Undrained Cohesion
Undrained Cohesion
(OP)
Hello,
I am new to this forum, and was hoping to get some help with a soil strength varible that is consfuing me.
I am conducting a hydraulic fracture analysis for a HDD project. To do this, I am using the program MDrill (from the Netherlands).
In their user manual, they refer to "Undrained Cohesion (Cu)" and define it with the following equation:
Cu = c*cos(phi)+p*sin(phi)
where p = (vert effective stress + horiz effective stress)/2
I am having difficulty understanding the logic used in this equation. It resembles Mohr-Coulomb failure criteria, but has a few differences that confuse me. I do not understand the "P" variable, and I do not understand why the cohesion would be multiplied by cos(phi) in the main equation.
I have not been able to find any other references with a similar equation. Any help would be very much appreciated!
Thanks.
I am new to this forum, and was hoping to get some help with a soil strength varible that is consfuing me.
I am conducting a hydraulic fracture analysis for a HDD project. To do this, I am using the program MDrill (from the Netherlands).
In their user manual, they refer to "Undrained Cohesion (Cu)" and define it with the following equation:
Cu = c*cos(phi)+p*sin(phi)
where p = (vert effective stress + horiz effective stress)/2
I am having difficulty understanding the logic used in this equation. It resembles Mohr-Coulomb failure criteria, but has a few differences that confuse me. I do not understand the "P" variable, and I do not understand why the cohesion would be multiplied by cos(phi) in the main equation.
I have not been able to find any other references with a similar equation. Any help would be very much appreciated!
Thanks.





RE: Undrained Cohesion
RE: Undrained Cohesion
RE: Undrained Cohesion
RE: Undrained Cohesion
Equation 63 has been confusing me for years. I finally decided to try this forum to see if anyone could provide an explaination.
Equation 63 closely resembles Mohr-Coulomb shear strength, but it has a few differences that really confuse me. I have not been able to find the logic behind the equation. I contacted the program developer in an effort to get them to explain this equation, but they said that they could not answer my question because my company does not have a maintenance contract with them.
Since the program was developed in Europe, I was wondering if they are using the term "undrained cohesion" in place of "shear strength"? Either way, I still don't understand what they are trying to get at.
RE: Undrained Cohesion
RE: Undrained Cohesion
RE: Undrained Cohesion
RE: Undrained Cohesion
the Cu term you state in your original question is the numerator of the parameter Q (see geobdg's link).
so, the good news is there's only one cohesion, but it varies up to a maximum value of the lab test i mentioned above.
RE: Undrained Cohesion
One thing that I keep getting hung up on is that in the MDrill manual, right above equation 63, it says "In case no data about the undrained strength of the soil is available, an estimated Cu value can be obtained using the subsequent formula" (see eqn 63).
Ok. So, wouldn't the undrained shear strength of the soil be determined using Mohr-Coulomb failure theory? If you have the information to solve equation 63 (cohesion, phi, unit weight), don't you have the information needed to determine the shear strength of the soil?
RE: Undrained Cohesion
Draw a generic Mohr circle (at failure) with its center at p' and MC envelope tangent to it. The failure occurs where the circle is tangent to the envelope (where the failure plane is 45-phi'/2 from the direction of sigma'1, which is oriented 90+phi' away from the horizontal axis on the plot). You can then calculate tau on the failure plane by trig, and I think you will find it matches the above.
Now, to get undrained strength from that seems trickier. Seems like they assume that sigma'1 at failure = sigma'v prior to drilling, and sigma'3 at failure = sigma'h before drilling, which seems like a crude assumption if you don't know whether the material is dilative or contractive.