I’ve never posted before so bear with me. I live in Florida where the new residents moving to the state are as thick as mosquitoes. There is a lot of development to provide homes for new residents. Development means more limerock mining (limerock is used in cement, road base, asphalt, etc) and often “waterfront homes” on deep ponds created by blasting the rock (and using it for fill to elevate the house pad). Without getting into the debate about Florida’s wetlands I have a question about blasting. Both limerock mines and developers blast the limerock that forms the water table aquifer. Limerock has varying porosity. (Think sponges holding water). Some parts of Florida get drinking water from this upper level aquifer. Additionally there are several layers of aquifers below separated by semi permeable clay layers. Florida is also Karst geology. Karst means there are “dissolution” features like caves (famous in north central Fla), fractures, conduits, faults, etc. (Think swiss cheese.) formed over lots of years by a complex chemical reaction. Blasting for mining and development “lakes” occurs between 15 feet and 100 feet. My question is this: Doesn’t this blasting fracture the rock below the site of the blast? If so this could disrupt the structure of the aquifer below. If there are already fractures and faults in the limerock, wouldn’t blasting open these fractures further? I’ve searched the web and library for several months now to no avail. The blasters and miners say the blast waves only go laterally, not down. This doesn’t seem possible. Also there is a high water table. Wouldn’t the dynamics of blasted water impact Karst structure? I’m skeptical of what the blasters and miners say because they also maintain homes won’t be harmed from blasting (which is a crock since I can see the cracks in my tile) and they get paid to blast. Response? Comments? Help?
Mining and blasting probably diverts water from its normal course, draining aquifiers. From there, the following scenario seems likely:
“According to Windham and Campbell (1981) and Foose (1981), the subsidence and resultant sinkholes in Florida may occur when several conditions are met. These include a well-developed joint-controlled subsurface drainage network in limestone, a thin to moderate thickness of unconsolidated overburden, and a potentiometric surface below the unconfined water table. The sequence of events that they envision begins with a lowering of the water table either from a drought or from excessive withdrawal of ground water from pumping. As the unconsolidated sediments and weathered debris that overlie the Tertiary limestones dry out, shrinkage cracks and cavities develop. Any subsequent vertical or horizontal percolation of water can enlarge and flush out these cavities through a process known as piping. Some of the unconsolidated sediment may drop directly into pre existing dissolution cavities in the limestone and be carried away by the subsurface drainage system. When the overburden dries, it develops an internal strength capable of supporting itself, even over fairly large cavities. However, when the materials become rewetted from a rainy season or an unusually wet storm, the water weakens the material and gravity stoping can enlarge the cavity until the roof fails. The result is a conical sinkhole, the size of which is largely dependent on the thickness of overburden. The thicker the overburden is, the larger the sinkhole. Most of these depressions develop rapidly and unexpectedly as in Winter Park and Bartow, Florida”
http://daac.gsfc.nasa.gov/DAAC_DOCS/geomorphology/GEO_7/GEO_PLATE_KL-4.HTML
So, while the blasting will not directly cause a sinkhole, the resultant conditions will eventually lead to one.
A semantic point to start with - limerock is a term used by engineers, but the proper geological term is limestone. If you were using “limerock” in your searches, you may want to try again.
I have a friend who monitors blasting at construction sites, and I asked about the mechanics of your situation. He said that the blasters do make every effort to shape their charges so that the maximum force of the blast goes laterally, and the force transmission downward is dampened - it’s in their best interests, as they don’t want to weaken the bedrock underneath their building foundations. That said, the downward force is not entirely eliminated, so it’s conceivable that existing fractures might expand, leading to other problems.
I don’t know how the laws vary from state to state, but I do know that New York and New Jersey require seismic monitors to be placed around construction sites where blasting is taking place, especially when the blasting is close to existing residences. Is the development near you being monitored adequately? How was this issue addressed in the environmental impact statement (EIS) filed by the developer? If you can prove that the cracks in your tile are happening/growing as a result of the blasting, the developer and/or general contractor may be held liable for damages. If you haven’t already, you may want to consult an attorney.