Not So Good Vibrations (Part I)
If your home is located in the general vicinity of blasting-type activities being conducted for mining, quarrying, or construction; chances are you or your neighbors have sensed vibrations within your home. It can be an uneasy feeling, the shaking and rattling of items on the walls can cause fear that your home is being damaged or it is just an annoyance that disrupts your daily routine. However, let me ask you a question, have you ever noticed these vibrations outside your home? Chances are the answer is no. So, what is going on?
Now, think back to the simple wave that you learned about in your middle school science class. Waves have key properties such as an amplitude or peak as well as a period, which can also be considered as the wavelength. How often a wave completes one cycle, one period, in a timeframe is known as the frequency of the wave which is measured in hertz or one cycle per second. When explosives are detonated in blasting-type applications, energy is released outwards from the blast location in the form of waves, which can be transmitted through the air as well as the ground. In the application of blasting, airblast is the wave energy that is transmitted through the air and the amplitude of the waves are measured in pressure or decibels. Ground vibrations are a type of wave energy that is transmitted through the ground, and the amplitude of the waves are commonly measured in terms of velocity or peak particle velocity.
Measurement devices known as seismographs measure the amplitudes of the wave energy as well as the frequency and time duration or the total time it takes the energy to move through the measurement location. Generally, as both forms of wave energy travel outwards from the blast location, their properties dissipate. However, as the waves are traveling outwards, their properties are variable and they can be influenced by many factors. In the case of ground vibrations, the geology, or the soil in which they are traveling through can affect them. Depending on the location of the seismograph used to measure the blast, different measurements of peak amplitudes and frequencies could be obtained if there is a change in the geology around the measurement locations. Weather influences such as wind or temperature inversions are an example of conditions that could influence the measurements of airblast at different locations.
How does my residence respond to these waves?
There are two primary modes of response for common residential structures as determined by the U.S. Bureau of Mines (USBM), racking and midwall bending. A racking response can be described as where the walls of your residence move as rigid bodies connected to one another. Racking response represents how the entire residence responds as a whole and can be measured at the corners. Mid-wall bending is a localized response of the walls within a residence at the middle of the walls, between the floor and the ceiling. Mid-wall bending responses are responsible for the rattling of windows and wall-mounted items.
The mid-wall bending responses from blasting produce noise from rattling of items on the walls which is often responsible for the human perception of the blasting activity. When we are inside our home during blasting activity, we notice the response of the structure to the vibrations created by the blasting and when we are outside, we typically do not notice the responses of nearby structures to the vibrations created by the blasting activity. So inside we are primarily noticing midwall bending responses of our home and outside we are not.
Can these vibration responses from nearby blasting activity cause damage to my residence?
Well, it depends. In part two regarding this topic, we will explore how our homes respond to blasting activity and discuss how damage may occur.
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About the Author
Matthew Shuler, P.E. is a Consulting Engineer in our Charlotte, NC Office. Mr. Shuler provides consultation in the areas of assessment of residential, commercial, and industrial structures including root cause investigation, scope of damage, building code compliance, construction evaluation, structural system assessment, and evaluation of structural components. You may contact him for your forensic engineering needs at firstname.lastname@example.org or (704) 523-2520.