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DISCUSSION ON SINKHOLES AND SUBSIDENCE MECHANISMS Page 1 of 2 In the geologic past, sea level has fluctuated significantly above and below its current elevation. As a result, limestone formations in Florida were exposed at the ground surface and subject to erosion and solutioning by rainwater, which is naturally slightly acidic. The erosion and solutioning caused cracks, cavities, and fractures to form in the limestone producing a weathered and irregular limestone surface. After subsequent rises in sea level, the undulating limestone surface was buried and filled by younger deposits of clay, sand, and silt. Even though the limestone is now covered with soils and clastic sediments, solutioning of the limestone by ground water continues. This chemical action tends to be concentrated along preferential paths for ground water flow such as joints, bedding planes, and pre-existing fractures and voids. The rate of dissolution is an imperceptibly slow process, requiring several thousand years for the formation of significant caverns or voids. Non-cohesive soils and sediments (sands and silts), which overlie the limestune, may move downward, or ravel, into the enlarged voids and cavities. The raveling may propagate upward to the surface as the overlying sediments move downward into the cavity. Sinkholes occur when the raveling reaches a point where the surficial soils cannot support the load at the surface, resulting in collapse or subsidence. In this part of Florida, sinkholes occur by several mechanisms: cover-collapse, cover-subsidence, limestone-collapse, or limestone-solution (Sinclair et al., 1985). Cover-collapse sinkholes form where a thick, competent, and generally impermeable clay layer overlies limestone bedrock. Initially, the clay has sufficient strength to bridge a developing cavity in the underlying limestone. A cover-collapse sinkhole occurs as a result of sudden failure of the clay unit and catastrophic downward movement of the overlying sandy soils into the cavity (i.e., raveling). Cover-subsidence sinkholes may form by the gradual downward raveling c►f non-cohesive sediments (usually sand and silt) into actively forming cavities. The raveling reduces soil density,which is manifested at the surface by an area of slow, gradual subsidence. Limestone-collapse and limestone-solution sinkholes typically occur in terrains where limestone bedrock is covered by a relatively thin (less than 25 ft.) layer of soil or overburden (sand and clay). A limestone-collapse sinkhole forms when a solution cavity expands to the point where the limestone roof collapses. The collapse is usually abrupt and may be catastrophic in nature. A significant factor in the formation of limestone-collapse sinkholes is a low water table, typically below the limestone surface. Limestone-collapse sinkholes are relatively rare oc;currences since dissolution is more likely to occur at the limestone surface. Limestone-solution sinkholes form through a mechanism of gradual solutioning of the buried limestone surface under relatively high water table conditions. Surface depressions generally form at a gradual rate as the limestone dissolves, usually without the formation of significant voids or sudden raveling of overlying sediments. Over geologic time, this process results in a general, regional lowering of the land surface. .�`�.i�i:�:�:�t_;iii:f:; �;1•,�".5�3��FtiC;?1 In Q ' �3C1 Pt�oject 1`,v. 0;� 1 t;,: r�s�xs��>e :��Ic�s�;c!�� a�:cl� /�ssoci{itic>�� 1��c}r�z�:�teci Aprii s�iG C'i�:!t. t�.I._,. �8�'.,%_3�b �j-s��F.z,�it,' �