Initial geostatic stresses
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The initial stress conditions can be generated also in two different modes: the first consists in an explicit finite element stress analysis taking into account the gravitational loads of the massive and second through the simplified K0 procedure. The K0 procedure may only be used for horizontally layered geometries with horizontal ground surface and, if applicable, a horizontal phreatic level.
In the case of linear elasticity we have the following relationship between normal stresses:

(209) 
where σx and σy represents the lateral and vertical normal stresses, respectively and ν is the Poisson ratio. This equation can be rewritten as:

(210) 
where K0 represents the coefficient of lateral earth pressure at rest. The thirs principal stress follows from the geometrical symmetry as:

(211) 
and the shear stresses are zero. The K0 variable in general ranging from 0 to 1. GFAS offers possibility for selection of arbitrary values for this coefficient wen generating an initial geostatic stress state prior to any construction stage.
In contrast to the simplified K0 procedure, the calculation of initial stresses by means of explicit finite element stress analysis based on gravity loading can be conducted on any shape geometry and determine the initial stresses in the massive using ether a linear anslysis or elastoplastic analysis. This analysis is performed in the first stage of construction prior to any staged constructions or loads. However when choose this option to generate the initial stresses results displacements, these displacements are not realistic because the soil is modelled as it appears in reality and the calculation of the initial stress should not influence the displacements computed later in the next stages of the analysis. These unrealistic displacements can be reset to zero at the start of the next calculation stage.

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