Most of the time, there is a “throat” to the sinkhole, an opening in the rock deep below that resembles a funnel. After the soil is removed and the size of the sinkhole is determined, Gibble says, a concrete plug is poured to fill and cover the opening. That keeps soil from continuing to filter down into the crack in the rock, eliminating any reappearance of the sinkhole.

“We then backfill with clay the material, compacting it between layers,” Gibble says, and finish with about 6 to 8 inches of topsoil for planting.”

Don’t be tempted to fill a sinkhole with junk you have lying around. Filling a sinkhole with waste materials is illegal, and may cause groundwater contamination for which the property owner would be liable.

SINKHOLE TIPS:

• Rope off an area larger than the obvious depression or hole.

• NEVER play in a suspicious hole. It may be the top of a much deeper sinkhole.

• Check for recent cracks in sidewalks, foundations and curbs for evidence of substantial movement.

• Call in a respected excavation company to handle questionable sinkholes.

• Filling holes and depressions with topsoil is like putting a Band-Aid on the problem, camouflaging the underlying problem.

 

Full article…here

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