Seismic Site Classification
Seismic Site Classification
(OP)
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.
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.





RE: Seismic Site Classification
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
RE: Seismic Site Classification
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.
RE: Seismic Site Classification
¡papá gordo ain’t no madre flaca!
RE: Seismic Site Classification
RE: Seismic Site Classification
RE: Seismic Site Classification
http://www
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
RE: Seismic Site Classification
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.
RE: Seismic Site Classification
RE: Seismic Site Classification
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/