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Sinkholes |
by Norman Aceron Garcia
A sinkhole is a ground surface crater that develops when an underground void reduces support of the overlying soil. Sinkholes can cause structural damage and weakness under roads, buildings, and bridges. Fixing them after failure is costly and calls for specialized skills. A pedestrian can be injured or killed when walking into a sinkhole or when the ground underneath it collapses. Sinkholes can also affect water supplies through draining water from lakes, streams, and wetlands directly into aquifers. The primary cause of the sinkhole must be fixed first, or else the effort may only be temporary.
There are two types of sinkholes based on how they are formed: natural sinkholes and human-induced sinkholes. Natural sinkholes develop when subterranean rock liquefies to form sub-surface voids. They are usually formed where circulating groundwater naturally dissolves the rock beneath, such as salt beds, carbonate rock, dolomite, and limestone. Sinkholes may also form after a drought, when the water table elevation goes down resulting in the exposure of an underground cavity.
Human-induced sinkholes are repercussions of land-use practices like construction and water pumping. When communities switch from septic systems to sewer systems, some of the old septic tanks are simply abandonned. Eventually, the concrete cover will deteriorate and permit the soil above to collapse rapidly, even with the just the weight of a person. If the house was constructed before sewer lines were introduced to the community, chances are there is an abandoned septic tank within its yard.
Decaying and buried organic materials such as compost and tree roots can also contribute to sinkholes. Over a long period of time, materials that have been dumped can form air pockets into which the soil steadily seeped, leaving an undermined surface that looks solid. The level of the aquifer goes down when there is drilling of additional wells that are too close to existing wells, or when there is over-pumping of current water supply wells.
If you observe or suspect signs of sinkhole formations:
• Barricade or fence off the hole, keep children away, and protect the area from waste and garbage;
• Notify the Water and Waste Department of the City of Winnipeg;
• Contact your insurance company;
• Document the incident by taking notes and photographs, but avoid getting to close to the edges. Record in your statement the actions you did, including communications and referrals; and
• Consult engineering firms specializing in the detection and evaluation of potential and evident sinkholes.
In summary, sinkholes are uncommon in most areas, but they can be very hazardous where they do happen. We should at least be knowledgeable about how sinkholes develop and how to identify them before they become unsafe, especially in high-risk locations.
Norman Aceron Garcia is a Professional Engineer of Mr. Peg Property Inspections Inc. Please email norm@mrpeg.ca for free technical consultation.