Blasting in Ohio's Quarries and Surface Coal Mines

The word "blasting" often conjures up visions of destruction - mushroom clouds, gigantic craters,  high-rise buildings collapsing, bridges falling and cars exploding.  However, each day in Ohio, nearly half a million pounds of explosives are safely detonated in quarries and surface coal mines. The Ohio Department of Natural Resources, Division of Mineral Resources Management is the agency responsible for regulating the environmental effects of mining and blasting.   This section will address some of the most commonly asked questions and misconceptions regarding blasting.

Why is blasting necessary?

Blasting is the most cost effective way to fracture rock so that it can be excavated by heavy, earth-moving equipment. This in turn, reduces the costs of building materials, such as gravel and concrete, energy produced from coal, and any other products derived from limestone, coal and other minerals.


Is dynamite still used?

People living near quarries and coal mines often express concern about the "dynamiting going on over there."  In fact, dynamite, a nitroglycerin-based explosive, is rarely used today in Ohio's quarries and surface coal mines.  The most widely used explosive is a mixture of ammonium nitrate (AN) and fuel oil (FO) called ANFO.  Nitrate is in the form of a pellet, as found in lawn and garden fertilizer, and is mixed with fuel oil at the approximate ratio of 94% AN to 6% FO by weight.  ANFO is far less hazardous than dynamite, doesn't cause nitroglycerin headaches, and breaks more rock per unit of cost.

How far do fractures extend from a blasthole?

Blastholes are normally drilled vertically and arranged in a grid pattern.  Typical blasthole diameters range from two to seven inches in quarries and five to nine inches in surface coal mines, with typical depths from 10 to 70 feet.  Upon detonation, fracturing of rock generally occurs no greater than 20 to 30 feet from any blasthole, depending largely upon hole diameter and the densities of the rock and explosive. A common misconception is that fracturing extends far beyond the mine property - even miles from the blast site.  If this were true, the blastholes could be placed much farther apart than the commonly used spacings of six to 19 feet in quarries and 12 to 25 feet in surface coal mines, and blasting would be much more economical since less drilling and explosives would be necessary.

Another common misconception associated with blasting is that significant fracturing occurs far below the bottom of a blasthole.  In fact, most of the gas-pressure forces created by the detonation of the explosive radiate outward along the length of the cylindrical blasthole.  Depending upon the hole diameter, type of explosive and nature of the rock, gas-pressure forces below the bottom of the blasthole are comparatively minimal and fracturing of rock is generally limited to several feet.  In most surface coal mines, a buffer of only three to five feet between the bottom of the blastholes and the top of the coal seam adequately protects the coal (which is brittle to begin with) from being fractured and contaminated by the rock material immediately above it.  Failure to protect the coal from fracturing can increase the cost of cleaning the coal and significantly reduce the mine operator's profits.

What causes ground vibration and how is it measured?

When a blast is detonated, some of the explosive energy not utilized in breaking rock travels through the ground in all directions as wave motion, similar to the ripple created in a pond when a stone hits the water.  This wave motion, or ground vibration as it is commonly called, travels mainly along the surface at speeds of 5,000 to 20,000 feet per second, depending upon the density and thickness of the rock and soil.  Its energy level decreases rapidly with distance from the blast and normally decays to levels undetectable by humans beyond a few thousand feet.  Because explosives are expensive and vibration represents wasted energy, it is to the blaster's advantage to utilize as much of the energy as possible in fragmentation, thereby minimizing vibration.

Blasting seismographs are used to measure ground vibration in terms of particle velocity, which is the speed at which each particle in the ground oscillates as the wave motion passes. This would be similar to measuring the speed of a fishing bobber in a pond as it moves up and down when a ripple passes under it. Particle velocity is measured in inches per second, but beyond a few hundred feet from a blast the actual movement of the ground, or displacement, is generally only a tiny fraction of an inch, about the thickness of a piece of paper, or less.  So it is important to understand that a particle velocity reading expressed in inches per second refers to the speed at which the ground moved, and not the amount of movement.