IBC Site Class Determination 2

[JAE]

The new IBC series of codes includes in their seismic sections a parameter for design called the "Site Class" (see IBC 2000 Table 1615.1.1). This involves the structural engineer using a particular classification (Class A through F) based on the soil shear wave velocity, the standard penetration resistance N, and the undrained shear strength. All of these are for the top 100 feet of the soil at teh building site.

My question is this....most geotechnical engineers drill between 20 and 50 feet for their reports. Many of you on this forum are geotechs and I was wondering how each of you deal with determining this for your structural buddies. Do you combine your borings with a general knowledge of the underlying geology of the site? Is there a potential that you could be wrong by doing this?

What I'm after is an understanding of what everyone out there is doing in terms of getting this classification properly set for the building design. It does make a large difference in loads.

 

[RJ62]

JAE - The procedure we use is this: We put in the budget for one "seismic" boring to a depth of 100 feet or auger refusal, which ever occurs first. SPT's are taken every 5 feet. We then calc the N(bar) using the shallower borings and the 100-foot boring and get the site class. Be sure to use N(bar) not N(avg).

If the site class, determined by N(bar), comes close to Site Class B, but is still a C, we will recommend doing a shear wave analysis, which will be more representative of the site than one deep 8-inch boring and several shallow ones.

Usually, if three consecutive spoons (15 feet) have N-values of 100 blows per foot, the boring will be terminated and N=100 used to calc N(bar).

Jefferys

 

[JAE]

Wow - finally a response! Thanks!

So are you in a specifically high seismic region? The reason I ask is that in moderate to low seismic regions, the idea of an owner paying for a 100 ft. deep boring isn't very attractive. I asked a local geotech in my city what his thoughts were and he simply said that they have a pretty good feel for the underlying strata and make an informed recommendation based on that.

That doesn't sound too professional in one sense, as he indicated that some sites near large rivers can jump up to an "E", but I couldn't pin him down on how he would "know".

 

[RJ62]

JAE - I'm in a moderate area, on Figure 1615(1) 40%, and on 1615(2) 15%. The code states that you can default to a Site Class D UNLESS exploration or suspicion dictate a class E or F. I don't know how the geotech can back up his "know".

Jefferys

 

[vtgeotech]

Jefferys,

How are you diferentiating N(avg) and N(bar)?

Also, we have those who try to average the properties in the upper 100ft even if there is not 100 ft of soil. and thrying to say the average values are great enogh to get a C.

 

[RJ62]

vtgeotech,

For N(bar) see equations 16-23 and 24 in the IBC.

And yes, the code does call for the top 100 feet of SOIL profile. So the thickness will be less if rock is encountered a depth shallower than 100 feet. The trick is to justify where "rock" begins if soft or firm rock is encountered. The only definition I've seen is a shear wave velocity of 2,500 fps. Blow counts?

Jefferys

 

[italicus]

Really in USA is cheaper to perform 100ft and relatives SPT (one each 5 ft) than perform MASW o ReMi shear wave measurements?

And at which dimension of the building or house become mandatory to dig to 100 ft ?

Thanks?

 

[cbosy]

We drill the borings to normal depths and provide a site class based on those and our knowledge of local geology. We also add a qualifier stating that a 100 ft deep boring is recommended to confirm our assumption.

 

[JAE]

cbosy - do you always suggest a 100 ft. deep boring for every project? Or just projects of a certain magnitude?

 

[jdonville]

JAE,

Good topic.

In NW Ohio, where the majority of my current work is, bedrock is usually within 150 ft of existing grade, often shallower.

Where rock coring has been performed (within 100 feet of the existing grade), I have estimated the shear wave velocity of the bedrock based on UCS testing of an intact specimen from the core, unit weight/density of the intact specimen and RQD values. AASHTO HB-17 includes guidelines for estimating the reduced Young's modulus based on RQD. Using an assumed Poisson's ratio, you can claculate G. From there, an estimate of the shear wave velocity is possible.

Obviously, in cases where the potential savings in materials, construction and engineering from a "better" seismic class might outweigh the costs of testing, we would recommend that in situ tests be used to refine the shear wave velocity numbers.

Italicus, it would be useful to learn from you how much the MSAW / ReMi tests cost (rough numbers) based on your experience, in order to compare with borings to various depths (up to 100 ft) to assess the relative costs. Of course, the in situ shear wave test costs for a given location will also depend on the availability of a (typically in the US) specialty contractor to perform the tests.

Jeff




Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[Mccoy]

Folks,
it's interesting to realize that your probs in the states are the same we are tackling here in ole Europe!
The IBC specs sound very like the recent eurocode -8 specs.
Debate is currently hot in Italy about the 100-ft exploratory drillings, wheter or not to (always) drill down to 100-ft, and alternative methods of site classification are being evaluated (see Italicus' post).

 

[italicus]

The cost of a Vs30 seismic profile, like MASW or ReMi (also crossed on the site) has a maximum of 1700 euros (1500$)and a minimum of 1000$.

 

[jdonville]

italicus,

When you say seismic profile, are you measuring velocity through the entire soil profile (like a CPT seismic) in a single boring, or is it between two borings for the cost you mentioned? Does this include the cost of drilling? Do you need to install casing to perform the tests?

We would expect to charge about $1000 for a single boring 100 ft of SPT sampling at 5-foot intervals, using 3-1/4inch HSA or 2inch SSA (if soils are stiff enough). ASTM 2166 tests on all spilt-spoon samples (if all cohesive) would probably run another $200-300ish.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[italicus]

These are superficial geophisical test, and require 60-100m long seismic alignement.

 

[kozera]

has anyone had difficulty in using the formula 16-22 in IBC for determining the average shear wave velocity. This equation appears to give values lower than the arithmatic average?

 

[italicus]

It's right, you mustn't perform an aritmetich average.

 

[vmirat]

I found this thread and hope you guys can help me with a question. The original post alluded to the table in ASCE 7 for determining site class. It is my understanding that undrained shear strength is 1/2 of ultimate compressive strength. Since we typically only get allowable bearing capacity values for building design, is it accurate to take half of this value to determine the Su value in the ASCE table?

 

[jdonville]

vmirat,

Allowable bearing capacity may be influenced by many factors, such as the depth of foundation, geometry of the footing(s), settlement tolerances, etc. and is not always a function of the undrained shear strength of the (cohesive) bearing soil. Additionally, there may be significant variations in soil strength laterally and vertically on a given site.

Moreover, allowable bearing capacities are based on ASD factors of safety. These factors of safety will vary somewhat from practitioner to practitioner depending on the type of foundation, intended construction, client's willingness to accept risks and analysis method used to determine the bearing capacity.

Also, the undrained shear strength is 1/2 of the unconfined compressive strength (UCS) - not ultimate compressive strength. Depending on the site conditions, it may be more appropriate to use a remolded unconfined compressive strength rather than the peak UCS.

So, to answer your question, it is not always accurate to take 1/2 of the UCS to determine Su.

Hope this wasn't too rambling.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[italicus]

Cu = Undrained strenght its a parameter of coesive soils (not rock); for categorize, you must know the values (Nspt, Cu or anything else usefull -Vs_) in 30m belove the subsoil.
Its completely different from the parameter for the bearing capacity of foundations (usually 2B from the subgrade).

 

[vmirat]

jdonville,

Thanks for the response. After I wrote my post, I realized my mistake of referring to ultimate versus unconfined compressive strength (q-ult vs. q-u).

My problem is that I am not getting Site Class determination from the geotech firms for our projects, so I have to figure out what it is from the information they do provide, namely allowable bearing capacity. Is 1/2 of qa a fair approximation to use with the table in ASCE 7? Should I multiply the allowable bearing capacity provided by the geotech by a factor of safety of 3 and then divide by 2?