Under the Analysis tab in the Project Settings dialog, you can choose the Analysis Type. There are two options available:
Plane Strain
Complete Plane Strain
For BOTH Plane Strain or Complete Plane Strain, it is assumed that:
the excavation(s) are of infinite length in the out-of-plane direction
the out-of-plane strain is ZERO (i.e. the material cannot deform in the out-of-plane direction).
The difference between Plane Strain and Complete Plane Strain is due to the orientation of the principal stresses, as described below.
Plane Strain
For a Plane Strain analysis, it is assumed that one of the in situ principal stress axes is aligned with the out-of-plane direction (i.e. parallel to the excavation axis). Therefore the other two principal stress directions lie in the plane of the analysis (i.e. in the cross-sectional viewing plane, perpendicular to the excavation axis).
Complete Plane Strain
For a Complete Plane Strain analysis, the principal stress directions do NOT have to be aligned with the excavation axis, and can be arbitrarily oriented. The analysis is still a plane strain analysis (i.e. out-of-plane strain = zero), but the in-situ stress field, and the computed stresses are effectively three-dimensional, with respect to the excavation axis. This allows you to define the true three-dimensional principal stress directions, and therefore should give more accurate analysis results (stress, displacement and strength factor), compared to the Plane Strain assumption.
NOTE:
The choice of Analysis Type determines the required Field Stress input parameters (e.g. you must specify additional Field Stress orientation parameters if the Analysis Type = Complete Plane Strain). Furthermore, you must also specify the orientation of the excavation axis (Tunnel Orientation), if you are using Complete Plane Strain. For details see the Constant Field Stress and Gravitational Field Stress topics.
The Complete Plane Strain option cannot be used in conjunction with Transversely Isotropic Elastic Properties. If you are using Transversely Isotropic elasticity, the Analysis Type will automatically be set to Plane Strain.
See the Examine2D Overview for comments regarding the accuracy of the plane strain analysis assumption.