Constant confining pressure throughout soil triaxial test

[macarojaime]

Hello,

I've been trying to figure out the way of keeping a constant lateral confining pressure throughout the shearing stage of a CU triaxial test on sand, without success. In other words, I need to maintain a constant sigmaXX and sigmaYY while I apply an increasing sigmaZZ on top of a cubic sample of sand. However, since this test is UNDRAINED, the process generates an excess pore water pressure that makes my lateral stresses decrease.

Is there a way I can fix the stresses that the material is "resisting" while there are other (pore) pressures and displacements going on? (like in the actual physical test)

Could anyone recommend me something in this regards?

I would appreciate any comment.

Thank you,

MARIA C. JAIME
Doctoral Student
Dept. of Civil and Environm. Eng.
University of Pittsburgh

[rjaeger]

Prof. Boris Jeremic has put together notes on his opensees elements and materials here http://sokocalo.engr.ucdavis.edu/~jeremic/CG/LN.pdf

I'm sure you'll find this useful.

[macarojaime]

Thank you rjaeger!

I had Prof. Jeremic's notes, however, it doesn't have any specification about the modeling in Opensees of a constant confining stress while axially shearing an undrained saturated soil sample.

Right now I use Brick8N_u_p_U elements and I've been running my model without such confinement (after I do isotropic consolidation), by purely applying an axial load on top and analyzing it with displacement control. With a small axial strain reached, it looks like the lateral stresses decrease too much, leading to a large drop of my mean effective stress. This doesn't look like the actual physical tests or the undrained models by Prof. Jeremic, so I am uncertain of the use of a confinement or not.

Thanks for the reference though.

[rjaeger]

Thank you rjaeger!

I had Prof. Jeremic's notes, however, it doesn't have any specification about the modeling in Opensees of a constant confining stress while axially shearing an undrained saturated soil sample.

Right now I use Brick8N_u_p_U elements and I've been running my model without such confinement (after I do isotropic consolidation), by purely applying an axial load on top and analyzing it with displacement control. With a small axial strain reached, it looks like the lateral stresses decrease too much, leading to a large drop of my mean effective stress. This doesn't look like the actual physical tests or the undrained models by Prof. Jeremic, so I am uncertain of the use of a confinement or not.

Thanks for the reference though.

Hi,

I'm sorry that didn't help. Could you post your tcl file and the output from a recorder? Have you also seen the verification example tcl codes which you can find through Prof. Jeremic's geomechanics notes pdf? If I remember correctly, he had provided 1 and 2 element triaxial loading examples. Have you had a chance to try those?

Robbie Jaeger

[cyz1654]

Hello,have you known how to simulate triaxial test and could you tell me how to do it