Soil Drainage Conditions

If the Time-Dependent Consolidation Analysis option is selected in Project Settings, then you will be able to define the drainage condition at the bottom of each layer in the Soil Drainage Conditions dialog. This allows you to define horizontal drainage at the interface between soil layers, as described below.

Drained at Bottom

For each soil layer, a Drained at Bottom checkbox can be selected in the Soil Drainage Conditions dialog.

If this checkbox is selected, then the layer is assumed to be freely drained at the bottom (i.e. excess pore pressure will be automatically set to zero at the interface between layers).

If this checkbox is NOT selected, then the interface will NOT be a drainage path (i.e. excess pore pressure can exist at the interface).

Below shows some examples of recommended settings with this option:

Case 1: Sand underlain by clay

• For the case with Sand layer and Clay layer underneath, clay type still has some excess pore pressure build-up at the interface, so it's advised to leave the 'drained at the bottom' option off as set by the default.
• The scenario where it is feasible to use this option is when the layer underneath has a drainage path which makes the excess pore pressure close to zero.

Case 2: Thick clay layer, H > 5 ft

• Many engineers will try to avoid defining thick clay layers in Settle3 as the tiny consolidation results from it are not reasonable. The reasoning behind this in real life is that you'll hardly see pure clay soils. There will always be some percentage of sand mixed with clay.
• Within the clay layers, there will be some drainage and thereby the consolidation will take place. The best way to model this thick clay layer is to have a thin (~0.1 ft) drainage layer every 5 feet increment. Drainage is allowed from the upper clay soils through the drainage soils. With or without defining these drainage layers, there will be significant consolidation settlement value changes.
• Using drainage at bottom option will allow excess PWP to reach zero between interface, creating drainage layer with high permeability assigned to it will allow drainage path between layers but will not cause complete drainage path. you can also add wick drains to add additional drainage conditions within soil layers.

Drained Ground Surface

By default, it is assumed that the ground surface is freely draining (i.e. excess pore pressure cannot exist at the ground surface). Consequently, the Drained Ground Surface checkbox (in the lower-left corner of the Soil Drainage Conditions dialog) is always turned on by default.

If for some reason you wish to define the ground surface as NOT freely drained, then you can turn OFF the Drained Ground Surface checkbox. If you do this, then excess pore pressure can exist at the ground surface (i.e. as if an impermeable membrane covered the ground surface).