Site classification of an excavated area 2

[geo77]

I am working on a foundation design for a property that has been excavated 25m below the ground surface. Would the site classification for seismic design be based on the soil properties from the ground surface or the soil properties from the foundation elevation (25m below ground)?

 

[csd72]

I would expect that it would be a combination of both.

 

[irawanfirmansyah]

I believe it should be from ground surface.

 

[cvg]

this sounds like a major structure, pick the most conservative one and use that

 

[geoman110]

The site seismic classification is considered from lowest building slab level (not foundation level). For example, for a building without basement, it is the ground surface.
We you say “a property excavated to 25 m below ground surface”, I get the impression that you are referring to a property much larger than the planned structure. In this case, the site seismic classification is based on the current ground surface (assuming no basement) which is 25 m lower than previous ground surface. If the footprint of the excavated area is small (relative of the 25 m depth of excavation), you are in a complicate situation which require detail analyses.

 

[msucog]

it depends. basically, if the soil touches the structure (or is reasonably close), you should use the original/higher ground surface if no additional dynamic analysis is performed. essentially, the soil at the ground surface is going to move like the rest of the ground surface while the deeper conditions will move less. therefore, your structure must be designed for the various accelerations and displacements. if you have tall basement walls, then i highly recommend performing additional analysis so that the wall design can account for the added seismic loads. being 25m in the ground, you should have varying values with depth therefore i'd perform a more sophisticated analysis. if you have a well-qualified engineer perform the analysis, they can help you optimize the analysis/design. there are many exploratory and analysis options available but finding an engineer that actually understands and can provide those services is the tricky part.

 

[six06]

My understanding is that the soil site classification from on the UBC (site as A,B,C,D,E,F) are based on borings (uncorrected SPT N values) up to 100 ft depth. But, it could be of any depth you have borings. I would go to the source from your site class comes from and learn a little bit more about it.

 

[msucog]

read nehrp 450, fema 451b, nehrp 611, or a few others (search eng-tips since i've posted the linked before). the language is in there. for below grade structures (basement walls, deep foundations, etc), you should use site class from the ground surface unless dynamic modeling is performed to assess the below grade structures. for a class, i recommend the asce seminar for ground motion assessment by dr. praveen malhotra.

 

[Mccoy]

Msucog,
I believe geo77's original post raised the question of shallow foundations cast upon the bottom of a deep excavation.

That would not be the same case as of deep foundations.

I also believe that many regulations take as a reference level for deep foundations (on piles, on slurry walls, on caissons) the foundations head near ground surface.

And I fully agree on the fact that with tall basement walls we might be in a more complex area requiring in depth analysis; site classification based on the deep level might be under conservative, whereas if referred to the ground surface it may be over conservative.

In major works with huge amounts of concrete involved I reckon design optimization may allow to save substantial amounts of money.

In some building regulations (like the Italian ones) unfortunately there are no suggetions for such a case.