Seismic Site Classification
Seismic Site Classification
(OP)
The problem is I am a new graduate and the other problem is a new structure is going to be placed on a site with a Class D seismic site classification. After running through the ASCE 7 classification, using data from our soil boring and SPT testing, it is a Class D and therefore would incur several hundred thousand dollars worth of structural improvements. I have been researching sub-grade modifications using pressure injected grout, geo-piers, etc, but cannot locate a specific code or specification that addresses how to handle an initial soil failure, and re-classification of the corrected site. If anyone can direct me to a book or a document (preferably online) that would assist me or has any advice after having gone through a similar circumstance, I would definately appreciate it.
Thanks in advance
Thanks in advance





RE: Seismic Site Classification
Why $ for improvement? Is this a seismic retrofit job? If the building was not designed to handle D then retrofit to handle D. Our job is to tell the truth even when it hurts.
Ground improvements are not used to improve site class. That would be a waste of money and difficult to prove the benefit. Ground improvements can be used to mitigate liquefaction for site class F.
RE: Seismic Site Classification
At this point in my 3 week long career, I just reasearch what they want and deliver the facts, which I already told them that based on our soil borings, the site falls under the Class D classification when using both Blow-Counts and undrained shear criteria but would need to verify this with the lead Geo-tech when she returns next week. When using ASCE 7, eq 20.4-4, the only way it would pass would be if you could re-classify rock as a cohesive material, therefore my dc value would be 100' instead of 66'... unless of course you are supposed to use dc = 100' - (all layers of non-cohesive material excluding the rock).
Basically I have a 72' boring depth after that point we hit a gypsum layer. From the boring log, we had 2 sand seams of 1' and 3' respectively, and a clayey-gravel seam of 2'. From my interpretation of the code, dc = 72' - 1' - 3' - 2' = 66'. If have made a mistake, I would like to get this corrected before I present it to her.
RE: Seismic Site Classification
RE: Seismic Site Classification
RE: Seismic Site Classification
You should be able to find an engineer who specializes in these studies. One is out of the University of Virginia, I believe, but it has been a while since I've had any dealings with this issue.
Google should be able to find something.
Of course, there is always the chance that the study will not increase the classification, but SPT is conservative in this area. My experience is that F class can often be upgraded to D, but D to C is less likely without the presence of dense soils or rock.
It also depends on how close you are. If you are a D marginal to F, you're probably stuck. If you're D marginal to C, the study may be worth it.
RE: Seismic Site Classification
In any regard, I have begun looking into the Seismic Refraction Study. I just want to give the client as much factual information as possible to allow them to make the best decision on what to do with there building.
Thanks again, I truly appreciate it.
RE: Seismic Site Classification
Just make sure the client knows he's taking a gamble.
RE: Seismic Site Classification
Seismic refraction can only ascertain "layers" when the refracted velocity increases with depth (i.e., you won't be able to "see" a lower velocity layer below a higher velocity layer).
The seismic cone can give you a profile that shows higher and lower velocity layers throughout the soil column. They're cheap and fast too! (No, I'm not an in-situ testing contractor!)
f-d
¡papá gordo ain’t no madre flaca!
RE: Seismic Site Classification
One other question, just for the purposes of seeing if this study was warranted, I had located a table of Mean Shear Wave Velocities in a Steel Design Manual(Borcherdt 1994), and was wondering how accurate these are or if anyone has had experience using a soil boring to develop the profile/characteristic, then using this table to determine the shear wave velocity? Again, I stress that using this table was just a quick evaluation to determine if using the shear wave velocity would be beneficial... and to help any future geotechs that may run across this problem
RE: Seismic Site Classification
I recommend that you have a ReMi survey performed. It is quick and based on what you have posted has a decent likelihood of rasing your site classification. Additionally, a site specific response specturm can be determined for the site. This would likely lead to a more effiecent design.
The firm I work for, Shannon & Wilson, and many others perform these services in the mid west.
Good luck.
Mike Lambert
RE: Seismic Site Classification
The reason ReMi gives you a higher site class is for the reason Fattdad said. Surface wave tests cannot see lower velocity layers under higher velocity layers. The site clss could be goverered by a thin layer of weak soil. Therefore, ReMi can often provide a bogus higher site class. The proper way would be with downhole tests. This seems to be the intent of ASCE 7 / IBC since they want a weigthed average velocity of each layer.
RE: Seismic Site Classification
RE: Seismic Site Classification
RE: Seismic Site Classification
I agree that downhole is better, however, ReMi is an acceptable method. Done properly by an experienced geophysist, ReMi provides an average value over the upper 100 feet.
Cost of ReMi is usually $2,000 or less for a site, while downhole is upward of $6,000 and cross hole even higher. There is no "right" answer, all depends on the site, type of construction, costs assoicated with extra detailing for a lower site classification, etc.
Mike Lambert
RE: Seismic Site Classification
The other methods of measuring shear wave velocities include seismic refraction, crosshole seismic, downhole seismic, and seismic CPT. However, seismic refraction typically does not "go" deep enough for site class purposes, while crosshole and downhole methods are more expensive than the surface wave methods, especially if you include the cost of drilling and casing the borings. In soft coastal plain soils, seismic CPT can be comparable cost to a surface wave test and you also get the typical CPT data also.
The cost for a surface wave survey generally starts at about $1500 US and will increase from there in proportion to the mobilization distance.
RE: Seismic Site Classification
RE: Seismic Site Classification
RE: Seismic Site Classification