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Coorelation between Direct and Triaxial Mohr Coloumb Parameters

Cansand (Structural) (OP)
31 May 07 14:48
Hello
I have Mohr-Coloumb strength parameters ,friction angle and cohesion, obtained based on Triaxial Test results ( axisymmetric loading case). Can the Direct shear test  parameters be obtained from the Triaxial Shear strength Parameters.If yes,  Please refer me to the related literature
Thanks
Chewi00 (Geotechnical)
31 May 07 17:26
what kind of tri-axial test was performed CU, CD, UU?
BigH (Geotechnical)
31 May 07 17:56
Don't have it with me but I think that Bowles does give a rough correlation - something like DSS phi' = 1.1 Triaxial phi' (assuming you are considering effective friction angles).  Please double check - it might be the other way around.  Why would you want the direct shear effective stress values if you have the triaxial?  

One reason might be you are aware of the general discussion indicated in Lambe and Whitman years ago that DSS is better values to use along the "bottom" - flatter - portion of the failure surface in a slope stability computation.  In this case they are right in that clayey soils show, many times, varying engineering properties in a horizontal versus vertical sense.  L&W indicated that the "slope" of the failure surface in the test should "match" closely the angles that would be expected in the analysis.  Years ago, there were some papers showing the values of undrained shear strengths at different "angles of shear" within a sample (i.e., at 0deg, 30deg, 45deg, 60deg and 90deg).
Cheers,
Cansand (Structural) (OP)
31 May 07 23:09
Let me explain to you my point:
Most of geotechnical engineers commonly overlook  an important matter when analyzing geotechnical problems under Plain Strain mode (Biaxial mode) .e.g.  2D deformation analysis of Slopes , Tunnels , embankment and foundation and so on. They implement shearing strength obtained from triaxial loading state tests into  analyses assuming biaxial loading situation. However, experiments show that the tests will lead to different results.
Loading case under Direct shear test doesn’t imitate the Biaxial plan strain test case but it is better to adopt than the Triaxial test in plain strain analyses
I attach below for your reference the following related materials
I need your comment please

http://www.ingentaconnect.com/content/els/01489062/1996/00000033/00000008/art87346

-Determination of plane-strain shear strength of sand from the results of triaxial tests
Author: Hanna A. Source: Canadian Geotechnical Journal, Volume 38, Number 6, December 2001, pp. 1231-1240(10)

rockiologist (Geotechnical)
1 Jun 07 9:33
Did you ever think of running a Direct Simple Shear Test instead of a Triaxial test?
GeoPaveTraffic (Geotechnical)
1 Jun 07 10:32
Cansand,

While the points you make may be valid, not saying one way or the other since I have not looked at the refferences; you are splitting a very fine hair.  Remember that even with a very extensive exploration and laboratory testing program, the geotech has seen and tested a very very small portion of the materials involved in the site.  

The geotech is making judgements that are orders of magnitude more important than the difference in measured shear strength between direct shear and triaxial testing.  Unless this a reseach project or an increadably exacting design the differences better not be important.

What you are suggesting is best covered by the old saying, "Don't measure with a micrometer to cut with a chain saw!"
dgillette (Geotechnical)
1 Jun 07 11:55
Are we talking about drained strength or undrained strength here?

The difference in phi' (drained) between triax and direct shear is pretty small - probably not worth worrying about.  (I think DS gives slightly higher phi', but won't swear to that, and anyway, I'm with GeoPaveTraffic on this.)

If we are talking instead about undrained strength, the differences among CIUC triax, CAUC triax, CAU plane-strain extension or compression, CIUE triax, direct SIMPLE shear, etc. can be huge.
BigH (Geotechnical)
1 Jun 07 20:47
See Bowles 5th Ed Section 2-11.3 for some discussion on this topic.

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