Trouble understand SPT N values

[crownmethod]

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

 

[oldestguy]

For this crude "test" some folks put great emphasis on exact numbers, for some unknown reason. In your case, I'd contact the drillers and get an explanation from them as to their system. We could go on and on about things that affect this "test". Hopefully you don't get too involved with exact details. Once you are out with several different crews and with different rigs, different tools, different degree of inspection and different soil conditions, you will see what I mean.

 

[crownmethod]

I am not quite sure what you are getting at, are you suggesting those blanked out, or missing numbers suggest there is an extra number there? (hence why you say it's "exact")?

 

[oldestguy]

I'm saying the system that those numbers are given in probably is best explained by going to the organization that gave you them. I am not trying to figure out what they mean, especially when not three are shown together. However, I am trying to say don't get too involved with trying to relate them precisely to any soil property, such as coef. of friction.

 

[Ron]

OG is correct. Go to the source to get their "shorthand" explained. Speculating on what they mean is not appropriate and the geotechnical engineer should be chastised for putting out an incoherent report. That is not appropriate.

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!!

 

[mikeofBelAir]

SPT--one of my favorite gripes.
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.

 

[oldestguy]

Then expect him to remember the numbers for the log 10 minutes later.

 

[mikeofBelAir]

Oldestguy--
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.

 

[GeoPaveTraffic]

With respect the discussion about logs and the numbers.

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

 

[oldestguy]

Mike: A great post. Yes an engineer on the job is best, but with competition out there, its not common any more. Result, look at the info as very general and be conservative with the recommendations. You get what you pay for.

 

[BigH]

Mike - I applaud your company's attention to the fine art of ensuring that the field work is done correctly. I started with a well known Canadian geotechnical company then went to the company in which they broke off of - and in both it was an absolute that a geotechnical engineer or technician (usually younger lads or lasses in the first case) be on the drill rig, log the data (blows / recovery / drilling observations), prepare the field log, care for the samples including the waxing of tubes and storage (sometimes having your car heater running full time when it was -20degC on the rig), work up the daily details on expendables and time spent, and ship the samples back to the company's laboratory. I've seen some very sad times where a driller's logging of a borehole led to a big big problem mainly because drillers do not understand the nuances of the strata in which they are "drilling" - as an example the importance of low permeability partings in an otherwise uniform sand stratum - which led to major ravelings during a large sewer installation.

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.

 

[fattdad]

after you figure it all out, then multiply the answer by 1.5 'cause they likely used an automatic hammer!

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!

 

[Ron]

I came from the same ilk as BigH and OG described. When I started my first engineering firm in 1983, we had no choice but to use subcontract drilling. Even though we only had three people at the time, two of us were engineers, we ALWAYS had an engineer on the rig, logging the hole. Nothing else was acceptable. As we developed a relationship with the driller over the years, and as we grew, we transitioned from an engineer to a Senior Tech with drilling experience. Interestingly enough, the owner of the drilling company was a picky as we were, and he was always on the rig as well!

 

[oldestguy]

With the inclusion of more and more geotech firms, and some drilling firms, competition is increasingly fierce. The firm I was with '63 to '83 never made any profits from drilling. Only rarely did we put an engineer there, due to the "bid price". In another post I described how my spying on a crew from a half mile away uncovered some real cheating that had been covered up by experienced crews that were pretty knowledgeable as to what to expect for soil conditions almost any where we worked. Somehow that situation didn't seem to show up with any job difficulties. It did shake up the crews however. Morale; don't take that N value as precise or even in the ball park sometimes. Even that "correction of 1.5" seems wishful thinking. Modern crews should be required to do a day with the old wash and chop method when 30 feet a day was real good.

 

[moe333]

I have always had a representative from my firm (geologist/engineer/technician) logging and sampling borings. I have never used logs and samples from drillers. I would not be able to rely on them.

 

[GeoPaveTraffic]

We see a lot of other firms that do not place an engineer or geologist on the drill rig. And we routinely loose project to them on price, but as others have said; you get what you pay for.

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

 

[emmgjld]

I have always had an experienced engineer, geologist or tech on the rigs, as I learned from my Father. Having our own rigs since '76, until last year, has also been a big plus. Performing a dynamic analysis on the Automatic Hammer also impressed upon the driller the need to keep the equipment clean & properly lubricated.

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.

 

[MDavidsonPE]

I read with sadness the thoughts expressed in the last two posts. Sometimes, I too wonder why, when we know the right thing to do, that we don't do it and insist upon it. My pet peeve is when I see a geotechnical report (even ones that cost $20K+) with plenty of borings and/or test pits, but there is no interpretive cross section presented in the report-- no subsurface profile that shows all the issues at play: ground surface,slopes, soils, water table, proposed/existing structures, etc. etc.

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.

 

[BigH]

@MDavidsonPE - I think the problem is that most of the "Old School" have now passed on and the "New School" can't see the forest from the trees . . .

 

[mikeofBelAir]

Looks as if I am not the only curmudgeon here. I am retiring soon, also.