Triaxial test implementation

Hello everyone,
I have been using Liggghts for a couple weeks and I have already done a simple shear test but now I want to do a triaxial test. I read about the servo control walls but since the test is a cylindrical moving boundary I dont think that is a proper boundary because of two reasons:
1. The walls are rigid boundary and in reality they are flexible.
2. I can not think of moving the cilindrical walls with out changing the shape or leaving a gap.

I was thinking on doing some sort of a membrane boundary, to achieve my idea I need to do two things:
1. Fix the displacement of one sphere type (I will call it type 1) in only one direction (z fixed but the x and y free)
2. If a type 1 sphere touches another sphere of the same type it should not produce any force (the overlap will be allowed), but if it touches a type 2 sphere it should work normally (I want to use just two types). I tried to do this by chosing some material properties so the Kn and Kt are zero but I couldn't make it work how I wanted.
I think that it shouldn't be so hard to implement these two thingsb(and it's the reason that I want to use this method, I thought about others but the implementation would be hard), just a couple of if in the right places, something like:
if balltype[i]==1 then disp_z[i]=0
and
if balltype[1]==1 and balltype[2]==1 then kn=0 and kt=0
else {work normal}

If someone can help me by telling me where is the right file to place this if in the code or telling me some function that already achieves these things i will be really greatful.

Thanks in advance, and congratulation for the excelent job that you all have done.
Antonio Salazar.
PS: I am sorry for my english redaction but i am from Chile and english is not my native language