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Kamis, 25 November 2010

Vs30 & Nspt

talicus (Geotechnical)
9 Mar 05 10:12
In Italy will be mandatory to determine Vs30 for projects. It will be admitted the use of SPT values to deterimine Vs30.

1) I' dont understand why according to USA, Nehrp, Nspt values are not to be corrected? Neither to (N60)ER. There are other raccomandations well explained ?

2) When is allowed to stop the boring before reach 30m.

ashjun (Geotechnical)
18 Mar 05 9:33
Hi,
Your views sound interesting. Would you please expand on the Vs30? I am not sure what it means.
Last month I was given a small assignment on the evaluation of volume of grout mix gone under a viaduct which runs over the new proposed underground in Italy. The project is near Bologna for Italferr. I recall SPT values being in the data sheets submitted by the contractor.
Correct me if I am wrong.
Thanks



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italicus (Geotechnical)
18 Mar 05 17:15
According to this Building codes:
Unified Building COde
NEHRP (USA)
EC8 (European Union)
OPCM 3274 (Italy)

For seismic caracterization, the site will be classified according to the value of the average shear wave velocity, Vs30, if this is available, otherwise the value of NSPT will be used.

Mi question was, is to be corrected the field Nspt ?
Why and why not?



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eric1037 (Geotechnical)
21 Mar 05 12:53
The IBC states that uncorrected SPT values be used. I am not entirely sure why you can't correct it.

It would seem that if you want a higher seismic site class, you would use a "donut" hammer with a rope and cathead instead of an auto hammer.



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ashjun (Geotechnical)
23 Mar 05 17:24
Hi italicus
You are correct when you use N60; that is apply "necessary correction" to the observed N value to make it "realistic observed N value" at 60% applied energy. This is probably because most of the field SPT values all over the world are now related to each other at standard N60 energy (a lot of work in SPT is done in America and their SPT hammer produces so called "60% energy"). You could probably find some literature by Seed who had done correlation between SPT values from some countries (including China and America) and liquefaction potential of the soil. He has also given relations to convert SPT from other countries to the standard "USA" energy.

The standard correction (e.g. overburden, presence of water table) is only necessary when you need to calculated the bearing pressure of the soil.

Your other question on stopping boring when the borehole has reached 30m - even I am perplexed. Here, SPT tests are carried out even at lower depths. But then again, your geotechnical engineer is more sensible because the energy required to drive the SPT tube below 30m is naturally lost (in rod joints etc) and God knows what energy is being transmitted to the SPT shoe.

eric1037 (Geotechnical)
24 Mar 05 11:36
There may be some confusion here.

The Internation Building Code, with respect to seismic site coefficient, states in section 1615.1.5 "Ni is the Standard Penetration Resistance (ASTM D 1586-84) not to exceed 100 blows/feet as directly measured in the field without corrections."

In addition, the seismic site coefficient is determined by taking the weighted average in the top 100 feet (30,480 mm).

As to why it is that way, there is currently much debate.

Personally, the I think the IBC does not accurately determine the site seismic coefficient due to their codes. It is much more representative to use cross-hole or down-hole testing to determine the shear wave velocity (Vsi).

In cohesive soils, the undrained shear strength is used to estimate the site class. There is confusion there as well, because table 1615.1.1 has a column of shear strength next to a column showing SPT values. The shear strength in this table does not jive with established correlations between SPT and shear strength. This is because they do not mean to imply this relationship in the table. The goal of the shear strength in the table is to estimate the stiffness of the soil, and thereby, the response of the soil to a seismic event.

It all comes down to trying to estimate the behavior of soil in the event of a seismic event. All of these methods are approximate. The ultimate goal is to estimate the modulus of elasticity of the soil. Unfortunately, the IBC does not allow for more direct measurement of modulus through the use of laboratory testing or field methods such as the dilatometer or pressuremeter.

I know this doesn't help much, but as geotechnical engineers, we are often put in a position by the owner and the structural engineer to provide a seismic site class higher than what is justified by simple SPT.



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italicus (Geotechnical)
25 Mar 05 2:55
I've understand thath the USA codes doesn't tell about ER60 because all the SPT apparatus teorically are ER60.
In general, according to many manuals SPT Vs measure are conservatives.

The strange is that for NEHRP 2003 and for your IBC no correction for litostatic pressure is to do (N1)60, and for Uniform Building Code (1997) cited by R.W.Day, Geot. Earthqk. Eng. Handbook (2002) this correction is to do.

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