Calculation of geostatic stress
Within the 1st calculation stage the GEO5 FEM program allows for setting up the initial geostatic stress either by performing a standard finite element analysis of a given structure or by using the Ko procedure.
The first option allows for considering an arbitrary constitutive model. By adopting a nonlinear model the analysis may take into account a potential evolution of failure surfaces already in the 1st calculation stage. This will be manifested by the evolution of plastic strains. Providing the soil response is elastic the ratio between the vertical σz and horizontal σx stress components is provided by:
where ν is the Poisson ratio.
The Ko procedure is typically adopted when a specific ratio between the vertical and horizontal stress components is needed. For example, when dealing with overconsolidated soils the actual horizontal stress can attain much higher values than found in normally consolidated soils. This option may also prove useful when adopting an advanced nonlinear material model that requires setting the preconsolidation pressure. While the analysis is elastic, the resulting stresses may violate the yield condition in the 2nd calculation stage when nonlinear material models of the Mohr-Coulomb type are used. Iteration of equilibrium is then carried out even if no changes occur in that stage. Parameters that drive introduction of initial stresses with this particular option are specified when defining the Material Models in Stress Analysis. Further details can be found here.