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RE: Site classification - IBC

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Thanks for pointing out the paragraph regarding estimated soil properties by
the PE - I missed that. It will likely come in handy some day. 

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

> -----Original Message-----
> From: Rick Burch [mailto:rburch(--nospam--at)conterra.com] 
> Sent: Friday, April 30, 2004 3:05 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Site classification - IBC
> 
> William,
> I agree that the layers are not necessarily equal, and should 
> match the 
> soil profile.
> 
> As far as rock goes, the IBC does allow some assumptions.  
> See 1615.1.1 
> where it says "Where site-specific data are not available to 
> a depth of 
> 100 feet , appropriate soil properties are permitted to be 
> estimated by 
> the registered design professional preparing the soils report 
> based on 
> known geologic conditions."
> 
> That matches the approach that I have seen in geotechnical 
> reports.  If 
> they get down to bedrock at 50 feet, say, and pound 45 blows 
> and only go 
> 3/4", I have seen them assume 100 bpf material from there on 
> down based 
> on their knowledge of the bedrock. 
> 
> Rick Burch
> Columbia, SC
> 
> Sherman, William wrote:
> 
> >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)cdm.com
> >
> >>-----Original Message-----
> >>From: Rick Burch [mailto:rburch(--nospam--at)conterra.com]
> >>Sent: Friday, April 30, 2004 11:38 AM
> >>To: seaint(--nospam--at)seaint.org
> >>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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