Seismic Site Classification

[Okiryu]

I am located in Japan and preparing the geotechnical report for an US structural firm. I have to provide the seismic site classification of the site based on ASCE 7.05, Chapter 20. The new building will be a 2-story school.
We conducted 11 soil borings with SPTs and verified the bedrock (fresh mudstone, SPT N-values more than 50) depth to be between 11m and 2m depths.
In some locations, very soft clay layers (SPT N-value = WOH, W%= 45%~60%, PI=52%, Su=23 KPa) of more than 3m thick overlie the mudstone layer.

4 of the 11 soil boring locations classify as site class "E" (based on an average SPT N-value less than 15 and W%, PI and Su parameters mentioned above). The other 7 soil boring locations have average SPT N-values between 25 and 50 and therefore classify as class "D".

My concern is about how to determine the site class for this site. If I take the average of the total 11 boring locations, it gives me a GLOBAL site class of "D" (Global average SPT N-value 24). However, if the very soft clay layers are considered as the parameter which governs the site class determination, I should classify the site as "E".

I will be preparing some soil profiles for this site and check how the distribution of the soft clay layers thru the site is.

Any thoughts? Just looking at the boring logs, the soft clays appear to cover approximately 40% of the site. I feel that since it is a school structure, I have to be conservative and classify the site as "E".

Any advice will be appreciated.

 

[Mccoy]

Okiryu, from what you write I'd say that your site is seismically inhomogeneous, exhibiting the characteristics of D and E soiltypes.
Considering all E is not necessarily conservative, depending on the demands of ASCE 7-05 (but why not the recent 7-10 code). for example, it may be that the code in such instances requires a specific procedure, to account for the differential seismic action imposed upon the same structure. As it may be that you can consider the most conservative soil type. What I would do I would consider both, worst soil type and differential seismic behaviour and adopt the worst scenario of the two. A similarity would be that sometimes differential settlements as worst than greater but homogenous settlements.

One curiosity: which is the present sesmic code in Japan and why are you using ASCE 7, the little I read of the Japanese code it's the best in the world as far as seismic site response is concerned. You don't have the limited concept of Vs,30 for example, which as a matter of fact does not govern much site amplification, as it has been shown in the technical literature and as it's evident from just a little reasoning.

 

[Okiryu]

Mccoy, the project site is inside an US military installation and therefore I have to use US guidelines. The US military has developed a criteria for installations located in Japan, so based on the site classification, I can provide seismic acceleration parameters for Japan to the structural engineer. As you mentioned, the Japanese code is one of the best in the world, but we have to follow US guidelines/criteria for US military installations although they are located in Japan.

Based on the ASCE code, for sites with soft clay layers more than 3m thick, Su<24 KPa, W%>40% and PI>20, the site should be classified as E. During our investigation, we found soft clay layers that meet these criteria, so I think that I might have to classify the site as "E".