Sinkhole repair

How to fix a sinkhole?

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Sinkholes are a fact of life in many parts of Florida. Whilst professional assessment of an area’s geology and history can estimate the risk of a sinkhole forming, there are no guarantees.

Given this, what options for remediation are there if a sinkhole does occur? Do they carry on growing indefinitely? Can they be filled in?

Can you fix a sinkhole?

The good news is that sinkholes can often be repaired, although it is a bit more complex than just filling them in.

The aim in any sinkhole repair is to reduce the danger of further collapse, support any surface structures, and avoid promoting new sinkhole formation nearby. The process to achieve this is designed on a case by case basis and generally requires professional advice, as every sinkhole has a unique set of conditions including shape, depth, surrounding geology, connection to groundwater storage, and safety requirements on the surface.

In remote areas where the sinkhole is not endangering anything on the surface, the decision is sometimes taken to fence off the area and leave the hole open. As a sinkhole is a natural geological feature relating to water flow, by interfering with it, drainage pathways can be altered, potentially increasing the likelihood of sinkholes forming elsewhere.

However, in many cases, leaving a sinkhole open is not safe or practical. As sinkholes can link directly into groundwater, it may also be necessary to protect underground water stores from pollution – especially if people start using it as a place to dump their trash, or unwanted agricultural materials. To repair a sinkhole, engineers have a number of options available.

Safety is paramount with sinkholes, so after the discovery of a hole, the area is usually fenced off to keep people away, and properties affected may be evacuated (the Florida Department of Environmental Protection has advice on what to do if you find a new sinkhole).

Once a sinkhole is safe, it can be professionally assessed for size and stability, along with the likely cause of the collapse and the surrounding geology. Small shallow stable holes are sometimes simply filled in with clean sand and soil.

Larger holes that are not underneath a house or other structure on the surface are often excavated down to the bedrock at the base and sides to remove any material that could collapse further. They can then be filled, frequently using a graded fill working from larger rocks at the bottom, up to soil at the top. By using a fill made of separate pieces of rock, rather than a solid mass like concrete, natural drainage can continue.

If the sinkhole is under a house, or other structure, then assessment and repair is more complicated. First the depth and spread of the sinkhole is assessed using remote sensing and geophysical techniques. Then a case-specific plan is developed by specialist engineers. As excavation is not usually practical in these circumstances, fluid grout based on either special chemicals or cement is often used as the fill instead. In many cases this grout will be injected into the sinkhole under pressure, so that it fills not just the main hole, but also penetrates into other cracks and linked holes in the bedrock, and through the overburden and soil to create a solid, stable mass. Sometimes injection of the grout can show that one sinkhole is linked to a much larger underground network, as the liquid cement disappears down into the sinkhole drain.

Sinkholes are often directly connected to groundwater, which in turn is connected to local rivers and wetlands, as well as being used for drinking water supply. It is therefore important that they are fixed in a way that minimizes environmental damage. In Florida an Environmental Resources Permit may be required before filling a sinkhole, depending on its location and nature – the Department of Environmental Protection provides advice on this. It is vital that sinkholes are only filled with clean approved materials, and not trash, or anything that might cause pollution.

Can sinkholes be prevented?

Sinkholes are a natural part of the landscape in any region underlain by limestone or evaporites. However, some activities can cause sinkholes to form or collapse more quickly.

One of these is pumping too much water out from underground. Many of the underground voids that become sinkholes are normally filled with water, and the pressure from this keeps them stable and intact. If the level of groundwater drops, either due to a drought, or increased extraction for use, then the void can become unstable and collapse. Furthermore, removing water from one place can change the composition and flow of groundwater in another, making sinkholes more likely. A famous and spectacular example of this is the Dead Sea in the Middle East, where the salty waters of the sea are receding at up to 1 m a year, due to water extraction from the River Jordan. As the salty waters recede, fresh water is pulled into cracks in the rocks, where it dissolves new and larger holes.

Another cause of sinkhole destabilization is changing the way water flows or drains on the surface. If water from a wide area is diverted into a much smaller area, then this can increase the drainage into the rocks at a particular point and speed up sinkhole formation. Water is also surprisingly heavy, so the creation of new ponds on the surface, or really heavy irrigation can cause a sinkhole to suddenly collapse. Excavation for construction projects, and development of new buildings can also increase the risk of sinkhole development, both by changing drainage, and by increasing the weight on the ground surface.

It can be seen then that whilst sinkholes can’t be prevented, there are factors that governments, companies and communities can take into account to reduce the risk. This is one of the reasons that fixing a sinkhole needs careful professional assessment and planning – the aim is not just to fill the current hole, but to reduce the risk of further problems later.

Guest blogger Dr Alison Blyth is an earth scientist with a particular interest in karst.

The content in this blog is an overview provided for general information purposes only, and does not constitute professional advice. The author does not accept any liability arising from the use of the information provided here or from the omission of information.  

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