Construction & Contractors/Ground Vibrations from Blasting
Are you aware of any studies, publications, or references regarding the effect that the soil or rock beneath a house has on ground vibrations resulting from rock blasting?
I am reviewing a case where blasts are below requirements (2ips), but the homeowner feels that because his house is on the same rock being blasted (about 300-feet away) the vibrations at his house may be greater than that measured at the seismograph. I am still trying to find out what type of surface the seismograph was placed on.
It would be great to know if vibrations would be expected to be higher or lower if the house is built on the rock being blasted, and by how much. I've found some info from the usual sources on this subject (Dowding, Siskind...), but no solid studies that could be referenced and stood behind.
Vibration amplitudes can be expected to be higher when the receptor site is in contact with rock, especially the same rock unit that hosts the blasts. This is fairly well-accepted in the industry. Energy passing from a high-velocity medium to a low-velocity medium (soil) will partly be reflected and the portion that crosses the boundary will be refracted, which tends to shift the frequencies downward. So in general, structures founded on soil overburden will experience lower-amplitude ppv motions, but at lower frequencies, than the structure would have if it had been sited directly on the rock. However, this is difficult to quantify without complicated analysis and modeling.
It is important to recognize that peak particle displacements, and therefore the magnitude of ground strain, are higher for low-frequency particle velocities. In practical terms, 2.0 ips at 10 Hz will induce higher ground strains than 2.0 ips at 60 Hz. Moreover, most structures will approach impedance matching for ground motions in the 10 Hz range and less, so not only are the peak displacements higher at low frequencies, but the likelihood that the structure will amplify the these higher ground motions is also higher.
For this reason it can be more conservative, from the standpoint of structural damage, to focus on areas of thicker overburden.
Back to the comparison of ppv -- in my experience the difference can be several times, depending on the magnitude of the rock ground motions. However it is totally site-dependent because the effect is related to the relative stiffnesses and densities of the rock and overburden, to the thickness of the overburden and to the nature and attitude of the rock-overburden contact. For that reason I doubt you will find much in terms of a plug-and-chug equation. I recall one instance where we had 4 seismographs around a residential blasting site in granite. The readings (in terms of ppv) were about the same except for one, which was twice the others, and the reason was that machine sat on overburden that was underlain by a shallow reef of nearly-fresh rock that projected into the excavation. Energy got in that rock lens and excited the surface all along it, even through the overburden.
If the homeowner wants to allege damage on the grounds that his structure is sited differently than where monitoring took place, and therefore experienced higher ppv, he will need to account for the frequency effect.