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Nationwide Insurance.- Center for Radiation Oncology, 7315 Green Slope Drive, Zephyrhills, FL Lab No: 1 90442 Page 14 The third type of sinkhole is called a subsidence sinkhole, which can then be subdivided into cover collapse and cover subsidence sinkholes. Subsidence implies that the overlying soils subside into pre-existing fractures and cavities in the bedrock. Cover subsidence sinkholes are generally slow developing and can take weeks or months for small amounts of surficial displacement to occur. Cover collapse sinkholes are much more rapid in terms of the rate of surface subsidence. This generally occurs where the bedrock is overlain by a thick sequence of cohesive soils. The cohesive soils overlying a void in the bedrock will gradually ravel or spall into the bedrock cavity and creating a cavity in the soils. The cavity will then collapse when the overlying soils become too weak to bridge the cavity. The overlying non-cohesive soils will then fill the cavity and a surface soil displacement is seen. The fourth type of sinkhole is known as a buried sinkhole. This is a sinkhole that has formed at sometime in the geologic past and has been infilled with younger sediments. This type can be an engineering hazard because it shows no surficial expression and is subject to re-activation. These buried sinkholes can be in stable or unstable condition. They may be termed as unstable if they are filled with very soft or loose sediments and changes in the hydrologic system may cause re- activation. They may also be in a completely stable condition and be filled with firm or dense sediments that are often more stable than the surrounding area. Determining that damage to a structure is the result of sinkhole activity is often a complex problem. Generally, the obvious open hole left by a "drop out" of soils is not seen. To determine that damage to a structure is the result of a cover subsidence type sinkhole, it must be shown with boring data that there is a very soft or loose strata (raveling zone) in contact with the limestone bedrock that would indicate some type of erosion, downward migration of soils and subsidence has taken place. To infer that the structural damage to a building had occurred as a result of this subsidence, the raveling zone should extend from the bedrock to or near the ground surface. If the raveling zone does not extend all the way to the surface it must then be determined if the overlying strata would "bridge" the raveling zone and support the structure. The presence of soft or loose zones encountered in a boring, or the loss of drill fluid circulation does not in itself prove that sinkhole activity is taking place. Many times drill fluid circulation will be lost at the interface between differing strata, which is not necessarily an indicator of a void. Soft or loose zones are a common occurrence in bore holes, to say that they are the result of sinkhole activity it must be shown that the soft or loose zone is in contact with the bedrock and extends upward. Damage to structures is often multivariate problems. Cover subsidence activity is many times combined with damage from shrink/swell clays, peat or muck. A comprehensive settlement investigation can determine the cause or causes of subsidence to a building. Compliance with Florida Statutes Section 627.707 In compliance with the above stated Florida Statute, this report was prepared Central Florida Testing Laboratories, Inc. 12625 - 40th Street North, Clearwater, FL 33762