Define Hydraulic Properties

If you are performing a groundwater analysis, the Define Hydraulic Properties option is used to specify the permeability characteristics for your materials. Material permeability is required by the finite element analysis, in order to solve the groundwater problem (i.e. determine the flow and pressure distribution throughout the model).

NOTE:

Saturated Permeability

A saturated permeability, Ks, must always be specified for a material. However, note that:

Anisotropic Permeability Parameters

Anisotropic permeability may be specified for a material (i.e. permeability varies with direction), by specifying a Factor and Angle, which defines the relative permeability in two orthogonal directions.

EXAMPLE: if K2 / K1 = 0.5 and K1 Angle = 0, then the permeability in the HORIZONTAL direction = Ks (in the saturated zone), and the permeability in the VERTICAL direction = 0.5 * Ks (in the saturated zone).

Permeability Models

The following models are available in Slide for describing the permeability in the unsaturated zone as a function of matric suction.

A user-defined function can be defined by selecting the New button. See below for details about each model.

Plot button image\gw_plot_button.gif

After entering the parameters for the selected model, you can select the Plot button to view the actual permeability function which is defined by the model and the parameters you have entered.

Pick button buttons\b_pick2.gif

If you select the Pick button in the dialog, you will see another dialog, from which you can select from a list of materials, with representative parameters for the selected model. (References for the data are provided in the dialog). If you select OK in this dialog, the currently selected parameters will be automatically loaded into the Define Hydraulic Properties dialog.

Simple

With the Simple model, an unsaturated permeability function is automatically determined by Slide based on the magnitude of the saturated permeability Ks and the selected Soil Type. To view the function determined by Slide, you can select the Plot button in the dialog. For the Simple model, you can choose one of the following Soil Types:

For the General soil type, the unsaturated permeability simply decreases by an order of magnitude within the initial range of matric suction values, and then remains constant for higher values of suction.

For specific soil types (Sand, Silt, Clay or Loam) average curves have been determined from typical literature values.

Brooks and Corey

The following equations define the Brooks-Corey permeability function (Brooks and Corey, 1964):

For  :

                                     Eqn.1a

For  :

                     Eqn.1b

where: = Pore size index

  = bubbling pressure

  = matric suction

 K = permeability

 Ks = saturated permeability

Fredlund and Xing

The following equation defines the Fredlund-Xing permeability function (Fredlund and Xing, 1994):

          Eqn.2

where: A, B and C are the model parameters

  = 2.7182818 …

  = matric suction

 K = permeability

 Ks = saturated permeability

Gardner

The following equation defines the Gardner permeability function (Gardner, 1956):

                           Eqn.3

where: a and n are the model parameters

 h = pressure head (suction)

 K = permeability

 Ks = saturated permeability

Van Genuchten

The following equations define the van Genuchten permeability function (van Genuchten, 1980):

                        Eqn.4a

       Eqn.4b

where:  = degree of saturation

 h = pressure head (suction)

  are the model parameters

 K = permeability

 Ks = saturated permeability

NOTE: the parameters m and n are related as follows:

                                   Eqn. 4c

However, you can specify a value of m which is independent from n, by selecting the Custom m checkbox, and entering a value for m.

User Defined

For complete flexibility in defining permeability functions, a User Defined Permeability Function can be defined as follows:

  1. Select the New button in the Define Hydraulic Properties dialog.

  2. You will see the User Defined Permeability Function dialog.

  3. Enter a Name for the function.

  4. Enter the Matric Suction and Permeability values which define the required function. As you enter the data, the graph at the right of the dialog will display the function you are defining.

NOTE: the data points on the graph can be moved graphically by clicking on them with the mouse and dragging.

  1. When all data has been entered, select OK, and you will be returned to the Define Hydraulic Properties dialog.

  2. In the Define Hydraulic Properties dialog, the Name of the User Defined Permeability Function you have defined will now appear in the Model drop-list. It can be selected at any time, just like any of the other permeability models.

Edit

To modify an existing User Defined Permeability Function, select the Name of the function from the Model list, and select the Edit button. You can then edit the data which defines the function, in the User Defined Permeability Function dialog.

Delete

To delete an existing User Defined Permeability Function, select the Name of the function from the Model list, and select the Delete button.

Copy

To create a copy of an existing User Defined Permeability Function, select the Name of the function from the Model list, and select the Copy button. Enter a new name in the dialog which appears, and select OK. A new function will automatically be created, and the Name will appear in the Model list.

Import / Export

User Defined Permeability Functions in Slide can be saved in their own separate file, if desired.