This blog post, the second in this webinar series, elaborates on five vibration monitoring best practices in depth. Subsequent posts will highlight a case study Dr. Storesund is actively monitoring in the field and cover the Q&A from this webinar.
Webinar Highlights
According to Dr. Storesund, there are five primary components to consider when it comes to best practices associated with construction‐induced ground vibrations:
- Pre‐project site screening: to ascertain your potential exposure to construction‐induced ground vibrations;
- Vibration mitigation opportunity assessment: Depending on your exposure, determine if there are mitigation opportunities to reduce or eliminate substantial ground vibrations;
- Management action: Identify what sort of management actions can you implement to control vibrations during the course of the project;
- Threshold vibration monitoring levels to enforce: Determine what sort of representative vibration thresholds might you want to enforce to mitigate risk;
- Data collection plan and vibration monitoring protocols: Identify the monitoring equipment, data collection, and dissemination protocols needed to help ensure your vibrations are minimized.
We’ll discuss each in detail below.
1. Vibration Monitoring Pre-Project Site Screening
“There are existing screening formulas you can use to evaluate the potential magnitude and spatial distribution of your ground vibrations, “Dr. Storesund states. “These formulas typically reference some sort of baseline or referencePeak Particle Velocity, or PPV, value based on a type of equipment and the corresponding attenuation of that PPV value at distances. An attenuation factor is used that generally varies from about 1 to 1.5. The attenuation factor is based in part on the subsurface soil conditions at and near your site.”
Dr. Storesund cautions that the list of vibration source equipment and reference PPV values is fairly limited, there are not established limits for every equipment type, as well as uncertainty exists in determining an accurate attenuation factor. He suggests that some initial testing at the project site with a vibratory roller and an array of vibration monitors may allow you to back-calculate a representative attenuation factor for your specific site (which would likely fall within the range of 1- 1.5).
2. Vibration Risk Mitigation Opportunities
Are there steps you can take to reduce or eliminate the risk of ground vibrations? Each site is unique, but Dr. Storesund outlines some potential mitigating steps you could take, including:
- Pre-drilling for driven pile foundations (depending on your subsurface conditions);
- Jet grouting vs. conventional pile driving to achieve ground improvements;
- Use of hydraulic crushers vs. conventional pile driving;
- Relocation of residents adjacent to the construction site to eliminate exposure to the sensations associated with ground vibrations.
3. Management Actions to Reduce Vibration Risks
Dr. Storesund outlines five considerations for vibration management of a site:
- Activities: Will the project include activities that may use equipment that generates high levels of vibration?
- Proximity: Are buildings located within 500 feet of vibration-generating activities?
- Screening: Use vibration formulas to establish vibration thresholds based on activities, equipment, and proximity to adjacent structures.
- Survey: Photographic documentation of pre-existing conditions within the vibration influence area can be very helpful for subsequent project-specific displacement analysis.
- Monitor: Monitor cracks with crack gauges and vibration via PPV values across the construction area and enforce threshold levels.
4. Effective Vibration Monitoring Threshold Levels
Are all vibrations people feel structurally damaging vibrations? Not necessarily; Dr. Storesund explains that, in almost all cases, human perception of ground vibration is much more sensitive than what is required to cause structural damage.
There are existing guidelines on generally acceptable and generally unacceptable vibration thresholds. However, Dr. Storesund cautions keeping the following in mind when considering threshold levels:
- First, these are general guidelines and do not account for any site‐specific nuance that may exist;
- Second, because the ability of individuals to feel vibrations regardless of the ability of those vibrations to cause damage, you may be faced with claims regardless of the selected threshold.
He concludes with recommending having a continuous record of recorded ground vibrations to enable you to assemble a more coherent and logical response to a potential claim. In addition, if you deploy crack gages, you can further refine the correlation between ground vibration and realized displacements or damage.
5. Vibration Data Collection and Protocols
Dr. Storesund describes the different types of vibration monitoring equipment available with comparative use cases of:
- Geophone-based vibration monitoring systems: Often deployed in what he describes as “manual read mode”, these instruments require an individual to be stationed at the device to review recordings and send out alerts if the project thresholds are exceeded. Dr. Storesund argues that this approach to vibration monitoring generates a lot of uncertainty for several reasons, not the least of which is its reliance on the attentiveness of the individual overseeing the vibration monitor.
- Automated digital monitoring systems: Newer technology, like Inzwa’s Veva III, is ready to deploy out of the box, is very easy to set up and activate, and provides automated data collection, near real-time data displays and alerts, and has pre-configured dashboards and data screens. The system is also scalable, so multiple sensors can be viewed and the data aggregated via your web browser. This ability to deploy a network of sensors and easily access and integrate the data presents substantial time-management benefits, especially for projects where the work can occur over large distances.
Subsequent blog posts will highlight a case study Dr. Storesund is actively monitoring in the field, and share the Q&A from this webinar.
Also, click here for a collection of resources Dr. Storesund recommends on best practices in construction-related ground vibration monitoring.
