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Avoiding Damage
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Nobody - contractors, project sponsors, or property owners - wants damage to surrounding structures to occur during construction work. Yet, it still happens - and FAR too often worldwide. I have discussed or mentioned ways of reducing damage probabilities throughout the CVDG. In this chapter of the CVDG, I will present a simple and economical, but eminently workable, approach to reducing the probability of damage caused by construction project work. Although the chapter is directed at project sponsors and contractors, home and property owners may find it useful for comparing what they see in construction around their properties with the safer ways of doing it as described here. The CVDG Pro's Mitigating Vibration chapter has considerable additional information on this topic. Pre-construction Preparation
The distances in the diagram should be doubled to provide a safety factor for variations in equipment size and use, local soil attenuation conditions and vibration interference effects which are not embodied in the calculation. Structures within twice the calculated safe distances for these construction operations should be considered as being in potential danger of damage from construction vibration. Mitigation measures should be used in such circumstances (see below). In some areas, like much of Florida, that "safe distance" might need to be as much as quadrupled or more, due to the low vibration attenuation conditions there. Better yet, the safe distances could be recalculated[4] with an appropriate attenuation exponent, typically in the range -0.6 to -0.9 for much of South Florida. The calculations behind the diagram above apply a conservative value of -1.1 for the attenuation exponent in the FTA attenuation equation (see Vibration and Distance). As the exponent gets smaller in areas of low attenuation, the distance at which damage might occur gets larger, because the PPV of the vibration decreases less with increasing distance, when attenuation is low. For more on calculation of "safe distances", see Vibration and Distance in the CVDG. Historic Structures If any part of the intended area of the work is near historic structures, whether designated in the U.S. National Registry of Historic Places or not, you will have a special responsibility to make yourself aware of those structures and use low vibration procedures and equipment to minimize vibration velocities. The generally accepted limit for such structures is the Swiss/FTA Class IV PPV of 0.12 in/sec. A simple search of the National Register of Historic Places database for historic sites in the locale of the job will tell you in a few minutes whether there are designated historic structures within 500 feet of the job site, the recommended screening distance for such buildings.[1] Such a search will also help you identify likely areas where other structures, not so designated, but able to meet the criteria for designation, might exist. This latter point is important, as historic structures are protected, even if not specifically designated in the National Register of Historic Places. More information on historic structures and the requirements of the National Historic Preservation Act, as amended in 1992, can be found in the CVDG's Preconstruction chapter.[7] If you live outside the U.S., there are historic preservation laws in many other countries which may affect a construction job. U.S. Federally-funded road and other projects often require a "vibration control plan" or similarly named document to be prepared prior to construction start. Some municipalities and states also require them. Even if a vibration control plan isn't required in your project, it's a good idea to have one in place. The aims of a vibration control plan are to identify problem locations and operations, circumvent damage and provide guidance for handling damage claims, if they occur. The plan should be assembled in close consultation with your vibration monitoring firm and consultants, who, in theory at least, know more about vibrations and their effects than a contractor probably does. A good vibration control plan sets out, among other items:
The goal of a vibration control plan is to allow a contractor to do necessary construction work, without needlessly damaging surrounding structures. That means that a contractor might have to make some minor revisions in the way he does things. He should still be able to do the job on time and profitably, while saving himself years in litigation over damage claims. I have reviewed a number of such plans, usually in the context of damage to surrounding structures. While they were variable in quality and utility, most of them have had at least some serious defects. The most common of these was setting vibration limits based on blasting standards - in construction jobs where no blasting was to be done. Since blasting limits are more than a factor of two higher than accepted construction limits for most building types, using blasting vibration limits in construction is a good way of virtually assuring damage. Also seen often are use of vibration monitoring seismograph trigger limits set at the, often inappropriate, vibration standard limit. If the seismograph only triggers at the standard limit, there is a chance that you will have already created damage before you are notified by the seismograph or have even created any record of it! Make sure that your vibration control plan explicitly accounts for any historic or culturally important structures, which demand lower vibration limits than do homes or engineered commercial structures. Finally, follow the provisions of the vibration control plan when you do the job. If you don't follow the limits set in the plan, or make ad hoc "adjustments" to it whenever your operations exceed the limits (often, many times), rather than adjusting the operations themselves, you're asking for trouble. A good opposing expert in any ensuing litigation will identify any missing documents in productions and failures to follow the plan. Such omissions and failures could make your vibration control plan effectively worthless. Vibration Impact Assessment
A simpler, but perhaps more approachable, tool for assessing vibration impact is offered in a report to the New Hampshire DOT.[10] It provides a means for assessing construction vibration effects pre-construction. It accounts for distance from work, duration of vibration, repetitions of vibration, equipment type, soil types, structure type and other variables, using a simple point scoring system to estimate the potential impact. A very high impact score is assigned to any work within 50 ft. of structures, for example. If your project has historic structures, whether designated or not, inside the recommended screening range of 500 feet, you should check out the NCHRP report's Appendix C for another risk evaluation flowchart.[7] A report for Caltrans has useful guidelines, both for assessing vibration impact and for mitigating vibration.[11] None of these tools is designed to predict the specific probability of construction damage occurrence. They cannot tell you whether damage will occur to a given structure in a given operation. Instead, they are intended to help contractors identify in advance those projects and areas in which construction vibration damage is most likely to occur, based on both experimental and theoretical understanding of vibration propagation and its effects on structures. This foreknowledge allows the contractor to take simple steps to reduce the probability of damage. Surveys Pre-construction surveys are often required in large construction jobs. In theory, they can be used to establish the pre-construction condition of buildings and properties, provide guidance regarding areas where mitigation
measures should be used, and help defend against vibration damage claims. However, the ones done are, most of the time, virtually worthless, either from a damage-prevention or litigation defense standpoint. Walk-by surveys
from the street curb, done as fast as the employee can walk with a video camera, will show no useful detail more than ten feet or so from the curb. Even large features are simply illegible in such surveys.
A video frame grab from an actual pre-construction survey, in which the subsequent work did damage to many homes, is shown at right. Just the most expensive homes in about a half mile section along the over 2 mile long project area were initially surveyed on one overcast day. Only after extensive damage was reported were the remainder of homes along the route quickly assessed by video - well after work start. Note that the 5" high logo on the trash can is completely illegible in this video. Pre-existing cracks in the home, if they had been present, would simply not have been seen in such a survey, even though the contractor alleged that all the subsequent extensive damage (e.g. over 600 cracks) was pre-existing.[5] Ideally, a pre-construction survey should be done inside the homes and other structures, or failing that, from close to the exterior. It should be carried out with sufficient time and video/photo resolution to allow the true pre-construction state of the home to be determined, at very least on the exterior. Either type of survey will require the permission of the homeowner. Problem Operations As discussed in the CVDG chapter, Vibration Potential, construction procedures and operations are not all created equal when it comes to damage potential. Generally, those operations or work components which involve impact with the ground have the greatest potential for damaging nearby homes and buildings. Work with certain types of heavy equipment, particularly vibratory compaction, pile driving, blasting and, in some circumstances, hoe ram (hydraulic rock breaker) use and heavy equipment traffic, may generate damage in structures, if not done with an eye to the vibrations they create. Of course, if equipment is misused (e.g. using an excavator bucket to demolish asphalt by pounding on it), damage can be done by other equipment types, as well. If whatever job you're doing involves ground impact of any sort, you should take precautions to avert vibration damage (e.g. change the method of the operation, lower the height at which objects are dropped on the ground, limit driving of tracked vehicles, use high frequency or oscillatory compactors, etc). Of course, no amount of knowledge of problem-causing operations will matter if you or your workers simply don't care if they do damage or won't believe that it is possible.13 Development activities, mostly compaction operations and demolition, follow very closely after road construction as the largest cause of damage reported to Vibrationdamage.com. Demolition operations by themselves figure prominently in our damage reports, too. Even small projects like additions/renovations to adjacent single structures can cause unintended damage to neighboring homes and buildings. The CVDG Pro chapter, Damage Statistics, is devoted to an extensive analysis of our over 1500 damage reports (as of this writing) worldwide, as well as to other aspects of vibration effects. The analysis includes project types, operations, equipment types, sponsors, damage types, distances from work, locale, structure type, professions, and many more. Carry Out Vibration Monitoring
Your monitoring should not only record each day's vibrations at the most critical sites (i.e. those closest to blasting, pile driving, demolition, heavy equipment traffic and compaction operations), but it should provide real-time feedback to your workers, in the form of a siren or flasher, email notification[14] or other alert, when their operations approach your appropriate limit values (see photo at left for one example of such a configuration). Often, that immediate feedback is the difference between causing damage, which will cost you and your insurers large settlement costs or years in litigation, and having a problem-free job. A vibration monitoring report which you receive weeks after the work is done is of almost no value in preventing damage. [14] There are many chapters in the CVDG Pro devoted to various aspects of vibration monitoring, including Hiring a Vibration Monitoring Firm, Reporting Vibration Data, and many others. The CVDG chapter, Vibration Monitoring, has a general introduction to vibration monitoring - what it can tell you, how to do it in accord with accepted procedures and standards and what not to do. Use Vibration Standard Limits Properly to Prevent Damage As I said earlier, one of the biggest contributors to damage during construction jobs is the application of the wrong vibration standard limits or, worse yet, no meaningful use of any at all. Blasting standards (from OSM or USBM RI 8507), which are based on damage probability estimates for single, short-lived blasting vibrations, are simply inappropriate for long-lived, repeated construction vibrations. For example, the 0.75 in/sec blasting limit of OSM is nearly four times that recommended for construction vibration in both the Swiss and U.S. FTA Class III standards (0.2 in/sec for timber-framed homes, 0.12 in/sec for historic, damaged or other "fragile" structures). Since damage is "expected" at twice the Swiss/FTA limit,[2] barely meeting the requirements of a blasting limit nearly four times the Swiss/FTA limit all but guarantees damage! Vibration standards are frequency dependent. If your standard limit is based on 30 Hz vibrations (e.g. the 0.5 and 0.75 in/sec USBM RI 8507 limits), but the ones actually observed in the monitoring, by FFT analysis of the vibration waves, are at 10 Hz or cover a broad range of low and higher frequencies, you must use a lower limit appropriate for the lowest frequencies actually seen in your operations. Of course, if you do no vibration monitoring at all, you will have no idea whether your work is exceeding vibration standards - until the damage reports start to come in. Even if you are correctly using the OSM limits for construction blasting, caution is advised. The OSM limits are based on probabilities of damage from single blasting events. The probabilities of occurrence for multiple, statistically independent, events are the sum of the probabilities for the individual events. Thus, a single event probability of 5% (i.e. the damage probability for blasting at the OSM limits) becomes at least 50% if you blast 10 times at the same distance from the structure and explosive charge weight (see Vibration and Homes in the CVDG Pro for more on this). Most people would consider that probability of damage FAR too high for comfort. This becomes especially scary when one considers that blasting damage probabilities relate to the number of homes damaged that experience a given PPV, not the probability of damaging a single home from the same blast. Thus, a 50% probability of damage means that half of a statistically significant number of homes which experience the blast vibration of that PPV would be damaged. See both Vibration Standards and Vibration Regulation in the CVDG and both Vibration and Homes and Statistics and Vibration Damage in the CVDG Pro for much more on these issues. Plan for the Unexpected Vibration velocities are hard to predict accurately even under the best of site circumstances (deep, non-layered soils, level topography, consistent underlying geology, no other nearby structures, no glacial deposits, well-trained and consistent use of equipment, among others), even if you have "calculations" or "guidelines" which suggest that your activities might "meet standards". Reflection and interference of vibration waves (see Vibration 101) can increase the actual vibration velocity in small areas over the predicted value by a factor of two or more. Similarly, unless you have intimate understanding of the local soil and its vibration attenuation properties, expected vibration velocities can be substantially exceeded. You can reduce these uncertainties by learning as much as you can about the local vibration attenuation. If you're very lucky, you'll be able to find studies in your area, available free on the Internet, which tell you the specific attenuation exponent which should be applied in your calculations (see Vibration and Distance). However, it is safest to give yourself a substantial margin below the nominal limit to allow for these uncertainties. It only takes one incident of a vibration substantially over construction limits to do considerable damage. I have suggested here and in the CVDG's Vibration and Distance chapter that a factor of two in vibration velocity would account for most of the expected variation, although even that factor might have to be adjusted in areas with low vibration attenuation - like most of the U.S. state of Florida. Thus, if you are trying to obey the FTA Class III limit of 0.2 in/sec, it is wisest to make every effort to meet a limit of 0.1 in/sec, to account for these unexpected, but common, variations. Do the Job Right
Vibration Mitigation Mitigation is the process of actively reducing vibrations in construction operations. Mitigation need not necessarily be undertaken in every locale of a job or for every operation, but only those operations where the work is close enough to have real potential for damage. Maximum distances where such mitigation should be undertaken depend on both the operation and the local vibration attenuation conditions. For example, in most locales, mitigation should be considered for any vibratory compaction operation within 50 feet of structures. That distance should be doubled or even quadrupled, depending on the local vibration attenuation, if any of the structures are "historic" or "cultural assets". OSM sets such distances (actually "scaled distances") for blasting. As long as the blaster meets the scaled distance requirements, he is not required to perform vibration monitoring, according to U.S. Federal regulations. Most mitigation methods for construction vibration are easily implemented at little or no cost. Make certain that your operators understand and follow the operator's manuals for their equipment. Any method of using such equipment which is dangerous for the equipment and/or the operator is probably also risky from a vibration standpoint. Vibratory compaction is the single most often cited damage cause in correspondence to Vibrationdamage.com. Vibratory compaction operations can produce up to a factor of two higher vibration velocities, if the operator fails to turn off vibration temporarily while he reverses direction. The effect of all these repeated short-duration vibrations ("transients"), along with the continuous ones, can be dramatic, both to those experiencing the vibrations and their effect on structures. Another easy mitigation step for compaction operations involves using a compactor with a higher vibration frequency (above 40 Hz) and lower drum weight near sensitive structures. Because the shape of the vibration velocity vs. damage probability curves is sigmoidal, making multiple passes at a low vibration level is much less likely to produce damage than a smaller number of passes at a high vibration level. Because oscillatory compactors produce in the range of 10-25% of the vibration velocities (PPV's) generated by vibratory compactors, they can be an important part of a vibration mitigation plan for compaction operations. In addition, they require fewer passes to produce the same degree of compaction, thus saving the contractor money and time.[8] Resonance (or vibratory) pile drivers produce much less vibration and faster driving rates than standard impact hammer drivers.[12] There is a substantial discussion of how to reduce compaction vibration in the CVDG's Vibration and Distance chapter and an even greater one in the CVDG Pro's Vibration Mitigation chapter. The principles behind vibration mitigation are discussed in Vibration 101. Dealing with Claims If, in spite of your best efforts, a damage claim is made, you should not dismiss it out-of-hand as false or "pre-existing damage". The first step in dealing with a claim of vibration damage is to admit that it is at least possible that construction work could be responsible for part or all of the damage. Assertions like "construction can't cause damage" have literally no support whatsoever in the scientific literature. Indeed, at any given ground vibration velocity, construction is more likely than blasting to cause damage (see Is Damage Possible? in the CVDG for much more on this). Damage claims which are discarded immediately as " pre-existing damage", with little or questionable scientific support, are likely to lead you directly to litigation. Even when there is real (as opposed to asserted without evidence), clear-cut, pre-existing cosmetic damage in the structure, that doesn't prove that all the damage is pre-existing. Most structures have a small amount of pre-existing, barely visible damage. But, an increase from 3 such sites to 300 obvious ones within half an hour during construction[5],[6] is not credibly attributable to environmental factors, construction of the home or movements of the residents within the home. Moreover, the proliferation of devices and systems that can record photos and videos (smart phones, security cameras, satellite photos) means that your ability to dispute what you did and how you did it are becoming more limited. If you don't do vibration monitoring, or want to keep the results confidential, that, too, may be impossible in the light of use of smart phones and tablet computers by homeowners to do their own vibration monitoring. In the face of such data, you may be forced to produce your own data, which you would otherwise keep secret. At Vibrationdamage.com, well over 1600 claims of construction vibration damage have been reported to us from all over the world, as indicated earlier. That number grows constantly. Many of those reports have been supplemented with video, photo and other evidence which lends additional credence to them. For those with an interest in understanding how to document scientifically and attribute damage, please see our Damage Assessment chapter of the CVDG Pro. Most construction vibration damage claims involve relatively "small" dollar amounts (a few tens of thousands of dollars) when compared to a construction project funded for millions of dollars. If you can settle a likely legitimate case with a minimum of lost time and bad feelings, you will avoid the unpleasant (and expensive) consequences of having a home or business owner (or even whole neighborhoods) angry at you and your project. Pursuing a Claim in the CVDG has much additional information for both homeowners and contractors about dealing with vibration damage claims. Constructing Profitably - and Responsibly The overwhelming majority of construction vibration damage is completely unnecessary and easily prevented at minimal cost. Given the cost and time commitments of handling (and litigating) damage claims, most contractors would probably choose to limit vibration damage, when they can do so simply and relatively cheaply. For the most part, avoiding damage in construction projects is about knowledge and acting on that knowledge in a timely fashion. All the steps advocated in this chapter of the CVDG can be performed at a tiny fraction of the total cost of a typical project and in a matter of a few hours at most. Construction vibration damage is almost entirely avoidable when you do your homework and monitor both the vibrations and how your work is performed on site. Doing all these things indicated in this chapter of the CVDG would add only a few hours to the job, but could save contractors years in litigation over damage claims, as well as jury awards to plaintiffs against you. It may not be possible to reduce vibration damage probability to literally zero in construction projects (e.g. those in which sensitive structures are unavoidably close to the work), but you can easily and dramatically lower its probability, its severity, and the number of affected structures by implementing simple and usually cheap vibration mitigation measures.
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E-mail to drzeigler@vibrationdamage.com or send online questions or comments about Vibrationdamage.com.
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There are a number of "tools" available for helping those charged with planning construction projects to estimate the potential for damage or other impacts
from vibrations generated during construction. Some of them focus on non-construction or specific types of construction-related vibrations. One of the best of these specific-use tools is the U.S. FTA Transit Noise and
Even though readily accessible in most towns and cities in the U.S., vibration monitoring of construction jobs is still relatively rare. Often, it is
only begun too late - after the first damage report. Yet, it widely considered to be essential by experts in 
Most vibration damage to homes and other structures is avoidable. Once you've done your "due diligence" in the ways described above, you cannot sit on the sidelines and assume that your employees will always follow your instructions
and policies. Even your supervisory personnel may be lax in their enforcement of your regulations. I have personally documented on video multiple examples of site supervisors watching operations, without any intervention, which were specifically
prohibited by the contractor's policies, e.g. pounding on asphalt with an excavator bucket to break the asphalt (see photo at right). The supervisor of that operation, banned by the contractor, is the man in the white hard
hat in the photo watching the operation as it occurred. The CVDG chapter, 
