## interpreting blow count to find friction angle

## interpreting blow count to find friction angle

(OP)

Hi. I have a very simple and maybe dumb question, but how do you interpret blow count N-value when there is a wide range in the blow count?

For example I am reading a boring report with N-values range from 18 to 50 blows which ranges from medium to dense. Can I just take the average and use N=34 to find my angle of internal friction?

For example I am reading a boring report with N-values range from 18 to 50 blows which ranges from medium to dense. Can I just take the average and use N=34 to find my angle of internal friction?

## RE: interpreting blow count to find friction angle

Even "IF" you

~~knew~~expected that the soil was perfectly uniform across the site, an average still would not make sense, since it varies so much from hole to hole.## RE: interpreting blow count to find friction angle

You've given no indication of what you will be using the friction angles for, but under most circumstances it would not be sensible to just assume an average- for example if you are looking at embankment stability, the low strength layers might be the controlling factor.

Even estimating friction angles from SPT N-values is risky unless you know the soils very well...I would only be confident in this correlation with relatively sandy materials. Perhaps ask your geotech. for his opinion.

The question is not at all dumb, but requires further information if you're wanting an intelligent answer.

Cheers,

Mike

## RE: interpreting blow count to find friction angle

## RE: interpreting blow count to find friction angle

## RE: interpreting blow count to find friction angle

If this is the case, I would try to have an idea of the preliminary size of the footings then just use the N-values within the zone of influence of the footings (2B of independent and 4B to 6B for continuous footings). Then, I would take the average N-value within the zone of influence of the footing minus one standard deviation (to account for soil variability). You can use two standard deviations but I think that it is too conservative (at the end you may be applying a FOS of 3 in your results)....

You can see here for the average minus the standard deviation approach (just statistics stuff): https://en.wikipedia.org/wiki/68%E2%80%9395%E2%80%...

## RE: interpreting blow count to find friction angle

## RE: interpreting blow count to find friction angle

As you look from boring to boring you may have some average for the yellow layer that ranges from 12 to 17. Some may advise you to take the lowest average N-value and use that site wide.

Hope that helps a bit?

f-d

ípapß gordo ainÆt no madre flaca!

## RE: interpreting blow count to find friction angle