GEOTECHNICAL INVESTIGATION PER IBC-2009 3

[leobramman]

I need some assistance with designing a geotechnical investigation per IBC-2006 or IBC-2009. The area where I am having problems with is collecting data for seismic design category. From what I understood, the easiest way to classify a site is by conducting a SPT (Table 1613.5.2). If this is true, how deep should the borehole be extended? Or, is classifying the site using soil shear wave velocity a better alternative? What is the current practice in the industry to determine shear wave velocity?

If everything that is listed in Section 1803.2 has to be met, the cost appears to be uneconomical.

I would like to get tips on how to design a geotechnical investigation per IBC and still be economical.

 

[VAD]

leobramman

Site Class is important to the structural engineer. There was an article written about the cost savings that will occur if the soil class can be improved. generally our first approach is undertaking SPT testing. Many of us contact a specialist company to ubertake shear wave velocity testing when rock material is encountered or suspected to exist within certain zones. As I understand the cost of doing the test is far smaller than what the cost savings would be to design of foundations. This is another area where
many Geotechs prefer not to dig into and give a one liner on the site class. I have noted that this approach is becoming less acceptable and that we need to sharpen up our understanding as many building officials are requesting this type of info especially for buildings that have large human habitation such as schools etc. I presume that this will become more inportant as time proceeds.

The drilling of 30 m deep boreholes required and undestanding that a certain thickness of overburden above bedrock can decrease the Site class amongst others needs to be understood as well. I am not an expert in this area but understand that there is alot more than putting in a one liner in the geotech reports and leaving it to our structural colleagues to do the math.

I am not sure this answers your question but your point raised would certainkly invoke many comments and by some who have examined this aspect extensively.

 

[leobramman]

VAD

How do the geotechs come up with the one liners on site class? Recently I came up with a program that is made available to the public by USGS to determine the site class and also to calculate the basic design parameters. I am yet to compare with site specific data in our area.

I am much concerned about the comprehensive list of tests and analyses required in any investigation per IBC. IBC does not differentiate between investigating for a residential building and a large project where sufficient funds are available for a detailed investigation. Is it a judgmental call of the geotech or the building official to decide how detailed should the investigation be?

Do you know any reference material that deals with liquefaction and related topics?

 

[JedClampett]

Unless you're in very mucky stuff, you can always defer to Site Class D. After numerous Geotechnical Investigations that gave that result, I pretty much stopped asking for special borings and just assume D.
As far IBC differentiating between residential and commercial based on available funds, there is a IRC which is more oriented toward housing materials and methods.
Are you sure the USGS site you found is not just for seismic parameters? The one I've seen

http://earthquake.usgs.gov/hazards/designmaps/

assumes you know the Seismic Site Class.

 

[Jalthi]

leobramman,

In my suburb the cheapest way to find out the site class is to perform Remi seismic survey(s). The number of survey depends on the soil variability, extent of the project site, and the confidence of the engineer.

There are other ways to determine the site including downhole seismic survey, and based on SPT from a 100 ft boring. However, some jurisdications do not accept certain methods.

I noted that you mentioned a program is available to classify the site class. I think you are wrong. The program calculates the structural design parameters, if you input the site class and the project location.

Hope this helps.

 

[msucog]

Google "FEMA 450" and "FEMA 450b" for some useful slides.

There are many approaches with numerous technical aspects to consider depending on the project specifics. There are also many limitations depending on the chosen approach. I have discussed many methods here before so search the eng-tips site. Shear wave velocity will give a refined estimation and generally removes some of the conservation built in to the Average N-value Method. I personally prefer ReMi. Regardless of the method, it is always good to correlate the data back to site boring data. Feel free to pass along any questions you may have and I'll see if I can put you on some reference material. I have posted several references in the past so be sure to search the eng-tips site.

Good luck!

 

[msquared48]

Jed:

We have to use site class C in all circumstances unless soils investigtions show us that D or otherwise is OK. Although D is the default in the IBC, it is not here.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msucog]

Oops I meant "FEMA 451" and "FEMA 451b"

http://nibs.org/index.php/bssc/publications/2003/

There you go. Sorry about the typo. I hope it helps.

 

[irawanfirmansyah]

Seismic classification is based on the upper 30 m soil data, hence you should drill the ground to the depth of > 30m.
Classification based on SPT N value is the rough one. If the result of using SPT N value shows that you might get higher class, than you should run geophysical test to get shear wave velocity in order to refine the classification. Down hole/Up hole test or Cross hole test can be carried out.

 

[Mickney]

Engage your project team to discuss the differences in building design between Site Classes. A single-story prefab building will likely not have much difference in cost between a design category D and a design category C. However, a large industrial warehouse building with tilt-up panel walls or a multi-story structure will probably have a huge difference between a D and C.

Also, keep in mind that a Site Class is only the first step in the Seismic Design Category. As a geotechnical engineer, I am typically only asked to provide the Site Class to the structural engineer. On certain projects, I am asked to go the extra mile and provide additional values per IBC. In my practice (piedmont physiographic province), I drill a deep boring either to rock or to a depth that will help me determine if additional downhole shear wave testing will improve the site class. I have an internal data base of over 100 sites that I have performed soil shear wave testing on with a seismic piezocone boring. The data base has confidence interval plots of soil shear wave velocity values versus SPT value, depth and soil type. If the deep boring suggests that soil shear wave testing will likely improve the site class, I will contact the owner to discuss and provide a proposal for the work. The cost for one seismic piezocone boring in my area is approximately $2K to $2.5K. Peanuts compared to the potential cost savings.

 

[KVgeo]

Since there hasnt been any recent activity, I can only assume that the original poster got what he needed.

However, I think this topic is a further example of the disconnect between structural and geotechnical engineering that exists in consulting practice these days.

In general these days it seems that structural engineers are looking for an input value to enter into their analysis programs and geotechnical engineers are providing advice on subjects, that in some cases, they have very little background in.