THAT SINKING FEELING!
During March 1998, a water well firm was drilling an irrigation well for a golf course under construction in Pasco County, Florida. The well was installed deep into the Floridian aquifer and the crew was developing the well using compressed air, when all of a sudden, a massive sinkhole opened up near the rig.  The crew and drill rig escaped unharmed, however, the surging weight of tons of earth collapsing into the sinkhole created a pressure wave through the aquifer that spawned approximately 200 smaller sinkholes around the big sinkhole. 

Clearly, sink holes are hidden liabilities that lie in wait below ground surface, threatening to swallow people, equipment and property.  Because of the serious consequences posed by sinkhole occurrence, detecting their presence in the early stages of planning and construction of a golf course is imperative, especially for areas with a history of catastrophic sinkhole development such as Florida.

Sinkholes (also known as dolines in geologic jargon), form in a variety of ways both naturally and manmade.  Generally they involve the collapse of weakened overburden into a large, subterranean void or cave.  Although sinkholes are found in many regions throughout the United States, it is the areas that are predisposed to sudden, catastrophic occurrences that are of  greatest concern.  Three common types of sinkhole development include: 1) solution from above, 2) undermining from below, and 3) removal of buoyant support.  Examples of these three conditions are illustrated below.

Origins of Sinkholes
Although conditions leading to sinkhole formation vary from site to site, the following factors are generally found in areas prone to their formation:

Certain activities at golf courses tend to aggravate sinkhole formation especially during the construction phase of the project.  These activities include ground vibrations through movement of heavy construction equipment, modifying the land surface drainage (which may cause surface water to stress structurally weak areas), and de-watering of the local aquifer during irrigation.

Sinkhole Exploration
Locating sinkholes before they collapse is a chancy proposition because there is no fool-proof method that can find them with 100 percent certainty (as yet).  However, there are a number of means to explore for them that should be considered because, as a rule of thumb, the bigger a subterranean void or cavern is, the greater the likelihood for collapse to occur, and the better the chance of detecting it using existing technology.

1)  Areal photographs (good for evaluating vegetative growth patterns and locating depressions that are too small to be included on topographic maps).

2)  Remote sensing techniques (also good for evaluating large scale patterns but sometimes difficult to obtain).

3)  Field reconnaissance survey (to identify small scale topographic/drainage features too small to be observed or features hidden by dense vegetation to areal photography).

4)  Subsurface exploration via soil borings (a tried and true method to define site geology but can miss subterranean voids entirely).

5)  Geophysical exploration including:
      -   seismic refraction surveys
      -   electrical resistivity surveys
      -   electromagnetic conductivity surveys
      -   ground penetrating radar
      -   gravimetric surveys

Dollar for dollar, geophysical methods have the advantage of evaluating larger parcels of land than drilling programs; however, due to their non-invasive nature, they work best when combined with a drilling program.  The geophysical survey can pinpoint subterranean targets that drilling can investigate directly through the collection of soil samples or rock cores.