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determining c and phi for clay fill

determining c and phi for clay fill

determining c and phi for clay fill

I have a cut/fill site in clay.  I need to look at short and long term slope stability.  The clays are quite stiff with unconfined values of about 5,000 to 7,000 psf and N=16 to 20.  Moisture contents are 16% to 18%.  

My short term analysis will use 1,500 psf cohession and 0 friction.  The stability should be quite high (overconsolidated clay).  For the long term case I'll use 250 psf cohession and 25 degree phi.  I'm pretty comfortable with those values but my question is regarding the fill areas.  Specification will probably be 95% standard proctor.  

For long term stability in fill I'll start with 200 pcf cohession and 22 degree friction.  I pounded a remolded sample in a mold at in-situ moisture (probably 4% over optimum) and completed a hand vane shear (.65 tsf), pushed a tube and completed an unconfined (steady yielding with a stress of about 1,200 psf at 15% strain).  

Any comment on the fill values I'm using?

RE: determining c and phi for clay fill

The tests you did for the fill are fine for short term values.  For long term you need to run a drained test (S test) or an undrained test with pore water pressure measurements (R-BAR test).  

If the slopes are not large or overly important, you could estimate long term strengths based on Atterburg limits or other index tests.

What is the basis of the 250 psf and 25 degree strength for the natural soils?  Don't get me wrong, I'm not saying they are right or wrong, just wanted to know the basis.  Also what is the basis of the 200 psf and 22 degrees for the fill?

RE: determining c and phi for clay fill

LL = 32 to 37 and PL = 12 to 17 so it is a good stiff to very stiff lean clay.  

Placing as fill to 95% standard proctor is my trouble.  I'm mulling about how do I assign parameters.  I don't have the comparative luxury of triax testing (sigh... such is life).  

I think the above numbers are actually conservative.  30 degrees for the cut and 25 degrees for the fill may be reasonable.  Does anyone have some defensible charts or empirical formula they like to reference?

Fill embanmkments are on the order of 12 feet high.  This being a fill water should not be an issue within the embankment.

RE: determining c and phi for clay fill

If I were doing the design, based solely on a PI of 20 to 25, I would use a phi of 25 degrees for both the cut and fill slopes based on Fig. 21.4 from Soil Mechanics by Lambe and Whitman.  That chart is for normally consolidated soils, but is the best avaiable link between PI and drained strength that I know of.  As for the difference between the cut and fill slopes, I would use the same drained phi angle.  The assigned cohesion may or may not be different, but I would not expect much difference.

If I understand you post correctly, the maximum slope height is 15 feet, therefore, I would say they are stable by inspection.

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