Stability in the construction stage
The calculation of stability in the construction phase is based on the current state of stress, plastic strains, and other state variables. This state is the result of a standard stress analysis, which always precedes the stability analysis in the given construction phase. Respecting the initial state of the construction ensures that after a successful stress analysis, the stability index FS is greater than or equal to 1.
During the course of stability analysis, the strength parameters of soils, namely the angle of internal friction and cohesion, are gradually reduced. This leads to the development of plastic zones and redistribution of stresses. The reduction of strength parameters continues as long as a stable (equilibrium) state of construction is found. When equilibrium cannot be achieved, the analysis terminates, and the program outputs the calculated factor of safety. A typical cause of instability is the formation of a global slip surface.
When calculating stability, it is important to note that some material models (elastic, modified elastic) and certain structural elements (beams, supports, rigid bodies) have an infinite strength and do not undergo plastic deformation or other modes of failure. Stability analysis in the construction phase allows for application, apart from the hypoplastic model for clay, for all material models.
Anchors
The finite element model to perform stability analysis is identical to that used in the stress analysis. There is only exception arising when anchors are newly added to the calculation in a given construction stage or when existing anchors are additionally prestressed. In such cases, anchors are considered in the stress analysis as pairs of forces with the prescribed magnitudes to prevent relaxation of the prestressed force in that phase. In the stability analysis, however, anchors are considered already as rod elements, and their prestress may change during the analysis. The anchor can be even excluded from the analysis if, for example, its tensile strength is exceeded.