SPT N values
SPT N values
(OP)
Hi all,
I am currently calculating SPT uncorrected N values for a variation of soils. I am getting a first 150 mm value (say 25 blows) fine which i understand you disregard. However i will get another 150 mm value (say 30 blows) but refusal thereafter. Will my N value therefore be 30? Also i have 150 mm value of 25 then the next value is 30 for 100 mm penetration. Is there a correction or calculation you have to undertake to utilise this value?
Cheers for any advice.
I am currently calculating SPT uncorrected N values for a variation of soils. I am getting a first 150 mm value (say 25 blows) fine which i understand you disregard. However i will get another 150 mm value (say 30 blows) but refusal thereafter. Will my N value therefore be 30? Also i have 150 mm value of 25 then the next value is 30 for 100 mm penetration. Is there a correction or calculation you have to undertake to utilise this value?
Cheers for any advice.





RE: SPT N values
Mike Lambert
RE: SPT N values
blows with no advance on the last spoon...which most would. As Mike indicated, you could report as 80/150mm assuming no advance on last spoon. If last spoon did advance, then add this value to the 150mm in example above.
If using the N-value in some analysis/correlation; will need to temper with geology and judgment. For example, you may have
sand over rock; where N=30 is more representative of sand and rock is better represented by say N=100+. Furthermore, when
dealing with rock, say for rippability analysis, then reporting individual blow counts may be of more value as you could
determine more closely where the strata breaks were occurring and judge the degree of rippability from the blow counts/spoon advance (i.e. rock with blow count of 50/6" is likely more rippable than 50/1"......again tempered with an understanding of local geology/correlations).
RE: SPT N values
usually reported the sum for the last 300 mm (150-450 mm) penetration so was not sure how to report if only getting 150 mm to 300 mm. Will report in line with what Mike mentioned, gives all data which is also good.
RE: SPT N values
RE: SPT N values
Now your question may be, "But how do I interpret geotechnical engineering parameters for that level of resistance? I don't know, but I'm sure your frictional resistance and cohesion would be pretty good. If you are in gravel, your penetration is likely being bias by the 1.4 in sampler driving against a 2 or 3 inch gravel/cobble.
f-d
¡papá gordo ain’t no madre flaca!
RE: SPT N values
Question - what is the reasoning for stopping at 100 blows? Worried about damaging the tip? Can always replace the tip. Many times in the Queenston Shale (weathered surface) we ended up with 100/4" - yes and we did stop and we did report this as 100/4" even though it was in the first 6". I wished a lot of you could read some of the Australian Standards - good and much easier to follow and comprehend - unless, of course, you are a lawyer - then ASTM is for you! (sorry all - just in a bit of a mood today!)
RE: SPT N values
RE: SPT N values
RE: SPT N values
do you guys ever use a conical closed tip in hard soils, instead of the standard open sampler? The conical tip doesn't get damaged. The danger is that the driller (yes, we are often at his mercy unless very very careful) is going to use it in soft soils as well, this entailing a value which is way too optimistic.
RE: SPT N values
RE: SPT N values
The conical tip we use instead of the SPT open barrel samplers is something different, very practical in rocks and dense gravels but potentially dangerous in soft soils (correlations are not the same as the SPT).
RE: SPT N values
http://www.scielo.org.za/pdf/jsaice/v53n1/v53n1a06...
RE: SPT N values
RE: SPT N values
RE: SPT N values
Can you please provide me with the link for TxDOT soils design manual ?
Or is there any manual for interpretation of soil properties from In-Situ testing?
Please let me know
Thanks in advance
Naga Talluri