Vibration monitoring involves measuring short-term movements in the ground or in a structure by using a seismograph like the one shown at left, or other instrument, to detect and record the movement. It is the most commonly-used method for identifying potential vibration damage problems pre-construction and addressing them post-construction. Most Federally-funded construction projects require pre-construction vibration assessment. Individual states and municipalities may require no monitoring or monitoring only in connection with blasting done during the project.
Properly done, vibration monitoring can be extremely helpful in understanding the nature of damage to structures and whether construction could have caused it. Improperly done, as shown in the photo above left, it can be worse than worthless. Since construction companies will sometimes present to those who have damage claims copies of vibration monitoring data or reports, this page will offer an overview of vibration monitoring and what can be learned from it.
Vibration Monitoring Instrumentation
A number of manufacturers make ground vibration monitoring seismographs, primarily for use in monitoring mine blasting, although they are also routinely used in construction vibration monitoring. Like earthquake seismographs, these detect and measure ground vibration by the movement of a magnet suspended in a surrounding by a coil of wire. According to the Lenz Law of physics, a current is induced in the surrounding coil in proportion to speed of movement of the magnet with respect to the coil (i.e. the speed of ground movement). Electronics in the monitor then measure this current, convert it to ground motion velocities and store the raw data in memory. Each of the three directions at right angles to each other (longitudinal or radial, transverse, and vertical) has its own separate measuring coil in the transducer head, since vibrations often differ in important ways along different measurement directions. For accurate measurements to be obtained, the seismograph measuring head ("transducer") must move as the ground moves, i.e. it must have full "ground coupling".
At right is a photo of a Blastmate III blasting seismograph1, manufactured by Instantel, in use. The silver gray transducer head at the lower right of the photo, which does the measuring, is sitting on top of the loose landscape rock and connected by the visible wire to the blue recording box. The blue box actually stores the data in its memory, much like a small computer. This instrument can also keep a paper tape backup record of the data as it is created, using its integrated printer on the left side of the blue control box. Although not seen in this example, blasting seismographs usually also have a microphone attachment which can be used to measure sound from the operations. This photo should not be taken as showing a proper installation of the seismograph, as there are numerous problems in the installation which contradict Operator's Manual instructions and other guidelines set by industry and government groups.
Memory Limitations of Blasting Seismographs
Blasting seismographs are capable of providing accurate recordings of ground movement when used properly in blasting settings. However, most standard blasting seismographs are not ideally suited for monitoring construction vibration, due to limited internal memory. The seismographs shown in the photos in the CVDG, in their standard configuration used, have a specified event capacity of "Up to 13 one-second events (1024 sample rate, four channels recording)"1 in the Histogram Combo mode employed for all recording on the project in which they were utilized.
Construction work can, and did, generate that many vibrations and many more, often filling the seismograph event memory in under a minute in a road reconstruction job. As repeatedly documented in that project, the amount of memory normally present in many blasting seismographs is often grossly inadequate for recording waveforms for an entire day of construction work. In that project, vibration events filled seismograph event memory hours before the end of the work day on 23 of 29 days for which seismograph data were produced (of 39 days monitored). Insufficient event memory means that, at best, very limited data will be recorded for many of the strongest construction vibrations, severely limiting the ability to draw meaningful scientific conclusions from such data and crippling the estimation of damage potential.
Such full memory conditions can be explicit, as shown by the "Memory Full Exit" (MFE) indication in the monitor log graphic above left. They can also be implicit, when vibrations are recorded in the histogram data which are above the trigger level of the seismograph, but are not shown on the log and, for which, no waveform data are recorded. When histogram data are not available, one can still identify likely memory full conditions by looking at the monitor logs to see if event reporting stopped well before the end of an active work day, especially when the recorded events are tightly grouped together in time.
While some of the latest blasting seismographs have greater event memory that may be sufficient in some construction situations, most earlier generation seismographs in use today have this severe limitation. These seismograph memory limitations can be so critical in construction vibration settings, and so prejudicial in legal proceedings, that their impact is discussed in more detail on a separate page in the CVDG Professional Edition, Seismograph Memory Limitations.
Other Ground Vibration Measurement Instruments
Other kinds of instrumentation can also be used to detect and measure ground vibration and its effects on structures. These include accelerometers, displacement gauges and strain gauges. They provide different, but complementary, information to that provided by seismographs. While these other equipment types are valuable and used in scientific studies, they are rarely seen in construction or blasting site vibration monitoring nor are their directly measured properties considered the best indicators of damage potential.
Recent advancements in sensor technology of cell phones and tablet computers, along with development of several software programs to access the sensors, have made it possible for homeowners to do their own vibration monitoring at very reasonable cost. The topics mentioned in this paragraph are discussed in more detail below and on the pages, DIY Monitoring and Vibration Measures, available only in the free CVDG PDF download and in the CVDG Professional Edition, not online.
Handling of Vibration Data
Once recorded, the vibration technician downloads the data stored in the seismograph memory to a personal computer for printing and analysis with appropriate software. In the case of the Blastmate III seismograph shown above, the software for Windows® is called Blastware®, and can be obtained free from Instantel. Other manufacturers of seismographs make similar software available for their products. Once the data are in the computer, the software can display those data in various "reports" (see at left for one such report type), similar to database program reports, that show them in different, complementary ways. Although the reports generated by software from the manufacturers of different seismographs will differ in both name and detailed content, they will mostly show the same data in much the same way.
Reading Vibration Monitoring Results
Construction companies and the vibration monitoring subcontractors they hire have a vested monetary interest in finding that vibrations are non-damaging to structures. Some may also be ill-prepared to understand the real meaning of vibration monitoring results. For these and other reasons, you may be the only person involved in a vibration damage claim who will take the time to read and analyze vibration monitoring results carefully. You should make time to do so, or, better yet, ask a qualified scientist to read and analyze them for you. Some of the problems you should look for are detailed on the CVDG Pro Vibration Data Issues page. Extensive additional information on vibration monitoring, report reading and analysis, interpretation and standards can be found in the Construction Vibration Damage Guide, Professional Edition.
Analysis and Interpretation of Vibration Monitoring Data
Proper and careful analysis of vibration data is laborious and time-consuming, but essential. At right are shown just four of the over 50 pages of tables which I prepared, relating to the detailed analysis of the partial set of produced vibration data from one road reconstruction project. Each table looks at the same data in different ways and provides a view and analysis of different elements of the data. Virtually all of these views proved important in understanding what data were present and what they really said, versus what was claimed by the construction contractor and its vibration monitoring sub-contractor.
For meaningful conclusions to be drawn from vibration data, they have to be properly obtained (i.e. following seismograph manufacturer and industry group instructions), analyzed and interpreted. All the data must be used, not just carefully selected parts, and they must form a consistent, or, at least, non-self-contradictory picture. Otherwise, the conclusions can have little or no scientific validity.
Vibration monitoring data analysis, interpretation, reporting and use is such an important and large area that many pages in the CVDG Professional Edition are devoted to various aspects of these and related topics. For help in reading and understanding vibration monitoring data, read the CVDG Pro page, Vibration Data Reports. For more on possible problems to look for in analyzing the data, see its page, Vibration Data Issues. The CVDG Pro page, Vibration Data Analysis, has a much longer and more detailed listing of specific vibration monitoring issues. For detailed tips in analyzing the large amounts of vibration data generated in monitoring, see our CVDG Professional page, Vibration Data Handling.
Standards and Their Proper Use
Even if vibration monitoring data can be read and understood, they won't be of much value in the absence of some independent, accepted means of linking them to the potential for causing damage. Because the mining industry has faced vibration damage claims related to blasting activities for many years, most of the data on structural effects of vibration, and standards for interpreting vibration data in that light, are based on studies done by or for the mining industry. Unfortunately, these blasting related studies and standards, while often used by construction companies and state agencies to justify their positions, provide little help in evaluating the potential for damage from construction vibration caused by heavy equipment.
Fundamentally, the reason for this is that blasting events differ dramatically from construction vibration. Blasting at a typical mine or quarry occurs perhaps once every day to every few days; it produces vibrations which last a few seconds at most (more typically, less than one second for the direct blast vibration). On the other hand, construction vibrations can go on for minutes, hours, days or even months. The diagram at right shows one of many examples where construction vibration repeatedly occurred and persisted at levels above the FTA Transit Administration (shown above) construction standards for minutes at a time. This difference brings into play resonance effects and amplification phenomena which are far less prominent or completely absent in most blasting environments.
A fundamental distinction between blasting events ("shots") and construction vibration is explicitly acknowledged in USBM RI 8507, the basis for the frequently used OSM blasting standard:
Thus, use of blasting standards for non-blasting, long-lived construction vibrations is ill-advised and scientifically questionable. Of course, even well-motivated contractors may simply not think about or understand these disparities. Proper use of vibration standards is a long and important topic which is investigated further on the CVDG page, Vibration Standards and in the CVDG Professional Edition.
Distance Makes the Vibration Become Lesser, ....
Vibrations die off with distance, although perhaps not as fast as most people would think. The lessening intensity with distance is the rationale behind what are called "scaled distance" calculations accepted and used by the mining industry to estimate vibration effects at distances other than those measured. The Federal Transit Administration's Noise and Vibration Manual (formally titled TRANSIT NOISE AND VIBRATION IMPACT ASSESSMENT) also provides a version of a "scaled distance" equation and necessary parameters for that equation to calculate expected vibration intensities at various distances from different types of standard construction operations.5 This equation may be useful when no vibration data exist, but damage is done.
Users of the equation should note that the benchmark vibration velocities for appropriate uses of construction equipment given in that reference and shown at right from that reference5 are indicated to be representative, not characteristic, of all such pieces of equipment or all methods of use of the equipment. Improper use or use of different equipment than that indicated in the standard could produce larger or lesser vibrations. You will need distances from your home to the vibration sources for use in this equation. You can either measure them or use Google Earth's Ruler feature6 to get them from satellite photos of your home and its surrounding area.
...But It's Not That Simple
Simple distance-based calculations do not tell the whole story of damage potential. As vibrations propagate through the ground, their frequency distribution changes from a typical broad spread of high and low frequencies to what are referred to as "low frequency wave trains" (see USBM RI 85072):
The frequency of the vibration wave trains is close to the resonance frequencies of the house. At the resonance frequencies, vibrations in the house grow with additional vibrations, rather than dying out. Thus, distant vibrations with resonant components can be more damaging than vibrations which originate nearby, especially when they last for longer than a few seconds, as is typical in construction vibration. Vibrations which may not be damaging close to the site of their inception, may become damaging at larger distances (typically, a few hundred yards for blasting vibrations), even though they are less intense at the greater distance.
For that reason, having your house at some distance from the work may not guarantee vibration damage safety nor can distance always be used as a legitimate scientific argument against damage causation. In our own case, the largest single crack that we observed developed when the construction work was a block away, although the cracks and other damage became more numerous and widespread as work approached the house. As a rough guide, if you can hear the construction operations in the distance, there might be reason to be concerned7, depending on what operations are being carried out and how they are being done. The effect of distance in vibration damage is discussed further on the CVDG Pro page, Applying Vibration Standards.
Locating and Using Blasting Seismographs
Vibrations spread out in all directions from the source, but usually not equally so, for reasons discussed on the CVDG page, Vibration 101. Because the frequency distribution, intensity and interaction of vibrations with structures depend so strongly on distance, a good vibration monitoring program will include more than a single seismograph, each well-located, calibrated and properly installed. Thus, good vibration monitoring will use two or more seismographs in tandem, one placed near the source of the vibration and one placed, perhaps, a hundred yards away. The second seismograph should be placed to avoid interference from vibrations from any additional operations going on at the same time as those producing the source vibrations. Of course, the intensity (velocity) of vibrations at the distant seismograph will be lower, in the absence of vibrations from other operations, but the point of the second seismograph is to detect and quantify the low frequency wavetrains2, as discussed above. Their frequency distribution and duration are at least as important as their measured velocities (peak particle velocities, PPV's).
Vibration Monitoring Data Reporting
It is common in the field of vibration monitoring simply to quote the maximum PPV observed for a given time period, location or operation, with, perhaps, a single "zero crossing frequency" (a simple approximation of the frequency of the largest vibration component - see Vibration Frequencies for more information) for the quoted vibration. While this simplistic approach may seem adequate to some, it has little scientific meaning or validity in the context of assessing potential for damage. As discussed on several of the CVDG pages, including Vibration Standards and Resonance and Fatigue, damage potential is dependent on both the maximum velocity of the vibration and its full frequency distribution.
Many types of construction vibration, particularly those resulting from impact (pile-driving, vibratory compaction) or impact-like events (tracked heavy equipment "drive-bys"), have broad frequency distribution spectra (see tracked excavator drive-by vibration frequency spectrum at right). These show significant intensity at the resonant frequencies of homes, even if the peak frequency and PPV of the vibration is not of any special concern. Such broad spectrum vibrations cannot be adequately or appropriately described by a single frequency PPV.
Thus, simple quotations of PPV's, even if accompanied by the zero crossing frequency, can be misleading and are of little value in estimating damage potential. One must take into account the complete frequency spectrum of the vibration, as derived from FFT analysis. Sadly, most vibration data are reported as single frequency PPV's, lacking any spectral analysis whatsoever. In all too many cases, FFT analysis of the frequency composition is not even possible, due to early filling of inadequate seismograph event memory with waveform data. These issues are further explained and developed, with actual construction vibration examples, in the CVDG Pro pages, Seismograph Memory Limitations, Reporting Vibration Data and Vibration Signatures.
Should I Allow Vibration Monitoring?
You may well get a request to allow monitoring on your property, or the technician may simply come onto your property without permission, as was done routinely in the case with which I am most familiar. You should think through whether or not you wish to allow vibration monitoring on your property. Your best aid in making such a decision is to find out as much as you can about any previous monitoring done on the project, prior to the request to you. It is probably wise to delay granting permission until you have seen and analyzed any previously generated reports and data, with an eye to the possible mistakes that can be made in vibration monitoring (CVDG Pro). Since the vibration technician probably works for the contractor, you can expect that any errors made in seismograph setup, use procedure and data interpretation will not favor you. Whether problems exist or not, delaying a decision on allowing monitoring is about the only leverage you will have in getting the monitoring data, short of filing a lawsuit.
If you don't allow monitoring, you may not be able to stop monitoring in the public right-of-way portion of your yard (i.e. that part of your yard bordering the street reserved by ordinance for street or sidewalk expansions - example at right). If monitoring proceeds anywhere in your yard or immediately adjacent to it, you are highly advised to videotape or photograph each and every installation of the seismograph, noting the time of installation and removal, to the extent you are aware of them. Your record may well be the only reliable documentation of the way the work was done.
Do-It-Yourself Vibration Monitoring
Vibration monitoring done on behalf of contractors can be so variable, both in execution (note "professional" installation at left for a good example of how not to install a seismograph properly) and interpretation, that it might be highly advisable to do your own vibration monitoring, as a check on the contractor monitoring. However, that may be beyond the financial capability of most homeowners. You can easily spend $10,000 or more to buy a seismograph vibration monitor or hire someone to perform the monitoring using a seismograph. However, recently this picture has, at least in principle, changed dramatically.
Many modern cell phones and tablet computers have an entire suite of sensors, including acceleration sensors and a GPS location sensor, which can be accessed by software ("apps") to turn the phone or tablet into a do-it-yourself vibration monitor (see example at right, using a small, generic tablet running a free vibration monitor app). With small tablet computers available for as little as 30 USD and cell phones with similar capabilities just about everywhere, anyone experiencing or concerned about construction vibration should seriously consider investing that much money to use the tablet or a phone as a mini vibration monitor. To learn more about smartphone and tablet computer sensors and how to use a phone or a tablet to do your own monitoring, see the free downloadable CVDG PDF version pages, DIY Vibration Monitoring and Vibration Measures (not available online) and the more extensive how-to information on this topic in the CVDG Professional Edition.
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