Trouble understand SPT N values
Trouble understand SPT N values
(OP)
Hi all
I am having some trouble interpreting some of the SPT results I have obtained from some boreholes. I have SPT results in various formats which is confusing me. These are my thoughts on the results I have:
N = 5/9/15 <==== obviously N = 15 + 9 = 24
N = 25/- <==== what does this mean? I am guessing the 25 is disregarded, and after that the SPT test is stopped? Does this mean N = 0?
N = 18/20 for 140 mm <==== Does this mean 18 for the first 150mm, then 20 for the second 140mm and then after that the SPT test has stopped? since the second set of "150mm" is under "150mm" then does that mean I would disregard the second set of blows and so N would be 0?
N = 25 for 130 mm < ==== Does this just mean it has not even reach the first 150mm ? Therefore I can disregard this, and N = 0?
Thanks in advance for the clarification
I am having some trouble interpreting some of the SPT results I have obtained from some boreholes. I have SPT results in various formats which is confusing me. These are my thoughts on the results I have:
N = 5/9/15 <==== obviously N = 15 + 9 = 24
N = 25/- <==== what does this mean? I am guessing the 25 is disregarded, and after that the SPT test is stopped? Does this mean N = 0?
N = 18/20 for 140 mm <==== Does this mean 18 for the first 150mm, then 20 for the second 140mm and then after that the SPT test has stopped? since the second set of "150mm" is under "150mm" then does that mean I would disregard the second set of blows and so N would be 0?
N = 25 for 130 mm < ==== Does this just mean it has not even reach the first 150mm ? Therefore I can disregard this, and N = 0?
Thanks in advance for the clarification





RE: Trouble understand SPT N values
RE: Trouble understand SPT N values
RE: Trouble understand SPT N values
RE: Trouble understand SPT N values
Also, as OG points out, the SPT values are not magic and they are not so accurate as to predict specific properties. Look at ranges of the SPT values and use appropriate engineering judgment to arrive at reasonable conclusions from the data.
I'm reminded of an old saying....
As engineers, we tend to measure with a micrometer, mark with a crayon and cut with an axe. Keep the assessment consistent with the accuracy of the data!!
RE: Trouble understand SPT N values
Have you ever watched as a driller performs this test? Ever see him already pulling on the rope before the hammer even hits the anvil? How many turns around the cathead? what speed is the cathead turning? Lots of variables. It is a rough estimate at best. Don't try to guild the Lily; you are just wasting the effort.
RE: Trouble understand SPT N values
RE: Trouble understand SPT N values
Yes another pet peeve. I do not trust driller's log when filled out at the end of the hole. He simply cannot remember the pertinent details that he (should have) observed. Add to that the illegible scraw on both the logs and jar lids. Sometimes its all but indecipherable.
We are at cross purposes with drillers--most of the time. The driller gets paid to make a hole. We want information about the making of that hole; the hole itself is of limited importance. I wish there was a way to pay based on the information provided.
RE: Trouble understand SPT N values
This is why we ALWAYS have an engineer or geologist with the drill rig. This person is not the driller or the helper, and is there just to log the boring. I would never trust a driller's log. Not that they might try to do the best job in the world, but because they do not have the training to do the log. Add to that human nature in wanting to get the job done as quickly as possible, and you have a bad situation.
As for the accuracy of the test, it can be very good or it can be bad. Automatic hammers make the test more consistent. You just have to remember to correct the blow counts when using old correlations.
As for the original question, asking the driller or however made the log is the best answer. If the driller were dead and could not be reached I would interpret the data as follows:
1. SPT = 24
2. not sure, this one does not make sense.
3. SPT = 20 blows for 140 mm, not sure why they stopped after 20 blows since standard is to drive to 50 blows for refusal.
4. SPT = 25 blows for 130 mm, not sure why they stopped after 25 blows since standard is to drive to 50 blows for refusal.
For cases 3 and 4 I would expect that spoon refusal was reached.
Mike Lambert
RE: Trouble understand SPT N values
RE: Trouble understand SPT N values
As for the recording of the N value - We normally did not report blows as 5/6/4 = 10; we actually just reported the "N" value on the log - in the case here - 10. Many times we would run into highly weathered shale where we couldn't even get past a few tens of millimetres. Let's say we had 100 blows for 4 inches (100 mm), we would report the N value as 100/4". if it was, say 30/6" then 70 for 2 inches, we would report it as 100/8". In this manner we were basically saying we are on weathered rock.
I fully understand why many have qualms and can get upset about this "ancient" test - but it is still a damn good test when an engineer uses judgment based on local conditions. Here in SE Asia - it is basically the only test done for many projects other than a tube sample in clays. One should remember that a good many very important projects have been designed and constructed based on this simple and "archaic" (in many's minds) test.
RE: Trouble understand SPT N values
In the realm of geotechnical engineering if the N-values are much greater than 60 does it matter whether it's 68 or 74?
We always require our geotechnical consultants have a full-time geologist or field engineer with the drill crew.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Trouble understand SPT N values
RE: Trouble understand SPT N values
RE: Trouble understand SPT N values
RE: Trouble understand SPT N values
For me it is just not worth the risk. If more engineers understood, that at least in the US, you are personally responsible for what you seal. I think there would be less corners cut. All in all, I'm just glad that I'm getting close to retirement. I do not have high hopes for our profession.
Mike Lambert
RE: Trouble understand SPT N values
For the last 15 years, I have made sure all final logs specifically state the type of SPT hammer. I have reported the blow counts in increments of 6 inch penetrations & unless near refusal is encountered, have a total of 18" of counts. To me, the 12" Modified lined (California?) sampler is usually atrocious. The -2" diameter lined samples are treated as 'intact' and a wealth of errors are introduced into the process. Shelby tubes are rarely attempted by my competition. A real pity.
As Mike expressed above, the profession appears to be entering some deep troughs. As I look to retire, I am saddened.
RE: Trouble understand SPT N values
I've seen a multi-million dollar project on a mountainside that had horrible slope stability problems and the original civil/soils engineer's report didn't have a single cross section. I've seen aggressively-terraced retaining walls used, with no section drawn-- just a typical single wall detail pulled off of a SRW manufacturer's website.
Similarly, in the structural and architectural world, I see the very same thing. Plan views only. Framing plan with no sections. I feel that even if we may be capable of picturing the section in our heads, not everyone is. And big mistakes get made. Aren't our reports intended to clearly communicate information to others, not just to summarize things for ourselves. And when we draw a section, we see issues that we don't see when only thinking in plan. So why is it that somehow it is no longer "de rigueur" to have a cross section at the site investigated? I'd love to hear other engineer's thoughts.
RE: Trouble understand SPT N values
RE: Trouble understand SPT N values