You may remember the story of a sizeable sinkhole forming on Route 2 in Newark and causing significant traffic delays. DelDOT confirmed an old sewer line that failed as the cause. Maybe you remember reading about the sinkhole in Guatemala City that sank a three-story building in 2010. Whether sinkholes are manmade or naturally occurring, they are equally terrifying. So what causes these voids to erupt and swallow the surface and everything resting on it? The answer is almost always – Water. The answer is water 90% of the time.

[Guatemala City Sinkhole: Photograph: Luis Echeverria/AP]

Natural Sinkhole Formation

Natural sinkholes form when rainwater penetrates topsoil and reaches the bedrock below the surface. Bedrock is sand, chalk, salt, gypsum, limestone, carbonate rock, etc. The type of bedrock you have depends on where you live. Imagine if you had a lizard tank. Some lizards live in moist climates like Brazilian forests. Some lizards live in the desert. The two types of layered terrain in the lizard tanks would be completely different due to the environments they need to survive.  

If water travels through the layers, the layers start to get ‘soggy’. What happens when you have wet sand and place a car on top of it? This is an oversimplification, but it makes the process that takes hundreds or thousands of years easier to understand. The weight of buildings, traffic, and general organic layer sinks into the soggy topsoil and soon, it collapses under the pressure and creates a hole or a void that swallows everything above it. Not all rock types are made equal; some succumb to water erosion sooner than others. Various ways encourage sinkholes to open up.

Weathering & Weather Changes

Recall our conversation that all rocks are not created equal. If the bedrock is limestone or gypsum, water can dissolve the material. The water leaves spaces and cracks in the rock as it continues to travel through, causing chasms. These types of rocks are common in:

• Texas
• Alabama
• Missouri
• Kentucky
• Tennessee; and
• Pennsylvania

There is also a type of erosion that does not include water dissolving the rock material. Instead, the currents of water passing through start chipping at the rock and carrying pebbles away. This type of erosion caused the building to be swallowed in Guatemala City. The bedrock was comprised of crumbling volcanic rock and ash.

Season changes also wreak havoc on the surface with freezing and thawing or heavy rains and droughts. As a result, weather and the groundwater table have a tense relationship.

Manmade Sinkhole Formation

Manmade sinkhole formation occurs with several different types of human activity. City development and infrastructure can compromise the bedrock’s integrity. Water can pool beneath the surface, such as when pipes leak.

Overloaded Surface

To be fair, this is a combination of human activity and weather. If there are torrential rain storms, a flood, or a burst pipe, the water will not follow the path it usually does. Instead, there is a burst of energy, like a dam breaking, and the water seeks out new ways that can have immediate consequences if there is insufficient rock support. If the water pools in one location and does not drain, it will seep into the ground, causing a sinkhole. Changes made to the surface such as mining, wells, drilling, excavations, or fracking impact the stability of the earth around the area and water quality; double whammy.

Aging or Damaged Infrastructure

Aged pipes are one of the most common reasons for sinkhole formation, like the Newark sinkhole quoted above. Sinkholes can also occur if repairs or excavations are not completed correctly beneath the surface. Burst pipes or incorrectly installed pipes can have significant consequences and water contamination.  

Mariner Pipeline

Throughout Pennsylvania, the Mariner pipeline contaminated hundreds of miles of water sources leading to criminal charges and Energy Transfer Operating paying for testing and remediating affected homeowners. In addition, several sinkholes developed, creating dangerous conditions throughout the state. As a result, the Pennslyvania Attorney General went to work constructing a case to prosecute and restore watersheds and streams that resulted in millions of dollars in damage.

[courtesy of Civil Engineering, Dong-A University, Busan]

Antiquated Practices: Debris Pits

An old practice banned in 1968 instructed developers to pile tree stumps and burn them. Once the practice had been prohibited, developers turned to burying the stumps. This practice wasn’t banned until 1988. There was a lot of land cleared during the industrial revolution and through the 1980s. Unfortunately, what was done is done and several dangerous sinkholes are a result of the buried stumps.  

These debris pits produce methane because of the decaying wood; some are narrow for easy entrapment. In addition, the debris pits can damage the foundation of structures built over them. The threat has been acknowledged and in response, Delaware.gov established a fund to assist in curing old debris pits. They have also posted signs to look for if a sinkhole may be settling in the vicinity, such as cracks, distressed vegetation, or small holes forming and having the potential to become much bigger. They do caution that:

“Sinkholes can also be formed by improperly channeled storm water or by abandoned septic tanks that have degenerated.”

Can Sinkholes Form Anywhere?

Sinkholes can form on land or underwater, such as in ponds or oceans. In areas with a condensed population, a sinkhole can (and has) cause loss of life and extensive damage. Like earthquakes, sinkholes can topple buildings, pollute water by bursting the web of pipes below and change the land’s topography. 

University of Delaware Geological Survey

The University of Delaware reported that most karst areas are confined to Piedmont and Hockessin Valley. The site features sinkholes, caves, and a hydrologic system supporting water wells. Interestingly, sinkholes didn’t exist in Delaware before 1978 and are believed to have emerged from significant weather conditions. According to the report, more emerging sinkholes have been identified in the area.

How Can Sinkholes Be Prevented?

Manmade sinkholes can be prevented through the maintenance of underground systems. Another way to avoid sinkholes is utilizing knowledgeable professionals committed to responsible practices when installing and working on infrastructure or anything underground including pumping and irrigation systems.

Recall that 90% of sinkholes are caused by water. Therefore, the best prevention is a competent drainage system that funnels water away from weak areas. One way to accomplish this is lining pipes and scheduled, timely infrastructure maintenance. Trenchless practices are ideal for minimizing intrusion.  

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