Critical state soil mechanics

[Mccoy]

I'm posting this after reading BigH in one of the open sections. Which aspects of CSSM can we put in practice in the routine getoechnical design?

I have some difficulties in studying a topic if I cannot link it to some practical purpose.

I believe we have already discussed phi_cs and Su_cs, but these are the only parameters related to CSSM that I've used so far. Can we benefit from the use of some other CSSM parameters?

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[BigH]

I have never had the urgency to use it since I've started working in 1975 but Dr Graham, Professor Emeritus of University of Manitoba, has done so especially with respect to slope stability problems. He gave a number of examples in his presentations - what he pointed out is the idea of "peak strength" especially with what appears to be strain hardening soils (soils that do not seem to reach peak . . . at large strains they still seem to be gaining strength. Yet the soils have reached CSSM at a much lower strain. Should peak strengths ever be used for stability analyses, especially for stiff clays?

DGillette will be much better at explanations than I would be . . . To use it, I would have to really sit down (probably for days) and really get into it.

 

[dgillette]

I've seen very little about CSSM in typical practice, although about a year ago, I heard Ken Been talk about stability of a massive tailings project in Africa, where they made good use of it. That was an ideal use for it, where they had very large volumes of relatively uniform material, beginning with very high void ratios. I'm not sure it would be of practical value for a variable alluvial foundation.

For post-liquefaction shear strength, we spend a lot of time talking about critical state, but for practical purposes, that's only an upper bound, because the gross-scale effects (void redistribution, partial drainage, nonuniformity) generally make the mobilized strength over any large area much smaller than that. That's why many of the top researchers in the field go back to correlations with how many times you have to drop a weight on a pipe to make it go 30 cm into the ground, when they have analyze a real structure.

Skempton suggested that softening of OC clays with time as water seeps in as the clay dilates would be a critical state phenomenon. Quoting from report from a workshop at Virginia Tech that fattdad and I attended a couple years back:

Some advocated the use of critical state soil mechanics to understand undrained strengths and the pore pressures that develop during undrained shear. Critical state soil mechanics indicates that, if clay has softened to a stress state on the critical state line, it will not tend to change volume during shear, and the drained and undrained strengths will be the same. This is supported by some of Skempton’s data. However, triaxial test results on specimens subjected to wet/dry cycles developed significant pore pressures during shear (Wright and Kayyal 1991), suggesting that this mechanism does not bring the clay to the critical state even though the strength is approximately equal to the normally consolidated strength.

I think Skempton was mostly talking about "fully softened clay" in slope failures in cuts in OC London clay, which is mechanically different from the wetting and drying that caused the "fully softened clay" in Wright and Kayyal's research (which I have never read).

BTW, in real life fattdad is not fat.

 

[fattdad]

DG's a funny man!

f-d

ípapß gordo ainÆt no madre flaca!

 

[Mccoy]

BTW, in real life fattdad is not fat.

Lol no, but it sounds good as a moniker, especially the Spanish variant, Papagordo.

http://www.mccoy.it

 

[Ron]

f-d is now stuck forever....Papagordo!

As for CSSM....is there really enough accuracy in geotechniques to establish critical states without the tempering of practical application? Is this a chicken or egg proposition?

 

[BigH]

I think a lot of the real push for CCSM is not, necessarily, in the routine geotechnical issues as most of us deal with where we use the "standard" geotechnical equations/judgment for footings, piles, slope stability (in practice) but in the need to develop and ensure the integrity of the models for 2-D or 3-D Finite Element analyses which are becoming increasingly prevalent with the younger engineers. I've seen the use of Plaxis for normal slope studies now rather than the use of Bishop. My concern is that as FE comes into play, it is becoming more of an IT issue and that there could definitely be a situation where not enough geotechnical oversight is given to the output.

I was in Singapore a few years back for their bi-annual Underground Singapore Symposiums. They have definitely moved, in the years when I first started going to 2-D FE and then 3-D FE - yet they are determining their "E" values from SPT results! They do these fancy computations (as they are required to by government now since the Nichol Highway Failure of 2008 (or 2007) - show that the movement is less than the maximum permitted and that is that. I asked if they ever went back to refine their evaluation of "E" based on a significant amount of real data - not computer models - and they said they didn't - "Don't have the time." Very interesting and, in my view, disturbing attitude.

 

[Mccoy]

...they said they didn't - "Don't have the time."

Now, that's funny, the virtual world governs over the reality, I've heard that in other occasions though. You guys saw the 'Matrix' film?

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