Peak Particle Velocity variation in soil and rock

[WHEngineer]

I am looking for references or information on peak particle velocity variation in rock and soil. We are working on a new commercial building which on two occasions showed damage immediately after blasting to the footings and masonry block walls. The building foundations are on rock, however, the seismograph was spiked in soils above the rock. The seismograph data was well within acceptable ranges although damage was observed that could only be attributable to the blasting (damage occurred immediately at time of blasts). Blast frequency was in 25 to 35 hz range, with PPV of .17 to .63 in/sec. Air Blast ranged from 115 to 137 decibels.

 

[ishvaaag]

You might proceed the forensic way: what actual displacement besides the damaged element has occurred, as inferred form the actual damages, or what loads are required to produce the observed damages?. Then you will have to decide how this relates to the displacement in the free terrain, because the foundation is something inserted therein, and distorts the free movement, and its peak at the interface. So I see investigating the loads that may be inferred from the damages is a sounder start.

 

[CarlB]

The following thread may give you some leads:

http://www.eng-tips.com/viewthread.cfm?spid=256&newpid=256&sqid=61566

 

[exploengineer]

To cause damage, the PPV had to be much higher. This would lead me to believe that the soil dampened the signal and you did not get a true reading.

Also, depending on how close you were to the blasting when setting up the seismograph, you could not be recording the correct data. The seismograph could have had its limits exceeded due to extremely high accelerations or frequencies and had aliasing or decoupling occur.

It is extremely rare to cause blast damage unless there was very high displacements or you had actual rock movement under the building.

We blast consistently within 1/2 foot of structures without problems, but, proper seismograph placement is the key to insure that we are getting the correct information during the test blast program to aid in actual blast design for the production blasting.

I have written a paper on close-in blasting. Go to

http://www.terradinamica.com,

the "Tech Talk" page. You can download it there in .pdf.

Frank Lucca M.I.Exp.E.

http://www.terradinamica.com

 

[sgsibob]

I'm with Frank, the velocities do not seem high enough on the face of it to cause damage. However I am wondering what kind of damage was experienced. If it's minor architectural cracking that would not be unheard of when nearby seismographs read below 1 ips. Also, if the building has large areas of windows or other structural flexible members the air blast (137 dB is not insignificant in close and makes me wonder what kind of protection was used) can give rise to minor cracking.

Bob Cummings
Geological Engineer

 

[Focht3]

I'd look at the air blast - seems to be the more likely cause from your description, although the readings may not be representative. Is most of the damage "high", or is it close to the ground? Look hard at where the damage occurred - as well as when.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[WHEngineer]

Thanks for the comments. The first evidence of damage was a cracked footing. The damage was noted immediately after tha blast. Both pre and post blast inspections had been performed. The footng had been poured approximately 7 days earlier. The footing was set partially on rock and partially on weathered residual soils. The crack was approximately 1/8" in width across the entire footing. This crack was deemed structurally insignificant due to the level of reinforcement. The second crack occurred in the masonry block walls prior to grout fill of the cells. Once again, the crack was observed in the post blast inspection. The crack in the masonry block varied from less than 1/8" at ground level to approximately 1/2" at the top of the 20 foot wall. The building inspector required evaluation by the structural engineer and the crack was significant enough to require minor reinforcement modifications. Our concerns with the blast monitoring is explaining the occurence of the cracks immediately after each blast. The basic data would suggest that the blast was not strong enough to induce structural damage. The rock on the site is a granitic gneiss with a RQD in excess of 85%. The building rests on an area where approximately 5 to 7 feet of rock had been removed. The soils occur across two bands of weathered material along old fractures. The damage was generally noted over the area of the soil zones.

 

[Focht3]

Sounds like the differing support is an issue; I have a hard time seeing attenuation as a factor. But others may disagree, or have other suggestions -

Where is the site? It sounds like you are on the U.S. west coast.

Give us a clearer picture of the footing and subgrade dimensions, particularly with respect to the crack. That might help us understand what's going on, and offer better observations and suggestions.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[sgsibob]

The fact that the crack in the masonry was wider at the top than at the bottom points to settlement, not the lateral shifting more commonly associated with blasting damage.

Lou Oriard wrote a prety good book on blast damage to structures, focusing on how to interpret the cause from the crack configurations. It is titled Effects of Vibrations and Environmental Forces, published by the International Society of Explosives Engineers (

http://www.isee.org).

also see Oriard's other book, Explosives Engineering, Construction Vibrations and Geotechnology, published also by ISEE.

Oriard is a realist and one of the clearest thinkers in this field.