How to fix boundary walls

Hi,
I'm carrying out triaxial tests using the script from [1], which uses rigid walls as boundary.
The whole process contains: (a) making sample with target confining pressure and porosity, (b) deviatoric loading.

I'd like to introduce another step for stress relaxation between step (a) and step (b). In this new step, I want to close the triaxial engine and fix all the six walls, and just let the simulation run for some steps with a higher damping coefficient. The way how I fix the walls is by blocking all the degrees of walls, as well as triax.dead=True:
####
triax.wall_bottom_activated=False
triax.wall_top_activated=False
triax.wall_front_activated=False
triax.wall_back_activated=False
triax.wall_left_activated=False
triax.wall_right_activated=False
triax.dead=True
for i in range(6):
 O.bodies[i].state.blockedDOFs='xyzXYZ'
###
However, I found that in some cases, the size of the sample changes a lot after running for a long time, which means the boundary walls are actually not fully fixed.

Do you have any idea about why this happened?

Thanks
Leonard

[1]https://gitlab.com/yade-dev/trunk/blob/master/examples/triax-tutorial/script-session1.py