applying sinusoidal loading under pre compressed state

Hello all,

I am trying to model a confined cylidrical box which consists of uniform particles. The particles have to be compressed to a stress of 100N and then under the stressed state, I have to apply sinusoidal load. I am using a mesh to represent the plate at the top. I use mesh/surface/stress/servo to apply the force 100N on to the particle. I cross checked the force by looking at the force values of the mesh plate.

I have problem when I have to apply a sinusoidal load over the precompressed state. I cant use move/mesh with the fix mesh/surface/stress/servo on. If I unfix the servo and introduce a mesh at the exact location obatained after pre compression, the forces in the new mesh is zero and not the precompressed value. How to handle this problem?

Kindly help.

Monica