Ground Vibration Definitions, Causes, and Important Characteristics

This is the first post in a series on blast vibration basics. Blast vibrations are an important concept to understand because all blasts produce vibrations and, in some cases, those vibrations will annoy neighbors or potentially damage nearby structures. However, in today’s environment, blast vibration damage is extremely rare due to the regulatory limits placed on blast vibrations from blasting and other construction activities.

This post focuses on why blast vibrations occur, some of the basic definitions concerning blast vibrations, and parameters used to describe blast vibrations.

If you’re in the blasting industry and don’t often deal with blast vibrations, this post should be helpful to you. If you’re not in the mining/quarrying/construction industries, but are curious why you feel blast vibrations from a nearby site, then this post could also be helpful to you as well.

Source: Applied Explosives Technology for Construction and Mining by Stig Olofsson

Why do Blast Vibrations Occur After a Blast?

A blast is a sequence of explosive charges that detonate in the ground with a release of chemical energy. The shock and gas created from the energy release fractures and breaks the rock. This process also produces heat, sound, and vibrations in the ground.

What are Blast Vibrations?

Blast vibrations are the portion of the detonation energy from a blast that is manifested as vibrations in the ground or as sound waves in the air. Typically, when the term blast vibration is used in the mining and construction industries, it refers to the ground component. While the air (noise) component is referred to as airblast or air overpressure.

If you want a more scientific or complex definition, blast vibrations can be described as elastic, oscillating, transient ground motion caused by a detonating blasthole. This simply means the energy from a blast vibrates as it moves through the ground, without causing disturbance to the ground, until it decays or reduces in energy to the point that that the energy dies out.

The 18th Edition of the International Society of Explosives Engineering Blasters’ Handbook defines blast vibrations as “the remaining energy exerted on the rock that causes no permanent displacement, this energy travels elastically as rock particles are temporarily displaced and return to the original position.”

Blast vibrations are complex waveforms that, depending on the blast, typically only last a couple of seconds.

**Analogy:**

Compare a blast to hammering a nail into a wall to hang a picture frame. In this analogy, the hammer hitting the nail is the source of the vibration. If you place your hand on the wall, you can feel the vibration of the hammer hitting the nail even though the vibration doesn’t damage your house. You can also hear it. A blast is analogous to the hammer hitting the nail. The blast causes vibrations in the ground and the blast can be heard by those nearby.

Blast Vibration Characteristics

Blast vibrations typically move along the surface away from a blast. Deeper in the ground, blast vibration energy typically reduces significantly with depth.

Blast vibrations exhibit energy in all directions, so some important terminology is used to describe the energy in each direction. The image at the right illustrates some of these concepts.

Wave Extension: As a blast vibration travels away from the shotfiring point (blast), the length of the vibration extends. Typically the frequency reduces and amplitude reduces as the energy decays along the travel path (more on frequency and amplitude in the next post).
Compression Wave: Illustrated by “P” in the image at the right and the left diagram in the image below. Compressive waves push and pull along the path of travel.
Shear Wave: Illustrated by “SH” in the image at the right and the center diagram in the image below. Shear waves oscillate perpendicular to the travel path. The movement can be vertical or horizontal.
Rayleigh Wave: Illustrated by “R” in the image at the right and the right diagram in the image below. Rayleigh waves travel in an elliptical motion on the surface producing vertical and horizontal movement. The elliptical movement is retrograde, meaning the movement is counterclockwise if observed moving from left to right.

Source: United States Geological Society (https://www.usgs.gov/programs/earthquake-hazards/earthquake-magnitude-energy-release-and-shaking-intensity)

Typical blast vibrations do not exhibit any difference between compressive, shear, or Rayleigh waves, except at large distance and in special geologic circumstances. Therefore, these terms are more commonly used to describe earthquake vibrations and are typically not used to describe blast vibrations. In fact, earthquake vibrations are quite different from blast vibrations because earthquakes create a significantly greater energy release than a blast.

Earthquakes have enough energy that the vibrations can damage structures in an entire community; whereas, a typical blast vibration will cause no damage but could cause minor annoyances to those nearby. This is illustrated in the image at the right, where energy release in kilograms of explosive (multiply kilograms by 2.2 to calculate pounds) is compared to the earthquake Richter Scale (RS). Blast vibrations occur around 2 on the scale, which is around the limit of energy that is felt by humans. If you’re close enough to a blast, you can feel the vibrations (slightly higher than 2 on the scale). However, property damage won’t occur. A much larger amount of energy will need to occur before any damage occurs (4 to 5 on the Richter Scale). Note: the RS is a log scale, so a jump from 2 to 4 is much more than double. You can see on the right that the a value of 4 on the RS (56,000 kg) is 1,000 times higher than a value at 2 (56 kg) on the RS.

Compressive Wave (left), Shear Wave (center), and Rayleigh Wave (right). Source: Blast Vibration Monitoring and Control by Charles Dowding

It is important to understand the nature of vibration characteristics when installing seismographs (blast vibration measurement tools), analyzing blast vibrations, and designing blasts to optimize blast vibrations. All of these practices will be described in later posts.

The next blog post will focus on blast vibration variables, such as amplitude, frequency, and duration.

Blast Vibration Basics Post #1