While not the most common occurrence in Pennsylvania, sinkholes are a problem that certain areas in the state face. Knowing how sinkholes form, what causes sinkholes in roads and how to detect an oncoming one will help you and your community stay safe. Looking for the signs and understanding how these formations occur will also give you the tools to evacuate the affected area and pass the relevant information on to those who need it.
What Is a Sinkhole?
A sinkhole is a depression in the ground that forms within minutes or hours and causes everything above it to fall in. Over time, this depression can expand and cause even more damage and casualties on the surface. Some sinkholes develop for hundreds of thousands of years before causing the earth’s surface to collapse, while others can occur more quickly.
A PDF from the Pennsylvania Department of Conservation and Natural Resources (DCNR) states that sinkholes in this state are often between 4 to 20 feet in diameter, though there have been reports of those that have expanded to several hundred feet.
How Do Sinkholes Form?
Sinkholes form on a specific kind of terrain called karst, which is an area that is stationed above certain soluble rocks like limestone, gypsum and marble. Karst terrain occurs when limestone is within 100 feet of the surface. This landscape is the most susceptible to sinkholes because rainwater and other moisture in the area soak through the bedrock and create acidic subsurface conditions when dissolving the soluble rocks. This chemical reaction creates a cavern that destabilizes the ground. Over time, the land collapses and the cavern expands.
Other natural conditions can create sinkholes, such as an increase in groundwater withdrawals during prolonged periods of drought. Heavy rains associated with droughts can dramatically change moisture content and water temperature on the ground, which can also cause sinkholes on karst terrain.
Certain man-made activities contribute to the formation of sinkholes, too. Drilling wells, creating artificial ponds and interfering with the natural flow of groundwater are examples of ways humans can accelerate the development of sinkholes. Plumbing networks that run underground in unstable ways, especially in urban areas with intricate highways, roads and home planning, are one of the most common man-made causes of sinkholes in any terrain.
What Are the Types of Sinkholes?
There are four types of sinkholes, three of which are naturally occurring:
1. Dissolution Sinkholes
These sinkholes are caused by a lack of vegetation or soil covering the ground. When rainwater falls, it drains through the earth’s surface and dissolves the soluble rocks in karst terrain. A depression forms in the ground due to the acidic reaction and gradually expands to form a sinkhole.
2. Cover-Subsidence Sinkholes
These sinkholes occur when caverns form on surfaces with sand covering the bedrock. Over time, the limestone below the surface dissolves and creates a void that sediment fills and blocks the flow of water, expanding the caverns. These sinkholes typically aren’t big but are quite deep.
3. Cover-Collapse Sinkholes
This type is what people normally think of when they hear the term “sinkhole.” They are the most dangerous sinkholes and occur on surfaces where the bedrock is covered by clay.
The depression forms underneath the clay from water dissolution, and the erosion creates a deeper cavity from the bottom. Eventually, the thin layer of clay covering the hole collapses and the land opens up suddenly, taking in everything on the surface. These sinkholes can expand far enough to swallow buildings and vehicles.
4. Man-Made Sinkholes
Man-made sinkholes are often caused by elements like vibrations from traffic on the surface, withdrawal of too much groundwater and excessive drilling on the surface, which undermine the ground’s stability. Thoroughly surveying land is necessary in high-risk areas because renovations, plumbing and land alterations can cause sinkholes. Installed water lines are susceptible to damage from soil erosion and weather fluctuations, which can also result in man-made sinkholes in areas such as streets.
What Can Prevent Sinkholes?
Sinkholes created by nature can’t be prevented because they take centuries to form, and by the time you become aware of them, it is already too late. Gravity combined with water pressure can trigger a dramatic collapse and, due to its abruptness, prevention is unlikely.
However, there are ways to avoid creating sinkholes from human activity. Many high-risk areas have measures in place to prevent the interference of water flow by people who want to alter the land. There are also efforts by conservation groups to ensure people who want to build on sinkhole-prone land have the right permits and documentation to prevent any surface collapses.
As a general measure, it is not advisable to throw off the balance of the soil and bedrock of a high-risk area, as man-made sinkholes often have little to no warning signs. In instances where sinkholes occur naturally, there are signs you can look for to get you and those in your area to safety before the collapse occurs.
What Are Sinkhole Warning Signs?
There are several signs that indicate a sinkhole is on the way or already started. If you can get familiar with these, you will be able to evacuate the area and avoid any significant harm. These signs include:
- A circular depression in the street: If one of these appears in your area with signs of getting worse, a sinkhole may be coming.
- A settling foundation: A sinkhole could have begun forming and is close to collapsing if the foundation of your home starts to settle and form cracks.
- Cracks forming in the road or sidewalk: Prevalent cracks in the road that weren’t visible before mean the soil is weakening and should warn you about an incoming sinkhole.
- Rainwater disappearing into the ground: The imbalance due to a forming sinkhole will cause rainwater to find its way into a void caused by the depression.
Learn More About Sinkholes From Union Quarries
Sinkholes are a naturally occurring phenomenon in certain areas of Pennsylvania and the United States. If you live on karst terrain, it’s important to get to know your area’s sinkhole risks and which warning signs to look out for to avoid harm. At Union Quarries, we prioritize safety and know how to identify sinkholes.
To work with a qualified quarry that understands the needs of the terrain, contact us today with your aggregate query!