do basements count in site classification

[spunkygirl]

In the past I have made site classifications based on the 100 feet below the building basement. (example, if there was a 20ft parking garage underground, the boring/CPT used to classify would be 120 feet, using the lower 100 feet to classify the site). I have recently switched companies, and am being told at my new company that we can do the site classification from the bottom of the foundation, whether it is a shallow foundation or a pile foundation....I'm having trouble rationalizing why you would want to classify your site from the bottom of the pile - wouldn't you want it from the base of the building? Yes, you could drive piles down to rock, but it would seem that the waves will still propagate up not only through the piles, but also through the existing soil? Any thoughts on this topic would be appreciated, or if there is a good reference I could use I would appreciate that also.

 

[msucog]

http://www.eng-tips.com/viewthread.cfm?qid=204075&page=1

i personally would not go from the bottom of the pile...maybe unless it's a short caisson socketed in rock. you still should consider the soils against the building since the motion will be transmitted through those soils. i typically go from the pile cap (basement level or otherwise). if you got class e materials behind the basement walls, then you (or perhaps the structural engineer) probably should try to evaluate whether the seismic forces transmitted by those soils will impact the lateral stability of the structure. i would keep it simpler than that and call it a class e if it's a class e (by ibc standards). to get around that, you could always undercut and replace to some distance around the building i suppose or maybe have a "separation" but i think you'd be pulling "the answer" our of your rear. i probably wouldn't bother with all that unless you've got several stories underground or are in a high seismic area (where high seismic areas might have their own rules). i'd take it from pile cap under most all circumstances and call it good.
and don't forget that code allows up to a 20% reduction in the seismic parameters if you go through the site specific analysis.