When explosives are used to break rock in a mine or construction project, the blast produces both ground vibration and air overpressure (noise). In most cases the atmosphere selectively absorbs the higher frequencies from a blast, leaving relatively low energy (5 hertz) sound waves to effect structures. If a structure has a natural vibration frequency around 5 hertz, it will respond to the air overpressure by producing higher frequency secondary noise on internal walls. It is this response from the middle of flat walls in a structure which causes much of the secondary rattling noise and other observed effects such as movement of pictures, clocks, etc.
Most concern about structural damage comes from people who feel the effects while inside their homes. They are actually responding to the structural motion that produces rattling and motion and not to the actual noise and ground vibration from the blast, which are often imperceptible when outside the structure.
The stress on a structure from a 131.7 dB overpressure produced by a blast is roughly equivalent to the stress produced by a 25 mph wind. The wind isn’t as noticeable as the air overpressure due to its slow rate of pressure change and the correspondingly minor or nonexistent rattling, in contrast to the relatively rapid pressure changes produced by air overpressure waves. The BOCA (residential building) code has a general requirement for vertical residential walls to be able to withstand a pressure of 10 pounds per square foot. Some areas have higher standards (such as Florida with its seasonal hurricanes). The 10 pounds per square foot pressure standard is equivalent to a gusty wind of 62.5mph. Thus residential structures are designed to withstand air overpressures well in excess of those produced by normal blasting operations.
Air overpressure produced by blasting is expressed in pressure units called decibels (dB). This overpressure can be measured accurately with specialized instruments called seismometers. The following table gives the decibel levels produced by some typical situations:
0 dB Threshold of hearing
20 dB Whisper
40 dB Hospital Room
65 dB Ordinary Conversation
95 dB Riveter
115 dB Threshold of Complaints
134 dB Bureau of Mines recommended “Safe Level” for Blasting
140 dB Historically Proven Safe Level
151 dB Occasional Window Breakage
171 dB General Window Breakage
180 dB Possible Structure Damage
The United States Bureau of Mines was the federal government organization responsible for research into the effects of blasting operations on residential and commercial structures. Extensive research by the Bureau of Mines over many years and in many different blasting situations showed that as long as the air overpressure produced by a blast did not exceed 140 dB, damage to structures was highly unlikely. As a result of these findings, the Bureau of Mines established a recommended “safe” level of 134 dB, well below the proven damage threshold, for commercial blasting operations.
Present day mining and construction firms typically perform their blasting operations in such a manner as to ensure that this “safe” level is not exceeded, thus protecting nearby structures from blast-induced damage.