A Neighbor’s Help Guide to Understanding the Effects From Nearby Blasting Operations
I’d like to address some issues that have come up frequently in conversations with neighbors concerned with the effects of the blasting at nearby operations. San Antonio wa blast and our neighboring cities along the I-35 corridor lie on the Balcones fault line, a major limestone deposit, critical for providing limestone materials for communities to the east, including major cities like Houston and Corpus Christi. For this reason you will find many quarries in this area, which crush the rock 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 nature of the effects produced 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. This is what I’d like to help you understand a little better.
Energy not used in breaking the rock travels either through the rock, or through the air. And that’s precisely what a seismograph measures. A seismograph records the intensity of felt energy that travels using a microphone to measure changes in air pressure (above normal atmospheric pressure), and a transducer buried in the ground, which measures waves of ground vibration (which travel in 3 dimensions).
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 over pressure travels in a wave form and much like a wind, it pushes on anything in its path. But unlike wind, this pulse comes and goes much more quickly. 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 have been established and are well published after repeated 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, which are invisible layers in the atmosphere where there is a noticeable temperature change. These thermoclines separate air with different temperatures and 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.