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.

 

[npthao121]

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,

 

[72shortbox]

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.

 

[BigH]

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).

 

[AZdinak]

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

 

[JGI]

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.

 

[72shortbox]

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.

 

[Focht3]

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!)




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?"

 

[connect2]

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.

 

[connect2]

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.

 

[Focht3]

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...



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?"

 

[connect2]

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...

 

[connect2]

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'.