In the Project Settings dialog, two different Analysis Types are available - Plane Strain or Axisymmetric analysis.
Plane Strain assumes that the excavation(s) are of infinite length normal to the plane section of the analysis. In most cases you will be performing a Plane Strain analysis. In a Plane Strain analysis Phase2 calculates:
the major and minor in-plane principal stresses (Sigma 1 and Sigma 3),
the out-of-plane principal stress (Sigma Z)
in-plane displacements and strains
By definition, the out-of-plane displacement (strain) is ZERO in a Plane Strain analysis.
In practice, as the out-of-plane excavation dimension becomes less than approximately five times the largest cross-sectional dimension, the stress changes calculated assuming Plane Strain conditions begin to show some exaggeration because the stress flow around the "ends" of the excavation is not taken into account. This exaggeration becomes more pronounced as the out-of-plane dimension approaches the same magnitude as the in-plane dimensions.
If you have an excavation which is rotationally symmetric about an axis, then you can use the Axisymmetric modeling option.