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
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
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
RE: SPT 'N' =0 in Clay
...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.
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: SPT 'N' =0 in Clay
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
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...
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: SPT 'N' =0 in Clay
RE: SPT 'N' =0 in Clay
RE: SPT 'N' =0 in Clay
RE: SPT 'N' =0 in Clay
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.
RE: SPT 'N' =0 in Clay
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 -
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: SPT 'N' =0 in Clay
RE: SPT 'N' =0 in Clay
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
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
Point well taken. Will be brief in future.
RE: SPT 'N' =0 in Clay
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
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: SPT 'N' =0 in Clay
RE: SPT 'N' =0 in Clay
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...
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: SPT 'N' =0 in Clay
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 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
RE: SPT 'N' =0 in Clay
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
RE: SPT 'N' =0 in Clay
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
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!)
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: SPT 'N' =0 in Clay
'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
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
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...
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: SPT 'N' =0 in Clay
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