Construction & Contractors/Blasting in Karst Formations
QUESTION: I have read many reports of blasting effects on underground formations but little to nothing about blasting in a Karst formation. I live in an area of Florida where sinkholes are almost a daily occurrence. Our surficial aquifer has been seriously depleted which contributes to the high rate of sinkhole formation. There are plans to put a lime rock mine within 700ft of local homes with its attendant blasting. My take is that the mine is playing Russian roulette with our neighborhood. Of course they refuse to take any responsibility for sinkhole formation. The costs of a neighborhood study to determine the risk are enormous. We have already experienced sinkholes right out neighborhood front doors.
ANSWER: I think the question reduces to whether the quarry activity would accelerate a limestone dissolution process that apparently is already under way in your neighborhood.
You are probably already aware that karst formation involves the dissolution of soluble rocks -- limestone in your case. There is an extensive literature on the formation and progression of karst, much of which comes from eastern Europe where they have been dealing with its effects in urban areas for hundreds of years. It has gotten a lot of news play in recent months in Florida but it is a problem that dates to the ancient Greeks. With changes in the ground water table the stress regime changes within and over the resulting voids. Ground water flow through those voids also presents the possibility for transport of the non-soluble overburden from below, leading to open voids bridged by unstable materials, resulting in sinkholes when the bridge can no longer stand. This is the critical aspect of the kind of collapse that has everyone is such a panic lately. There must be a mechanism for transport of subsurface materials -- ordinarily by rapidly flowing water. The most common mechanism is through man-made features like storm sewers, rather than vast networks of subterranean caves.
Most sinkholes do not occur catastrophically like the one in the news recently. The mechanism of their formation is such that they are usually expressed as progressive settlements.
You mentioned blasting vibrations. Intuitively, one would think that when the ground shakes it could disturb those fragile land bridges over the karstic voids, causing an accelerated risk of collapse, and could also result in enhanced dissolution by widening ground water pathways and making more rock surfaces available to attack by ground water, much like stirring sugar into tea. It is easy to make too much out of this mechanism. In an urban area the suppression of blasting vibrations for the sake of abating public nuisances and structural damage will also greatly reduce the likelihood that vibrations would exacerbate the dissolution process. Moreover, the vibrations that occur in the subsurface are of different frequency and magnitude than those that occur on the surface. If there is any manifestation of blasting vibration on the occurrence of sinkholes it would most likely be the expansion of a progressive process, as opposed to causing sudden collapse everywhere.
For this reason it is not likely, in my opinion, that socially responsible quarry blasting practices would have a measurable effect in enhancing a process that is taking place naturally anyway. That is not to say it cannot occur, just that blaming blasting for making the process worse is most likely not a provable proposition.
There is, however, the potential role of dewatering for quarry operations. Artificially depressing the water table in the quarry proper will affect the water table in the surroundings, the extent of which may or may not be trivial depending on the local hydrogeology. If, however, significant changes in flow pathways occur, there could be new opportunities for karstic progression.
The cost of determining whether sinkhole development is under way, and if so, what stage it is in, can indeed be high because it entails hydrogeologic modeling, tracer tests, geophysics to identify dissolution zones, and confirmation borings. Even then, the vulnerability of those processes to modification by quarry operations would be even more difficult to pin down. I can see why the quarry operator would be reluctant to engage that kind of study. If it is favorable to him, he could be accused of partiality and endlessly involved in confirmatory studies and technical defense which in a public forum is basically unwinnable. If it is not favorable to him he might never recover the cost of the study through quarry operations.
Perhaps the best solution is to demand that the quarry operator defray the cost of insurance, acceptable to the homeowners, that they would otherwise carry against the occurrence of sinkholes.
---------- FOLLOW-UP ----------
Thank you for responding to my question.
This is more a comment than a question. Recently Hurricane Sandy blessed our state with copious amounts of rain. While significant, we have had other larger rain events. The situation was that within a few days, 10s if not 100s of sinkholes opened up many catastrophic in that they opened in minutes, hours at most. Almost all had a common denominator. They were located proximate to recent construction.
Two winters ago we had a severe cold spell. Farmers to save their crops, especially in one county pumped record amounts of water to cover the crops in ice. The result was that 140 sinkholes opened up, again many catastrophically, including two which resulted in homes being condemned.
These two events speak more to rapid collapse from either erosion of the bridge or weakening of the bridge due to vibrations. Even the Florida DEP has a brochure that states that sinkholes can be affected by vibrations, although they do not quantify this.
Lastly, recent studies indicate that the flow models for the Florida Aquifer are seriously flawed. Where current models predict subterranean flows in months, testing has shown these flows to be hours.
I am aware of the sinkhole problem and the association of various natural and human-caused activities with sinkhole formation. It is certainly a growing issue in Florida and there is a need for the authorities to comprehensively address the very difficult technical aspects of sinkhole prediction, site evaluation, and associated land uses. As it is, the adjudication of the issue is playing itself out in the courts and the legal system is a very bad place to do science.
However your question related to a specific issue at a specific place. The issue is, as I said above, whether the proposed quarry would cause or exacerbate the likelihood of sinkhole formation in your particular case. Given the available information in your first question I could offer nothing better than a general response. It is understandable that you don't want to incur the cost of a scientific study to determine whether that quarry will increase sinkhole likelihood and/or severity, but that's where these cases typically end up.
Yes, sinkholes can be affected by vibrations, but the rock bridge can also be competent enough to withstand vibrations. It all depends on the site-specific characteristics of the rock bridge, the hydrogeology, and the kind and intensity of the blasting that will be taking place.
You criticized my answer as being out of date, and stated that you were not comfortable with it. Perhaps you were looking for affirmation that the quarry will be so likely to damage the neighborhood that it should not be permitted to operate. Your accumulated circumstantial information would support at least asking the question, but it does not prove the point. At risk of repeating myself, all that circumstantial information has to be demonstrably relevant to the particular site conditions where you are. In this case what you need is a professional engineering investigation of such a conclusion. That is not the business of this forum.