BS-Theory: Buoyancy safety using global safety factors
The buoyancy safety for each soil layer within the excavation is determined via a comparison of the soil weights to the water pressures at the respective layer bases. The self-weights of site plant and structures, frictional forces etc. are not included.
ηN = buoyancy safety of layer N
Gi = self-weight of layer i
PN = water pressure at base of layer N
layer 1 (i = 1) is the uppermost layer
The minimum value of all ηN is the buoyancy safety of the system.
If the same permeability has been defined for the whole system, safety against buoyancy is not determined. In certain cases, for example, when permeability on the passive side is much greater than on the active side, the calculation of safety factors for hydraulic uplift is meaningless. If, in such, or similar cases, the message "Buoyancy safety could not be demonstrated" appears, you can either ignore it or set the safety against buoyancy to 1.0, which suppresses the message.