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Sinkholes

Sinkholes. What are sinkholes? A farmer may view them as naturally forming holes that occasionally open up in the fields. Some people see sinkholes as sites for dumping trash. In urban areas, the sudden appearance of a sinkhole is

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Sinkholes

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  1. Sinkholes What are sinkholes? A farmer may view them as naturally forming holes that occasionally open up in the fields. Some people see sinkholes as sites for dumping trash. In urban areas, the sudden appearance of a sinkhole is a hazard that can disrupt utility services, hamper transportation, and cause severe damage to nearby structures. In anyone’s backyard, a sinkhole is a safety risk to the curious who may find it exciting to explore this new “cave.”

  2. Carbonate Bedrock • Carbonate bedrock includes limestone, dolomite, and marble. • Ocean-dwelling organisms such as corals, clams, and algae use the various elements within seawater to form a hard, rigid skeleton composed of the carbonate mineral calcite. When these organisms die, their skeletons accumulate on the ocean floor as sediment.

  3. What causes sinkholes? • Subsidence occurs naturally due to the physical and chemical weathering of certain types of bedrock. Subsidence can also occur as a result of underground mining, excessive pumping of groundwater, or subsurface erosion due to the failure of existing utility lines. • Carbonate rocks have a shared chemical property; they are basic (pH>7), and can be dissolved by certain acids. • Rainwater has a pH of 4.5-5.5, making it slightly acidic • Combines with CO2 in atmosphere to make carbonic acid • H2O + CO2 H2CO3 (carbonic acid) • CaCO3 + H2CO3 Ca2+ + 2HCO3-

  4. Threshold of sinkhole collapse • Sinkholes and sinkhole-related features in west-central Florida are commonly identified from surface penetration test (SPT) borings, • SPTs are made by repeatedly vertically dropping a 63.5 kg hammer for 76.2cm length until a total penetration of 45cm is reached. The number of blows required to penetrate the last 30cm is called the N-value. • The N-value, or blow count, is related to the density of granular soils or stiffness of cohesive soils. Zones with low N-values are expected in association with raveling into a sinkhole cavity or a dissolution cavity itself. • The odds ratio depends on how the threshold criteria for low N-values are defined, with a maximum observed odds ratio of 2.89. • As the minimum SPT value in the shallow zone (0-4.6 m) ranges from 0 (very loose soil) to 20 (compact soil), the probability of finding a GPR anomaly will decrease by 84% (from 70% to 11%).

  5. Cenotes • Cenotes are a special type of sinkhole filled with water and interconnected • Underwater cenote cave systems can stretch for many kilometers • Mexico’s Yucatán Peninsula is internationally known for its extensive cenotes

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