Servo command

Hi everyone, I am struggling with the following simulation.

I want to simulate a rectangular cavity (of 15.6x5.2x1 cm3) fully filled with 1mm spherical particles with an outlet orifice. The particles should be exposed to a constant pressure of ~1000psi(equivalent to 7e6 Pa) while water is injected on the other side of the cavity. For that reason I settled a servo command like:

fix cad2 all mesh/surface/stress/servo file ../DEM/cad/mwall.stl type 1 com 0.075 0.025 0.0 ctrlPV force axis 0. 0. 1. target_val 56000 vel_max 0.005 kp 5

Additionally, because the packing fraction of spheres is close to 75% the max number of particles that can be arrange in the cavity should be close to
(0.75x156x52x10)/(4/3 x 3.14) ~ 15 000

I would like to know why when I visualize the simulation (in paraview) I observe an awkward splitting effect of particles. The half part of them collapses to the upper z plane while the other collapses to the opposite side, leaving an empty channel in the middle.

I am using an unresolved method and the fluid is simulated as a 2D laminar one.
Should I use a resolved one? I am worried that the size of the system might too big.

Cheers.