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Intense Rain Events and Sinkholes

by Jim Olson, P.G.

September 01, 2016

Intense rainfall events that occur over a short period of time can be a triggering mechanism for the development of cover collapse sinkholes in Florida.

 

Due to Florida’s danger of being affected by tropical weather events, Geohazards would like to shed some light on the geologic concerns associated with intense rain events.  The unique geologic landscape of Florida, shaped by millions of years of coastal and oceanic processes, has created a porous, highly variable karst landscape that is particularly prone to sinkholes.  Especially during tropical weather systems, abundant amounts of rainfall, in short periods of time, can trigger the formation of sinkholes and cause erosional problems.  These events can significantly impact your home and property.

Where limestone is observed at the surface down to 100-200 feet, it is common to see surface expressions of karst topography, such as sinkholes.  The geologic map below illustrates areas where soils overlying limestone are thinnest, and thus prone to sinkhole development.  While many cavities exist within Florida’s limestone, and since many are below the water table, they are filled with water.  Under those conditions, cavities are typically stable.  Problems arise when large volumes of water or water-saturated soils overlie an air filled cavity.  Under these conditions, the ceiling of the cavity can collapse, allowing surface materials to flow into the void.  This type of sinkhole, commonly observed during or after heavy rain-producing tropical events like hurricanes, is known as a cover-collapse sinkhole.

 

Extreme caution is advised when approaching newly-formed sinkholes.  The soils directly adjacent to the feature are often highly unstable and additional rapid settlement of these soils should be expected for some time after the initial collapse.  If noises such as pops and creaking are suddenly heard inside one’s home, it can indicate the development of abnormal stresses on the structure and careful monitoring should be taken.  A visual inspection of the soils surrounding the foundation of the home should be conducted as soon as possible to ensure that no obvious sinkholes have formed adjacent to the structure.  Even if a sinkhole is not visible, it does not guarantee that one has not formed beneath the footprint of the house, as was the case of a recent one resulting in a death in Seffner, Florida.

 

Multiple approaches can be taken to remediate sinkholes and there is no single solution that applies for all cases.  Site-specific geologic and environmental conditions dictate which remediation effort should be employed.  Professional sub-surface testing is always prudent in order to assist in developing a plan to safely remediate sinkholes.

 

Fla Geo 1

 

Image from:  http://sofia.usgs.gov/publications/maps/florida_geology/Florida_geology-med.jpg

Figure illustrates the shallow limestone surface area of Florida with the greatest probability for cover collapse or cover subsidence sinkhole formation.

 

 

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