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Since October, the city of Pittsburgh, Pennsylvania has been dealing with the fallout from a huge sinkhole that became a national story. On October 28, 2019, a Port Authority bus in Pittsburgh was suddenly swallowed up in the middle of rush hour when a massive sinkhole opened in the heart of the city, causing lengthy delays, power outages and costing the city nearly $100,000 to lift the bus out of the 20-foot deep hole. The sinkhole caused damage to underground utilities that are still being repaired.
For many Pennsylvania residents, the sight of a huge sinkhole in the middle of Pittsburgh came as no surprise. Pennsylvania is one of the biggest hotbeds in the United States for sinkhole activity. Along with Florida and Tennessee, the state is a leader in the not-so-glamorous department of sinkhole damage. The Pittsburgh sinkhole came out of nowhere with little warning sign, but the problem was likely brewing for months or even years before finally breaking open and swallowing the bus.
Because sinkholes are so unpredictable, it can be very difficult for property owners, prospective homebuyers, real estate agents and municipalities to predict when and where they will appear, but there is some science on our side. It will take some legwork and research on your part, but that effort will be worth it in the long run if you are able to avoid opening yourself up (pun intended) to potential sinkhole damage.
Why do sinkholes form in Pennsylvania?
For anyone living in Pennsylvania, it is important to have a strong understanding of why and where sinkholes form because they do not affect every area of the state. Pennsylvania has very varied subsurface geology, and much of the state has never seen a single active sinkhole. Two factors influence the likelihood that a sinkhole will form in a given area – geology and human activity. Let’s first touch on the sinkhole that swallowed the bus in Pittsburgh. That sinkhole was not caused by geology, which is the case for most sinkholes that develop in urban settings.
The majority of new sinkholes in cities are caused by human activity. Their formation boils down to the fact that humans have made the decision to either impeded the flow of water or force water to flow through pathways that nature never intended. Aging infrastructure also plays a big role in new sinkholes in heavily-developed areas of Pennsylvania. Most sinkholes that form in cities like Pittsburgh occur because water has slowly eroded the soil that supports our buildings and roads. Because so much of the area has been paved over or built up, stormwater no longer flows as nature intended.
When water is not able to flow on its desired path and is directed to sewers, there are consequences. The pipes that carry our drinking water, runoff or steam for heating age quickly and leak. These leaks erode the soil surrounding the pipes and create large pockets or voids below the surface. This is why so many sinkholes form in the middle of city streets, as was the case in Pittsburgh. Most of our pipes and utilities are laid out to follow the centerlines of our streets. It is less likely to have a sinkhole form below a residential building in a big city, but those sinkholes that open up in the middle of the road can still have big consequences.
Sinkholes caused by geology have an entirely different formation process. Most types of rock that underly Pennsylvania are extremely hard, durable and resistant to erosion. Limestone, however, is none of those things. It is a carbonate, sedimentary rock that experiences a chemical reaction when it comes into contact with the slightest amount of acid in water. Even the purest water is slightly acidic and will react with the carbon-based limestone rock to emit carbon dioxide, in the process, taking away a tiny bit of the solid rock with it. If this happens enough over a long period of time, huge sections of rock will be washed away. Now, combine that process with the increased presence of acid rain and contaminated water, and it becomes clear why sinkholes can be such a problem in areas where limestone is the underlying bedrock.
The mechanism that leads to sinkholes develops slowly over time as the rock washes away, leaving huge underground tunnels and voids through which water and soil are gradually sucked through. The actual opening of a sinkhole is violent and sudden, but that hole has likely been in the works for dozens of years.
How to locate karst topography in Pennsylvania
To have an understanding of whether or not there is a high probability that the property you are considering in Pennsylvania is in an area prone to sinkholes, all you need to do is consult a geologic map of the state. Carbonate rocks are confined to several narrow bands in Pennsylvania, and if you are located outside of those bands, chances are slim that sinkholes will ever have an impact on your property.
The United States Geologic Survey has a treasure trove of resources available related to the geology of every state. The Pennsylvania Geologic Survey has been working for over three decades to fully map the carbonate rocks and potential sinkhole hotspots in the state, and ultimately produced a very useful map.
There has been immense progress made in mapping the location of sinkholes and subsurface depressions in Pennsylvania, led by the Department of Conservation and Natural Resources. The PA DCNR has a dynamic dataset of sinkholes in the state that has been digitally presented and made usable for GIS mapping.
As you can see in the display above, taken from the interactive DCNR map, the bands of orange and green data points, which represent sinkholes and surface depressions, are almost entirely concentrated within the bands of folded carbonate rocks shown on the first map. The risk of sinkhole development on your property is confined to several very narrow regions of the state.
Pennsylvania is unlike Florida, ground zero for sinkholes in the United States, in that only certain parts of the state are prone to sinkholes. If your property is located within one of the bands of limestone rock, you will need to be prepared to identify potential sinkholes. There is no one true way to predict with 100-percent accuracy whether or not a sinkhole will develop under your house or in your backyard, but previous geologic studies, past damage reports, and large datasets will help give you a better idea of how much risk you may be taking on.
How to look for sinkhole damage at a property
If you are considering buying a property in a region of Pennsylvania that is prone to sinkholes, you’ll need to know what to look for to protect your investment and avoid costly damage to your home or business. The available records of sinkholes and geologic maps can only do so much for you. Sinkholes are unpredictable and can happen at any time when carbonate rock is the predominant subsurface geology. The subsurface topography can vary wildly in limestone, with 50 feet or more of solid rock located a few feet away from a huge void.
Luckily, there are ways to monitor your property or evaluate it critically before making a purchase. Any visible surface depression is a sinkhole just waiting to happen. Look for depressed areas in the yard, especially close to the foundation. A surface depression forms during the early stages of sinkhole development as soil and rock are being washed out through voids below the surface. Surface depressions are particularly easy to spot on harvested agricultural fields, as they are typically left unplanted with trees or bushes growing out of the collected surface runoff. In a typical backyard, a surface depression may be difficult to see due to the presence of grass and other landscaping. However, depression can be more easily seen and identified by the presence of ponding water after heavy rain events.
A surface depression in your yard is a clear warning sign that a sinkhole could one day pose a problem to your property, but there are also other signs to look for. Cracks in the ground may indicate a future sinkhole, as the surface begins to be pulled and stretched under the extra stress of supporting itself over a weak foundation of bedrock. Cracks in the building’s foundation are another big red flag. These cracks form as buildings experience differential settlement. Some settlement is normal, especially in older buildings, but if you live in an area that experiences sinkholes, these cracks should be constantly monitored for further propagation. You can also look for cracks in your interior walls, around windows and doors. Additional signs of potential sinkholes to monitor outside your home include tilting trees, walls and fences, as well as actual holes opening.
How to protect yourself from sinkholes
Unfortunately, home insurance companies in Pennsylvania are not required to include coverage for sinkholes, as they are in Florida. If you want that extra protection, you will have to pay a premium for it in your annual insurance bill. As you go through the process of buying a home, push your realtor to inquire about previous insurance claims made at the home for sinkholes. If it has happened once or more in the past, it will likely happen again. Walk all areas of the property, especially outside and look for signs of surface depressions and cracks in the foundation. Pay extra close attention to the foundation because this is the most important part of your house and where you will see sinkhole damage first.
Hundreds of thousands of Pennsylvania residents live in the regions of the state where sinkholes can be a problem, but few experiences a catastrophic event at their house. Take steps to make yourself knowledgeable about how and where sinkholes form before buying a property in the areas of the state where sinkholes can be a problem. Be proactive in monitoring your property for signs of sinkholes and take steps ahead of time to remedy the problem before it becomes too big to fix.
Guest blogger: Joshua Sadlock is an experienced professional civil engineer who has worked on geotechnical and structural projects in areas of Pennsylvania with limestone geology and active sinkholes. He has designed bridge and retaining wall foundations to limit the impact of sinkholes, as well as developed plans for ground improvement techniques like compaction grouting.