The procedure for defining the cross-section of a volume is similar to that used to model a gravity dam or a spillway. 

The Close perimeter command must be used to close the cross-section. Only convex sections can be defined when modeling volumes and voids. In the case of a concave section, it must be divided into two or more volumes with convex sections. By definition, a convex polygon has all interior angles equal or less than 180 degrees.

Figure 99

The first segment created will necessarily be connected to the origin point defined previously. The user must define the perimeter of the volume by proceeding with segmentation from the origin point and by returning to this origin point. In accordance with all other perimeter definitions, the perimeter of the volume is generated through the usual four available segments: the linear segment, the circular arc, the power equation and the polynomial.  The last segment returning to the origin point must be generated using the Close perimeter button to avoid zero length elements.

Figure 100

Once the perimeter defined, the user must proceed with the extrusion of the 2D contour (polygon) to generate the volume using the button Extrusion options.

Finally, the user must specify the type of volume: Solid or Void.  The solid type will add mass while the void type will remove mass from the model.

in the case of a "void" type volume, different options are proposed to establish the condition inside the void:

    1. No water (not a drainage condition): This condition will consider that the inside of the void will be at atmospheric pressure.  However, The void will not be considered as a drainage .(default option)
    2. Upstream reservoir pressure: This condition will consider that the inside of the void will be connected to the upstream reservoir condition.
    3. Downstream reservoir pressure:  This condition will consider that the inside of the void will be connected to the downstream reservoir condition.

The "confined water" option correspond to a condition where the water in the void is believed to be confined and will be considered as a rigid seismic mass.

The voluminal mass must be specified for both Solid or Void types as an absolute value.  Since a Void is somehow a negative mass, it is important to note that the voluminal mass for the Void should not be set to zero or negative, but to the voluminal mass corresponding to the volume that has to be removed. Please note that a zero or negative voluminal mass may not be specified by the user.