Position of Mesh

Hello,
I am working on Triaxial Compression test on granular particles. I use servo walls for the compression.
The problem is initially the forces are high and my servo walls are in the input positions but as the forces approaches to the target val the particles push away the walls and my desired volume changes.
How to keep the walls in position with the target val. I don't want the walls to be fixed in the position through out the test as I want to observe a Volume increase or decrease during isotropic compression and under shearing.

Thanks,
Venkataraman