Undrained Shear Stength 2

[scottiesei]

I am trying to get a grip on soil properties. I have seen many tables listing typical phi, gamma and E values for different soil types. However I have not seen one for Cu. Does such a table exist? Does anyone have a link?
Thanks!
SS

 

[jdonville]

scottiesei,

There is no typical Cu for cohesive materials. Some are soft, some are stiff. And you never know for sure what the shear strength is until you sample and test them. That's why we perform investigations.

Jeff

 

[MRM]

Undrained shear strength includes the contributions of both phi and pore pressure response for the type of undrained loading considered, mineralogy, etc. Also, Cu is specific to a given depth below grade and the effective stress on the soil at this depth. These are some of the other reasons you won't likely find a table like that.

You may, however, find some generalized curves for c/p, i.e., undrained shear strength normalized for insitu effective stress. For design purposes, site specific testing is the way to go as Jeff mentioned.

 

[scottiesei]

Thank you gentlemen, this subject is new territory for me and I am just trying to qualtaively group things.

 

[BigH]

Just like you will see different "phi" values for different levels of relative compaction (e.g., loose, compact, dense, etc - which must be determined during your site investigation), there are ralationships between consistency of clayey soils and undrained shear strengths - you will see very soft as being 0 to 250 psf (0 to 12.5 kPa); soft from 250 to 500 psf (12.5 to 25 kPa); firm (or medium in some circles) as 500 to 1000 psf (25 to 50 kPa); etc. These are well given in text books - see Terzaghi and Peck (oops, add Mesri as of 1995!!); but, many times it is found in the "investigation" section. Again, the undrained shear strength is related to the consistency - and this, as with cohesionless soils, is determined, as jdonville points out, from an investigation.

 

[scottiesei]

Thanks BigH, that is the kind of info that I was looking for.

 

[MRM]

scottiesei,
I'm sorry that I didn't understand your question right off. I thought you were asking about Cu for different types of soil as in illite-dominated clays vs. lower plasticity kaolinte type clays, for example. I don't know how he does it, but BigH always seems to understand exactly what is being asked! Good job once again, BigH!

 

[dgillette]

Just posted part of this on Moe333's thread on a related topic.

See Chuck Ladd's Terzaghi Lecture:

Ladd, C.C. (1991), "Stability Evaluation During Staged Construction," the 22nd Terzaghi lecture, ASCE Journal of Geotechnical Engineering, vol.117, no. 8, pp. 1540-615.

He includes a lot on c/p ratios, and gives typical values. It's a good summary of undrained strength issues in general (in addition to SHANSEP), including the importance of triaxial compression vs simple shear vs triaxial extension in the undrained strength, i.e., c/p is not a constant for a given clay - it varies considerably with stress path

Better yet, get a vane or cone, and remember it's just a bunch of dirt and we can't get terribly precise in trying to determine its engineering properties.