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Mohr-Coulomb Envelope and Friction Angle

Mohr-Coulomb Envelope and Friction Angle

Mohr-Coulomb Envelope and Friction Angle

(OP)
Hi All,

I have a series of data (residual direct shear results - normal stress vs. shear stress), and have plotted the data in Excel.  I'm trying to find a way to automate the process of determining the friction angle based on the data, bouning it by the 95 percentile lower bound.  I can do it manually, but this requires updating it whenever I update the data set.  Does anyone have a spreadsheet that does this automatically, or have an idea of how to do this?

Thanks...

RE: Mohr-Coulomb Envelope and Friction Angle

You first need to define the failure criteria for each stress-strain data.  Be careful on this.  If you have one data set that has a peak strength at 3 percent strain and another data set that has peak strength at 15 percent, do you use the peak for both in assembling the failure enevelope?  I think this is a case where the pencil is your friend and would encourage you to use it for assembling the failure envelop.

Just the opinion of a pre-computer professional, slowly getting up to the modern times, that is. . .

f-d

¡papá gordo ain't no madre flaca!

RE: Mohr-Coulomb Envelope and Friction Angle

(OP)
The data I am using are all residual strength values (i.e. the sample has been sheared, then the test is re-run to get residual values.

The pencil approach is fine (and what I am currently doing), however, I have alot of data, and I would like to automate the process, then review it.  Don't get me wrong - I understand the importance of the 'pencil approach'.

RE: Mohr-Coulomb Envelope and Friction Angle

I have seen this automated in an excel spreadsheet using linear regression equations, and trig. functions.  Excel probably also has a statistical function where you could automate the percentiles.  However, you may be fitting a best fit line to a relationship that is non-linear.  

RE: Mohr-Coulomb Envelope and Friction Angle

DG5487:  At what strain are you assigning "failure"?  For the practical purpose of assigning strength, are you willing for the real-world to strain 12 or 15 percent to mobilize the residual strength?  Back in college, we would take the slope of the initial tangent modulus and shift it to 2 percent on the x-axis.  At the point that the slope of the initial tangent modulus intersected the stress-strain curve, that'd be the "failure" strength.  This way you don't assign too much strength to any given point.

Hope these words make sense. . .

f-d

¡papá gordo ain't no madre flaca!

RE: Mohr-Coulomb Envelope and Friction Angle

Dirtyguy,
do you mean to build a 95° confidence level to the linear regression (as the Eurocode literature suggests)?

 

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