ground vibration 2

[Forensic1]

I need to know the effect of a bedrock layer on transmital of ground vibrations caused by a vibratory compactor used in road construction near a house.

I hope someone here has some experience with this type of vibration application.

Thanks!!!

 

[Hatch]

Most of the vibration transmission is via a surface wave if the ground is stiff enough. However some of the vibration energy is propagated downward and maybe to "hell". But, a bedrock layer would result in a reflection of the vibration energy back to the surface if it is not too deep and the ground is ralatively hard. If the house is located at the right angle and all the other factors line up, you could have a problem. C. Hugh

 

[Forensic1]

Thanks Hatch! That's what I thought.

BUT, do you have any idea how to quantify it?????

Later!

 

[nbucska]

measure it with an accelerometer. <nbucska@pcperipherals.com>

 

[Hatch]

There are several methods, some consultants pound a 6 in. spike into the ground and then mount an accelerometer on top. You can do this at the source and near the house. Prediction methods exist for other distances etc based on the measured values, but they are beyond my abilities since I have always had the services of an earthquake engineer (Phd) to do the predictions for me.

Another method is to use a small wooden plate (2&quot; x 2&quot and nail it to the ground with several nails. Then mount the accelerometer to the plate. This provides much better surface wave coupling of the accelerometer.

The final method is to do readings right on the house structure or in the basement. I have taken measurements on the basement floor and even on window frames if the resident is complaining of rattaling windows. There are standards for such measurements but I don't have them available right now. Try an internet search. C. Hugh

 

[Forensic1]

Thanks Hatch - I appreciate your advice. But I have a problem - it's something I left out of my original question. The construction was completed almost a year ago!!

So, I guess what I need is a lead to something more theoretical that I might use to estimate effects.

Thanks again!

 

[Hatch]

In order to predict the ground progagation level you would need so many assumptions and probably need to drill a test hole to check the soil/rock layers. It can be done but it would be much cheaper and more accurate to perform a controlled test.

I'll assume that there is a damage claim involved after the fact. You are better off with the controlled test to show either excessive or acceptable levels at the receptor. C. Hugh

 

[exploengineer]

What is the distance to the structure in question and the energy source (compactor)?

I specialize in blast vibration and prediction and occassionally deal with vibration relating to compactors and vibratory rollers.

Normally, with a vibratory roller, at a distance of 10 meters, you can expect a Peak Particle Velocity of 12mm/sec. Even with extremely low frequencies, this is not sufficient to cause hairline cracking in foundations. It could cause cosmetic damage to some plaster-on-lath construction or shake pictures off the wall, etc.

In the future, if you need to monitor, use a blasting seismograph setup in histogram mode. Frank Lucca

http://www.terradinamica.com

 

[Forensic1]

Hi Frank,
Glad to get a chance to talk with you. This is an unusual situation. Most construction/blasting vibration situations I hear about are two or more story structures with most damage on the upper levels. This is an earth home, single story dwelling with ground level about 0.6 meters from the soffet. The house is about 21 meters from the construction where a heavy vibratory compactor was used.

Cracking in the reinforced concrete foundation walls was shown with vertical cracks about 1 to 2 mm wide. There was also extensive cracking of the interior wall and ceiling sheetrock.

Judging from the extent of cracking, I estimate vibrations in the range of 120+ mm/sec. But, at 21 meters distance, this doesn't seem to make sense unless the bedrock layer is reflecting/magnifying the vibration effect. I guess the bedrock is about 3 to 4 meters below the house.

We're not talking about a little rock layer. There are limestone quarries on property adjacent to two sides of the house.

What do you think about the effect of the bedrock on vibration transmition?

 

[exploengineer]

Hi Hatch,

Normally in limestone, vibration attenuates or decays to about 1/3 of the peak everytime the distance is doubled.

Many international studies and my own studies have shown that to cause hairline cracking (75 microns or less) in concrete foundation walls requires a minimum of 120 mm/s but would normally take close to double that before I would even worry.

Also, if there was vibration on a magnitude high enough to crack the concrete, there would be considerable cosmetic damage to the house itself (even earthen) and damage to the contents as well.

It is extremely hard for construction vibrations or blast vibrations to be amplified.

I would recommend the book &quot;Explosive Vibrations, Construction Vibrations, and Geotechnology&quot; by Louis Oriard and &quot;Construction Vibrations&quot; by Charles Dowding. These books explain many things it has taken me years to learn. Oriard and Dowding are from different camps on damage criteria, but they both agree on many points.

Feel free to ask any other questions:

Frank Lucca

http://www.terradinamica.com