Mohr-Coulomb Envelope and Friction Angle

[Dirtguy4587]

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...

 

[fattdad]

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!

 

[Dirtguy4587]

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'.

 

[moe333]

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.

 

[fattdad]

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!

 

[Mccoy]

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