Interesting Ground Vibration Measurements 1

[WARose]

I got a guy to take some ground measurements for me at a site where a piece of machinery is producing unacceptable vibrations in nearby structures......and one thing that is jumping out at me is the fact the vibration amplitudes (of the ground) are actually higher within several feet away from the source than (at the top of concrete) at the source.

Has anyone else run into something like this? I am use to the vibrations at the source being higher than anything away from it (even at a short distance). I suspect some of this may be because some of the soils in the upper layer (at the site) are fairly loose.

 

[cvg]

shear wave velocity is higher in rock and hard materials than it is in softer strata. Pretty well documented seismic phenomena. Structures located on bedrock fare much better in earthquakes than those on sandy soils due to the higher velocity

https://earthquake.usgs.gov/regional/nca/soiltype/

 

[WARose]

Yeah but this is within several feet of the source. They aren't sitting on different stratas. My guess on it is: the foundation is just making a stiffer system (ergo, harder to excite) than the adjacent, loose soils. (Also have to consider the fact the body waves have yet to attenuate (so close to the source).)

 

[Ron]

WARose.....the machinery is exciting the concrete which in turn is exciting the soil. Your source (for the soil) is no longer the machinery, but the support system as well. If you look at a spectrum of the vibration, you will likely see that the frequency at the machinery is higher than at the soil, but the amplitude is lower. In the soil structure, you will likely see the opposite....the amplitude is higher but the frequency is lower.

 

[oldestguy]

A likely reason is that the natural frequency of the slab as well as the wave length locate the highest waves as directed by the slabs natural vibration characteristics.

 

[WARose]

[blue](Ron)[/blue]

If you look at a spectrum of the vibration, you will likely see that the frequency at the machinery is higher than at the soil, but the amplitude is lower. In the soil structure, you will likely see the opposite....the amplitude is higher but the frequency is lower.

Good point Ron.