Good citizenship begins with open and honest discussion. Our commitment to collaboration means we’re dedicated to answering your questions and concerns.
What is Vibration Mitigation?
Ground vibration comes from many sources in the construction, mining and energy development industries. When dealing with man-made vibration, you must understand and quantify the amplitude, frequency and duration of the vibration event. You must also understand the unique engineering properties of the structure to understand its dynamic response and ultimate susceptibility to vibration. Read more about GeoSonics/Vibra-Tech’s vibration monitoring services.
Would you please define and explain what peak particle velocity is?
Peak particle velocity refers to the maximum speed of a particular particle as it oscillates about a point of equilibrium that is moved by a passing wave. It is a term used to describe vibration, or elastic movement, resulting from excitation by seismic energy as it passes a particular point. We usually measure this speed in “inch or inches per second.” The reason we use this measurement is that it has been shown in numerous studies over the years to be the best indicator of whether damage to a residential structure is possible or likely.
What is Scaled Distance?
Scaled distance is a relationship used to relate similar blast effects from various explosive weights at various distances. Scaled distance gives a blaster an idea of expected vibration levels based upon prior blasts detonated at a project. It is calculated by taking the distance in feet from the blast (source) to the receiver and dividing it by the square root of the maximum pounds of explosives detonated per delay. It is for estimation and may supplement actual seismograph measurements.
Why isn’t the seismograph reading taken inside the home?
The interior measurements represent “structure” motion and are not considered the same. In many cases the vibration levels measured within the home are less. Research on the effects of blast vibration upon structures has been conducted while recording the blast vibration on the ground, outside of the structure, and usually in the area of the closest portion of the structure to the blast. Vibration recording by the United States Bureau of Mines and many other individual researchers, over many years, with many structures, has allowed those researchers to observe, and predict with a very high degree of certainty, how typical residential-type structures respond to blast vibration. Further seismograph standards are based upon exterior ground vibration levels which take into account the structure’s response to vibration. The measurements on the exterior also allow for consistency. The geophone or transducer (sensor of the seismograph) is attached and secured by spiking, burying or other physical attachment that cannot be accomplished within someone’s home. The outside measurement is consistent and allows comparison of the measurements on a day by day basis.
How can you guarantee that my property will not be damaged by the blasting at this site?
Regulations that govern most quarries, mines and construction projects place limits on vibration to protect nearby structures whether that structure is ten feet or thousands of feet away from the blast. The blast is designed to meet these regulatory limits, and monitoring of every blast is done in order to ensure compliance to the limits. Maintaining levels below limits provides off-site vibration levels that will not cause new defects or aggravate existing defects.
What are the vibrations limits recommended within the U.S. Bureau of Mines reports?
The USBM recommended limits are published in Report of Investigations No. 8507. The “Z-curve or Siskind curve” is the information most often cited, which was published in 1980 as a result of an extensive study conducted in the late 1970s. The study was completed using numerous blasts, throughout several states, while monitoring at different types of structures. The graph shows recommended limits to reduce the damage potential for structures when comparing ground velocity with its associated frequency. The USBM curve is like a speed limit. If vibration levels do not exceed the curve, damage is not likely. For those cases over this limit it does not automatically mean vibration has caused damage; however, it does indicate a thorough evaluation should be made. The graph shows the limits recommended by the USBM to preclude cosmetic damage to plaster and drywall, the most fragile building materials.
It was determined by the evaluation that each of the structure types (single, 1 ½ and 2-story homes) have a varying natural frequency. Blast vibration matching the frequency of the structure of concern may amplify the shaking within the house. If the particle velocity is high enough, cracks can occur or may be extended. This cosmetic, threshold damage potential is alleviated when the limits are maintained. As the frequency of the ground motion changes from low to high (1-40 Hertz) the structure responds less and the limits increase. A safe level of 0.50 ips for plaster and 0.75 ips for drywall that exists for 4 – 15 Hertz increases as the frequency increases to a maximum velocity of 2.00 ips.
I know there has been research on safe blasting levels, but that study was not done in the state where I reside.
Building materials that are in your home did not necessarily come from your state either. The criteria in place are designed to protect building materials regardless of where those materials are located. Those materials don’t change as they are designed to meet minimum ASTM published standards which are incorporated into City, State and Federal Building Codes. While building practices may change to some extent the materials and the types of construction are consistent and would apply.
You can’t tell me everything will be fine when they will be using dynamite 1,200 feet from my home.
Explosives are used every day in very close proximity to manmade structures. For example, blasting for swimming pools next to an existing home, utilities next to roads and buildings, foundations, or a basement for a slab on grade home are typical. It is the level of vibration that creates damage within or to structures and not the distance. Blasting, pile driving, compaction or any other vibration source is reduced or limited the closer to a structure the operation progresses. This is all designed to limit vibration leaving the project site with vibration monitoring equipment.
I feel vibration. Is that not causing damage since I feel it?
People feel vibration and are very perceptive to a very small amount of ground motion. However, people cannot put a value on the amount of motion created, which is necessary to assess whether damage occurs. That is the key issue in determining damage potential. Was there sufficient vibration to create damage? There is no disagreement to feeling vibration, but homes take quite a lot of vibration before damage occurs. Homes are built to move and react to everyday activity and environmental motion. Door slamming, thunderstorm activity and wind all produce vibration that we feel and take for granted since they are “cultural” events. The level of vibration in these events equals or exceeds the vibration from blasting or other vibration activity. Building materials can withstand specific vibration levels before damage occurs.
One blast may be low and below regulated levels, but blast after blast will slowly add up and eventually wear my house down and damage it after several years.
The United States Bureau of Mines studied the repeated effects of vibration on structures. They concluded that as long as the vibration levels were below regulated limits, cumulative or repeated effects that produce fatigue were not seen. Their conclusions summarized that a home could withstand blasting twice per day at 0.50 inch per second for 28 years before the first crack would be expected to be extended, let alone a new crack created. In the same study, they found that there were forces acting on the structures that were greater than the levels from the blasting operation. These factors were changes in temperature and changes in humidity. These forces occur every day and are constantly putting strain on the structure in excess of the vibration. Since these occur over many years daily during the life of the structure, with greater effect there is little vibration will do within the regulatory limits.
Why is it that some blasts feel stronger while others are mild in comparison?
Human perception is not a good measure of blast effects. Blasting and other vibration producing activities do not take place in the same location each time. The change in distance may affect the actual vibration levels and this would be supported by seismograph measurements. Whether or not the person is expecting the blast or whether they are inside or outside at the time of the blast may affect perception as well. In the case of construction blasting the actual blasts may change the explosives type or amount necessary to break rock being excavated. In addition, the size of the blast, orientation and direction of initiation may also affect perception. Also, the airblast may cause an additional effect. The ground vibration travels faster than the airblast (noise) associated with the detonation, and each affect the structure differently. As airblast is highly affected by environmental conditions, mainly wind, the effects change with each blast.
Why do I feel two vibration “jolts” a few seconds apart?
When a blast is detonated, there are two primary effects – one is ground vibration and the other is air overpressure, or airblast. The ground vibrations move very quickly, arriving at nearby structures within a few seconds typically. The airblast waves move comparatively slowly, at about 1,000 feet per second. This can be compared to seeing lightning and then hearing the thunder. The light travels extremely quickly compared to the sound wave. In the case of vibration, the ground motion is the faster of the two components. This creates two separate pulses as the vibration affects the structure and then the airblast.
This blasting is going to ruin my well. How are you going to protect my well?
There are two major areas to consider with wells and natural water supplies. One alleged cause of well problems commonly claimed is from rock fracturing. To cause fracturing or shifting of rock strata, blasting operations must be within close proximity to the well. Many people believe that if the energy can be felt then it is also fracturing the rock at that distance. This is not the case. Fracturing of the rock takes place within about 10 feet of the drilled borehole. Outside of that radius, the energy that remains is elastic in nature and is felt as ground vibration. For the overall blast the fracture zone is a bit bigger but not past a few hundred feet, depending also upon the size of the blast. This is the reason drillers put boreholes on a specific spacing pattern and use multiple holes in a blast. In other words, the fracturing of the rock only occurs within a very small area around the drilled blast hole.
For the vibration component of a blast, well studies dealing with ground vibration produced from surface mining provide solid research. Numerous studies were developed to analyze the relationship between ground vibrations, well components and the aquifer. These studies all concluded that vibration damage to wells and well components requires much higher velocities than the regulated levels currently set by state and local governments. Research shows that wells are resilient to vibration and levels at the surface are reduced at the depth of the well. This precludes damage to submerged pumps and plumbing and the well casing.