Vibration Monitoring

Vibration Monitoring

Glossary of Terms:

PPV: Peak Particle Velocity

How does a wooden structure pass vibration differently than a concrete foundation?

As a general rule, more rigid building materials like concrete will conduct vibration more efficiently than less rigid materials. Conversely, less rigid materials like wood will dampen vibration. The concrete foundation is expected to vibrate with the surrounding subgrade soils and vibrations and pass the vibration on to the wooden part of the structure with little dampening.  It should also be noted that wooden house structures are relatively forgiving from a structural perspective and are more likely to experience cosmetic damage well before the structure would be considered damaged.  This is true of building deformation due to foundation settlement and swelling as well as vibration. 

It is noteworthy, that when providing criteria for assessing damage potential, the entire residential structure is treated as a general class including the conventional concrete foundations and above-grade wooden structures (i.e. non-engineered timber buildings).  The sensitivity of the wood structure is understood to govern.  So, there is a general increase in vibration sensitivity for wooden residential structures, which may result in a reduced PPV versus frequency threshold.  In some guidelines, like the FTA criteria, there is more likely to be a differentiation between old and new residential construction, or between sensitive historical buildings versus normal residential buildings.

Are objects shaking in a house indicative of damage to the wooden or concrete structure?

The shaking of objects in a house is not indicative of damage to the concrete and wooden portions of the house due to the very different natures of the objects. The concrete foundation and attached wood frame are a fixed structure with vibrations passing through the materials. The strength and flexibility of the materials will affect the limits on damage to the building. Objects within the building are not fixed in place and when exposed to vibrations can move/shake when exposed to vibrations.  

Vibration becomes noticeable at a PPV of about 0.25 mm/sec. This conservative PPV threshold is over 20 times higher than the amount of vibration noticeable to humans.

When would damage be expected, and is the vibration monitoring data below this?

Each set of vibration monitoring equipment has been set up to measure vibrations at two different distances away from the construction activity.  Our typical setups tend to be closer to the construction activity than to the nearest structures.  In all cases, the PPV in the geophone furthest from the construction is noticeably lower than the damage threshold.

How long could it take for damage to show in a finished basement? 

Damage due to vibration can show up weeks after the completion of construction.  The typical practice is to assess potential damage up to 6 months after construction.  This gives the structure time for possible relief of stresses caused during construction to equalize within the structure. 

The potential for damage to concrete foundation walls behind the drywall in the basement is considered to have a very high threshold to blasting vibration, at PPV of more than 250 mm/sec before cracks will normally appear.  Even using half of that PPV threshold to account for the steady-state vibration of construction activity the threshold is still greater than 125 mm/sec.  The PPV threshold for cosmetic damage to the interior drywall of a house basement due to vibration transmitted through those concrete foundation walls would be much lower at 5 to 25 mm/sec for the vibration frequencies observed during construction on the DRFM project. If vibrations were so great that structural damage in the concrete foundation wall was to occur, visible cosmetic damage in the basement drywall and finishes would be observed well in advance. 


Why are there only a couple of vibration monitors in each area? 

In general, the transmission of vibrations in similar soils does not change drastically over short distances.  If the area has the same basic soil and groundwater profile, then the behaviour of vibrations in the subgrade will be relatively consistent.  The subsurface profile and conditions along the berm alignments in both Newcastle and Midland are similar.  Similarly, the construction activity is also relatively consistent in terms of the type of equipment, vibration settings, number of compaction passes, etc. We have had two sets of equipment at Newcastle, Midland and Willow Estates.  On a day-to-day basis, we have moved the equipment around both communities and varied the distance from the construction activity to meet access restrictions.  The vibration response in terms of PPV versus distance from the construction activity at Newcastle, Midland and Willow Estates has been consistent between the two sets of equipment at each site on both a daily and ongoing basis.   

To address the level of concern expressed about this issue we are adding another set of vibration monitoring equipment for Newcastle and Midland.

How are the monitors representative of the vibration experienced by houses when they are a distance away from the structures?

The frequency range of the vibration observed has generally been within the 20 to 50Hz range, but the PPV reduces with distance from the construction vibration.  The purpose of the monitoring program was to monitor the construction for vibration so that construction activity producing excessive vibration could be identified and modified as soon as possible.  If we are looking for excessive vibration situations it makes sense not to be too far away from the construction activity.