The mild winters are an important growing season for west-central Florida citrus, strawberry and nursery farmers. However, occasional freezing temperatures can result in substantial crop losses. To prevent freeze damage, growers pump warm (about 73.... F) ground water from the Upper Floridan aquifer and spray it on plants to form an insulating coat of ice.

Extended freezes have required intense and prolonged ground-water pumping, causing large drawdowns in the Upper Floridan aquifer and the abrupt appearance of sinkholes.


MANY NEW SINKHOLES
Ground-water pumping for crop freeze protection tends to induce sinkholes during the month of January in Hillsborough County.


FREEZING AND PUMPING

During a 6-day period of record-breaking cold weather, ground water was pumped at night when temperatures fell below 39° F.

The new sinkholes were attributed to the movement of sandy overburden material through a breached clay confining unit into cavities in the limestone below.

"A giant sink hole opened up on Thursday, September 19 [1975] at a drilling site near Tampa, Florida and swallowed up a well-drilling rig, a water truck, and a trailer loaded with pipe all valued at $100,000. The well being drilled was down 200 ft when the ground began to give way to what turned out to be a limestone cavern. Within 10 minutes all the equipment was buried way out of sight in a crater measuring 300 ft deep, and 300 ft wide. Fortunately, the drilling crew had time to scramble to safety and no one was hurt."
-from National Water Well Association newsletter


"Construction practices often 'set the stage' for sinkhole occurrence."
-J.G. Newton, 1986
One factor confounding the relation between pumping wells and the distribution of induced sinkholes is the nonuniform hydraulic connection between the well and various buried cavities. The development of secondary porosity is not uniform. Dissolution cavities often form along structural weaknesses in the limestone, such as bedding planes, joints, or fractures places where water can more easily infiltrate the rock. The distribution of cavities can be controlled by the presence of these features and thus may be preferentially oriented. It is not uncommon for a pumping well to have more impact on cavities that are well-connected hydraulically although farther away from the pumping well than on nearby cavities that are less well connected hydraulically. Proximity to pumping wells is not always a reliable indicator for predicting induced sinkholes.


When structures such as buildings and roadways are constructed, care is usually taken to divert surface-water drainage away from the foundations to avoid compromising their structural integrity. Associated activities may include grading slopes and removal or addition of vegetative cover, installing foundation piles and drainage systems, and ditching for storm drainages and conduits for service utilities. The altered landscapes typically result in local changes to established pathways of surface-water runoff, infiltration, and ground-water recharge. Pavements, roofs, and storm-drainage systems can dramatically increase the rate of ground-water recharge to a local area, thus increasing flow velocity in the bedrock and potentially inducing sinkholes. A common cause of induced sinkholes in urban areas is broken water or sewer pipes. Pipelines strung through karst terrane are subject to uneven settling as soils compact or are piped into dissolution cavities. The result can be cracked water pipes or the separation of sewer line sections, further aggravating erosion and perpetuating the process.

Loading by heavy equipment during construction or, later, by the weight of the structures themselves may induce sinkholes. A number of engineering methods are commonly used to prevent this type of sinkhole damage (Sowers, 1984), including drilling and driving pilings into competent limestone for support, injecting cement into subsurface cavities, and construction of reinforced and spread foundations that can span cavities and support the weight of the construction. Compaction by hammering, vibratory rollers, and heavy block drops may be used to induce collapse so that areas of weak-ness can be reinforced prior to construction.

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