Excavation can only be defined in the second or the following construction stages.
The degree of deactivation [%] is specified in the dialog window "New Excavation".
The excavation feature enables to partially deactivate of the forces by which the soil within the excavated profile supports the surrounding soil. This technique allows for modeling the successive excavation within a 2D (plane strain) analysis. The standard use case consists of partial deactivation of the excavated soil in one computational stage and installation of the primary lining (beam) and deactivation of the remaining supporting stress in the following stage.
In literature, this procedure is known as the Convergence Confinement Method.