Sinkholes and Underground Cavities Due to Human Activity

Did a cavity just open in your yard, as if you were in Jules Verne's Journey to the Center of the Earth? How could this happen? A sinkhole is one of several things that can lead to land subsidence, where the ground sinks gradually or suddenly. Sinkholes can look the same on the surface regardless of the cause. This article addresses some of those collapses caused by human activity.

In the Department of Conservation and Natural Resources (DCNR) Publication ES11, Sinkholes in Pennsylvania, Kochanov states that geologically, a sinkhole is a subsidence feature. The material supporting the surface moves downward and leaves the surface to collapse. This often occurs naturally when the underlying carbonate bedrock (such as limestone or marble) dissolves. That's how caves come into being. But sinkholes can also be a result of mining, washing away from leaking utility lines, or decay of buried material. Such cavities are an unfortunate but common problem that may occur five to ten years after construction. Like natural sinkholes, these underground cavities form gradually, but can appear in your lawn overnight.

Such cavities typically develop where a construction crew buried organic debris such as tree branches, brush, stumps, and construction materials. After a few years, this buried debris decays, leaving behind a large underground cavity hidden by a relatively thin surface layer of soil and vegetation. The soil layer eventually caves in, causing a sunken area or hole that can seem bottomless.

If a sinkhole appears in your lawn, do not go near it unless you work with a partner able to assist and to contact rescue in case of an emergency. Subsidence voids may initially have a more stable steep side and a side with a more gradual slope. The shape and size change over time as water continues to flush unconsolidated material into the hole.

Inspect the collapse carefully to determine if the problem was caused by buried debris or a utility washout, or a natural occurrence. Enlarge the surface opening so that you can inspect the entire cavity with a flashlight. If you can see traces of decayed trees or building materials, you may have a trench or pit that was used to bury construction debris. You can confirm your suspicions by pushing the bottom and sides of the cavity with a long pole. If you find solid earth on the bottom and sides, the cavity is probably man-made. This can be treated with one of the remedies listed below.

Some Treatment Suggestions for Small Underground Cavities

1. Check the location of buried utility lines before you dig. Call *811 within Pennsylvania for utility locating services. This may involve paying a fee, but it could save your life.
2. Enlarge the surface opening to inspect the whole cavity if you can do so safely. Look for decaying debris and confirm the cause of the problem.
3. Remove pieces of debris that you can handle or can find a contractor to lift out.
4. Fill the entire cavity by placing a geotextile fabric, then adding fill gradually with smaller stones in 12-inch layers (often called lifts). The final lift should probably be a material similar in texture to the surrounding surface. Failure to refill and pack the entire cavity may cause another sinkhole in the future.
5. Once the cavity is filled, establish grass or other permanent vegetation on the bare soil. No large growing trees should be planted there until the void is stable.

If you find a pipe in the cavity, a leaky pipe may have caused your problem. The best course of action is to consult the proper water authority before proceeding.

The cavity may be so large that you cannot reach the sides or the bottom. You may detect water or soft, mucky soil inside. If so, you could have a more serious problem. Call your County Conservation District, as they administer federal and state earth and water protection regulatory needs in your area. Once this step has been taken, call your local Penn State Extension office for research-based advice that the national university network may have. For large issues, a private geotechnical or soil consultant should be consulted to inspect the site and recommend an engineering solution.

Helpful Resources

Pennsylvania Department of Conservation and Natural Resources Middletown, Pennsylvania  717 - 702–2017

U.S. Geological Survey  Reston, Virginia

Information on environmental regulations and permitted activities, including underground and surface mines

Pennsylvania Department of Environmental Protection Harrisburg, Pennsylvania,  717–783–2300
*Contact the regional or district DEP office for assistance.

Pennsylvania Mine Map Atlas 

Pennsylvania Historic Underground Mine Map Inventory System 

National Mine Map Repository Office of Surface Mining Pittsburgh, Pennsylvania, 412–937–2833

Article Information From

Kochanov, W. E., 2015, Sinkholes in Pennsylvania (2nd ed.): Pennsylvania Geological Survey, 4th ser., Educational Series 11, 30 p.

Sjolander, SA; Michael R Lewis, and Stacy Wolbert. Pocket Best Management Practices for Pennsylvania Homeowners. Penn Soil Resource Conservation and Development Council, 2005. 39 pp.