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Guest mikdrago

Soil spt test and N values

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Hello all,

I am a college senior that is in the last semester of civil engineering program. I am working on an assignment that requires calculating the N values for a given boring log. I know that you ignore the first 6 inches and then use the next two sets of data of the next foot to get the N value, but how do you begin the N table? Do I record the N value at 6" or at 1 foot? I am starting from ground level and test goes to 102 ft. I am assuming that I would take the value for 6" to 18" and enter it for a depth of 6". Then my next depth would be 3' and and would enter the data at 2', using the blow counts from 2' to 3', continuing to the end. If anyone can help clarify this that would be great. Thank you very much.

Michael Drago

Engineering Student

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Attached is a cropped copy of the boring log we are using. I have circled the data that I believe is the correct data for the N values. If anyone can help to clarify this that would be greatly appreciated.

Thanks again.

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Michael,

If you take a look at your report, please note the following:

1. You are using a split spoon sampler device as indicated by the SS notation in Sampler Type;

2. The recovered cores are based on 2-ft intervals (e.g. S1, S2, etc) which is consistent with SPT ASTM;

3. Measure intervals are on 0.5-ft (6") intervals, which is again consistent with SPT ASTM;

So, if you are following the ASTM as described by Old as Dirt Geo, you are supposed to take the 2-ft interval and knock off the first and last blow count and use the middle two blow counts for the N-value.

That N-value will be attributed to the 0' - 2' length of the soil profile.

You will repeat this procedure for each 2-ft interval so your table will look like:

0' - 2'

2' - 4'

4' - 6'

6' - 8'

and so on

If you are interested in reading more about applicability of the SPT, you can try these two documents:

Unified Facilities Criteria - Soil Mechanics (Replaced NAVFAC 7.01)

Subsurface Investigations - Geotechnical Site Characterization (FHWA NHI-01-031)

I hope this helps!

JR

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It seemed very confusing to me when it was explained by my professor since he told us to use 1' 6" intervals. He also told us to use the values for second and third 6 inches intervals that came after the 1'-6" increment. It seemed as though we were backpeddling and when we reached the end of the report the data just did not work out. I follow what you are saying and will try it out. Thanks for your reply.

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It seemed very confusing to me when it was explained by my professor since he told us to use 1' 6" intervals. He also told us to use the values for second and third 6 inches intervals that came after the 1'-6" increment. It seemed as though we were backpeddling and when we reached the end of the report the data just did not work out. I follow what you are saying and will try it out. Thanks for your reply.

hey guys see >http://www.novotechsoftware.com/spt/ for SPT correlations program (FREE).

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