Part 1: What Happens During a Blast?
By Charles G. Spencer, Ph.D.
Boreholes, typically spaced a few feet apart, are drilled into a rock layer. The explosive is placed into each borehole and all of the holes are attached to an electronic system that sets off the explosives. Detonations are set off in sequence, each separated by a timespan of several (typically about 8) milliseconds (called the delay). The amount of explosive used in each borehole is called the charge.
When an explosive detonates, the chemical reaction produces rapidly expanding gas, which exerts pressure on the rock surrounding the drilled hole in which the explosive was placed. The pressure imparts energy into, and fragments, the rock around the borehole. Detonation in sequence fragments the rock along the line of the boreholes, and pushes the rock outward, usually toward the open face of a rock ledge.
The energy of the explosion that enters the air and surrounding rock and soil travels away in all directions. This energy, called seismic waves, causes vibrational motions in whatever it travels through. These are identical to the seismic waves produced in earthquakes, though at different wavelengths, frequencies and energy levels.
The vibrations cause temporary and cyclical displacement of whatever they move through. In solid rock the displacements are very, very small. At the surface they are greater. In all cases, once the seismic wave has passed, whatever is vibrating returns to its original position; there is usually no permanent displacement.
Some of the energy is transferred upward into the atmosphere, compressing the air as a sound wave; that’s called overpressure, or the air blast. It produces the sound you hear and it shakes your house. It is very much like thunder, travels at the same speed (1100 feet per second), and certainly gets your attention.
Some energy travels into and through solid rock layers above, adjacent and below the explosion. The displacement produced by these vibrations is very small within the rock mass. These so-called body waves travel very quickly (up to 20,000 feet per second) and have relatively-short wavelengths (high frequencies). In general, these vibrations don’t cause damage to structures.
Typically, the most damaging seismic waves to structures are surface waves which, as the name implies, are produced and travel at or near the ground surface. These travel at about 5,000 feet per second and have lower frequencies (longer wavelengths). They cause the ground vibrations you and your house “feel,” and can also produce complicated rolling motions in structures.
While common sense may suggest that the farther you are from a blast the less vibrations you should feel, it may not always be that way. In addition to distance, the energy that reaches any given point, and how it affects your house, may depend on other factors including the charge per delay, geologic materials, the path body waves take, constructive or destructive interference, and construction variables. There’s no way to accurately predict effects at any particular location.
The recent approval by the Missouri Mining Commission for the use of explosives at the Star Excavations site on Quarry Park Road in Lee’s Summit has led to concern on the part of many nearby residents. The Tribune asked Dr. Charles Spencer, a local geological consultant, to provide some basic information on the nature of blasting and its effects. His article will appear over the next four weeks.