I understand that the upper 10 feet of the soils are 'very weak'. It does not necessarily mean that the upper 100 feet is weak.
In my opinion you can do a shear wave test to determine the site class. Generally these tests consider the soils in the upper 100 feet.  

Thank you Jalthi.  The soil layer in question is about 10 feet below foundation grade.  I believe that shear wave testing is appropriate to upgrade the site class, but there are firms in my region that interpret the code as Site Class E regardless of shear wave velocity testing since it exceeds the moisture content, shear strength, and Atterberg Limit criteria.

Which section of IBC2009 the soil seismic classification is described in ? I couldn't find it.

Even by producing an S-waves profile by adequate geophysical investigation, we should reason what it is which governs the site amplification, the overall Vs30 or the soil characteristic described, or both. In the latter 2 cases it would be useless to employ geophysical investigation with the mere purpose of upgrading the soil class.
Also, you describe many firms rule the first opportunity out.

Mccoy,  Section 1615.1.1

I'm interested into a comparison with IBBC and European-Italian codes, but the site I consulted did not visulize the section you cite.

Here in Italy some reliable authors are showing the fallacy of the amplification concept based on the S-wave velocity in the upper 30 m or 100 feet.
 

Many experts in the field (Idriss and others) state that Vs30 is not a good measure for amplification.  It depends where the soft layers are; upper, lower or middle portion of the 100' soil profile being considered.  I think you would really need to do a dynamic site response analysis to determine this.

moe,

what do you mean exactly by dynamic analysis?

Measure of macrotremors or microtremors (HVSR by Nakamura method) ?

Do you guys use something else in your place?

Mccoy, I mean at least a 1D shake analysis or equivalent.  2D analysis with effective stress may be warranted for some geometries/soil conditions.  

Moe,
Are you happy with your numerical site response analyses?

I've started to use that method (equivalent-linear numerical simulation )and I'm realizing it can be sometimes off the beam.

The theory is an s-wave coming from the bedrock and travelling upward, wheras the input, a true seismic signal, is mainly made up of surface waves which are subject to multiple reflections (and refractions), with very little of the simulated upward propagating S-wave. The numerous approximations used propagate a final error which is unknown and probably significant.

I see many spurious values (probably function of the input motion values). I see shifted frequencies. I see peaks where they should be and I don't see peaks where they should be. I see pretty high and suspicious values of motion amplification.

My experiences are not always encouraging so far, although I'm going on since the codes reccomend them in some cases. I'm only using 1-D methods, those drawbacks I observed in laterally homogeneous situations.

I don't know if your results are more satisfactory than mine.

Mccoy, the site response analyses are definitely not "accurate" methods, but I think they give a reasonable first order approximation when used carefully with good data.  The method does have limitations and input motions from rock sites are best. Model calibration is a valuable tool if you are fortunate enough to have response from previous EQ's in a similar soil profile.

The input ground motion and how you apply it is a huge factor.  There are a few different opinions out there on what the best way to apply the input ground motion.    

I absolutely agree on using bedrock motions.

What are the other relavant aspects you find critical as far as the application of input motions go (I'm speaking about real EQ motions)?

Here the building code requires 3 to 7 input motions, which must be normalized to meet the code area's spectral features (the design spectrum imposed by the code in those coordinates). This probably takes a little away from reality.


 

Using a code based target response spectrum does have issues.  The code based response spectrum includes response from both close and far EQ events (a few different EQ's), so matching an input motion to the code based specttrum will generally require a lot of modification to a real time hstory.

Some of the issues with regard to application of input motions are related to opinions on the use of deconvolution of the bedrock motion, and to the depth at which the bedrock level is set at, particularly if you are modeling a deep soil site.  I think there is at least one other issue I can't think of right now.  I will try to get my thoughts and references together on this topic.  

Actually moe, this topic would deserve its own thread since it has wide implications and angles.

It's a powerful tool, but it's all too easy to come up with unlikely results.

i agree with moe on this one. i would probably start at performing geophysical survey to better define the shear wave velocity and perform a site response analysis. index testing and dynamic lab testing could also be performed to assist in the analysis. the geophysical surveys are relatively inexpensive but the lab testing will help to confirm the model. for high order projects, i'd suggest expanding the assessment. i've done this on chemical plants before and the results were always positive...but the documentation has to be there to justify the recommendations.

also, crosshole is a high order geophysical survey. other surveys may be better applied depending on the site conditions. in other words, for large sites, i'd suggest surface performed surveys correlated to a downhole or crosshole survey. i'm not a fan of taking test results from one location and applying it to vast sites. i typically reserve crosshole for very specific locations/foundations due to the high costs. i often perform surface methods and correlate with a select number of downhole surveys.

if you need additional help off the board, feel free to message me at my user name at gmail dot com. good luck.