SPT N values

[fraser01]

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.

 

[GeoPaveTraffic]

I would report the value as 80 blows for xx mm where xx will be 150+whatever penetration you had on the last portion of the drive (between 0 and 150 mm). The 80 comes from the 30 you had for the second 150 mm and the 50 you have for refusal.

Mike Lambert

 

[nbr1]

The N-Value is the sum of the last 2 blow counts, which in your example (25-30-50/0mm) would be N=80 if you stopped after 50
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).

 

[fraser01]

Thanks for the responses, The tests were within gravel, sands, clay's and some rock. The clay's are stiff (field examination) and responsible for refusal around 300 mm i would imagine and rock is a Rhyolite and getting values below 100 with refusal so the values are not too representative of ground conditions, unless heading down a fracture of some zone of weakness it should be a lot higher i would imagine. My understanding is the SPT is not an overly great test in cohesive soils and rock.

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.

 

[BigH]

I always used to report as >100/8" (if 2nd and part of 3rd 6") or 100/2" - sometimes if hammer was "bouncing" we would make a special note on the log. One problem occurs in, say, you have 100/2" in the "first" six inches. I didn't worry - just reported it as 100/2".

 

[fattdad]

according to ASTM D1586, the default units are feet with incremental penetration measured to the nearest 0.1 ft. The obligation is to drive the sample each 0.5-ft interval or for a total of 100 blows. So, if you had 40/40/40, you'd have driven the sample too much (never knew that until today). If there is no penetraiton after 10 blows (any increment) you'd stop. If you had 25 blows for the first 0.5 ft, 40 blows for the second 0.5 ft and then 20 blows for the next 0.2 ft (and the last 10 blows provided zero penetration, you'd report 20/0.2'

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!

 

[BigH]

Yes, and again, ASTM . . . but not normal practice - at least in Canada! What did the profession do before ASTM codified a standard for something that the industry had been doing for years?

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!)

 

[fraser01]

Ha ha indeed. Yeah I am running off the Australian standard, out in Western Australia. It does have the same 'stop after 100 blows' but running on metric not imperial. Out here you can be at the mercy of the driller but guess that goes most places. The Aus standard is just a little light on the reporting part if you are not getting the full test etc. guess reason for stopping after 100 is in soils you probably have all the information you need if you hit 100.

 

[oldestguy]

It's just a waste of time and time is money.

 

[Mccoy]

Just a related question:

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.

 

[BigH]

Mccoy - in Canada we use the Pentest (at least we did when I was there) - it is a conical tip 51 mm driven with the same energy as SPT. We actually had a disposable tip and would drive the tip from ground to practical refusal or intended depth of investigation - would give a blow count profile.

 

[Mccoy]

BigH, In Italy we have the same equipment and it is very popular, 51 mm diameter conical tip. I wonder whether Italy exported it to Canada or the other way around!
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).

 

[BigH]

It is also used in South Africa - saw an article about it the other day. India has a spec on it - although they have revised it to a 63 mm dia cone. Not sure if it was imported from Italy but I've seen logs using it from the 1950s in Canada.

http://www.scielo.org.za/pdf/jsaice/v53n1/v53n1a06.pdf

 

[oldestguy]

Hey that conical tip probably was patterned after a contract driller that Wisconsin had for some Interstate work in 1959. I happened on the job one day to find the split spoon tip was so buggered up that the opening was only about 1/2". It seems the sample jars were not very full as a result, yet blow counts didn't seem to be badly affected. Inspector didn't seem to know the work was questionable. That firm is still in business!!!!Their 140 pound weight was hollow steel filled with lead.

 

[aeoliantexan]

TXDOT uses a 3" diameter cone driven with a 170-pound hammer falling 24 inches. I was told that it was developed about the same time as SPT. It is used in rock as well as soil, and there are empirically-developed design charts for drilled shaft design. Google TXDOT for their soils design manual.

 

[nagatalluri]

Aeoliantexan:

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