Seismic Site Class

[zaqqaz]

Hello All,

I am using shear wave velocity profiles for seismic site classification per IBC. However, I am not sure that if we drill a 30 m borhole and use the Nch or Su method it will give the same result as shear wave velocity measurement. Any experience or comment is appreciated.

Regards

 

[msucog]

the average n-value method has conservatism built in since you don't get "credit" for the rock. also, small rocks influence the blow counts. i like the shear wave velocity method better than using the n-values. in my area, i've done enough of shear wave testing and drilling to put together some correlations. the correlations hold true the vast majority of the time. there is some test variance that should be recognized depending on which method you use to determine the shear wave velocity.

don't forget that there's a few other things that might kick the site class to E or worse.

 

[civilperson]

Shear wave is most dependible and gives the best values, (best for site classification), and thus is used if the seismic design is so expensive that a considerable savings may be realized with the wave velocity test producing a different classification.

 

[msucog]

ditto to that.

 

[zaqqaz]

I do like shear wave velocity method and am using it a lot. However assume that for an investigation you recommend a 30 ft BH plus shear wave velocity measurement to 100 ft bgs. The borehole does not show any problematic soil specified in class E. The shear wave velocity measurements show average shear wave velocity of 350 m/s with a layer of soft soil about 3 m thick (Vs<150 m/s) at a depth of 20 m.

Under this condition do we need to drill another borhole to 20 m to verify the soil condition at 20 m? or we can rely on the obtained average shear wave velocity? What happens if the unsampled soft layer has properties of the soils stated in class E?

 

[msucog]

keeping in mind that you're concerned more with the average velocity for the 100 foot profile, i'd be a little concerned from the geotechnical side too. however, being that deep in the profile, i'm not sure what that does to you foundation support. i'd probably want additional borings (or another shear wave velocity assessment) to verify or discount the lower velocity layer. keep liquefaction in mind too. it would be beneficial to make sure you have soil samples and test data to back up your final decision. check the moistures and PI's and go from there...if nothing other than the low velocity layer forces it to class E, i suppose you'll just have to make an engineering judgement one way or the other. are you in a geology known to have soft layers that deep below relatively hard soils?

what method are you using to evaluate the shear wave velocity? you might double check and fine tune your analysis as best you can to make sure the 3m layer isn't more or less thick. this may help you sleep better at night once you make a final decision.

read the entire chapter(s) in asce7 to make sure you haven't overlooked anything and rely on the information you have in front of you. good luck.

 

[zaqqaz]

msucog thanks for the response. My point is that based on the code a shallow BH and average shear wave velocity measurement within the upper 30 m is sufficient. There is condition on shear wave velocities for example if low shear wave velocities observed in a leyer do this or that.