How is ground vibration controlled?

Blasters control ground vibration mainly by limiting the weight of explosives detonated within any eight-millisecond period of time.  They do this by using millisecond delay detonators (blasting caps) to separate the firing time of each hole from adjacent holes. In a typical 50-hole blast, the result would be 50 smaller and separate explosions instead of one large blast.   A common misconception is that the number of blastholes determines the resulting intensity of vibration.   However, given the same charge-weight per delay (pounds of explosive detonated within any eight-millisecond period) and the same distance, a 100-hole blast could be designed to produce no more vibration than a 10-hole blast.

What is airblast and how is it measured?

When a blast is detonated, some energy is lost to the atmosphere in the form of noise and/or concussion.  This phenomenon is caused by the venting of gases through cracks and fissures and the upward and outward movement of the rock on top and in front of the blastholes.  The resulting increase in the air pressure is commonly called airblast. Like ground vibration, airblast levels decrease rapidly with the distance from the blast. However, airblast travels only at the speed of sound, around 1,100 feet pet second, depending upon air temperature, and can be greatly influenced by wind direction and speed, and by atmospheric temperature inversions which can bend it back toward the earth and focus its energy several miles away.

Airblast is usually measured with a special microphone connected to the same type of seismograph that measures ground vibration. The most common units of airblast measurement are pounds per square inch (psi) and the decibel (dB), which is based on a logarithmic sound-pressure scale related to human hearing.  The threshold of hearing begins at zero decibels (See Figure I). An increase of six decibels represents a doubling of air pressure.  As an example, an airblast measured at 126 dB would have twice the air pressure of an airblast at 120 dB.


How is airblast controlled?

Airblast is controlled mainly by the proper use of stemming material (the drill cuttings or crushed stones that are shoveled back into the blasthole after the explosive material has been loaded to a predetermined depth from the surface) and by not loading explosives into portions of holes with cracks, voids or mud seams.  These techniques minimize the escape of gases and confine the explosive energy where it is needed to efficiently break rock.

What are the ground vibration and airblast limits?

The United States Bureau of Mines (USBM) has conducted extensive research during the last three decades on the effects of blast-induced ground vibration and of blasting on residential structures. This research produced recommended limits that, if adhered to, will effectively protect residential structures from damage, even if the blasting is repeated on a daily basis over a period of many years.  In their simplest form, these recommendations state that ground vibration should not exceed one inch per second and airblast should not exceed 133 decibels at any dwelling.

In meeting its obligation to the Federal Office of Surface Mining to regulate coal mining, the State of Ohio adopted the USBM limits for surface blasting in coal mines.  In Ohio quarries, however, a different situation exists.  There is no airblast limit, the applicable ground vibration limit is twice as high as the limit in surface coal mines, and blasters need not be examined and certified by the Mine Examining Board.  Fortunately, most Ohio quarry operators have been motivated by economics, safety concerns and good neighbor policies which have resulted in voluntary compliance with the more stringent requirements applicable to coal mines.  The quarry industry is currently cooperating with the state in an effort to upgrade its antiquated blasting regulations.


If the dishes rattle, is my home being damaged?

Homeowners should not be surprised that even when blast vibration levels are far below the legal limit, highly perceptible vibration can be experienced inside the home; windows and dishes might rattle, knick-knacks and pictures might move or fall if not securely fastened, and swag lamps or chandeliers might sway.  These effects can be generated by ground vibration or airblast acting separately or together, and can last from one to three seconds or more depending upon the distance from the blast, geologic influences and other factors. Despite these sometimes startling effects, there is absolutely no correlation between how a blast "feels" and its potential for causing structural damage to a home. In fact, cultural stresses (e.g., doors slamming, kids jumping, people ascending or descending stairs) and natural stresses (e.g., sunlight, wind, rain, temperature and humidity fluctuations and changes in soil moisture) place greater stresses on a home than legal blast vibrations.