What is a Sinkhole?
A sinkhole is a natural hole that forms in the Earth's surface as a result of the chemical weathering of carbonate rocks like limestone, as well as salt beds or rocks that can be severely weathered as water runs through them. The type of landscape made up of these rocks is known as karst topography and is dominated by sinkholes, internal drainage, and caves.
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Sinkholes are dramatic because the land usually stays intact for a period of time until the underground spaces just get too big. If there is not enough support for the land above the spaces, then a sudden collapse of the land surface can occur.
A sinkhole is a naturally occurring geological hazard that can worsen rapidly and cause devastating damage. It can cause extensive damage to roads and structures resulting in costly repairs. It can also compromise water supplies by draining unfiltered water from wetlands, lakes and streams directly into the underground water supply.

Sinkhole Formation:
The main causes of sinkholes are weathering and erosion. This happens through the gradual dissolve and removal of water absorbing rock like limestone as percolating water from the Earth's surface moves through it. As the rock is removed, caves and open spaces develop underground. Once these open spaces become too large to support the weight of the land above them, the surface soil collapses, creating a sinkhole.
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Naturally occurring sinkholes are most common in limestone rock and salt beds that are easily dissolved by moving water. Sinkholes are also not normally visible from the surface as the processes that cause them are underground but sometimes, however, extremely large sinkholes have been known to have streams or rivers flowing through them. 

Sinkhole Prone Areas:
​They mostly occur in karst terrain. This is a landscape formed from the breakup of soluble rocks, including limestone, gypsum and dolomite. They form when a fracture in the sub-surface rock enlarges, which can increase the risk of groundwater contamination. Twenty percent of the United States’ land surface is karst. One of the most famous karst areas in the country is Kentucky.
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Recognizing a Sinkhole:
Some of the warning signs that can signify that there is a sinkhole include structural cracks in floors and walls, windows and doors that do not close properly and cloudy or muddy well water. Signs of a sinkhole on the property include exposure of previously buried items, such as fence posts and foundations, vegetation that wilts and dies as the sinkhole draws away essential water, and a circular pattern of ground cracks.
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Other signs include the formation of small ponds after heavy rainfall, falling or slumping fence posts or trees, interrupted electrical service or plumbing due to damaged utility lines, and gullies that form as soil is carried towards the sinkhole.
​Many natural sinkholes cannot be prevented. However, those caused by human activity may be avoided, especially those caused by over-pumping groundwater. During dry conditions, water tables drop in the limestone and cavities under Florida’s sand and clay soil. The combination of gravity, loss of buoyancy and water pressure can activate a collapse.

​​Sinkholes can be triggered by human activities such as:

• Over withdrawal of groundwater
• Diverting surface water from a large area and concentrating it in a single point
• Artificially creating ponds of surface water
• Drilling new water wells

Humans can also cause sinkholes to develop by changing water drainage patterns through diversion and industrial water storage ponds. In each of these instances, the weight of the Earth's surface is changed with the addition of the water. In some cases, the supporting material under the new storage pond, for example, may collapse and create a sinkhole. Broken underground sewer and water pipes have also been known to cause sinkholes when the introduction of free-flowing water into otherwise dry ground weakens soil stability. 

Sinkhole Repair Methods:
Sinkholes can result in loss of real property damage to transportation infrastructures and cause ground water contamination. As sinkholes and other ground movement issues can result in denied insurance claims and costly repairs, some property owners have resorted to find alternatives ways of dealing with sinkhole issues.
As there are different types of sinkholes, repair may be possible; however, treating these geological nightmares will vary, because repair will depend on how extensive the problem is. Some of the repair methods are enlisted below.
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Compaction Grouting:
Florida ground is full of limestone which is highly soluble in water so the acidic water can easily make its way through it. Mostly rainwater is slightly acidic and becomes more acidic as it moves through decaying plant debris. This acidic water becomes a key part in the erosion processes in Florida’s limestone. With enough erosion, the ground above will start to sink. The stiff mortar-like grout used in compaction grouting will mix with the soil and firm up the ground around the foundation.
Grouting involves the use of concrete or other chemical substances to fill subsurface defects. If expertly done, these solutions for sinkholes will seal off breaks in the continuity of the underlying bedrocks and restore stability to the affected areas.
Compaction grouting is the only proven method of repairing a sinkhole and stabilizing the soil. The process involves drilling holes in the ground to access the underground cavities. Casings are then inserted, and a grouting compound is injected into the voids, to fill voids and stabilize the soil.
Compaction grouting can increase capacity of soils under slabs and spread footings, stop foundation settlement, level structures, and control settlement over sinkholes.
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​Underpinning:
Underpinning is a very common foundation repair method that utilizes piers to lift and stabilize settling foundations. It can be used to repair nearly every type of foundation issue, including cracks, uneven floors, sticking windows or doors, and foundation leaks. Steel Push Piers, Helical Piers and Drilled Piers are effective for varying soil types and depths to stabilize and support foundations, especially important if your foundation has been damaged by weak soils underneath your home.  They can be installed down to variable depths as required to transfer a foundation’s weight to the load bearing stratum or bedrock. They can also be installed under existing foundations in difficult locations with minimal disruption.
underpinning is required when the original foundation is not strong enough to support the house. This is usually a result of:

• the soil supporting the foundation has changed in some way e.g. through subsidence, expansion/contraction due to moisture, large trees nearby, damaged plumbing left unrepaired.
• the properties of the soil were not adequately understood during the original design of the foundation - meaning the foundation is not adequate for the conditions.

• The soil beneath the structure has shifted or run off with heavy rains or sinkhole activity.

In less common cases, underpinning is also required for the following reasons:

• The way the structure is used has changed e.g. following a major renovation
• New construction nearby resulting in the excavation of soil supporting existing foundations
• To increase the capability of existing foundations e.g. to support another storey to the building
• Natural disasters such as earthquakes, floods or droughts that have caused the structure to move or become unstable.

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​Geopolymer Injection:
Geopolymer injection is the newest form of ground stabilisation, offering an innovative alternative to more traditional methods such as underpinning. A geopolymer resin is injected into the ground, where it expands, following the path of least resistance, until all voids are filled and soils are compacted. Once expanded, the resin quickly hardens, creating a stable and durable subsurface on which to build.
Geopolymer injection is a popular method as it is quick to carry out compared to other methods and is non-disruptive, requiring only a few small holes for injection to be created in the ground. 
It is also suitable for projects with multiple varying issues, such as different soil types and voids, as it is easily able to combat all of these issues at once. If the site is near a water source, this method also helps to avoid future water ingress that could lead to washout. Another advantage of geopolymer resin injection is that it does not affect the groundwater at the site and causes no environmental threat