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Silt-Clay Testing: What approach do you use? 2

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coneboy

Geotechnical
Oct 20, 2001
79
Greetings all.

I would appreciate some feed back regarding what testing procedure you would use for determining the soil strength and OCR for a soft silt/clay.
Personally, I have had good experiences using the NKt procedure and the electric cone. Schmertman's procedure using a (Su/overburden stress) value for NC soils from a plasticity index also seems to give good correlations. If my project budget will permit, I would also choose a few quick vane tests.
In addition, I have never been a big fan of lab consolidation tests as corrections for sample disturbances can vary, especially in soft, NC soils.
Thanks for the feed back.
 
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Coneboy,

I have had better success using the excess pore pressure response than the tip resistance, in very soft clayey or silty soils. I work a lot in the offshore and this is usually the case at the seafloor.

The various Su/P methods work well at estimating OCR in soft NC to lightly OC soils, once you have an accurate measure of undrained strength. The in situ vane or maybe self-boring pressumeter gives the best reference data on strength (but SBP tests are likely not feasible on any small onshore project).

I agree with you on consolidation testing of very soft soil specimens, unless they are of high quality (e.g. piston samples). I would also do some miniature lab vane tests on the end of the shelby tubes immediately after sampling.

I would like to hear what you would recommend in non-cohesive silts. In cases where we are evaluating soil liquefaction susceptibility or in designing piled foundations, we still are not always able to deal with silt-rich materials properly due to severe sampling disturbance effects and the propensity of silts to behave in a partially-drained manner during cone testing. Maybe we need to have a discussion started on silt behaviour, in addition to your questions, as this discussion could become volumous.
 
Dear Coneboy,

What I usually order, right upon being encountered with a soft clay, is field vane shear test (bigger blades are usually more sensitive for this case). At the same time you should pay attention to the test assembling system (i.e. having lowest friction between the rotating rods and the casing wall, using ballbearings etc).The other very important point is to have some sort of calibration (i.e. doing the test when the blade is not attached)for friction determination, this is very important when the soil is soft to very soft.
I insist on these matters as I have been able to get useful results. Don't forget to crrect the values for overburden effect and ..., by this test you can also get a good idea about the sensitivity of the soil.

For OCR, even if high quality undisturbed samples are recovered,The oedometer test results do not give an exact view about the history of soil (at times you can not even have any enguineering judgment about, applying conventional graphic methods). I usually compare them with the graph of Su/P'o, and also survey the Triaxial CU test results (don't forget that the sample quality recovered for the cell should be reliable enough)from which the Pc could usually be estimated.
I have focused on these points as I have experienced the compatibility of the results gained with the results obtained by preloading control tests.
Using pocket penetrometer would be useful, aswell.

I do not know how much I could help.
Regards
Cmorgh
 
Good point(s) Cmorgh

I have also found that determining the rod friction on the vane assembly is very important. On bigger jobs where I can justify the cost I use the Nilcon shear vane to determine the soil Su as the Nilcon determines the rod friction during the test.

On jobs with small budgets I use a standard drillers soil vane. Ideally, I try to use this style of vane in conjunction with hollow auger in order to eliminate/reduce the effects of rod friction. As you mentioned, having the rods threaded togeather tightly is very important.

On other occasions I have pushed the vane into the soil from the surface and performed vane tests down to 40 + feet. The rod friction becomes a very important consideration with this approach so I usually do a adjacent vane hole with out the vane on the rods. This enables me to measure just the rod friction. I can then subtract the rod friction values from my vane values to get more of a true reading.

I have found good results can be obtained with all three of the above mentioned procedures.

I know many people like to use a vane correction factor based on the soil P.I. but I have found pretty good correlations with out the vane factor.

One final note: On projects where my budget will not let me do any vane testing at all I simply do a CPT test and use a Nkt of appox 13 to determine the Su values.

Thanks for the comments
Coneboy
 
Dear Coneboy,

I have also performed(in some projects) the correlation for Nk versus Su, which has led to (for the clay texture I dealt with)about 18 to 20 for OC clay.
However you seem to be completely familiar with the procedure.
By the way,in the area where I work, CPT is more expensive than the Vane test.
Nice communication.
Regards
Cmorgh
 
Thanks for the additional comments CMORGH

Yes, I agree, CPT testing ($275 - $310/hr) is much more expensive than using a simple vane unit supplied by the drilling contractor ($165/hr?). It usually comes down to budgets in consulting dosen't it? Use the cone and the exploration costs become greater and you risk loosing the project to someone else who is doing a less in depth investigation. The pros and cons go on for ever.

Have you tried using the cone data interpretation program developed by UBC? It's called UBCPTINT. In the past UBC sold the program but I am pretty sure they are now offering it free of charge. It can be downloaded at the UBC web site for free or do a search on Dr. Campanella. I have had good results using this program (the interpreted results correlate nicely).

What type/name of CPT data interpretation program do you use?

Coneboy

 
Dear Coneboy,

Thank you for comments about "UBCPTINT". Several days ago, I had been searching UBC for the latest print of SD1 (i.e. for chemical topics related to soil, cement, etc.). This time, I'll search for the free copy of "UBCPTINT".

Actually I have prepared the draft of CPT standard for my country (which is to some extent beyond the general concept of the majority of international standards, apart from ... say Eurocode series,and that covers the interpretation of CPT results as well as the test procedure, equipment specification etc.). It is now distributed to the proffesional and academic centers for comments and will be probably finalized in 6 months. Regarding this, I have started to prepare some sort of interpretation program with cooperation of some computer experts. However too busy to be able to have some definite idea about the formal commencement and termination of the activity!!!!!!!!!
For my own practice, I have prepared some programs (very simple, but considering the well-known interpretation methods, which are more practical for me rather than others!!).

However I'll take advantage of your friendly comment and will search UBC, as I said before.

In the world of Geotechnical activities (which at times seems to be as small as a village- specially when you have to defend your geotechnical report against engineers not familiar with this ART), It is so nice to find people who understand each other, professionally.

Regards
Cmorgh
 
Cmorgh / Coneboy,

I have used CPTint 5.0 for many years, which I got from Prof. Campanella at UBC (call Scott Jackson at 604-822-4143 for info on how to get a free copy).

It is a DOS-based program and does not work very well in Windows XP, so you will likely need to run it on an older OS. It is also no longer being upgraded, although it works well enough as is.

There is a companion program called DepthPlot which works in tandem with Grapher (again use the DOS version) to produce profiles of whatever CPT parameters you want - as defined by the output file from CPTint). I have even been able to take digital HPGL output files from DepthPlot into CAD drawing software for report presentaion (e.g. preparing fence diagrams etc.).

Technical support for CPTint and DepthPlot is available directly from Prof. Campanella. Scott can give you his current contact info, as he is now Prof. Emeritus at UBC.

By the way, I have obtained excellent results using the Nilcon vane inside BQ rods, to a depth of 130 ft. ConeBoy's suggestion to run a rod friction calibration test w/o the vane is an excellent idea; this would be especially useful when using casing.

Regards,
Toolpusher
 
Dear Toolpusher

Thanks for comments

Regards
cmorgh
 
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