The hydraulic heave safety for each layer below the excavation base is determined via a comparison of the soil weights to the flow forces at the respective layer bases.
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ηN = hydraulic heave safety of the layer N
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G'i = buoyant self-weight of layer i
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SN = flow force of layer N
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layer 1 (i = 1) is the uppermost layer
The minimum value of all ηN is the hydraulic heave safety of the system.