Picture this: Gigantic holes in the ground swallow up entire buildings, houses, trees, and even people. While such a description sounds like a scene from a disaster or science fiction film, incidents like this actually can and do occur around the world as sinkhole events.
Essentially, a sinkhole is a depressed area in the ground that contains no natural external drainage. Because of the lack of drainage, all of the water that collects within the sinkhole remains and then drains down to the subsurface.
Sinkholes vary in size from 3 feet to 2000 feet wide, and are found throughout the world. In addition to their destructive impacts, they can also lead to the contamination of local water resources .
What causes sinkholes?
Sinkholes occur most commonly in areas known as karst terrain, where there are dissolvable types of rocks below the ground’s surface that can be eroded by both groundwater and rainwater. Such soluble rocks include salt beds and domes, gypsum, and limestone.
When it rains, the rainwater (which is naturally acidic) dissolves the soluble rock and forms spaces and caverns below ground. Over time, the underground spaces continue to grow larger and larger while the land above remains intact. Eventually (such a process can take hundreds or thousands of years under natural conditions as the water slowly dissolves the rock), the spaces get so large that the land above them can no longer be supported. Without this necessary support from below, a collapse of the land surface can occur, sometimes quite suddenly. Thus, a sinkhole is “born”.
Sinkhole collapses occur more frequently after intensive rain events, but they can also occur during times of drought when groundwater levels have significantly decreased, leading to a greater risk of a collapse of the ground above. In a world with a greater variability in rainfall and drought events due to climate change, sinkhole events may increase in number around the world .
Types of sinkholes
Geologists classify sinkholes in three major types: dissolution, cover-subsidence, and cover-collapse sinkholes. But the truth is, we also have artificial sinkholes, caused by different human activities.
Let’s have a detailed look at the most common types of sinkholes and what makes each different.
As the name describes, dissolution – the process in which a solid substance moves into a solvent to make a solution – is the driving factor for this type of sinkhole.
Dissolution sinkholes start to form when limestone or dolomite is very close to the soil surface, usually covered by thin layer of soil and permeable sand which washes away or is eroded. Rainwater then progressively dissolves the exposed limestone and as a result, a bowl-shaped depression gradually forms.
The process of dissolution can be faster in places where water flow is more concentrated in preexisting openings in the rock, such as along joints or fractures .
In the case of a sinkhole suddenly opening up and swallowing a car or an entire house, it formed because the surface soil layer was no longer stable enough to hold itself. This well-known type of sinkhole is known as a collapse or cover-collapse sinkhole and can be the most dramatic and potentially catastrophic.
A cover-collapse sinkhole forms in areas where limestone bedrock is covered by a thin layer of soil with clay sediments. Over time, groundwater and rainwater infiltration into the soil dissolve the limestone and forms a small cavern underneath the surface.
As the cavern gets bigger, the soil layer from the surface will no longer be able to support the weight on top (could be a house, a car, even a person) and it will eventually collapse into the cavern below, creating a large sinkhole .
Cover-subsidence sinkholes occur in areas where unconsolidated material such as sand covers limestone bedrock.
When surface water penetrates the sand, it will gradually dissolve the limestone underneath and eventually leaving a void that will be filled with the sand from the top. This inflow of sand can stop the outflow of water by blocking the fractures and passages that connect the sinkhole to underground water channels. As that water has nowhere to drain anymore, many of these sinkholes end up becoming ponds.
Cover-subsidence sinkholes are usually just a few feet wide and deep. They also don’t create the dramatic kind of cave-in as a cover-collapse sinkhole would do .
In addition to naturally-produced sinkholes, there can also be man-induced sinkholes. Such sinkholes can be caused by old mines, leaky faucets, when sewers and waterlines crack and collapse, during groundwater pumping, drilling, and during construction .
- A broken pipe can lead to sinkhole formation by flooding and weakening the soil, but it can also act as an outlet for the soil that surrounds the pipes.
- Industrial and runoff storage ponds may place too much weight on the subsurface below and can lead to a collapse of the ground beneath.
- When groundwater is pumped in order to provide urban water supplies and for use in irrigation, the balance of the groundwater fluid pressure can be disrupted and the groundwater level can be lowered. Both of these processes can lead to a collapse of the soil above, forming a sinkhole.
- Abandoned mines can cause sinkholes regardless of the rock type, because the underground cavity was created by excavation. When the ceiling of an old mine is unsupported, it will slowly lose its resistance and it could crash all the sudden. However, this process is not that common. Thanks to modern mining technology, an abandoned mine is not left open to later collapse anymore, it’s actually collapsed in a controlled environment to eliminate any risks in the future.
If you are a property owner, it is important that you regularly check your landscape if you live in an area that is vulnerable to sinkholes.
Detecting and dealing with sinkholes
If you are unsure whether or not you live in a sinkhole risk area, you can check with your local, territorial, or national government offices and geological surveys such as the United States Geological Survey (USGS). However, it is not always easy to predict where a sinkhole event will occur.
Scientists are currently working on developing effective methods to find underground cavities that might develop into sinkholes, including the use of radar, seismography, and electrical resistance. Since 10% of the world’s land area is estimated to contain karst topography, the development of such detection methods could go a long way in preventing the catastrophic damages that are associated with sinkholes .
Signs to watch out for in terms of sinkhole development include small holes in the ground, if a structure is found to have cracks in its foundation, slumping trees or fence posts, water collecting in areas where it has not previously collected, or the wilting of small circular areas of vegetation. These warning signs can potentially indicate if there is a sinkhole that is currently forming on your property.
If you have discovered a sinkhole that is threatening a house or another structure, be sure to get out immediately to avoid a potentially dangerous situation. Then, contact your local emergency personnel and a building inspector. If you discover a sinkhole in the middle of a road, be sure to contact local law enforcement right away.
What about insurance and permits?
In most cases, sinkholes that occur on a person’s private property are considered to be their own responsibility. Insurance companies are much more likely to cover an evaluation and a repair if the sinkhole was naturally caused, such as by a subsidence event, rather than if it is caused by people, such as a collapsed or broken sewer pipe and drainpipe, broken septic tanks, buried trash, or improperly compacted soil following construction activities .
Typically, permits are not required to fill a sinkhole on private property (check with your local authorities to be certain). If the sinkhole contains groundwater (and thus, is connected to a local aquifer), a sinkhole fill permit will likely be required by the local Environmental Protection Authority or Water Management District to ensure that proper actions are taken to prevent groundwater contamination.
How to fix a sinkhole
Before you put your hand on that shovel, you need to distinguish a do-it-yourself sinkhole fix from a more challenging problem, which only an expert can handle. If the sinkhole is not affecting a house or other structure, and has a reasonable size — 2 to 5 feet in both diameter and depth — then you can repair it yourself.
A large sinkhole will most likely require excavation and a more complex filling operation.
Here are six steps to repair a sinkhole:
- With a sturdy stick, determine the depth of the sinkhole and whether the bottom and the surrounding ground are stable. Monitor the sinkhole to make sure it does not grow any longer.
- Shovel away the outer edges of the sinkhole if they are unstable.
- Add a layer of dry-mix concrete or a concrete plug to the bottom of the hole to provide a solid base.
- Add clayey sand on top of the concrete. This will help to keep water from leaking out of the hole to prevent the sinkhole from growing larger over time. A local contractor should know where to obtain the clayey sand. If clayey sand is unavailable, pure clean sand works as well. Rocks can also be used to help fill in the hole at this stage.
- Add sand and topsoil on top of the previous layers in order to help things blend in with the surrounding landscape. Pack the soil down with an iron bar or the top of a sledgehammer.
- Water the filled-in sinkhole thoroughly. Watering helps the fill to settle and lets you know whether you will need to add more soil. That’s it!
After few days check the filled-in sinkhole. It is possible that you will need to add additional soil because all layers are well packed by now, but eventually the hole should become stabilized.
But what about the very large… huge… and mammoth-sized sinkholes?
Large sinkholes typically require engineering professionals to properly repair them and to follow the necessary environmental requirements concerning local aquifers.
In some cases, cement and rocks are used to fill the very large sinkholes. However, it is possible that by simply using concrete alone to fill these big holes, the water may actually concentrate in other nearby areas, potentially leading to additional sinkhole formation.
The most preferred way to fix these very large sinkholes is similar to the method that is outlined above to fix smaller sinkholes, called the graded-filter technique, where there are layers of boulders, smaller rocks, and then gravel that are placed into the hole. The advantage of this technique is that it fills the hole while allowing for water drainage.
However, if the sinkholes are caused by older and decaying infrastructure, such as a leaky sewage system in urban areas, more sinkholes are likely to develop until such problems are fixed. This is especially the case if you live in a geographic area that is already vulnerable to sinkholes.
Fortunately, the dramatic cover-collapse type of large sinkholes that instantly swallow up buildings and cars are fairly rare.
Some caution is advised!
Because many sinkholes are connected to aquifers, it is important that you do not fill them with any materials that could potentially be harmful to the groundwater supply, such as trash or chemicals.