Undrained Triaxial Tests - BBM

[bnkolev]

Dear CB community,

does anyone know how to set up a simulation of an undrained TX test in CB?
My steps are:
1. Define geometry 1x1m
2. Create material with BBM constitutive model and assign to surface
3. Fix bottom of model in y direction
4. Assign initial porosity and initial suction (0 for full saturation)
5. Set up 2 intervals: 1st for consolidation and 2nd for shearing
6. During interval 1, assign to bottom boundary a Seepage Line Flux condition with Gamma liquid = -1e6 (to simulate drainage during the consolidation)
7. During interval 1, assign isotropic stress of 0.5 MPa on 2 side boundaries and the top boundary
8. During Interval 2, I apply a vertical line displacement rate to the top boundary (strain control test) and I make the bottom boundary impermeable
9. Mesh using 1 quadrilateral element with 4 nodes.

For some reason, this strategy has failed me and I am very stuck. I would appreciate it very much if anyone can help me fix this problem. without this, my masters research cannot continue.

Best regards,
Boris

[bnkolev]

I have attached my CIU test with the described steps above.
Please help.

[code_bright]

Dear Boris,

There might be more problems, but the main problem of your simulation is that the whole model is free to move in the x direction, so even if there is a slight difference in the stresses applied to the right or left, the model will move to the infinite (or -infinite). One possible solution for that is to fix the bottom line of the model both in x and y directions.

Regards,

Alfonso

[bnkolev]

Dear Alfonso,

thank you for the input. Yes you are right, if only y is fixed the whole model starts moving. I fixed both x and y and the problem still persists.

Best regards,
Boris

[code_bright]

That's probably because you are using a single element, and you cannot simulate a single element in Code_Bright; you need al least more than one due to the algorithm used by Code_Bright to reduce the matrix bandwidth.