If you live in a large city like Toronto, you have no doubt heard or felt the subway system running underground. It probably doesn’t bother you if you’re just passing by—but for people who work or live in the area, transit vibration is a much bigger concern.
As municipalities build more transit systems through denser neighbourhoods, there is significant pressure to ensure there is minimal impact on the surrounding community. In fact, the noise and vibration impacts are elements that must be considered in any transit project, and often govern the ultimate layout and design.
Predicting Transit Noise with Surface Vibration Testing
For transit running at grade, engineers perform surface vibration propagation testing. We use these measurements to develop an accurate prediction of how loud a proposed transit line might be in a quiet theatre or whether you will feel rumbles in the basement of your home. Surface propagation testing involves repeatedly impacting the ground along the future transit line and measuring the resulting vibration nearby.
If the transit system runs underground, this practical approach is more difficult to apply, and it’s common for firms to instead rely on theoretical models of the soil. But imagine if you could get a more accurate result by carrying out propagation testing hundreds of feet underground. Our team wanted to explore how we could achieve this vibration testing for underground transit systems.
Borehole Vibration Testing is Born
Earlier this year, our team developed a vibration testing capability as part of our service offering and it has been a game changer. Our measurement system relies on combining our new proprietary sensor with borehole drilling rigs, which are already commonplace in most infrastructure projects.
The borehole drill operator simply swaps their drill bit for our proprietary sensor and feeds it down to the desired depth. When the sensor hits the bottom, the drill operator carries out a Standard Soil Penetration Test which impacts the ground at the bottom of the hole. We measure the force that is transferred to the soil at the depth of the borehole, and the vibration response nearby.
This new vibration testing capability means we can deliver site-specific, realistic predictions of underground transit impacts that are just as accurate as surface vibration testing for transit running at grade.
The most effective way a design team can accommodate noise and vibration mitigation is by flagging potential impacts early in the project. If you’re working on a transit project that might need propagation testing, here are a few things to consider:
1. Add vibration testing early in the project scope:
Incorporating propagation testing in the plan from the outset has both time and cost efficiencies. The results of surface or borehole propagation testing can significantly impact the amount of mitigation your transit system design might need to accommodate.
Though it may seem premature, it can even be valuable to carry out vibration testing prior to submitting a bid. Your team will be able to deliver an accurate budget which includes realistic amounts of noise and vibration mitigation and will have an advantage against competing firms who may be budgeting to accommodate the worst-case scenario.
2. The ground responds differently at different depths:
Surface vibration testing and borehole vibration testing deliver different results, and you should always use the appropriate test method for your project. While you might be able to feel the ground shake when transit is at grade, you might instead hear the noise and vibrations in your walls when it travels underground, or vice versa. These details are important when choosing the right mitigation types for your project.
3. Be patient:
Coordinating vibration testing can take time, and our team is only one of the many moving parts required. Drill contractors with appropriate testing equipment are often booked well in advance, especially during construction season. Permits for drilling and coordination with residents and operators of other sensitive spaces are often required. And after everything else is in place, we are ultimately at the mercy of Mother Nature. If the weather isn’t right, we might not be able to test. But in a best-case scenario, we could be testing at your site within a couple weeks.
Building Better Vibration Testing for Transit
Borehole testing is not a new method, but thanks to our new proprietary tool, we are able to use it as a new way of predicting noise and vibration. It’s a new capability for our team and one that we’re very excited to implement as we work on major transportation projects in the months and years to come. We hope this new approach will contribute to better vibration testing for transit, helping you improve how to plan for noise and vibration mitigation in your projects.