The Science of Sinkholes: Formations, Types and Global Hotspots

Suddenly, the ground under your feet starts to sink into a vast, deep crater that can swallow up your vehicle, the highways, and buildings in mere seconds. It is not a scene from a science fiction film; it's the actual geological occurrence of a sinkhole.

Some sinkholes develop over several hundred years, but others can form suddenly and without warning. It is critical to modern urban planning and safety to understand the geographical zones at risk, the types of architecture likely to suffer sinkholes, and the conditions that can precipitate their appearance.

What is a Sinkhole?

A sinkhole, also known as a shake-hole, swallow-hole, or doline, is a hole or depression in the ground caused by the collapse of the upper layers of the earth. They are found mostly in landscapes with karst topography, where the underlying bedrock is composed of rocks that can dissolve in water, such as limestone, dolomite, gypsum, and salt.

Acidic rainwater percolates into the ground, over millennia, eating away the soluble bedrock to form vast underground caves. Once the roof in one of these concealed caves becomes too thin to withstand the weight of soil and buildings above, the ground collapses. Sinkholes are a common occurrence, but can form rapidly and catastrophically due to human activity such as heavy groundwater pumping, leaky underground water mains, and improper construction drainage.

The Three Basic Classes of Sinkholes

Sinkholes can be categorized according to the manner in which the mechanical failure takes place:

1. Cover-Collapse Sinkholes

They are the most serious and dramatic of sinkholes. They occur when the earth above, in the form of a cohesive soil (clay), is thick enough to cause a cavern to develop below the surface. As time passes, sediment comes in from below and forms a void in the cave in the shape of a hollow dome that progresses upward. The surface ground looks completely solid until the remaining roof is very thin and suddenly collapses without warning.

2. Cover-Subsidence Sinkholes

These voids are created over time in permeable, non-cohesive surface materials, such as sand. Groundwater slowly dissolves the underlying limestone, and the sand grains slowly fill the cracks in place of the lost limestone. This forms a surface depression that is noticeable over a few months or years, usually without any structural catastrophes.

3. Solution (Dissolution) Sinkholes

Solution sinkholes form when the water-soluble bedrock is exposed to the surface or is thinly covered with soil. Rainfall and surface runoff directly accumulate in the natural fractures and erode the rock from the top to the bottom. This creates a depression that spreads out and will settle into a slow pond, sometimes forming a small, natural pond.

Where are Sinkholes Found?

While sinkholes can occur in any place where karst formations occur, there are locations on Earth that are particularly affected by these underground voids:

• China: Home to the world's largest and deepest natural sinkhole, the Xiaozhai Tiankeng (also referred to as the "Heavenly Pit"), located in Chongqing, plunging over 600 meters deep, housing its own isolated ecosystem at the bottom.
• United States: About 20% of the landmass is very prone to sinkholes. Florida, Texas, Alabama, Missouri, and Kentucky are among the states that are built on highly karstified platforms.
• Central America and Mexico: The Yucatán Peninsula is renowned for its cenotes, which are water-filled sinkholes. Furthermore, the Taam Ja' Blue Hole in Chetumal Bay has been charted by sea-going explorers to be a record-breaking underwater sinkhole that goes a thousand feet deep.
• Other Vulnerable Zones: Human-induced sinking of piping holes in unconsolidated volcanic ash is a common issue in Guatemala City, and in parts of Japan, Croatia, and the Khammouan mountains, there are massive karst structures.

Underground Detection Methods: How Geologists and Engineers Find Subterranean Voids

Most people assume that when sinkholes open up, they happen all at once. But they don’t. There is a lot of degradation happening below the surface long before a sinkhole event actually manifests on the surface. Today, most geophysical and geotechnical engineers use subterranean imaging techniques to find sinkholes rather than fix them after they occur.

Geophysical Imaging Techniques

• Electrical Resistivity Tomography (ERT): Electrical resistivity tomography involves passing an electrical current through the ground and measuring its resistance. Since voids (whether filled with air or water) will have different electrical resistance characteristics than the surrounding rock or soil, ERT allows geologists to build detailed two- and three-dimensional images of subterranean features such as Karst features.
• Ground-Penetrating Radar (GPR): GPR transmits high-frequency radio waves into the ground and measures the reflected signal. GPR works particularly well for shallow (less than 30 feet deep) surveys to detect voids and cavities in roads, airports, buildings, etc.
• Microgravity Surveys: Earth has a gravity field that varies locally based on the density of the soil above it. If there is a cavity or void in the subsurface, the gravitational force measured at the surface will be reduced.

Early Warning Signs of a Developing Sinkhole

Several surface indicators may be noted before a significant collapse occurs, and these can be observed by a homeowner and/or a municipal engineer, but are not visible from a distance:

• Structural Anomalies: New, deep cracks in interior walls, tiling, concrete foundation, or outdoor sidewalks.
• Surface Depressions: Subtle, circular tilting or slumping in lawns, fence posts, or backyard trees.
• Hydrological Changes: Abnormally large collections of water into small rainwater ponds in places where water never pooled before or abnormally dry soils in localized areas where water is draining into new underground crevices.
• Infrastructure Stress: Sudden changes in the water utility line pressure or uncovered underground pipes due to movement of the underlying earth.

Insurance, Remediation and What to do During Ground Collapse

The other half of the equation is understanding what happens after the fact, both structurally and financially. 

How to Fix the Problem 

Once a void has been discovered in the proximity of a structure, there are three primary ways to deal with it, according to civil engineering professionals. 

1. Compaction grouting: A thick paste of cement, sand, water, and sometimes additives is pumped at high pressure into the space between the surface and bedrock. It acts to compact the area and plug the top of the cavity. 
2. Cap grouting: Usually done for solution sinkholes that are fairly close to the surface, engineers dig a hole straight down to the bedrock, and then fill it with concrete, cover it back up with clean gravel, and then dirt. 
3. Underpinning (steel pinning): Steel pins are placed in the ground all the way down to the bedrock below the property. They become the support for the house (since they’re anchored into the bedrock) instead of the surrounding soil. 

What to do if you suspect your property may be affected by a sinkhole? First of all, if there’s a large crack in your foundation, or the doors in your home start to stick, you should probably evacuate immediately. Call local emergency services, as well as the local water department (a leaking pipe will make the issue worse). 

What Type of Insurance Do I Need in the Sinkhole-Prone Area? 

In many places around the world, homeowners insurance doesn’t cover land subsidence. In some parts of the US, however, such as Florida, state law requires insurance companies to offer catastrophic ground-collapse coverage as part of a homeowner policy. To be eligible, the damage must have rendered the main dwelling uninhabitable (meaning you have to have lost a bit of your structure), and it had to happen suddenly. It also needs to have been declared uninhabitable by local government. 

However, if you do live in a sinkhole-prone area, it might be worth purchasing a separate sinkhole endorsement. This would allow you to fix any problems before the house collapsed.

Conclusion: Balancing Progress and Preservation on Unstable Ground

Sinkholes serve as a sobering testament to the ever-evolving nature of the ground we traverse, be it the natural weathering induced by acidic rain over eons or the accelerated rate caused by modern-day urban growth and aquifer extraction. However, as geologists and civil engineers use advanced radar from space, soil scanning technology, and contemporary groundwater management, we may be able to detect subterranean caves far in advance of surface failures.

With vigilance to subtle signals of impending environmental instability and thoughtful construction practices adapted to karst regions, we can confidently grow our urban centers without compromising these subterranean environments.

Also Read: Into the Heart of Hang Son Doong: Exploring the World's Largest Cave