Material Unit Weight

The Unit Weight in the Material Properties dialog works in conjunction with the Initial Element Loading, and is only applicable if Body Force is included in the Initial Element Loading. Unit Weight is only enabled if Initial Element Loading is either:

• Field Stress & Body Force or
• Body Force Only

Unit Weight is disabled if Initial Element Loading = either Field Stress or None

If applicable, the Unit Weight is used to determine the Body Force applied to each finite element.

In RS3, There are two ways to apply unit weight to a material, and each is explained below:

• Uniform unit weight
• Account for Moisture Content in Unit Weight

Uniform Unit Weight

Enter a value for Unit Weight and keep the Account for Moisture Content in Unit Weight option unselected. The inputted unit weight will be uniform for the entire material. Soil conditions will not affect the unit weight. The default value is 20 kN/m³.

Account for Moisture Content in Unit Weight

To Account for Moisture Content in Unit Weight, activate the corresponding option. With this option enabled, the dialog requires two user inputs for unit weights: Moist Unit Weight () and Saturated Unit Weight (), representing the unit weight of soil or rock, partially saturated and fully submerged, respectively. The Dry Unit Weight (), representing the unit weight of soil matrix with 0 moisture content, is calculated using the equation below:

= - n ·

where n represents the soil porosity and represents the unit weight of water.

More accurately, the degree of saturation can also be factored in, in conjunction with various soil conditions. Scenarios are described below.

1. When unsaturated components are not considered in calculating effective stress (i.e., Unsaturated Behavior = None or Unsaturated Shear Strength in Strength tab), the unit weight of the material above the ground water level (i.e., the pore pressure is less than 0) is the moist unit weight (). The unit weight of the material below the ground water level is the saturated unit weight ().
2. When unsaturated components for calculating effective stress are considered in calculating effective stress (i.e., Unsaturated Behavior = Single Effective Stress in Strength tab):

The unit weight will be calculated as:

= - n · S ·

where is the soil unit weight, and S is the degree of saturation. Or:

= - ·

where is the volumetric water content of the sample.

For more information about the Unsaturated Soil Mechanics, see the linked topic.

Unit Weight vs. Unit Weight of Overburden (Gravity Field Stress)

If you are using Gravity field stress, you should understand the distinction between the Unit Weight of a material entered in the Material Properties dialog, and the Unit Weight of Overburden entered in the Field Stress dialog.

• The Unit Weight of a material in the Material Properties dialog is used to determine the Body Force applied to each finite element of a given material type.
• The Unit Weight of Overburden in the Field Stress dialog, is used to determine the Gravity Field Stress for all finite elements in the model.
• For a multiple material model, these two unit weights will not necessarily be the same, since the Unit Weight of Overburden is an average unit weight representing all materials.
• For a single material model, the Unit Weight in the Material Properties dialog should be the same as the Unit Weight of Overburden in the Field Stress dialog.