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Re: Site classification - IBC and

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Hello Gentlemen,
I just happened by and saw that some "Geotech" typically only do borings to
25 feet. This is definitely absurd. If the Structural actually provides the
anticipated loads for th eproject, typical borings go to 10 per cent or less
of the soil pressure bulb...which can vary substantially dependent entirely
upon the earth materials encountered. N values are a judgement call as a
minimum...and to drill 100 feet into unweathered Granite is absurd. My
point?
The depth should be based upon the actual loads and the earth materials
encountered. Excuse my rambling.
Chris Lillback PE

----- Original Message ----- 
From: "Adam Vakiener" <avakiener(--nospam--at)southernae.com>
To: "SEAINT" <seaint(--nospam--at)seaint.org>
Sent: Monday, May 03, 2004 4:30 AM
Subject: RE: Site classification - IBC


| >From: "John C. Jones" <john(--nospam--at)struct-engr.com>
| >To: <seaint(--nospam--at)seaint.org>
|
| >Maybe that's really what my question was.  When you look at the method =
| >used to get the average it has "N" or blow count and depth.  Does rock =
| >really have a blow count?  If not, the formula blows up.
|
| >John C. Jones, PE
| >Barnett Associates
| >Pell City, AL
| >205-884-5334
| >205-884-0099 (fax)
|
| I have been out of the office for a few days - sorry for the late
response.  Section 1615.1.5 of the 2000 IBC spells out exactly how to obtain
the average blow count.  In this section you will note that N cannot exceed
100.  Therefore, the N value for rock is 100.  According to the geotechs in
my area this really limits the site class to C or worse unless you bear
directly on rock.  In fact, due to the averaging formula, with about 25 feet
or more of soil above rock you can do no better than site class D.  Keep in
mind that geotechs do not regularly do borings to 100 feet.  You have to
specify a 100 foot boring or they will assume soil below the normal boring
depths (about 20 to 25 feet).
|
| Adam Vakiener, PE
| Southern A&E, LLC
| Austell, GA
|
|
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