• Scott Allen

Vibration Monitoring: Your Best Construction Insurance

Have you ever been stopped in traffic by a flag person while a vibratory drum roller passes you by in the opposite lane, bouncing on the sub-surface soil or newly placed Asphalt (https://www.allrockconsulting.com/post/behind-the-black-asphalt-101) and leaving in it’s a path a lingering smell of heavy equipment fuel exhaust? We’ve all been in a similar situation, and undoubtedly the same thought has crossed our minds – “hurry up and turn that sign around so I can get away from this construction site.”

While you were sitting there, it’s likely that you also experienced another common sensation caused by that annoying piece of heavy equipment, as the relentless hammering of the ground caused vibrations to travel through the drum of the machine, into the soil, through multiple lanes of traffic, and into your car. It may have even shaken your coffee all over your center console.

There are many mechanisms that cause construction vibrations – some common sources include demolition of existing structures, installation of piles or caissons by means of pile driving or drilling, transportation of materials (concrete, soils, etc.), shoring works, compaction of backfill materials, blasting operations, and the list goes on.

The vibrations are typically caused by the equipment that is being utilized to complete the work which include heavy machinery such as excavators, drum rollers, jackhammers, pile hammers, bulldozers, and nearly any other piece of equipment that one can imagine on a construction site. What differentiates these sources are the intensity, frequency, and speed of the vibrations caused by the equipment.

Vibration is defined in terms of particle displacement, velocity, or acceleration, which are related through the vibration amplitude and the frequency of oscillation. For vibrations affecting structures, vibration levels are commonly described at the peak level, or peak particle velocity (PPV), in millimeters per second (mm/s), while vibration frequency is described in cycles per second or Hertz (Hz).

These parameters are measured using sensitive equipment called seismographs, which can detect very small movements within a medium such as soil or concrete. AllRock has amassed an inventory of these devices to assist our clients with any vibration monitoring needs.

For the public, vibrations are often disregarded as they are typically of a magnitude too small to notice. For engineers, construction managers, workers, and business and homeowners, construction vibrations can be troublesome, and introduce a unique and challenging set of problems on construction projects. It’s for this reason that the City of Toronto has incorporated Vibration Monitoring in Toronto into the city bylaws. The City of Toronto’s By-Law 514-2008 indicates that potential vibration impacts from construction activity must be assessed by a Professional Engineer to determine requirements for engineering controls where impacts on neighboring structures are anticipated. AllRock has a team of professional engineers to help you implement your vibration monitoring in Ottawa, Toronto or Newfoundland and Labrador. See https://www.allrockconsulting.com/leadership to contact our team.

Although it has not yet been incorporated into Newfoundland and Labrador’s legislature, Vibration Monitoring in Newfoundland is becoming more popular as owners and engineers are developing higher standards for construction quality control in the province. In recent years it has become more common to come across project specifications on larger-scale projects that require monitoring of construction vibrations – and for good reason! Vibration monitoring during construction can save all parties involved from serious headache.

Vibrations generated by construction activities can have a wide range of negative effects, from minor cosmetic issues to large-scale structural failures. If vibrations are generated at high intensity and close proximity to a building, for example, the vibration waves can be transmitted directly to the structure through the building’s support points (foundations), or they can cause displacement of soils beneath the structure which can result in loss of support and settlement.

Vibration monitoring can act as a great insurance for your construction project to protect against claims made by the public or other stakeholders. If a priceless family heirloom happens to fall off the shelf at a property next door, guess who is going to be standing outside of your gate with the broken pieces. Even though your construction project had nothing to do with it breaking, you have no protection or grounds to argue without a vibration monitoring system in place.

Vibration Propagation Through Soil Caused by Heavy Equipment
Vibration Propagation Through Soil Caused by Heavy Equipment

Consider this scenario: You’re a homeowner that lives 50 meters away from the site of a new bridge crossing the river behind your home. Perhaps your home is 50 years old, the foundation is sound, and you haven’t noticed any significant cracking in your foundation or garage floor slab for the 25+ years that you’ve lived there. The bridge is being constructed in an area with deep, poor subsurface soils and therefore large piles are required to support the structure. In order to install the piles, they must be driven deep into the ground by means of a heavy hammer. Each blow to the pile causes intense vibration to propagate from the pile, through the soils, and towards your home. A week after construction of the bridge is complete, you notice a large crack in your garage concrete floor slab running from one end to the other that wasn’t there before. Where did the crack come from? Why now? It’s very possible that the intense vibrations caused by the pile driving equipment exceeded the magnitude required to compromise the structural integrity of the slab.

Unfortunately, you have no record of the pre-condition of your floor slab and thus weak legal grounds to speculate that the construction activity was the cause of the crack.. If there is no vibration monitoring in place, there is no data to support that the construction vibrations were sufficient to cause the structural damage.

Concrete Floor Slab Cracking
Concrete Floor Slab Cracking

Alternatively, imagine that you’re the contractor on the same project. If the project is in a densely populated residential or commercial area, there is a reasonable probability that the construction vibrations may affect many structures near the project site. However, if there is a quality control monitoring plan in place – there can be little question as to whether the vibrations were sufficient to cause structural damage as there are data records.

Furthermore, on-going monitoring ensures that vibrations do not reach the maximum threshold, or if they do, further controls can be put in place to prevent this from occurring again. In any case, if a complaint of structural damage is made to the contractor following construction, it’s crucial that the contractor and property owner have the vibration data to find common ground on these matters to prevent headache and the potential for costly legal troubles.

To avoid these types of issues, the City of Toronto By-Law 514-2008 addresses in detail the steps that are to be taken prior to any construction project when determining vibration monitoring requirements. Unfortunately, there is no “one size fits all” solution for vibration monitoring due to the vast differences in project sites and construction schedules.

Commonly, a preliminary study or vibration assessment is completed for a new project site. This assessment will include: a plan of the construction site and adjacent land and structures, a zone of influence (ZOI) for vibrations and whether this ZOI extends beyond the legal boundaries of the site, and it will identify whether there are any buildings withing the ZOI that are designated as Heritage Structures under the Ontario Heritage Act. Determination of the vibration ZOI is a complex task and should be completed by a vibration engineer as it must account for the specifics of the site including soil conditions, adjacent/ heritage structures, location of the water table during construction, potential weather conditions, vibration source locations, etc.

Toronto By-Law 514-2008 defines this Zone of Influence as the area adjacent to the construction site where the peak particle velocity (PPV) is equal to or greater than 5mm/s at any frequency, or as determined by other specific site conditions. Other special conditions, for example, may be a nearby hospital that houses vibration sensitive equipment that must be protected from construction vibrations at certain times over the course of the project at a level below 5mm/s.

Once the vibration engineer has determined the project site vibration zone of influence, a Pre-Construction Survey in Toronto is completed by a qualified testing and inspection firm such as AllRock if the ZOI extends beyond the legal boundaries of the construction site. A pre-construction survey typically will include baseline vibration readings within the ZOI to determine background vibration levels.

Additionally, a pre-construction inspection will be carried out on the adjacent buildings and structures within the ZOI to identify and pre-existing abnormalities such as cracks in walls, floors, exterior cladding etc. The data is then compiled in a manner that allows for comparison with conditions revealed in the post-construction survey. A Post-Construction Survey in Toronto is a very similar process, whereby a qualified firm will re-evaluate the same ZOI once construction activities are complete to ensure no damage as been caused due to construction-induced vibrations.

Implementing a vibration monitoring and pre/post-construction surveys are your best insurance to protect yourself and your construction site from claims.