Seismic Site Class - Liquefaction

[NBRY1]

If a site has soils (loose clean sands + high groundwater) that are susceptible to liquefaction, however the ground accelerations (amax less than 0.04g) are too low to produce liquefaction....then is it still assigned a seismic site class 'F'?

ASCE 7-10 says assign site class F if soils are 'vulnerable' to potential failure under seismic loading.

 

[EireChch]

Site class is independent of PGA.

Have you used N values or Vs to classify teh site? Or as you have none, you are just assuming F?

Do a 30m bh and confirm it yourself.

 

[NBRY1]

EireChch - not sure you understood my questions. I understand completely how to derive site class by different methods (N-value, Vs, Qu).
My question is, does one assign a site class F if the site has liquefiable 'vulnerable' soils; however accelerations are too low to cause liquefaction.

 

[EireChch]

I think you will still be site class F regardless.

You can do a liquefaction analysis and confirm that there is no risk of liquefaction however the site is still underlain by potentially liquefiable soils and be classed as F.

Your max PGA may be 0.04g, however, if there is seismic activity in your region this may change. For example, the PGA in Christchurch NZ prior to their 2011 earthquakes was no where near the current 0.35g ULS design PGA.

 

[GeoPaveTraffic]

I wouldn't assign it site class F. Any non-plastic silt or sand will liquefy if you shake it hard enough. I would run a liquefaction analysis and if the material liquefies, then site class F, otherwise it would be one of the others.

Mike Lambert

 

[NBRY1]

According to EireChch it is a site class F regardless.
According to GeoPaveTraffic it should be classified as another site class if susceptible to liquefaction, but does not liquefy during earthquake.

So, I'm back to square one. It must be one or the other??

 

[EireChch]

To me site Class F is a last resort when you have no testing that confirms otherwise. All loose sands with a high groundwater table are potentially liquefiable as GPT says so technically your site is site class F.

You seem to be stuck on site Class F because you have no testing? In that case you cant expect to have no testing but then try adopt a less conservative site class. Now if you have experience in the area and know that genearlly they sites are site class D, then you could take the(small?) risk adopt this for design but it eventually has to be verified.

I dont think you can base the site class on whether it liquefies or not. We have lots of sites class D and some liquefy and some dont, the site class doesnt change as it is solely based on Vs or N value data (preferably Vs)

 

[MTNClimber]

My interpretation is that 7-10 as saying it is not a Class F if the sand is not liquefiable:

" 20.3.1 Site Class F: Where any of the following conditions is satisfied, the site shall be classified as Site Class F and a site response analysis in accordance with Section 21.1 shall be performed.
1. Soils vulnerable to potential failure of collapse under seismic loading, such as liquefiable soils, quick and highly sensitive clays, and collapsible weakly cemented soils. EXCEPTION: For structures having fundamental periods of vibration equal or less than 0.5s, site response analysis is not required to determine spectral accelerations for liquefiable soils. Rather, a site class is permitted to be determined in accordance with Section 20.3 and the corresponding values of Fa and Fv, determined from Tables 11.4-1 and 11.4-2.
2. Peats and/or highly organic clays [H>10ft] of peat and/or highly organic clay where H= thickness of soil.
3. Very high plasticity clays [H>25 ft with PI > 75].
4. Very thick soft/medium stiff clays [H > 120 ft] with su < 1,000 psf."

So I would just use Table 20.3-1 and classify it as E or D, depending on your average N.

 

[EireChch]

I agree with you MTNClimber, but if there is no testing/data your only option would be to assume F. Even though it may be highly conservative? Agree, no?

 

[GeoPaveTraffic]

EireChch, actually the code, Section 1613.5.2, calls for Site Class D to be assumed when there is insufficient information to otherwise determine the Site Class.

Mike Lambert

 

[MTNClimber]

I'm not sure what information the OP has but that's (my previous post) how I would approach it if the soil was not susceptible to liquefaction. I'll add that if the soil was borderline liquefiable (say FOS 1.1) and its a sensitive structure, then I would re-think my approach to be more conservative.

 

[MTNClimber]

Also make sure you know that you're required to work in 7-10 because the acceleration values are larger for 7-16 in the areas where I practice.

 

[EireChch]

I stand corrected, thanks GPT. I actually remember that now that you say it!

 

[msucog]

PGA is not independent of site class...well the PGA used isnt...PGAm is what you use to evaluate liquefaction which is the site modified PGA. "PGA" is at bedrock level which likely tell you much other than you have to apply the amplification. If liquefaction potential is present, then run the calcs to see if it will trigger and the magnitude of potential. And defaulting to site class D just because you dont want to deal with the issue doesnt mean it is legit...you cant simply ignore it away. That would be like me (as a geotech) not wanting to do 2way slab calcs on a slab so I just ignore that the deflection is all out of whack. If you want to refine the site class, then measure shear wave velocity since this is more appropriate than SPT, and will often provide the "better" site class. This also helps liquefaction calcs and, if needed, supports the site specific seismic assessment with site response.