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RE: Site classification - IBC

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Rick, I agree with you on item 1 (below). And I agree "in general" with item
2, except that the 'n' layers need not be equal layers. The layers can vary
in thickness and should correspond with changes in soil properties. Thus you
could use 1 layer of 20-ft with N=15 and 1 layer of 80-ft with N=100. 

However in item 3, there are problems with using N=100 for rock, at least
based on a "literal" reading of the code. First the code states that 'N'
must be based on blow counts "directly measured in the field" - so if you
didn't do blow counts in the rock, you don't meet this requirement.
Furthermore, Table 1615.1.1 lists the Standard penetration resistance, N, as
"Not applicable" for rock under Site Classes A & B; however, it does permit
'N' values for "soft rock" under Site Class C. 

I guess that you could ask the driller to pound on the rock to confirm the
100 blow counts and then argue that the rock is at least as good as "soft
rock" with N>=100. Without actual values, you would have to argue that the
rock properties are well enough understood to "assume" N=100, even though
the code doesn't explicitly allow "assumptions". 

William C. Sherman, PE 
(Bill Sherman) 
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)

> -----Original Message-----
> From: Rick Burch [mailto:rburch(--nospam--at)] 
> Sent: Friday, April 30, 2004 11:38 AM
> To: seaint(--nospam--at)
> Subject: Re: Site classification - IBC
> John,
> Here is my understanding on how the avereraging is done:
> 1.  The IBC formula doesn't use blow count, shear wave velocity, AND 
> undrained shear strength,  you (or actually the geotechnical 
> person) use 
> only one of the three.  See 1615.1.1.  It says that when shear wave 
> velocity data is not known, you can use N values OR undrained shear 
> strength values.
> 2. See 1615.1.5 for averaging.  The top 100 feet is divided in to n 
> layers, for example 20 at 5' each, and the test values at 
> each of these 
> points are averaged.
> 3.  Section 1615.1.5 covers what to do with the rock N 
> values.  On page 
> 352, it says N is ... not to exceed 100, as Thomas Hunt said.  So you 
> might be averaging in a lot of 100s for the layers of your 
> 100 feet that 
> are in solid rock.  In other words, you might have:  5 6 8 7 9 12  13 
> 15  22  33  35 ... 100 100 100 100 100.
> Rick Burch
> Columbia, SC

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