Webinar post #3: Dr. Rune Storesund, D.Eng., P.E., G.E., S.NAFE, F.ASCE
This blog post, the third in this webinar series, covers 16 questions raised during this webinar. Answers have been edited for clarity. To view the webinar in full, please go here.
Q: How can we determine if cracks in a structure are caused by settlement, compaction, or construction vibration?
A: The most reliable way to differentiate crack initiation/enlargement from construction activities is by physical mapping prior to start of construction and monitoring during the course of construction activities, but this can be cost-prohibitive for most construction projects. For purposes of this question, ‘settlement’ is typically a longer-term phenomenon that is the result of compression of underlying subsurface soils as a result of loading (dead + live loads) stemming from the structure’s foundation. ‘Compaction’ would be those external activities (not from the structure) that compress the structure’s underlying soils. Construction vibrations may result in visible cracks in a structure from temporary elastic differential deformations within a structure and/or from plastic deformations (such as compaction of underlying soils).
If you have a crack monitor installed on the structure as part of an active construction monitoring program, you can correlate crack occurrence with the location where and when the construction activity(ies) took place.
Q: What vibration levels are considered high before implementing a vibration monitoring program?
A: The threshold for high vibration levels varies depending on the site and structure. References and guidelines provide guidance on potential damage based on soil conditions. For sensitive structures, vibrations as low as 0.1 inch per second may be significant, while more robust structures can withstand vibrations of one or two inches per second. The specific susceptibility of the structure and the propagation characteristics of the site determine the acceptable vibration levels.
Q: Can non-damaging vibrations during construction monitoring become damaging if they occur for a longer duration?
A: There is no clear correlation established between the duration and magnitude of vibrations causing damage. While longer-duration events may be a concern, the research literature does not provide specific guidelines on the impact of duration on structural damage. It would be prudent to track and monitor vibrations of any duration to ensure they remain within acceptable limits.
Q: How can the response time of geophones be associated with piezometer readings for pore pressure build-up?
A: By coordinating the date and time information from both the ground vibrations recorded by geophones and the pore pressure readings from piezometers, you can establish correlations between the two. This requires having time-series data for both the ground vibrations and pore pressure measurements. The common link between the two sets of data is the date and time, enabling analysis and correlation.
Q: How many vibration monitoring devices would be needed for a 200 ft by 200 ft site to obtain a realistic plot of the Peak Particle Velocity (PPV) distribution?
A: It is recommended to have a minimum of two vibration monitoring devices, but three would be preferable for a 200 ft by 200 ft site. With just one sensor, it is challenging to perform accurate triangulation. Having at least three vibration monitors provides redundancy and backup. In case one device malfunctions, you still have two functioning vibration monitoring devices. Although three devices are recommended, it is possible to work with as few as two sensors.
Q: When assessing human annoyance to vibrations, do you consider VDB (Vibration Dose Value) values instead of cosmetic or structural damage?
A: Each site has unique factors to consider when assessing human annoyance to vibrations. Ground vibrations are typically felt by people before they cause cosmetic or structural damage. To manage expectations and mitigate annoyance, it is recommended to establish a communication program associated with the construction work. Communicating with residents and addressing concerns in advance can be an effective risk management strategy regarding vibration perception. Given the large portfolio of factors and site-specific nuances, a sole reliance on a site-agnostic parameter such as VDB may leave you vulnerable to preventable claims had an effective communication strategy, coupled with a robust vibration monitoring program, been deployed in the larger work area.
Q: If vibrations are not directly measured on structures, how do we consider the effect of Peak Particle Velocities (PPV) on them?
A: It is highly recommended to measure vibrations directly on structures, as this provides site-specific and actionable data. The soil conditions, foundation types, and other factors vary across different structures. While there may be some existing correlations, having vibration monitoring instruments on both the ground and the structure allows for a better understanding of the soil-structure interaction and provides valuable data for assessing the impact of vibrations on the structure. Absent structure-specific data, correlations would need to be used. These correlations have a number of implicit and explicit assumptions, and you would need to be able to demonstrate that all the correlation assumptions are valid for your particular site. This can be challenging.
Q: Have you explored using historical SAR and lidar data to optimize the placement of vibration monitors for increased redundancy in high-risk areas?
A: As part of the pre-project planning assessment, utilizing all available data, including SAR and lidar, is helpful. These data sources can provide valuable information, such as elevational details, which may not be available from aerial photos alone. For an effective construction monitoring program, it is important to start with a thorough understanding of the project plans, site limits, access, and equipment expected on-site. Overlaying this information with SAR and lidar data can enhance the optimization of vibration monitor placement, especially in high-risk areas. However, these types of data are not essential for configuration of a vibration monitoring program. The most important element to consider is the location of the work generating the vibrations (and magnitude of anticipated vibrations) relative to the adjacent structures. An initial site walk by the entity configuring the construction monitoring program is probably the most important aspect for optimizing a monitoring program.
Q: How does a non-vibratory backhoe sheep's foot roller affect nearby structures and close-in displacements?
A: While a non-vibratory backhoe sheep's foot roller generates some vibrations as it rolls over the soil, the magnitude of these vibrations is typically lower compared to other compaction equipment. Using equipment with lower magnitude vibrations, like the sheep's foot roller, can be an effective strategy to minimize the impact on nearby structures and reduce close-in displacements.
Q: Where should analog crack monitoring instruments be installed on structures? How do we identify potential cracks in the structure?
A: During a pre-site assessment, you should walk around the structure to identify existing cracks. Obvious cracks on walls or other visible areas can be good candidates for installing crack monitors. The assessment should consider the layout and configuration of the site to determine additional opportunities for crack monitoring. This may include cracks on sidewalks, back sides of buildings, or any other areas where cracks are visible and can be monitored.
Q: What is the recommended depth for a vibration isolation trench, if possible to construct one?
A: The recommended depth for a vibration isolation trench will vary depending on site-specific considerations and the type of construction being conducted. It is worth noting that isolation trenches are not commonly used for vibration isolation purposes, as space limitations on construction sites often pose challenges for implementing such techniques. A more effective alternative may be to modify the construction approach (i.e. drilled piers instead of driven piles).
Q: Do you consider vibration frequency and resonance potential when designing a vibration monitoring plan? For example, the difference in resonance impact between rock chipping at 100 Hz and hammer pile driving at 15 Hz.
A: Yes, it is essential to consider the frequency and resonance potential of vibrations in the design of a vibration monitoring plan. Analyzing the construction equipment and the vibrations they produce, including their frequency and duration, should be part of the construction monitoring assessment. By understanding the frequency and characteristics of the vibrations, you can evaluate the potential response of surrounding structures, including the resonance effects. This question illustrates the importance of site context. You would not typically expect pile driving at a site requiring rock chipping as piles are generally installed at sites with compressible foundation conditions, whereas rock chipping would occur at sites with generally incompressible foundation conditions.
Q: Can soil vibration data collected during a site investigation program, such as during SPT hammer testing, be used to correlate with future construction pile driving?
A: Yes, it is a valuable opportunity to collect soil vibration data during a site investigation program, such as SPT hammer testing, and correlate it with future construction pile driving. This data can provide useful information for designing a construction vibration monitoring plan. It allows for understanding the site-specific soil response to different types of vibrations, aiding in the prediction and assessment of potential impacts during pile driving and other construction activities. This enhanced initial subsurface exploration campaign would need vibration monitors deployed and monitored during the subsurface exploration campaign.
Q: Can vibration monitors installed on neighboring structures be used to triangulate the source location, similar to ground-mounted devices, or does soil-structure interaction affect the calculation reliability?
A: Triangulation can be performed using vibration monitors installed on neighboring structures, as well as ground-mounted devices. However, it is important to account for soil-structure interaction and conduct site-specific analysis and cross-checking. Soil-structure interaction can introduce uncertainties, so performing sanity checks and verifying the reliability of the calculation is crucial. Having vibration monitoring devices both on structures and in the ground allows for utilizing established relationships for ground vibration monitoring while incorporating additional data points from structures.
Q: If requested by the owner of a neighboring multi-story building, would you place vibration monitors inside the building on the top level or ground level?
A: It is advisable to install vibration monitors at both the top and ground levels of a multi-story building if requested by the owner. Different levels of the building may exhibit different behaviors and responses to vibrations. Monitoring both locations provides comprehensive data and allows for better understanding of the vibration impacts on the structure. Installing vibration monitoring devices at multiple levels increases the reliability of the vibration monitoring program and provides additional data points for analysis.
Q: Should vibration measurements be taken every second instead of every five minutes?
A: It is important to have a sampling rate that captures the necessary information for analysis. A higher sampling rate ensures the capture of critical events, including exceedances or high/low vibration events. The typical sampling rates recommended are at least a thousand samples per second or even higher, depending on the anticipated maximum frequency. A general guideline is to have a sampling rate of at least 10 times the anticipated high frequency. Most digital vibration monitors have the ability to easily capture and store high-frequency recordings and then report maximum values at desired ‘reporting’ intervals.
The subsequent blog post will highlight a vibration monitoring case study Dr. Storesund is actively monitoring in the field.
Click below to view the webinar in full.
Also, click here for a collection of resources Dr. Storesund recommends on best practices in construction-related ground vibration monitoring.