Mohr-Coulomb Failure envelope - help please

[alienrobot]

Hi all,

I am stuck in my attempts to calculate M-C envelope as well as uniaxial strength parameters from example stress data:

Sigma_1 (psi) Sigma_3 (psi)
0 -892


Any tips on how to get started? FYI, I am not an engineer but am familiar with the principles of some of it. Thanks in advance.

 

[dgillette]

Hello, alienrobot.

I'm afraid you don't have the kind of data you need. Looks like all you have is an unconfined compression test.

(BTW - the convention in soil mechanics is compression is positive stress.)

DRG

 

[alienrobot]

Oops, I left off some of the other data.

Sigma_1 Sigma_3
0 -892
8850 0
14925 1000
20900 2000
30900 3000
62887 6000


I need to find the tensile strength, compressive strength, shear strength, internal friction and angle of failure by rewriting the equation somehow.

Any ideas?

 

[dgillette]

Where in the world did you get these data? The lab should have given you more than just raw data.

Start by assuming the tensile strength is pretty darned close to zero. It is a rare project indeed that requires any attention to tensile strength at all. (I can recall once in ~23 years of practice, and that was a situation where tensile strength would be a problem if too high!)

Then, check the units. They don't look like plausible psi numbers. Also, verify that those are effective stresses and not total stresses.

Next, draw the Mohr circles for the data pairs. (Don't bother trying to rewrite the equation.) If you don't know Mohr circles, you'll have to look it up. I don't want to explain here - it would take too long.

Finally, get a basic soil-mechanics text book and see how to interpret strength parameters from the Mohr circles, and to verify that they make sense with respect to the nature of the soil, overburden stresses, etc. This is much too complicated to explain here. Better yet, find a geotechnical engineer to do this.

Bon chance!

 

[Dirtguy4587]

Can I ask a silly question - was this data from a Soil sample, or are we dealing outside of geotechnical work here? 800 psi in tension for soil? Never heard of that before.

 

[aeoliantexan]

Assuming these are stresses at failure from a series of triaxial tests, plotting with a compass and measuring with a protractor, I get roughly phi = 52 degrees, C = 1800. What is the material, perhaps dense, hard angular crushed stone with a little clay binder?

 

[alienrobot]

Actually, these data are for rock. I plotted a p-q diagram in excel and ran a "best fit" line and came up with similar values as aeoliantexan did.

 

[debpriya1984]

Try using Roclab programme.................it plots all the parameters you required..............its free.............try from

http://www.rocscience.com

 

[Jandres26]

if this information is in PSI units, and we convert it to Megapascals, it gives more reasonable numbers for a rockmass.

s1 s3
0 -6
61 0
103 7
144 14
213 21
434 41


very high S1