Don’t Get Sunk: Everything You Need to Know About Sinkholes (Part Two)

Sinkhole in Frederick, Maryland. Credit: Randall Orndorff, U.S. Geological Survey. Public domain.

Sinkholes can be quite terrifying. We see them on the news, on television and in movies seemingly appearing out of nowhere, swallowing up cars and creating calamity in towns across the world. In this two-part blog series, our experts uncover the mystery around sinkholes and arm you with the facts you need to make them less scary.

In part one of the blog series, we discuss what a sinkhole is, three different types of sinkholes, and what causes them to form. In this second part, we explore how to detect sinkholes, what to do if you detect a sinkhole, and the steps taken to repair them.

WELCOME TO PART TWO: DON’T GET SUNK: EVERYTHING YOU NEED TO KNOW ABOUT SINKHOLES
How to Detect a Sinkhole:

Cover-collapse sinkholes (outlined in red) in eastern Bullitt County Kentucky. Photo by Bart Davidson, Kentucky Geological Survey.Not all sinkholes are Hollywood-style monstrosities capable of swallowing your whole house. But even a much smaller, less noticeable sinkhole can do its fair share of harm, compromising your foundation and damaging utilities.

Although sinkholes can be scary to think about, you can take comfort in knowing there are ways to detect them, both visually and experimentally. Often, you can spot the effects of a developing sinkhole before you can spot the hole itself. If you live in an area with characteristics common to sinkhole formation (i.e. “karst terrain,” or types of rocks that can easily be dissolved by groundwater), there are some things you can do to check your property for signs of potential sinkhole formation.

According to the American Society of Home Inspectors, there are key signs you should be on the lookout for in and around your home:

Inside:

  • structural cracks in walls and floors;
  • muddy or cloudy well water;
  • interrupted plumbing or electrical service to a building or neighborhood due to damaged utility lines; and
  • doors and windows that don’t close properly, which may be the result of movement of the building’s foundation.

Outside:

  • previously buried items, such as foundations, fence posts, and trees becoming exposed as the ground sinks;
  • localized subsidence or depression anywhere on the property; in other words, an area that has dropped down relative to the surrounding land;
  • gullies and areas of bare soil, which are formed as soil is carried towards the sinkhole;
  • a circular pattern of ground cracks around the sinking area;
  • localized, gradual ground settling;
  • formation of small ponds, as rainfall accumulates in new areas;
  • slumping or falling trees or fence posts; and
  • sudden ground openings or ground settlement, keeping in mind that sudden earth cracking should be interpreted as a very serious risk of sinkhole or earth collapse.
Actions to Take if You Believe You’ve Detected a Sinkhole:

If you spot any of the signs listed above, or you suspect that you have a sinkhole on or near your property, you should contact your township, public works, or the local engineering firm that represents your municipality right away. If you have discovered a sinkhole that is threatening your house or another structure, be sure to get out immediately to avoid a potentially dangerous situation.

Also, it is highly recommended that:

  • Credit: USGSIf a sinkhole expert can’t get to the area relatively quickly, you ensure that kids and animals keep away, fence/rope-off the area while maintaining a far distance away from the actual sinkhole, keeping in mind that doing so requires extreme caution and is always best left to the experts when possible;
  • Notify your neighbors, local Water Management District, and HOA;
  • Take photos to document the site;
  • Remove trash and debris from around the suspected area in order; and
  • Keep detailed records of all the actions you took.

If you’re trying to determine whether or not you have a sinkhole on your property, there are a few physical tests that can be conducted to determine the best course of action.

In Australia, a courtyard formed a sinkhole. Credit: Earth-Chronicles.comElectro-resistivity testing: This extremely technical test can best be summed up by saying it uses electrodes to determine the conductivity of the soil. Since electricity can’t pass through air, this test shows any pockets where the current didn’t pass through. This is a fairly accurate way to determine if there is a sinkhole and where it is.

Micro-gravity testing: Another incredibly technical method, this test uses sensors that detect the measure of gravity. Since the gravitational pull in a given area should be the same, you can see if there are minute differences in the measurement. If there is a difference, then it’s likely that you have a sinkhole in that area.

If you are still unsure whether or not you live in a sinkhole risk area, you can check with your local, territorial, or national government offices; review geological surveys such as the United States Geological Survey (USGS); and contact an expert.

How a Sinkhole is Repaired?

There are three main techniques experts utilize to repair sinkholes. The type of sinkhole and landowner’s aesthetic preferences determine the methodology used to repair the sinkhole.

The three common methods are:

  1. Inject grout with a drill rig: This uses a piece of large drilling equipment that pierces the ground and goes down into the sinkhole, injecting it with grout/concrete. This method stops the filling of the carbonate crack with sediment since concrete and grout do not break down into such small particles (no piping).
  2. Inverted cone: With this method, the construction crew digs down and finds the bowl-shaped opening. They then open up the surface so that the entire sinkhole area is exposed. To stop the draining of sediment into the crack in the carbonate rock, they fill the hole with bigger rocks first, then gradually fill in the seams with smaller rocks until the sinkhole is plugged.
  3. Filling it with concrete/grout from the surface: This is a combination of the prior two methods. The construction crew opens the surface all the way up so the entire hole is exposed. Then, they bring in a big concrete pourer and fill the sinkhole with concrete.

Missouri Dept of Natural Resources, Inverted cone repair sinkhole mitigation diagram

Our engineers regularly go out in the field to oversee and inspect sinkhole repairs. If you detect a sinkhole, or what might be a sinkhole, on your property, our experts strongly advise immediate actions be taken. Ignoring a sinkhole will only cause it to get larger and more dangerous as time passes, and putting topsoil over a sinkhole will only exacerbate the symptoms.

What Can You Do to Prepare for a Sinkhole?

While there’s really no way to prevent a sinkhole, you can never be too prepared! Here are three easy steps you can take to determine if you live in or around a sinkhole-prone area and what to do in the event of a surprise sinkhole:

  1. Find out whether or not you’re living in one of the sinkhole-prone states, which includes Pennsylvania, Texas, Florida, Alabama, Tennessee, and Missouri. You can do so by visiting USGS.com and searching for Bedrock Geology maps of your area. If your town is underlain by carbonate rocks, you are likely in a sink-hole prone area.
  2. Contact an engineer who’s certified to deal with sinkholes to determine if your property is at-risk.
  3. Develop a plan for what to do in the event of a sinkhole. Do you grab your family, pets, and leave immediately? Do you have a safe zone somewhere near (but not too near) your property? Do you have the appropriate emergency contact numbers in your phone? Does your car have a safety kit? These are some of the things to consider when making your emergency plan.
  4. Speak with your insurance company to see if they have sinkhole coverage, especially if you live in an area where they’re known to occur.

Although scary, sinkholes are a manageable threat if you’re informed and prepared. After all, it is possible to do something about sinkholes – if they can be detected in time.

Special thanks to Princeton Hydro Staff Engineer Stephen Duda, Geologist Marshall Thomas, and Communications Intern Rebecca Burrell for their assistance in developing this blog series.

Revisit Part One of this blog series in which we provide a detailed look at what a sinkhole is, three different types of sinkholes, and what causes them to form:

Don’t Get Sunk: Everything You Need to Know About Sinkholes (Part One)

Don’t Get Sunk: Everything You Need to Know About Sinkholes (Part One)

Photo by Steven Reilly/New Jersey Herald

Sinkholes are a phenomenon that tend to baffle and frighten most people. How is it possible that the ground beneath our feet could just drop? How do we know if we’re nearby a sinkhole? What should we do if we see one? How are sinkholes fixed? The mystery of the unknown around sinkholes can be quite unnerving.

Have no fear, we’ve got answers to all of those questions and more! In this two-part blog series, our experts share their knowledge and provide important information about this scary occurrence. In part one, we provide a detailed look at what a sinkhole is, three different types of sinkholes, and what causes them to form. In part two, we explore how to detect sinkholes and the steps taken to repair them.

What is a Sinkhole?

Sinkholes are a common phenomenon around the world. They result from both man-made and natural causes. Marshall Thomas, a Princeton Hydro geologist, describes sinkholes as “depressions observed from the surface, caused by dissolution of carbonate rocks.” In other words, sinkholes form when the rock below the land surface gets dissolved by water that penetrates the surface and continues to move downward, further into the subsurface.

Most common in areas with “karst terrain,” or types of rocks that can easily be dissolved by groundwater, sinkholes can go undetected for years until the space underneath the surface gets too big or enough of the surface soil is washed away. Sometimes the holes are small, measuring a few feet wide and ten feet deep. Sometimes the holes are hundreds of miles wide and deep. However, all of them can be dangerous.

Sinkholes are found throughout the world. States like Pennsylvania, Texas, Florida, Alabama, Tennessee, and Missouri are at higher risk for sinkholes because they tend to have more soluble rocks like salt beds and domes, gypsum, limestone, and other carbonate rocks. People living in these states are recommended to have professionals look at any property they intend to buy to make sure it isn’t in an area above soluble rock.

Types of Sinkholes

Not all sinkholes are the scary, earth-falling-out-from-underneath-your-feet events. Some occur slowly over time and are very evident from the surface. Geologists classify sinkholes in three major types. Their formation is determined by the same geological processes, barring a few differences. Let’s dive in!

1. Dissolution Sinkholes

Illustration by USGSDissolution sinkholes start to form when limestone or dolomite is very close to the soil surface, usually covered by a thin layer of soil and permeable sand which washes away or is eroded. Rain and stormwater runoff gradually percolate through crevices in the rock, dissolving it. Consequently, a bowl-shaped depression slowly forms.

Sometimes, dissolution sinkholes become ponds when the depression gets lined with debris, which traps water inside. Dissolution sinkholes develop gradually and are normally not dangerous. However, the ones that become ponds can drain abruptly if water breaks through the protective bottom layer.

Fun fact: Most of Florida’s lakes are actually just large sinkholes that filled up with water!

2. Cover-Subsidence Sinkholes

Illustration by USGSThis type of sinkhole, which starts with the dissolution of the underlying carbonate bedrock, occurs where the covering sediment is permeable (water can pass through it) and contains sand. First, small pieces of sediment split into smaller pieces and fall into openings in the carbonate rock underneath the surface. With time, in a process called piping, the small particles settle into the open spaces. This continues, eventually forming a dip in the surface ranging from one inch to several feet in depth and diameter. Again, these aren’t the sinkholes movies are made about.

3. Cover-Collapse Sinkholes

Illustration by USGSThis type of sinkhole is the one making headlines and causing fear. In order for cover-collapse sinkholes to happen, the covering soil has to be cohesive, contain a lot of clay and the bedrock has to be carbonate. Similar to the cover-subsidence sinkholes, the cohesive soil erodes into a cavity in the bedrock. The difference with this is that the clay-filled top surface appears to remain intact from above. However, underneath, a hollowed out, upside down bowl shape forms. That hollowing gets bigger and bigger over time until eventually, the cavity reaches the ground surface, causing the sudden and dramatic collapse of the ground. Just like that, poof, we have a sinkhole that appears to be surprising and abrupt but really has been brewing for many years.

What Causes a Sinkhole?

Sinkholes can be natural or man-made. The most common causes of a sinkhole are changes in groundwater levels or a sudden increase in surface water.

Intensive rain events can increase the likelihood of a sinkhole collapse. Alternatively, drought, which  causes groundwater levels to significantly decrease, can also lead to a greater risk of collapse of the ground above. In a world with a greater variability in rainfall and drought events due to climate change, sinkholes may become a more common occurrence around the world.

Humans are also responsible for the formation of sinkholes. Activities like drilling, mining, construction, broken water or drain pipes, improperly compacted soil after excavation work, or even significantly heavy traffic (heavy weight on soft soil) can result in small to large sinkholes. Water from broken pipes can penetrate through mud and rocks and erode the ground underneath and cause sinkholes.

Most commonly, human-caused sinkholes are the result of:

  • Land-use practices like groundwater pumping, construction, and development
  • Changing of natural water-drainage patterns
  • Development of new water-diversion systems
  • Major land surface changes, causing substantial weight changes

In some cases, human-induced sinkholes occur when an already forming sinkhole is encountered during construction processes such as excavation for stormwater basins and foundations. Dissolution of bedrock generally occurs in geologic time-frames (thousands of years). In these cases, the excavation process has removed the covering soils, decreasing the distance between the top of the void and the ground surface.  

In other cases, voids in the bedrock are generated due to rock removal processes such as hammering and blasting. Hammering and blasting can generate fractures or cracks in the bedrock that soil can then erode into. A void in the bedrock may already exist, however, the process of removing the bedrock by hammering and/or blasting can speed up the meeting of the upside-down bowl and the surface that much quicker. One site where this happened has experienced over 35 sinkholes in 4 years.

Overall, it’s generally not a good idea to pump groundwater or do major excavation in areas that are prone to sinkholes. According to the USGS, over the last 15 years sinkhole damages have cost on average at least $300 million per year. Because there is no national tracking of sinkhole damage costs, this estimate is probably much lower than the actual cost. Being more mindful about the subsurface around us and our actions could help lower the average yearly cost in damages and even save lives.

Photo by Barbara Miller PennLive Patriot News

Stay tuned for Part Two of this blog series in which we explore we explore how to detect sinkholes and the steps taken to repair them! For more information about Princeton Hydro’s Geotechnical Engineering services, go here: http://bit.ly/PHGeotech

Special thanks to Princeton Hydro Staff Engineer Stephen Duda, Geologist Marshall Thomas, and Communications Intern Rebecca Burrell for their assistance in developing this blog series.

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