In the foundation module, ground water is considered only as a factor that induces Archimedes pressure. No effects, neither these concerned with the water movement in the soil nor those related to the change of physical properties of the soil (cohesion, internal friction angle) induced by the presence of the ground water, are taken into consideration.
The Archimedes pressure acting on a foundation is calculated on the basis of the Archimedes' principle. Buoyant force is equal to the weight of the water displaced by the foundation.
The Archimedes pressure acting on the soil immersed in the water and an increment of density of the wet soil are analyzed as in the case of a porous body. For these calculations the soil parameter called: Unit weight of soil solids is applied. In the calculations it is assumed that the unit weight of the soil is comparable to the unit weight of the dry soil, which enables the following relation to be used:
Where:
ρsw - Unit weight of the soil below the ground water level
ρs - Unit weight of the dry soil (unit weight)
ρss - Unit weight of soil solids
ρw - Water density.
For the Polish code there is a possibility to determine only one ground water level, others have two levels: high and low. The high water level is treated as variable, occuring only when this impact is unfavorable (it also refers to the ground water level defined for the Polish code). The low water level is treated as invariable and occuring in each of the analyzed states.