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SPT 'N' =0 in Clay
7

SPT 'N' =0 in Clay

SPT 'N' =0 in Clay

(OP)
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.

RE: SPT 'N' =0 in Clay

3
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.

RE: SPT 'N' =0 in Clay

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

RE: SPT 'N' =0 in Clay

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?

RE: SPT 'N' =0 in Clay

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 Su = 50 psf.  (The low Su 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 Su = 20 kPa could be off by a factor of 8 - or more.

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



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RE: SPT 'N' =0 in Clay

3
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.

RE: SPT 'N' =0 in Clay

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

BigH 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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RE: SPT 'N' =0 in Clay

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.

RE: SPT 'N' =0 in Clay

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.

RE: SPT 'N' =0 in Clay

Interesting discussion! Good thread.

RE: SPT 'N' =0 in Clay

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.

and good luck with your project.

RE: SPT 'N' =0 in Clay

Good post, VAD.  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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RE: SPT 'N' =0 in Clay

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.

RE: SPT 'N' =0 in Clay

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!!

RE: SPT 'N' =0 in Clay

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.

RE: SPT 'N' =0 in Clay

Hello Ashjun:

Point well taken. Will be brief in future.   

RE: SPT 'N' =0 in Clay

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 "buy low and sell high, if it don't go up, don't buy it".  <G>

RE: SPT 'N' =0 in Clay

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



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RE: SPT 'N' =0 in Clay

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 . . .

RE: SPT 'N' =0 in Clay

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 "matter-of-fact" 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 "pearls of wisdom" 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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RE: SPT 'N' =0 in Clay

Dear All,

Nice thread, it is regret to join so late. VAD, BigH & Fotch 3 pointed out many good things.


To ashjun, off course it is good if one post provide valuable info in short passage. But, i never be annoyed with the long one if it shows a good idea & supporting background (text book, history case ...). As jheidt2543 said, some great post is more valuable to pay than a short course. I wish that i can read a lot of good ideas like this thread everyday.

I, and i think all of you, never believe fully in a recommendation that provide a experienced data from one person if that guy could not show a strong evidence demonstrating that his data is reasonable.

regards,

RE: SPT 'N' =0 in Clay

I just finished a in a drilling in a river bottom in CT
I encountered WOR/24" material 60 feet below the river bottom. The matrial is an MH (LL=61, PI=29 M%=54)
I performed hand vane shear tests on the spoon sample and found shear strengths ranged from 100psf up to 500 psf(@ 50'depth).  

I am currently running triaxial test to verify the vane results.

Thought this might be helpful.

RE: SPT 'N' =0 in Clay

Don't forget at 60ft the weight of rods (even A-rods) can be pretty heavy - why you get Su values higher than the undrained shear strengths (at least the 500psf value).

RE: SPT 'N' =0 in Clay

I also just skimmed the discussion, but I can offer two pieces of advice based on my experience with two very interesting projects involving soft clay soils (Route 1/I495 interchange on the D.C. Beltway and Craney Island in Portsmouth, VA).

1) If you have soft soil conditions and are building something expensive, don't rely on SPT and UC testing to develop your shear strength profiles. Think about it up front and integrate the use of FVS, high quality sampling (6" tubes is a start), and laboratory testing to define discrete shear strength "points" in the profile. Use the other methods of in situ testing (CPTu, DMT) to fill in the "in betweens" and the spatial variation at the project location.

2) If you have soft clays causing concern with shear strength, don't overlook the consolidation parameters of these materials. They consolidate slowly and often have secondary compression issues. Usually shear strength testing and consolidation testing go hand in hand when dealing with soft clays under embankments.

On the note of long complex posts: keep them coming. If I don't want to read it I won't. I often find different perspectives worth knowing once someone spends the time to put their thoughts on a page. While my experience is surely a great deal different than Focht3's, my perspectives are similar. I have had the pleasure of working with some of the best "soft soil" minds out there on the two projects referenced above (Mitchell, Duncan, and Ladd). Their insight and technical knowledge was invaluable and ultimately saved many $$ in potential construction costs by not taking an "overly conservative" approach because of the uncertainty of the data used in analysis. Get them involved and get them involved early.

Z

Zdinak

RE: SPT 'N' =0 in Clay

I came across your forum and found your information relating the SPT and clay.  I am reviewing a previous consultants report that recommends a mat foundation with a soil bearing pressure of 8,000 psf.  The geotech testing showed SPT of 10 - 15 and hand penetrometer >4 tn/sq ft.  The soils are clay.  I was concerned about the low SPT  until I saw your comments regarding SPT.  Please comment if anyone thinks this is a concern.    

RE: SPT 'N' =0 in Clay

the blow counts seen low for that type of bearing pressure, by I wouldnt too concernd if the report was writen by a compitent Geotech.  Some clay formations have low SPT valuse but high shear strength (i.e. 4tsf unconfined comprsion strength from the penetrometer)  there also could be some sand in the mix which bumps up the Nc, Nq and Ny.

As a follow up about the WOR/24" clay I am dealing with.  I have some of the lab test back.  The hand vanes correlated well with the UU triaxial compression loadings within 10%.  I am also running UU and CD triaxial compression unloadings to simulate a cut slope.  The two results I have so far are from the UU unloadings one (684psf) as 2 times the UU loading strength (360psf) and the other (144psf) was 1/2 of the UU loading strengths (240psf).   we will see how the rest turn out.

RE: SPT 'N' =0 in Clay

Hmmm,

How large is the mat?  Where is the site?  What will the mat support?  What is the soil profile, and what are the relevant soil properties?  GWT information?

I would ask you to start a new thread - include these details in your problem statement.  You will get responses (well, at least one - mine!)



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RE: SPT 'N' =0 in Clay

A very good post indeed from a structural engineers perspective.  We are are often involved designing foundations in a deep marine clay deposit in Eastern Canada based on geotechnical parameters provided to us.  Often Bins, Silos, or Tanks.  The tanks aren't so bad, none the less have great interest to us, but the bins and silos often are 1,000 to 3,000 tons in very small areas 20' to 60' diameters.  The design of the foundation structure, mat or ring and finally 'lets use piles', for bearing and settlement can get pretty hairy. While not much has been discussed about Modulus of Sub-Grade Reaction with respct to testing criteria, it has always seemed to me that in the design of the foundation structure, whether using 'Winkler Spring Models' or some other form of mathmatical degree of confidence building thought process, this value would be very useful to the 'guy' trying to figure out how thick a foundation and were exactly to put the reinforcing for the expected settlement, differential settlement, bending, torsion that will occur.  I of course refer to;

       'Evaluation of Coeficients of Subgrade Reaction'
         Karl Terzaghi, Geotechnique Vol. 5, 1955.

Who was Poisson any ways?  Isotropic, Anisotropic, linear elastic half space, semi-infinite/infinite/homogeneous etc...., Boussinesq what?  Finite Element, Finite Difference, finally lets get a chart ...

You can not do structural design mathmatically without E, and Poissons Ratio.

Im sure I didn't mention OCR and settlememt.

These guys should get a mathmatical symbol library set up for us when we submit a post.

RE: SPT 'N' =0 in Clay

Just re-read this and to the point, need ks from your soils investigation, work thru a distance, the spring constant is quite important.  Often  load is not uniform on a mat or footing, or for that matter give a pile group a shake. N=0 is crappy soil.

Excuse me if Sunday is my day of 'Relaxtion', having a beer, or 3, and cruising the best engineering site on the web.  For sure i know after i've read this post to date that most of the soils we design foundations on are extremely difficult even from the 'Geotechnical' point of view.

RE: SPT 'N' =0 in Clay

Terzaghi's work on this subject, while a real classic, has been superceded by much more recent studies and observations.  Do a search of this site for 'plate load test' or 'modulus subgrade reaction' to get a flavor of those discussions.

A soil's response to load is non-linear - and strain softening.  We can model soil as a linear system only when we know the soil properties over a narrow range of behavior, or where we can model the variations in stiffness with succeeding deflection (like p-y curves.)  This is a pretty complicated area of geotechnical engineering - not textbook stuff...



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RE: SPT 'N' =0 in Clay

allright,
I always start at the beginning Focht3, it is rare to not see this paper referenced, a classic, by any soil structure interaction equations, texts,  or 'canned' computer models running under the guise of 'Beams on Elastic Foundations'.  There I said it 'Elastic'.
My point being the settlement properties and related characteristics of these types of soils are way more valuable to me than the SPT #'s in the design of the foundation structures that sit on them.
More to follow on this subject been away for abit,  seems to have gotten quiet on this threads front though, hope it wasn't me.  I was enjoying the comments on the weight of the drilling rod...

RE: SPT 'N' =0 in Clay

Sorry just cracking open a cold one talking to my 16 year old wanna be a rock star son and it occurred to me that should have said 'consolidation properties' not 'settlement properties'.

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