Calculate N (Blows per foot) to get Site Class

[greggriffin]

I have a geotechnical report that gives N (blows per foot) values down to 20 ft. One boring went down to bedrock at 23 feet. All borings record N measurements at 1, 3, 5, 7, 10, 15, and 20 ft. In order to obtain site classification, I need an average N value for the top 100 ft. IBC 2006 allows me to assume N=100 for bedrock. Currently, I have used the bisection method to assume values between the measurement depths, and extended the last given measurement to the known bedrock point. From this point on, i have taken N=100 for every foot of rock between 23 and 100 ft. Using this method, I get a site class C (N>50), instead a site class D (15<N<50) as recommended by the geotechnical report. Am I using the right method? Are my assumptions incorrect?

 

[msucog]

be sure to use the correct approach to average

Nave=100/(d1/n1 + d2/n2 + di/ni)

from 23'-100' use N=100

without knowing the blowcounts, i'm guessing the number will be greater than 100. also be aware, there are other caveats to determining site class. you will have to refer to IBC/ASCE7 for details. also, one boring does not necessarily decide the classification. it's more appropriate to look at multiple borings across the area. and the classification could be for say the building area whereas you might be looking elsewhere on the site. a "site class" does not mean the entire site is say Class C.

you should pose your question the geotechnical engineer that provided the recommendations/report. there is likely reasoning behind the recommendations.

also, there are more sophisticated approaches that sometimes provide a "better" site class than the N-value approach. the cost of the more sophisticated approach may be offset by the savings from the improved site class.

 

[greggriffin]

Thank you very much. Your Nave equation is exactly what I am looking for. I am aware of the other methods for determining site class, but they were not used in this geo report. There are multiple borings for the building footprint and I will take the total average of the site area. I would love to just ask the engineer that provided the report but I am trying to have his report thrown out, which will cost him money now and in the future. If I can not prove his analysis wrong, then I may have to pay another company to provide second geo report, using better methods.

 

[greggriffin]

MSUCOG,

I have tried your Nave method and found drastically different results. I have attached an excel file with all of my given borings. Can you help me out with determining a total N_ave for the site?

 

[msucog]

are those R&C or auto hammer values?

looks like a class D to me if based purely on blowcounts (but kind of sort of close to C--unfortunately, close don't count). measurement of the shear wave velocity could help get to the C. in my particular area, those blowcounts would yield shear wave velocities in line with Class C. keep in mind other caveats could kick classification to E or F depending on your soils which doesn't sound like an issue if the geotech recommended a class D.

along with the shear wave measurement, it's possible to get additional reduction of the seismic design parameters through a site specific seismic assessment which evaluates the seismic potential of the site. IBC limits the reduction to 80% of the code map values. but that much could get you to a different seismic design category or simply reduce the design parameters for use in calculating the seismic forces.

good luck.

 

[greggriffin]

Muscog,

I do not know what R&C is. The methods mentioned for obtaining N values are either solid or hollow stem continuous flight augers. I have attached an updated file with my interpretation of the equation you gave me above. With the current method I am using, I do not understand how it applies to the top 100ft. It seems it is only applying to the top 23ft. I would hate to lose 73 ft of bedrock in my calculation. Please tell me I have made a mistake. What N_ave value do you come up with? By the way I appreciate your help with this matter.

 

[msucog]

no you have mistakes. you assign n-values to layers...not to depths. so say the first value at 1 foot (B-9) would be 2/11. the first layer is 2 feet thick. the second would be 2/14. here's the B-9:

100 / (2/11 + 2/14 + 2/20 + 12/16 + 5/11 + 77/100) = 42

R&C means rope & cathead (type of hammer). there's a difference between auto hammer and R&C hammer so it changes the N-values. rely on the geotechnical engineer to do their job and stick with your job/expertise. since you're not experienced with the topic, you should not attempt to disprove anyone. instead, discuss (with the geotech) how they may be able to assist you and the project...also expect to pay for the services as i have rarely seen services provided for free. we (geotechs) can often come up with many alternatives to "situations" but it requires more money (quite often much more money) for testing, engineering analysis, field work, etc. and most/some times it's not cost effective to the project.

the point: talk to the geotech

good luck...

 

[greggriffin]

Thank you so much