In Settle3D, as a point moves downwards, it descends further below the water table. This causes a pore pressure increase at the point and therefore an effective stress decrease. In addition, if soil that was originally above the water table settles below the water table, then the soil become saturated and its unit weight may increase. This causes an effective stress increase at all points below the submerged soil.
By default, we ignore this effect in the settlement calculations, as it is usually quite small. In cases where the buoyancy effect may be significant, it can be included by selecting the Include Buoyancy Effect checkbox in the Advanced Settings dialog.
There are several assumptions associated with this option, and these are outlined in the Settle3D Theory Manual, available in the Online Help.
Note that we do not recommend applying buoyancy as a default setting (it is off by default). To determine if this setting is appropriate it is best to compare the Settle3D results with field measurements.