Tips to Minimize Adverse Effects of Construction Vibrations on Historic Properties
- posted in: Knowledge Center
Projects that involve ground-disturbing activities oftentimes go to great lengths to avoid adversely affecting historic properties that are significant cultural resources such as buildings, structures, and even sites protected by Section 106. Some activities result in very obvious effects; others not so much. Thorough evaluation during the pre-construction planning process identifies most effects and makes recommendations to avoid, minimize, or mitigate adverse effects. Occasionally, some effects fly under the pre-construction planning radar.
Construction vibrations are phenomena that are sometimes overlooked, yet these unseen products of blasting, demolition, pile driving, and compaction can travel well beyond a construction right-of-way and cause cosmetic and, in severe cases, structural damage to significant cultural resources. Discovering their effects after a project is underway can have costly consequences. Certain project elements may be delayed while a vibration monitoring plan is prepared. Disgruntled property owners may file damage claims. Project proponents may receive bad press. Time, money, and project supporters’ good will are potentially wasted.
If your project includes ground-disturbing activities, then it is advisable to consider whether construction vibrations will adversely affect historic properties. LSD’s cultural resources specialists can estimate two important vibration values, amplitude and frequency—using theoretical and empirical models and basic project information. If the values are likely to exceed safe limits, then a vibration monitoring plan can be implemented before a project begins. A good plan will include at least three components.
Existing Conditions Inventory. Prior to construction, the existing condition of at-risk properties needs to be thoroughly documented. Structural and cosmetic deficiencies should be recorded using quantitative rather than qualitative descriptors. Why? Because most people can agree that a crack initially measuring 2 mm wide either maintained or increased its size, but getting two individuals to concede that a “small crack” became a “big crack” or remained unaffected is nearly impossible, especially when competing interests are at stake.
Seismic Monitoring. Measuring construction vibrations with portable seismographs is also important. To collect meaningful and accurate data, equipment placement and programming must be carefully considered and the construction activity being monitored needs to be identified. Continuous vibrations, like those produced by vibratory compactors, have much lower limits for excessive vibrations than impact-type vibrations, such as those resulting from pile drivers. Moreover, limits for historic properties are significantly lower than those for newer buildings and structures.
As monitoring results become available, they should be shared with construction personnel who can adjust equipment or methods. Conscientious supervisors understand that having an operator make more passes over an area at a lower vibratory setting is not a hardship when compared to work stoppages and damage claims.
Settlement Analysis. Some soils are more conducive to settlement than others when vibrated; therefore, it is smart to consult with an engineering geologist or geotechnical engineer about potential risks. And because geotechnical studies are typically required for planning and design purposes, questions regarding settlement risk can be embedded in the upfront evaluations. Settlement analysis—survey of selected points on at-risk properties, before, during, and after a project—may prove useful if the potential for movement exists. Construction aids, modification of construction techniques, or a combination of both may be beneficial when the potential for movement is high.
Construction vibrations have the potential to adversely affect historic properties, yet they occasionally are overlooked. Project delays, damage claims, NHPA Section 106 noncompliance, and blemished reputations may result if their damaging effects are detected after a project begins. LSD’s cultural resources specialists can assess your project’s potential to adversely affect historic properties and then develop a plan to reduce that risk. The dividends derived from a well-executed plan will far outweigh the costs associated with its preparation and implementation.
Historic properties are districts, sites, buildings, structures, or objects included in, or eligible for, nomination to the National Register of Historic Places. 36 CFR 800.16(l)(1)