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Sinkholes

by Nick Gromicko

What are Sinkholes?

Sinkholes are ground-surface depressions that result when a subterranean void weakens support of the overlying earth.

Why are Sinkholes a Concern for Inspectors?

They threaten water supplies by draining unfiltered water from streams, lakes and wetlands directly into aquifers.
They kill and injure people. A person can be harmed when stepping into an existing sinkhole or when the ground beneath gives way during a sinkhole’s collapse.
Sinkholes can cause structural damage and instability under buildings, roads and bridges. Repairing them after collapse is expensive and requires specialized knowledge. The underlying cause of the sinkhole must be addressed first, or the repair may prove to be only temporary.

Formation

Natural sinkholes are formed when sub-surface rock dissolves to create underground cavities. They are most often found where the rock below the land surface is limestone, dolomite, carbonate rock, salt beds, or rocks that can be naturally dissolved by circulating groundwater. In the U.S., natural sinkholes are most common in Florida where karst (limestone) geography is an ingrained part of the landscape. Other states where natural sinkholes are likely to be found are Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania. Natural sinkholes often form following a period of heavy or prolonged rain. They may also form following a period of drought, which can lower the water table and expose cavities.

Human-induced sinkholes are consequences of land-use practices, especially water-pumping and construction. Other types of human-induced sinkholes result from:
- abandoned septic tanks. Even as communities modernize and switch from septic to sewer systems, the old septic tanks may remain in place. The concrete cover may eventually crack and break down, allowing the earth above to drop suddenly, especially beneath the weight of a person. In one week in 2004, two such incidents occurred in New Jersey and in Texas, claiming the lives of a 2-year-old girl and a 92-year-old woman. Check with the local zoning office, which should know whether the house was built before sewer lines came into the neighborhood, which would indicate the possible presence of an abandoned septic tank;
- decaying, buried organic material, such as tree roots or trash. In 1993, a 7-year-old New Jersey boy fell to his death in his front yard after the ground beneath him gave way. A vein of tree debris, which had been dumped there many years before, had formed air pockets into which the soil gradually seeped, leaving a weakened surface that appeared solid;
- collapsed mines;
- over-pumping existing water supply wells, or drilling additional wells in close proximity, thereby lowering the aquifer; and
- the period following housing development, which adds pressure to the supporting earth.

Sinkhole Warning Signs

Signs that indicate sinkhole formation, especially in regions where they are most likely to occur, should be interpreted by inspectors as a serious safety hazard.

In buildings, look for:

structural cracks in walls and floors;
muddy or cloudy well water; and
doors and windows that don't close properly, which may be the result of movement of the building's foundation.

On the property, check for:

previously buried items, such as foundations, fence posts and trees becoming exposed as the ground sinks;
gullies and areas of bare soil, which are formed as soil is carried towards the sinkhole;
a circular pattern of ground cracks around the sinking area. Sudden earth cracking should be interpreted as a very serious risk of sinkhole or earth collapse. The first sign that a sinkhole was developing in Daisetta, Texas was the opening up of cracks in the ground and in the roadway on the morning of its collapse;
localized, gradual ground settlement;
formation of small ponds, as rainfall accumulates in new areas;
interrupted plumbing or electrical service to a building or neighborhood due to damaged utility lines;
vegetation that wilts and dies as essential water is drawn away by the sinkhole;
slumping or falling trees or fence posts;
sudden ground openings; and
sudden ground settlement.

What to Do or Recommend If a Sinkhole is Observed or Suspected at a Property During an Inspection:

Notify all parties: occupants, owners, real estate agents and buyers.
Notify the local Water Management District.
Fence or rope off the hole.
Keep children away!
Take photographs for documentation, but do not get too close to the edges.
Protect the area from garbage and waste.
Contact the homeowner's insurance company.
Inform the parties that there are engineering firms specializing in the detection and evaluation of potential and evident sinkholes.
Record in your report the actions you took, including notifications and referrals.

Sinkholes and Insurance

Florida law requires insurers to cover “catastrophic ground cover collapse,” which may or may not cover sinkholes, depending on the type and extent of the damage. Law in that state does, however, require insurers to offer optional coverage specifically for sinkholes. Coverage in other states varies.

In summary, sinkholes are rare in most places, but they can be very dangerous where they do occur. Inspectors should learn how sinkholes are formed and how to spot them before they become dangerous, especially in prone areas.

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Sinkholes

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