Hundreds of sinkholes ranging in diameter from 1 foot to more than 150 feet formed within a 6-hour period on February 25, 1998, during the development of a newly drilled irrigation well (a procedure that involves flushing the well in order to obtain maximum production efficiency). Unconsolidated sand overburden collapsed into numerous cavities within an approximately 20-acre area as pumping and surging operations took place in the well.

Sinkholes induced during the development of an irrigation well affected
a 20-acre area and ranged in size from less than 1 foot to more than
150 feet in diameter.


The affected land is located near the coast in an upland region that straddles parts of Pasco and Hernando counties. A 20-foot-thick sediment cover composed primarily of sand with little clay is underlain by cavernous limestone bedrock. The well was drilled through 140 feet of limestone, and a cavity was reported in the interval from 148 to 160 feet depth, where drilling was terminated. Very shortly after development began, two small sinkholes formed near the drill rig. As well development continued, additional new sink-holes of varying sizes began to appear throughout the area. Trees were uprooted and toppled as sediment collapse and slumping took place, and concentric extensional cracks and crevices formed throughout the landscape. The unconsolidated sandy material slumped and caved along the margins of the larger sinkholes as they continued to expand. The first two sinkholes to form eventually expanded to become the largest of the hundreds that formed during the 6-hour development period. They swallowed numerous 60-foot-tall pine trees and more than 20 acres of forest, and left the well standing on a small bridge of land.

TEST BORINGS AND HYDROGEOLOGIC DATA INDICATE SUSCEPTIBILITY TO SINKHOLES
The affected land contains several ponds formed by sink-holes long ago (paleosinkholes). Because west-central Florida is susceptible to sinkhole development, stability was tested along the margins of these ponds to determine if the site had higher-than-normal risks of sinkhole occurrence. Many test borings were made to measure the structural integrity of the bedrock, revealing a highly variable limestone surface. Two of the borings, approximately 100 feet apart, were made within a few hundred feet of the well site. One boring indicated that there was firm limestone at depth, whereas the other never encountered a firm foundation.

Irregularity in the limestone surface is typical of much of west-central Florida. Cavities, sudden bit drops, and lost circulation are frequently reported during drilling in this area. These drilling characteristics indicate the presence of significant cavernous porosity in the underlying limestone and, while commonly noted in drilling logs, only occasionally cause trouble during well construction.

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