Use the Constant Field Stress option if you wish to specify an initial in-situ stress field (prior to excavation) which is constant, and does not vary with depth. Only the direction and magnitude of the principal stresses need to be defined.
See the Field Stress topic for general information about using the Field Stress option in Unwedge.
Sigma 1, Sigma 2 and Sigma 3 are the major, intermediate and minor principal stresses respectively. By definition:
the Magnitude of Sigma 1 >= Sigma 2 >= Sigma 3
if Sigma 1 = Sigma 3 then this represents hydrostatic stress conditions (equal stress in all directions)
By definition, the orientations of Sigma 1, 2, 3 are orthogonal to each other, therefore the Trend / Plunge of Sigma 1 and Sigma 3 must be defined accordingly. The orientation of Sigma 2 cannot be independently specified because Sigma2 is orthogonal to BOTH Sigma 1 and Sigma 3.
Note: Because Unwedge uses a Complete Plane Strain formulation, this means that the principal stress orientations do not have to be aligned with the excavation axis. Although the excavation model is 2-dimensional, the defined Field Stress (and the stress analysis results) can be 3-dimensional.