Vibration monitoring for residential home

[coneboy]

Many times in the past our company has performed vibration monitoring for buildings close to pile driving operations.
Most often we are out on site on behalf of the pile driving company in order to provide documentation in case adjacent building owners pursue a damage claim.

For monitoring project of this nature I have always used a simple vibration/acoustic recording device that provides a strip chart output. The vibration monitoring tool is simply placed on the floor or pegged into the soil and then programed to record at preset intervals.

Recently I heard mention of a much better method of conducting a detailed vibration monitoring program. Could some one give me some advice on a more accurate approach. What type of equipment is needed?


Thanks for the help
Coneboy

 

[Kevin01]

Hi Coneboy,

There are a wide range of vibrographs on the market. The instrumentation that I use logs the recorded information to memory for downloading to a PC at a later date. It can also be set up to allow remote retrieval of data via a cell phone, or to send SMS messages to defined individuals when defined thresholds are exceeded. Information is logged according to user defined criteria. There are options to record either entire vibration waveforms (for a defined duration) when a pre-determined threshold is exceeded, or to simply record the maximum vibration that was experienced over a defined interval (say for example 1 minute). It is also possible to set the sample rate – important because under sampling will potentially give spurious results. The instrumentation also is able to interface with different types of sensors, for example geophones or accelerometers.

When you are recording vibration it is important to decide what information is needed. For example are you interested in the frequency component? What aspect are you interested in recording – for example peak particle velocity, acceleration or displacement? This will determine how the vibration instrumentation is selected and configured.

In your short note you have made brief mention to the set-up of the sensors. This is a very important part of the monitoring. It is critical that the sensors are fully coupled to the ground, or surface being monitored. There are codes of practice that describe what the set-up should include. It is important to note that a structure has the potential to impact on the measured vibration and the set-up should reflect the anticipated influence as well as the reasons for monitoring.

Hope that the above is useful. There is some very good instrumentation out there.

Kevin01

 

[FSS]

Don't underestimate the importance of a good pre-construction survey of existing structures. Good documentation of pre-existing conditions does wonders towards reducing damage claims. Plus it lets you identify areas that may be more susceptible to damage.

 

[boo1]

Include detailed photographic documentation of the structure.

 

[Focht3]

Good comments, all. When selecting your equipment, be sure it can read in the 3 Hz and above range - a two story house can have a natural frequency of 4 or 5 Hz. If you get a 10 Hz unit, you will completely miss the critical vibrations...

 

[coneboy]

Many thanks for the comments. I have also found that a complete photo and video record of the pre-existing condition of the building helps in resolving complaints and claims for damage.

Thanks Focht3 for your comments about the correct Hz.
I am currently checking to ensure that the recording equipment we use has the correct frequency.

Does anyone with more of a geophysical background have any info about using accelorometers for this type of project?
The peak particle velocities are mentioned in many research papers on this subject, but is there a different (more elaborate = more $)method?

Coneboy

 

[Kevin01]

Coneboy,

As mentioned by others – a condition survey including photographs and video is in my books a given. One other thing I also consider to be a given is communication. When owners see you taking the trouble to measure something and understand just how small say 25mm/s (a common environmental limit or standard) is then potential problems often just go away.

PPV is used for historical reasons - there are numerous correlations between PPV and damage. In my experience most engineers relate to acceleration more readily.

There are drawbacks to measuring acceleration though. If a signal is recorded directly from an accelerometer and needs to be interpreted in terms of velocity, then the low frequency stability of integration presents a challenge. Integrating velocity to displacement has similar difficulties. However, the reverse process (i.e. differentiating) velocity to acceleration is generally stable. I would, therefore, only use an accelerometer under special circumstances (for example I have used them under water and in very confined spaces). One other factor that is worth considering is of course the cost and durability. Velocity sensors are better on both accounts.

We routinely measure strain or movement across a “defect” in conjunction with vibration. This approach has proven to be very successful in defending a claim or justifying work. For example for a recent project that required blasting within several feet of an existing foundation we recorded full vibration and displacement waveforms for blast events. Doing that we were able to show that the effects of the daily and seasonal weather cycles (including temperature, wind and humidity) had a far greater impact than the blasting. Some of the icons in vibration engineering now seem to be advocating measuring displacement in real time.

Kevin01

 

[exploengineer]

Hi,

Most regulations and studies have been completed using peak particle velocity as the key indicator.

To monitor pile driving, it is best to purchase a commercial seismograph built for monitoring blast vibrations and construction vibrations in peak particle velocity. To monitor on-going construction vibrations, use the instrument in histogram mode.

These instruments will also report the peak acceleration and controlling or dominant frequency in hertz.

You can also convert from ppv to acceleration fairly easily, but since the vibration wave is usually not symmetrical, its only a rough approximation. The Imperial formula is: ((2pi)(ppv)(hz))/386.4 = g

Remember, you must have the geophone or sensor coupled extremely well to the ground. It is best to spike and bury the sensor at least 1 foot deep. Almost all studies have been performed with the sensors outside the structures. So, do not monitor inside the structure unless you have completed an analysis of correlation between inside and outside.

It is extremely rare that pile driving will cause any signaficant damage. It may cause cosmetic type things due to continued low frequency vibration, but that is equivalent to natural forces.

It is always imperative to do preconstruction surveys just before starting work and do on-going surveys. Daily environmental forces cause huge equivalent peak particle velocities:

Inside humidity change (19%) = 3 in/sec or 76 mm/sec
Outside humidity change (35%) = 5 ips or 127 mm/sec
Inside temp change (12 degree F) = 3.6 ips
Outside temp change (27 degree F) = 8.2 ips
Wind 23mph = 3.5 ips
walking = 0.3 ips minimum (wall corner)
Jumping = 1 ips minimum (midwall)
Door slam (next room) = 0.5 ips minimum
Sliding door (wall above) = 1.5 ips minimum
Pounding picture nails = 2 ips (midwall nearby)

As you can see, pile driving is not that bad especially at a distance of over 30 feet because of wave attenuation.
Frank Lucca

http://www.terradinamica.com

 

[coneboy]

Thanks for the comments Kevin01 and exploengineer. I will let you know how the project works out.

Coneboy

 

[Focht3]

I agree with most of Frank Lucca's (exploengineer) comments - except those about low frequency vibrations being equivalent to "natural" events. (The specific quote is, It may cause cosmetic type things due to continued low frequency vibration, but that is equivalent to natural forces.) I disagree.

Low frequency vibrations can cause serious damage - particularly if they are 'continuous' rather than 'instantaneous'. And most of the blast monitoring equipment operates above the relevant frequencies, so the PPV measurements at low frequencies by most blast monitors is simply wrong. In defense of the typical blast monitoring equipment, it was not designed for these types of measurements since mining explosions typically have frequency spectra that are well above 10 Hz.

On the issue of acceleration vs. displacement or velocity measurements: Frank is right when he says that most of the published information references PPV. However, this has more to do with the equipment available and used by those early researchers. We now have better tools and improved knowledge - and we should make use of them both.

 

[TJWATKINSPG]

I had a project where we were working with a construction firm and were called at the last minute to set up " something" to monitor the adjacent structures. I found a neat simple device that basically was two overlapping grids on glass slides. In Fl. the are always existing cracks in every building of size. I set up these "crack monitors" by super gluing them one each side of a crack then setting the grids to match. Each day i photographed the monitors. This measured not the dynamic motion but the effect of that motion on the structure. Very effective in calming fears that construction was causing, increasing, or otherwise affecting the buildings. The monitors are still in place five years later and have not shown any movement to date.

The truth will set you free. Best of luck. Geodude

 

[JamesDean7]

Clients sue the blasting company because of foundation cracks not present before blasting commenced, based upon the pre-blast survey. Although the cracks are hairline and follow joint lines and therefore would not be considered structurally significant, clients want to be "made whole". The vibrations felt during a blast are substantial, yet the PPV is not exceeded.
When the Fed Gov says that PPV below a certain level cannot cause structural damage, are they referring to significant damage and dismissing aesthetic damages?

 

[Focht3]

[blue]JamesDean7[/blue]:

Why not start a new message thread? You'll get more respondents that way -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"