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Sinkholes in Florida - Florida sinkhole house buyer.
24. Sinkhole Activity? - How Florida Sinkholes are formed.
Florida sinkholes occur due to the nature of the limestone bedrock, which is primarily calcium carbonate and is subject to very gradual dissolution in the ground water.
This process originates as rain dissolves carbon dioxide from the air, forming a very weak carbonic acid. As rainwater infiltrates into the ground, it also may pick up organic acids and additional carbon dioxide. The calcium carbonate bedrock dissolves over long periods of time as it comes into contact with the very weakly acidic infiltrating water. because this reaction is fairly rapid, most of the corrosion/dissolution takes place at the surface of the limestone. Accordingly, loose/soft conditions are often found at the soil-limestone interface. The infiltrating water may preferential paths toward fractures or porous zones in the limestone, creating more dissolution localized areas.
In addition, portions of the limestone may be more susceptible to weathering processes than other due to composition and physical variations, which may take place on a regional as well as local scale. Further, it should be noted that the limestone in some areas has been exposed and subject to weathering processes in the geologic past, resulting in an irregular surface with holes, depressions and pinnacles that may very tens of feet or more over relatively short distances. Low spots in the limestone surface may contain soft sediments that never consolidated due to bridging of the overlying soils across the low spot. Accordingly, soft zones at the soil-limestone interface and variations to the depth of the limestone are often evident in soil borings. Some geologists refer to this condition as epi-karst.
Different types of Florida sinkholes are thought to occur. One type is a buried ancient or relic Florida sinkhole, which is a sinkhole that occurred in the geologic past and has been filled with sediment that often is organic stained or organic. A relic sinkhole may be stable or may show indications of re-activation.
Another type of Florida sinkhole is more dramatic. It involves formation of a sudden circular depression at the ground surface as the result of rapid erosion of soil into a void or cavity in the limestone bedrock or collapse of the roof of a cavern near the surface of the rock. These collapse events are often triggered in periods of heavy rainfall following drought conditions, and are due to a reduction in buoyancy support/increase in overburden soil weight due to low ground water table, weight added due to wet soil above, and an increase in infiltration into the limestone, which can tend to erode the soils into voids. Similarly, installation and/or operation of a well can draw down the ground water and initiate groundwater flow, also triggering a collapse sinkhole. In a third type of sinkhole affect, gradual subsidence of the surface may occur as the soil follows dissolution of the limestone surface or gradually slumps or erodes into a void.
The very gradual, sinkhole-related subsidence process can result in slowly appearing settlement distress in an overlying structure. However, many non-sinkhole related subsurface conditions (compressible organic deposits, for example) as well as structural conditions and construction material behavior can create similar distress. In fact, multiple causes that may have contributed to development of the observed distress in the structure often are identified.
The soil conditions capable of resulting in differential movement of the structure and material behavior are often more readily identifiable and their likelihood of causing distress more directly evident or quantifiable than the presence of sinkhole activity. (It should be noted that the statutes dealing with insurance claims refer to damage caused by 'sinkhole activity' rather than a sinkhole, which has, in our experience, further enabled claims to be made when minor settlement has occurred) The contribution of non-sinkhole related conditions to the appearance of the distress can obscure any affects of the sinkhole activity, If any and makes the determination of whether any subsidence-related settlement distress has occurred as the result of sinkhole activity difficult to discern, should indicators of sinkhole activity be present in the boring, especially given the legal context of sinkhole activity.
The potential existence of active or incipient sinkhole activity is usually inferred from SPT boring data by the presence of very weak, possibly disturbed (Apparently) reveled) soils extending upward from the surface limestone bedrock. This condition is indicated when the SPT sample advances under the weight of the drill rods or sampling hammer, and by the often random consistency or composition of the soil that is returned in the sampler. In some cases, residuum remaining from the corrosion of the limestone is mixed with the overlying sediments that might have migrated downward into the corroded zone. The very soft/very loose soils are often combined with the loss drilling fluid circulation in soils that would normally retain the drilling mud. These very soft/very loose conditions are more common in some areas (western Pasco and Hernando Counties Florida) due to the geologic conditions, and also occur in varying degrees of severity or vertical extent.
It has been argued that some of the very loose/very soft conditions are due to a residuum or rubble zone remaining from erosion/weathering of the limestone surface when it was exposed in the geologic past. In addition, portions of very soft conditions at the soil-limestone interface may be residuum from active weathering processes as the shallow groundwater infiltrates the limestone, as discussed above. However, the very soft/very loose zones may also indicate soils that might have migrated downward into voids in the limestone raveling, and the more highly weathered area may eventually develop over geologic time into sinkholes. The very soft zones are more clearly indicative of sinkhole activity that is capable of affecting a structure when they extend significantly upward from the suspected (or in some cases confirmed) void-laden limestone bedrock toward the ground surface, and thereby would directly affect the shallow sub grade soils supporting the home. Often, however, there is no detectable disturbance or loosening of the soils directly beneath the home, and the upper soils in borings with possible sinkhole activity are similar to those boring where no evidence of sinkhole activity is intercepted.
Accordingly, where very loose/very soft areas of minor to moderate vertical extent are found over the limestone without strong evidence of upward progression of the weak soil zone (commonly called raveling) , and building settlement distress is evident, it is difficult to assess the contribution of the very loose/very soft zones to the distress, especially when other soil conditions that are capable of causing settlement are present and the soils overlying the very soft zone appear unaffected and consistent with those at other locations. This difficulty in distinguishing the source of any particular settlement damage has frequently resulted in all alleged damages being paid, even though other, more obvious, concurrent causes of the distress are also present. In our experience most insurance claims for sinkhole coverage that require assessment of the structure and subsurface by an engineer and geologist are for distress that has occurred over a period of time, and do not involve the sudden appearance of a localized, detectable, circular ground depression (a sinkhole).
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