Seismic Site Classification

[NMTexEngr]

I have a client that determined during negotiations not to bore to 100' for seismic site classification. All holes were drilled to a depth of 32'. Out of the 21 boreholes, 14 have mean N values greater than 50. The soil classifications in the area are SM and SC. I do not have any additional geotech information in the area whereby to estimate the remaining 70'.

The structural engineer for the client is pushing for a site class of C, and states that since more than 50% of the test holes have a mean N value of greater than 50, I should be able to classify the site as a C. I stand firm at Site Class D, however I question if there is an SOP that if more than 50% of the holes have a mean N greater than 50 to upgrade site class to C?

As a side note, even if I estimate the remaining 70' at a N value of 50 or whatever the bottom of the test hole is, only two of the seven come up above a mean greater than 50.

 

[jdonville]

NMTexEngr,

The cost difference in construction should easily pay for the deep boring. Ask your client to pony up whichever way he feels best about - Site Class D based on the available data or possible Site Class C based on further site exploration.

Jeff

 

[JAE]

Without the 100 ft. holes, you are left with your own professional judgement, other past 100 ft. holes in the vicinity, or some other local source information as to the geology of thesite to set the site class.

I'm a structural engineer and I agree with jdonville that the site class can mean a significant difference in costs.

I would stand firm in what you professionally believe, but also suggest the extra hole.

The structural engineer can and should provide the owner with the potential savings. The only catch would be you at least conveying any estimate of probability that class C is obtainable - but that may not be reasonable.

 

[fattdad]

I don't like being pushed around either. That said, using Site Class "C" is likely appropriate, based on your original post. What do you know abou the overall geologic setting? Is the area glacial, is rock likely to be within 100 ft, are you in the Coastal Plain? What is the regional seismicity? I'd also consider these factors in my assignment of seismic class.

¡papá gordo ain’t no madre flaca!

 

[msucog]

i'm a geotech and i'd refuse to give it to him without a boring to 100' or shallower auger refusal. if he wants it so bad, then he can give it the site class C and leave you out of it all together.

 

[DTJ]

To me the question is if the Site Class is a "D" than what is the Seismic Design Category? If going from a Site Class D to a C does not change the Seismic design category, than there is no reason to perform any additional testing. If going from a Site Class D to a C does enable you to lower the SDC than it would be worth the clients time and a little bit of money to do the proper testing.

 

[msucog]

DTJ, let me pose this question to you. i posted this a few weeks ago on the structural threads but you guys post so much over there, it's gotten buried on page 4.

http://www.eng-tips.com/viewthread.cfm?qid=189524&page=4

msucog (Civil/Environme) 12 Jun 07 20:25
for you structural guys/gals, can you ballpark what you think the $$$ savings would be in terms of total project cost if the site specific seismic assessment gets the maximum 20% reduction allowed by the code for the following (assuming the project is at least moderate sized):
1. if the seismic design category does not change?
2. if the seismic design category bumps up one category?

(example: the reduction saves the owner $25,000 on a $5mil project--0.5%)

thanks

 

[Mickney]

Good Thread, sorry I stumbled on it late in the game. One question I have is - What geographic area are you in?

Here in the Piedmont (residual soils), I have found that Site Class determination by SPT values is VERY conservative. This is particularly true for soil conditions where the upper 20 to 30 feet of soil is firm to stiff soils (Nvalues<20BPF) underlain by dense soils (Nvalues>50BPF), weathered rock and bedrock. We are given a limit of 100 BPF to calculate Nmean for the Site Class determination. This often results in a Site Class D determination.

For such soil conditions, I have performed down hole seismic testing to evaluate the soil shear wave velocity profile. This approach has resulted in a Site Class C determination on nearly all of the sites. I believe that this is due to the high shear wave values in weathered rock and rock, and the fact that I am not given an upper limit value for shear wave as I am for blow counts.

I hope this helps.

 

[ThingsLearner]

Is it costly to measure Vs30? I guess this is the most reliable way of determining the site class. I think it is cheaper than boring isn't it?

 

[msucog]

mickney, i'm seeing the same the thing on my sites in the piedmont. i also see a significant reduction with the PSHA on most sites in the same area. for my area, if i'm able to get only half of the reduction allowed by code, i usually see a change in the SDC.

you might pay special notice to the new maps that are coming out. the maps appear to be lowering Ss and S1 again in our area (IBC 2000 vs 2006 changed fairly significantly and the new preliminary maps appear to have a similar reduction in values).

http://earthquake.usgs.gov/research/hazmaps/