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Estimating c and phi for Overly Consolidated clay in drained condition
3

Estimating c and phi for Overly Consolidated clay in drained condition

Estimating c and phi for Overly Consolidated clay in drained condition

(OP)
Hello,

I ran into overly consolidated clay and I need to provide drained strength parameters. Triax is not available. I got Atterberg, Vane Shear, Pocket Penetrometer and SPT.

All my reference only provide phi for Normally consolidated clay. For Overly consolidated clay, they only say things like "They got a cohesion but with less phi".

Any suggestions?

Thanks.

RE: Estimating c and phi for Overly Consolidated clay in drained condition

refer to the work of Tim Stark at the university of Illinois, Urbana. He has correlations that realate to the determination of fully-softened and residual shear strength friction angle. I think his correlations require that you do a hydrometer for grain-size distribution as he uses the percent clay fraction in his correlations. Bear in mind that clay size is much smaller than the No. 200 sieve and requires the use of a hydrometer.

You may be inclined to base your design on the peak friction angle. In some instances that may or may not be appropriate. It's often likely for fat clays and elastic silts for the peak strength to fatigue over time to some lesser value - fully softened or residual. Stiff-fissured clays that undergo wetting and drying, freezing and thawing and such temporal changes often lose their cohesion and often revert to some fully-softened shear strength. This process can take years (decades). So, be mindful of your assignation of friction angle. Peak may not be appropriate.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Estimating c and phi for Overly Consolidated clay in drained condition

fattdad points out many good points. I recently attended presentations by Dr. James Graham, Professor Emeritus at the University of Manitoba - an area of heavily overconsolidated clays. Basically, he states that one should be basing their design on Critical State conditions, i.e., c'=0 and phi'cs. I attach his "message" on overconsolidated clays.

RE: Estimating c and phi for Overly Consolidated clay in drained condition

Both the points made above are good ones and should be considered.

If you want to determine peak strengths, there is a relationship based on plasticity index that is published in a book by Lambe and whitman and is included in a NAVFAC DM-7. I'm traveling and do not have the exact reference with me.

Mike Lambert

RE: Estimating c and phi for Overly Consolidated clay in drained condition

(OP)
Thanks everyone. You guys had some really good suggestions here. I guess the conclusion I am getting at is that design based on the virgin curve with c=0 and phi.

RE: Estimating c and phi for Overly Consolidated clay in drained condition

The critical state condition versus the peak condition is an extremely interesting subject. It begs a few questions though.

To avoid unduly caution though (and excessive economic burden) we should perhaps be more precise about the instances when peak values are no more advisable and critical state or residual values are plausible.

How many projects on OC london clays or Boston Clays have been verified by critical state or residual parameters? Has most of the design been done by peak values? If so, what's the overall failure ratio?

FAttdad's discussion implies that OC clays are often in a metastable state. He says they may take tens of years to turn to the stable, critical state condition. Tens of years from the onset of a stress-strain field? Is the time stress & Strain dependent? Is there a stress & strain treshold below which the metastable state does not alter?

Are there any conditions (for example, landslides, horizontal thrust of foundations) when the critical state design is Always advisable? Which ones?

I understand that the precautionary principle would suggest to design Always based upone residual or cs, but what about the economic optimization? Related of course to probability of failure?

www.mccoy.it

RE: Estimating c and phi for Overly Consolidated clay in drained condition

Since i do possess Terzaghi and al book , i can attest that OC clays shear stregth depends highly on its intact state . If the clay is fissured or has suffered in geological times from erosion/ removal of overburden then the frequency of fissures control its resistance . Under drained conditions , OC fissured clays with high frequency of fissures develop no cohesion intercept and only phi is required , but the problem is whether you should use peak , fully softened or residual phi .

RE: Estimating c and phi for Overly Consolidated clay in drained condition

to mccoy, I don't believe that o/c clays are metastable at all! I'm just echoing the thought that o/c clays can revert to fully softened state owing to temopral processes of wetting/drying and freezing/thawing. These temporal processes can/do occur in the near surface (e.g., top 10 to 15 ft). So, if you are evaluating a proposed cut slope, today's strength (i.e., as measured by N-value, CU-bar, direct shear, etc.) may not survive the test of time. DOTs typically use a 75 year design life, so we have an obligation to the people to consider this attenuation of strength over time. And, we do just that. It often is a struggle with those that are more focused on commercial development work, where the design life is more like 20 years.

I mean to make no sweeping generalizations or proclimations. I'm just saying todays estamation of c and phi (peak values, large-strain values, etc.) may not be appropropriate for the near surface or projects with extended design life. I"m also mindful of our sensitive, stiff-fissured clays that we find along the Fall-Zone of Virginia. They are (can be?) a problem! Shale fills too!

f-d

ípapß gordo ainÆt no madre flaca!

RE: Estimating c and phi for Overly Consolidated clay in drained condition

Fattdad, right, in your previous post you cited wetting and drying cycles, freezing and thawing so I should have understood sooner that you were referring specifically to the case of cuts and artifical slopes. My bad, old age approaching makes up a convenient excuse in my case.
By the way, by metastable I mean exactly what you say about changing mechanical properties when conditions change and another point of stability must be reached.
And conceptually it seems pretty much reasonable.
Also, as killswitch reasons about the more appropriate phi overall, if we measure phi-c on an intact specimen, that's undoubtedly a phi_peak, but if fissures govern the stability, we have already existing sliding planes along which phi_cv sounds more of a technically sound choice.
The sliding plane may cross thru fissures and intact clay, so the averaged shear strenght might be some post-peak value which is not as low as the critical state. In this case phi_cv would be a cautious lower bound,.

The OP did not specify the kind of geotechnical problem I believe.

In some cases the regulations call for specific values, as in Fattdad's DOT, or in European building codes, where the sliding design of shallow foundations is required to be carried out by phi_cv.

Also phi critical state is assumed to govern when a soil-concrete interface exists, or when there is significant soil disturbance, as in drilled piles (some authors suggest phi_residual at this point).

The peak versus critical state parameters choice is probably different in different geotechnical analyses.

www.mccoy.it

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