SPT 'N' =0 in Clay 7

[peek]

I have SPT 'N' data in clayey soils. For N=0 case, (or weight of drill rod)I need to find out what su value I can assign, say for a fully saturated sample (PL=39, W=76, LL=87) located under 1 tsf effective overburden pressure. (This is part of an exercise for slope stability analysis). I look forward to getting enlightened from the opinions of the forum members. Thanks in advance.

 

[VAD]

Hello peek:

Based on work that I have done in such soils, I suggest that the su would be around 14 to 20 Kpa. Your soil seems to be a marine clay. For such soils you can have a crust strength as well due to drying and sometimes you may be surprised that the material stands up at a steeper angle than what your slope stability analysis would suggest. One of the things to do in any environment is to observe natural features/ behaviour of such materials. This will help considerably in your exercise.

Thats all for now. I am sure you will get a number of responses.

 

[ashjun]

Hi,
You could forget about the SPT values in clay.
Perhaps you could use the relation between the undrained shear strength and the liquidity index (you could do an internet search on it). This could be conservative unless your clay is highly fractured.
Regards

 

[GeoPaveTraffic]

If all I had to go with was the SPT value of zero, I'd assume 100 psf. However, if this is a layer in a slope failure or significant slope analysis - you need a lot more data. How about redrilling and getting an undistrubed sample or two or three and running some real tests?

 

[Focht3]

Hmmm,

...I suggest that the su would be around 14 to 20 Kpa.

14 kPa ~ 300 psf

I'm very uncomfortable with that recommendation - have seen WOH (weight of hammer) "readings" in materials with S_u = 50 psf. (The low S_u values were made in the field {Nueces Bay near Corpus Christi, TX} on high quality samples with a miniature vane device while the sample was still in the sampler.) Using a S_u = 20 kPa could be off by a factor of 8 - or more.

You need to follow [blue]GeoPaveTraffic[/blue]'s advice - get high quality samples and run appropriate tests. Those tests would not include unconfined compression tests.



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[BigH]

For very soft clays here in West Bengal, I developed a curve of SPT N values to undrained shear strengths (field vanes - corrected - and qu tests). I don't have a value for N=0 because, well, it is sometimes hard to sample especially with crude equipment. But for N=1, the undrained shear strengths I correlated are about 15 to 20kPa. Taking my "curve" down to the SPT=0 intercept, I would estimate undrained shear strength = 10kPa. Given this, I still would be somewhat more conservative and look at a bit lower value, say 5 to 8kPa - or use 10kPa and require a higher SF. For very soft clays we had in northern Quebec, we used to use 200psf (10kPa). One of the things to determine is the sensitivity of the clays - even if the "peak" strength is 10kPa, any remoulding might drop it down to much less.

Secondly, you indicate the N=0 is under 1 tsf (50kPa) of effective pressure. This is about 2.5m of normal fill, say. If you still have N=0, you probably haven't had much consolidation yet (there is an increase, of course, with the strength with consolidation). This is important to realize. In the West Bengal study I have done, the increase is about double for 5m of longterm consolidation.

If you are putting anything on this - say an embankment, you should be careful in either using low embankments with stabilizing berms, or better yet, use vertical drains (wick drains, sand drains, etc.) to permit faster consolidation and hence quicker increases in undrained strengths.

There is also the old standby that undrained shear strength is about 0.22 x effective o/b (see Terzaghi, Peck and Mesri, among others). Using this and your 1tsf, gives Su of about 450psf (22kPa) - you'd be looking at about N=3 or so for that after consolidation.

Hope these comments help. Any further information on the nature of the project - what are you doing in or on this soft soil, lab tests you have, etc. would be useful in refining some comments given by the various participants.

 

[Focht3]

Don't forget to test for the sensitivity of the clays!

[blue]BigH[/blue] is dead-on correct; you have to do it, or you may miss a critical factor for design - and construction. I have an ex-partner that made that mistake...




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[DRC1]

I agree with ashjun - you can't use SPT's for clay, especcially soft clays. There is a lot of literature out there that says at its best, spt's can only give you a very rough approximation of the clay's strength. This is because SPT's measure the relative density of the soil which is related to the strength of cohessionless soils, but not necessarily cohesive soils, especially soft soils. As Big H pointed out, time and remolding can change the strength of the clay. My experience has been that you can't design on the basis of SPT's for clay. Drillers should take shelby tubes of clay and the clay should be labratory tested.

 

[BigH]

DRC1: You are correct to a point. But, many organizations and regional areas have developed pretty good relationships of clay shear strengths and SPT values. Of course, piezocones are better; but, in many parts of the world, you don't have the luxury. My point in soft clays - is that with N=0, 1, 2, 3 - sheisse, it's pretty soft and you need to be very very cognizant of the ramifications of foundations and embankments on such materials. IF N=20, I feel pretty confident that I have good foundation conditions. It is based on years of practice, good judgment and experience. So, yes, shelby tubes are so much better; but, if you only have SPT values (maybe the client did the investigation prior to your hiring on), then, the low SPTs give you some information and, as I have shown, you can develop reasonable correlations at site specific areas.

 

[PEinc]

Interesting discussion! Good thread.

 

[VAD]

I would like to step in again as my values seemed to be of some concern. The range of values were obtained from Flat Dilatometer testing and had some information that colud have been of higher blows. However, I had done some additional work on one borehole which showed N=0 and specifically under the weight of hammer and rods.

I questioned the drilling techniique which was done using wash boring. Yesrs before rather than SPT the holes were cased and insitu vanes were done. SPTs in those clays came afterwards and now this is the preferred type of testing.

All samples were Shelby tubes up to 6 inch diameter. The brief study used UC tests, pocket pen and a Pilcon Vane along with the Flat Dilatometer. The results were as follows for a soil with Liquid Limit on average 85 %. Plastic Limit 35 % and natural water content 73 %, the Pilcon Vane gave results of around 3kPa, the Unconfined test gave on the average about 8 Kpa and the Dilatometer 20 kpa. The very soft zone was over a depth varying from 3 to 15 metres.

Why did I choose the values provided earlier. Well , I have often said that in this business there is something that we all use that is experience and judgement. I base a lot of my decisions on field observation of behaviour and reasons for such. I may not always be correct but I would also use optimistic and pessimistic values despite my feelings before making a final conclusion.

After correlating a large number (323) of soft clay results over the years for this particular deposit irrespective of N values I came up with this relationship Su = 4.3 + 2.3 times depth. The range of values were from 8 to 35 kPa over a depth of 20 metres. I also found that the Flat Dilatometer provided exceedingly good correlations.

Other values I have seen range from 9 to 14 kPa for this type of deposit.

For N=1 our UC and Pocket pens were around 10 kPa which sits well with what big H has found. However our dilatometer results were in the 14 to 25 kPa range. Generally it seems that we obtained about 4 times that of the UC tests and twice that of the vane shearwhen using the Dilatometer.

Certain things make me trust the dilatometer results a bit more than the others. The pocket pen one can critique as well but the important thing is the trend of data.

If you are not looking to achieve a certain factor of safety say 1.5, these soils can sustain with about 3:1 slope a height of embankment of about 11 ft without the need for berms.

In certain situations the choice of too soft numbers without observations can result in the recomendation of very wide berms and one would look foolish if only stability analyses were addressed.

As a geotechnical engineer observations of things around us must be addressed and sometimes looked at before we leap. This would be the call of the person who introduced the thread.

As Harry Poulous had to admit to Ralph Peck in the prediction seminar on spread footings at Texas A& M conference in 1994 Geotechnical Special Publication 14 on why some of his predictions failed to correlate well with the actual results. His answer was that one is really only an expert in one's own turf. Despite the large variations in predictions Ralph said that this was to be expected and everyone did well. I will await the 20 year prediction on setlement as this is of interest.

It is of interest to read this GSP I provided a prediction and it is contained on the last four pages of the GSP publication as I was late being overseas at the time. This was quick and dirty but not bad for the time spent. The oversight made was discovered later and I am richer by this exercise.

Hence no matter what results I provide they are only pertinent to the particular geology of the area where the work was done. Nontheless, we see some remarkable similarities of results from various parts of the globe and one then must ask why and dig deeper. Perhaps there is indeed a geologic similarity. This is why we must also be careful when we pick numbers from the literature etc.

To BigH I would like to have a copy of your correlation of the West Bengal study if you do not mind as I am planning to some day reflect the information I have gathered to add to the published database.

[Cheers] and good luck with your project.

 

[Focht3]

Good post, [blue]VAD[/blue]. I now understand why your results were so much greater than mine. Not only were you using a dilatometer, my LI values were a bit higher (~80 - 90 v.s. 76.) My LL and PL values were a bit higher as well. Just enough to make a difference; and besides, "WOH" isn't particularly precise.

As a general comment to the less experienced readers of this thread: you need to beware of two things. First, beware of the trap of using empirical factors developed using test X (say, unconfined compression tests) with "better" strengths obtained using test Y (in situ vane, dilatometer, SBP, CPT, CIU, DSS, etc.) If the correlation was developed using unconfined tests, then you'd better "feed" the procedure "unconfined-like" data - or risk a nasty failure. Second, remember that strength variability is a big issue for linear projects like slopes, embankments, transmission lines, pipelines, etc. The "average" value may get you in trouble here, too -



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[MRM]

Good point, BigH. You said that N=0 in clays usually indicates soft soils and poor supporting capability. I think that’s a true statement in almost all cases. To me, that's valuable information obtained from conducting a very inexpensive test! I'm always shocked by the number of engineers I hear say that they would never be caught dead using an SPT in clays and immediately discard the values as if there is nothing that can be gained. You can always follow up with Atterberg limits and other lab tests to evaluate the clay, but to me, that's good information to start with. With N=0, you could also say, with some confidence, that the clay is normally consolidated or perhaps underconsolidated depending on site history, i.e. fill deposition, groundwater lowering, etc. You might even evaluate the sensitivity as Focht3 suggested and do some simple tests in the lab to remold the clay to get an indication of that. It's been my experience that most soils containing clays that were deposited in water are going to be sensitive to some small extent at least. I haven't come across one yet that "felt" the same before remolding and after remolding.

 

[BigH]

VAD - send me your email address at bohica@indiatimes.com
I'll forward on the data.

I am impressed with VAD's argument on the undrained shear strength of the clays - he has a good point that there are very good in situ techniques these days to get "good" test results - and, with VAD being a practicing Canuck, I am sure he is quite versed in soft northern glacial lake clays; his own work is showing his great interest in the subject of very soft clays.

It also shows the necessity of the regional experience as he pointed out with respect to Polous. I would expect Canadian glacial lake clays to act somewhat differently from West Bengal alluvial clays or John's expansive Texas clays as Focht3 pointed out.

My correlations may not be the most accurate, but, they have been developed based on the means and methods available at this time in this particular location. And, I would rather be a little pessimistic (now that we are in construction) than optimistic.

As experienced engineers VAD, Focht3, MRM, many others too, and, hopefully myself, realise the necessity of looking a bit more than just the standard textbook rendition of dealing with soft clays as many starting engineers might do. It is a complex subject and one that can have serious ramifications if not looked at in detail. Stiff clays are a bit more forgiving, I would venture to say.

I, for one, have been pouring over Grogory Tshebotarioff's classic 1951 edition of his Soil Mechanics, Foundations, and Earth Structures. It is awe-inspiring (in 1951) the insights that he is putting there in print. I most heartily recommend that younger (say those under 40) engineers find themselves a copy. I have the 1973 edition and I think that some things were edited out - but read his 1951 chapter 13 "The Selection of a Suitable Type of Foundation" and Chapter 9: "The Stress Distribution in Soils. The Bearing Capacity of Soils" - and a whole new world will open up. Put your Bowles and Das away and take a ride on real case histories at the time that geotechnical engineers were "inventing" the art. Wow! I assure that you will not be disappointed. (Also his chapter 14, Spread Foundations. Excavations.)

This has been a good thread and I hope that more input is forthcoming!!

 

[ashjun]

Hi friends,
Would it be nice to write short replies to the forum? May be I am wrong (my apologies) or it is just my opinion, but many a times after I start to read a reply (which are exceptionally good most of the times...cheers to VAD and BigH), I loose track or simply loose interest in the whole thing. Correct me if I am wrong.

 

[VAD]

Hello Ashjun:

Point well taken. Will be brief in future.

 

[jheidt2543]

Good thread! When these threads get to the nitty-gritty of real engineering problems and personal experiances it is like attending a seminar. I have gotten more out of some of these threads (like this one) than some of the short courses I've had to pay for. We should get credits just for reading some of these! It is particularly helpful when responders give references to papers and books on the topic they discuss.

ashjun, some threads do seem to get too long, they are usually the ones I'm not interested in, so I don't read them. I believe it was Will Rodgers who said about the stock market &quot;buy low and sell high, if it don't go up, don't buy it&quot;. <G>

 

[Focht3]

Why don't one of you distill this thread into a FAQ?



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[BigH]

I admit that I get, at times, verbose (maybe because I can type at my thinking speed!). Still, I tend to skim long ones, pick out the points to be taken and if I got the itch, I print it out for future reading. I would rather have contributors like VAD, Focht3, jheidt2543, etc. provide details and especially personal case histories than quick short answers. Usually with quick short answers you get only the bread of the sandwich without the meat. I admit I've learned a 'ell of a lot from the long winded discourses by others. So, sorry, but . . .

 

[Focht3]

Okay, I'll chime in - briefly.

I find the process of the discourse itself can be quite enlightening - I learn a lot about the other members of the fora as well as what their opinions are on a particular issue. You can't discern that from an abbreviated discussion - 'ell, you don't even get all the necessary info if you're too &quot;matter-of-fact&quot; about the issue! Some off-topic meandering is needed from time to time...like this post of mine -

I also have a somewhat different perspective on our profession than some of you. I have had the good fortune to meet - and get to know fairly well - at least five Terzaghi lecturers and two Terzaghi award recipients as well as many of the well-known leaders in geotechnical engineering. I certainly value the technical information that I have garnered form my conversations with them, but the greatest treasures I have received are those &quot;pearls of wisdom&quot; that you won't find in any geotechnical textbook.

While my time is valuable, too, I appreciate the discourse. And I think we all add something of value - even if it is a little verbose (like this post.) At least most of the time...




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