lateral stress in periodic cell

Hi
I am trying to consolidate a periodic cell. the cell is a cubic one which right and left sides are defined as periodic boundaries.
During the consolidation phase that is done by moving the upper platen, the lateral pressure exceeds the normal stress which means that the specimen has ko=sigma(zz)/sigma(xx)>1. however, replacing the periodic boundary by rigid walls results in ko=0.5 that is reasonable.
I was wondering if the problem is related to the periodic boundaries or this phenomenon has a physical explanation. I do need to model a specimen with ko=0.5.
Furthermore, when I replace the periodic boundaries by rigid walls during consolidation and unfix the walls before shearing, ko>1 happens immediately.
thank you in advance
m.a.