Let’s address some issues that come up frequently in conversations with neighbors concerned with the effects of blasting at nearby mining operations. San Antonio and our neighboring cities along the I-35 corridor lie on the Balcones fault line, a major limestone deposit which provides limestone materials to communities lying to the east, including major cities like Houston and Corpus Christi. Quarries in this area mine the rock, crush it into various sizes and send it to areas that cannot mine it themselves locally.
The vibrations produced by quarry blasting have been a source of concern and frustration for neighboring communities as long as people have occupied homes or businesses near these active aggregate producing operations. In order to ensure that the environmental impact from blasting on neighbors and businesses is not negative, instruments called seismographs are set in the field to record the intensity of energy that is felt wherever the instrument is located. Using data from these seismographs, or seismometers, we have learned much about the effects from blasting.
It is important to begin by explaining that what people living near a quarry may feel is a combination of ground vibration and air over pressure. In a perfect world, 100% of the energy produced by explosives loaded into the ground would go in to breaking the rock. If that were the case, no energy would be felt by anyone immediately outside the quarry. However, this is not possible. That being said, a well executed blast uses as much energy as possible in the fracturing of the rock, and leaves very little to escape into the surrounding environment. It is this escaping energy that is the topic of much neighborhood conversation and concern.
Energy that isn’t used for breaking rock travels either through the remaining rock, or through the air. A seismograph records the intensity of escaping energy using a microphone to measure changes in air overpressure (that is, over normal atmospheric pressure), and a transducer to measure ground vibration.
Escaping energy from a blast that travels through the air produces a temporary increase in air pressure much like a clap of thunder or a jet engine from aircraft traveling overhead. This increase in air pressure, called air overpressure, is measured in decibals. Air overpressure travels in a wave form and much like a wind, pushes on anything in its path. However, this pulse comes and goes much more quickly than a gust of wind. It is this wave that is “caught” temporarily by surfaces in its path, like the sides of structures, before it is quickly released. Air pressure can be an annoyance even at low levels and once it reaches very high levels, can produce the potential for damage to structures. The criteria for safe blasting levels of air overpressure have been established and are well published after extensive testing and research by the United States Bureau of Mines. Air overpressure is produced where energy escapes through fractured rock and primarily travels in the direction that the rock being blasted moves. In this image below you can see the movement of rock at detonation. Air pressure increases in proportion to the amount of energy released between the fractured and moving pieces of rock. Therefore, changes in air overpressure are more discernable along this path and can sometimes be perceived miles away. There are several factors that make controlling and predicting air overpressure more difficult than ground vibration. Some of these factors include atmospheric conditions that change constantly, such as wind speed and direction, or thermoclines. These invisible thermoclines separate air with different temperatures or air traveling at different speeds. Because of this, it is universally considered optimum conditions for blasting when there is a clear cloudless sky with no wind. However, weather conditions can change very quickly and conditions that were perfect only moments before, can degrade, resulting in undesirable changes in air pressure for neighbors.
Ground vibration is produced by energy escaping through the remaining solid rock, so it tends to be more discernable behind the blast. Unlike air overpressure, the intensity of ground vibration tends to be more predictable since it travels through a more solid medium.
The human body is a very sensitive seismograph, but many people are confused by what they feel, misjudging air overpressure to be vibration, and vice versa. Because energy travels through the ground more quickly than it does through the air, seismographs and neighbors alike will perceive the vibration before the air overpressure. The greater the distance from the blast, the larger the gap in time between the arrival of the two. That is why some neighbors correctly describe feeling “two blasts”. They first perceive the ground moving, then the air moving.
Neighbors who are unfamiliar with these dynamics of blasting cannot understand why what they feel tends to be different from one blast to another. It is not uncommon for a neighbor to feel very little on one blast, and then find another blast significantly more intense. The most obvious conclusion that can be reached is that a larger amount of explosive energy was used whenever the blasts are more perceptible. In fact, the amount of explosive energy used by rock producers is tightly controlled and selected as a result of much research. Seismic data collected from each blast is analyzed, and offers valuable information to ensure that the impact on neighbors is minimized. Despite this, factors like the weather and the orientation of the blast to neighbors, make predicting the effects difficult. That’s why conscientious operations have a blast monitoring program which offers them immediate feedback on every blast and protects them from overlooking variables that can produce undesirable results.
Nick Buccihttps://www.firmatek.com/wp-content/uploads/2017/08/Firmatek-Logo.pngNick Bucci2013-05-20 09:42:072013-05-20 09:42:07A Neighbor's Guide to Understanding the Effects From Nearby Blasting Operations