Define Hydraulic Properties

The Define Hydraulic Properties option, in the toolbar or the Properties menu, is used to specify the groundwater/hydraulic parameters for each material in the Define Material Properties dialog.

Define Hydraulic Properties is only applicable if you are considering pore pressure in your Phase2 analysis (i.e. using Piezometric Lines, a Water Pressure Grid, Ru values, or finite element seepage analysis).

If you are NOT considering pore pressure in your analysis, then Define Hydraulic Properties is not applicable.

The options available in the Define Hydraulic Properties dialog, correspond to the Groundwater Method specified in the Project Settings dialog:

If the Groundwater Method = Piezometric Lines, then you will be able to assign Piezometric Lines to each material (or specify an Ru value). See below for details.
If the Groundwater Method = Water Pressure Grid, then you will be able to turn the effect of the grid on or off, for each material (or specify an Ru value). See below for details.
If the Groundwater Method = Finite Element Analysis, then you will be able to define the saturated and unsaturated permeability characteristics of each material. See the Hydraulic Properties (Seepage Analysis) topic for details.

NOTE: the materials in the Define Hydraulic Properties dialog correspond to the materials in the Define Material Properties dialog (i.e. the dialog in which you define the strength properties of your materials).

The material names and colours which appear in the Define Hydraulic Properties dialog are those which are specified in the Define Material Properties dialog. The names and colours CANNOT be edited in the Define Hydraulic Properties dialog -- if you wish to edit the name or colour of a material, this must be done in the Define Material Properties dialog.

The available options are described below for Groundwater Method = Piezometric Lines or Water Pressure Grid.

If the Groundwater Method = Finite Element Analysis, see the Hydraulic Properties (Seepage Analysis) topic for information about the input parameters.

Piezo to Use

Piezometric Lines are created with the with the Add Piezometric Line option. Each Piezometric Line has an ID number associated with it. These ID numbers are automatically created by Phase2, when the Piezometric Line is added to the model.

The Piezo to Use option in the Define Hydraulic Properties dialog, will contain a list of ID numbers of all Piezometric Lines which currently exist in the model.

By default, the Piezo to Use option is initially set to None. This means that NO Piezometric Line will be assigned to that material, and pore pressure at all points within such materials, will be zero.
However, if Piezo to Use = None, you can specify an Ru value to calculate pore pressure, see below.
In order to assign a Piezometric Line to a given material, select the ID number of the desired Piezometric Line from the Piezo to Use drop-down list. The selected Piezometric Line will then be used for the pore pressure calculations for that material.

NOTE: the Piezo to Use option is applicable if you want to use ONE Piezometric Line for the pore pressure calculation for a given material, AT ALL STAGES OF THE MODEL (for staged models). If you require different piezo lines to be applied at different stages, for a given material, then use the Stage Piezo Lines option described below.

Stage Piezo Lines

If your model consists of multiple stages, and you require different piezo lines to be applied at different stages, for a given material, then use the Stage Piezo Lines option. This allows you to simulate, for example, the draw down of a water table at different stages.

To assign the staging of Piezometric Lines:

In the Define Hydraulic Properties dialog, select the Stage Piezo Lines checkbox.
You will notice that the Piezo to Use option becomes disabled, and the area below the Stage Piezo Lines checkbox becomes enabled.
Now select the Add Stage button, until the number of rows is equal to the number of stages at which you need to specify a Piezometric Line. For example:

if you need to assign 3 Piezometric Lines at 3 different stages, then select Add Stage TWICE, so that a total of 3 rows appears in the data entry area below the Stage Piezo Lines checkbox.

NOTE: if the Add Stage button is disabled, this means that you have not set the Number of Stages in Project Settings. In this case, go to the Project Settings dialog, and enter the correct Number of Stages (greater than 1) for the model.

In each row, enter the Stage number at which the Piezometric Lines are to be assigned.
In each row, select the Piezo ID number representing the Piezometric Line that you wish to assign at the specified stage.
It is important to note that you only need to define the Stage and Piezo ID information, for the stages at which the Piezometric Line will change. You DO NOT need to define this information at stages where the Piezometric Line does not change.
If you accidentally add too many rows with the Add Stage button, click the mouse in a row and select the Delete Stage button, to delete the row.
When you are finished entering the Stage and Piezo ID numbers, if your model consists of multiple materials with the same staging of Piezo Lines, you can use the Copy To button to automatically copy the Stage / Piezo ID information to other materials in the dialog (see below).
When the analysis is run, Phase2 will use the Stage and Piezo ID information in the Define Hydraulic Properties dialog, to determine the correct Piezometric Line to be used at each stage of the analysis, for each material.

NOTE: you can use the View Piezos by Stage option to view only the piezo line(s) which are applicable for the current stage. This allows you to easily verify that the correct staging sequence of the piezometric lines has been applied.

Hu Value

The Hu value is simply a factor between 0 and 1, by which the vertical distance from a point (in the soil or rock), to a Water Surface (i.e. Piezo Line) is multiplied to obtain the pressure head. The Hu value is used to calculate pore pressure as follows:

where:

= pore pressure

= the Pore Fluid Unit Weight (entered in the Project Settings dialog)

= the vertical distance from a point to a Piezometric Line

= the Hu value for the material

NOTE: if the distance h is negative, (i.e. a point is above the Piezometric Line) then the pore pressure is set to zero at that point.

There are two ways of defining the Hu value - Auto or Custom.

Custom Hu

With the Custom option, the user can enter their own value for Hu. A value between 0 and 1 must be specified. For example:

Hu = 1 would indicate hydrostatic conditions. This can be used where the Water Surface is horizontal. Where the Water Surface is inclined, setting Hu = 1 will (in general) provide a conservative (high) estimate of the true pore pressure.
Hu = 0 would indicate a dry soil. Pore pressure will be zero. Setting Hu = 0 can be used to turn "off" the pore pressure for a material, although this can also be achieved by setting Piezo to Use = None.
Intermediate values of Hu can be used to simulate head loss due to seepage. This would be applicable where the Water Surface is inclined. NOTE: the Auto Hu option, described below, can be used to automatically account for the inclination of the Water Surface.

Auto Hu

With the Auto Hu option, Phase2 will automatically calculate a value of Hu, based on the inclination (angle) of the Piezo Line, above a given point (i.e. a mesh node at which the pore pressure is required). This is based on the assumption that the equipotential line which passes through the point, is a straight line between the point and the Piezo Line (strictly applicable for an infinite slope case). This is illustrated below.

= the inclination of the Water Surface (above a given point)

= vertical distance from the point to the Water Surface

Simple geometry can be used to show that the pressure head, as illustrated in the diagram below, is equal to . The automatically calculated Hu coefficient, is therefore equal to . For a horizontal Water Surface, = 0, and Hu = = 1.

Automatic Calculation of Hu coefficient

The Auto Hu option is a useful method of estimating pore pressures, based on the inclination of a Water Surface. In the absence of more accurate data (e.g. Seepage Analysis results), this is a simple but useful method of approximating head loss due to seepage.

Ru Value

An Ru value simply models the pore pressure as a fraction of the vertical earth pressure. The vertical earth pressure is estimated from the unit weight and height of each material directly above a given point. The Ru Value can range between 0 and 1.

An Ru Value can be specified for a material under the following conditions:

If the Groundwater Method = Piezometric Lines in Project Settings, and Piezo to Use = None in the Define Hydraulic Properties dialog, then you can enter an Ru value for the material.
If the Groundwater Method = Water Pressure Grid in Project Settings, and the Use Grid Values checkbox is NOT selected in the Define Hydraulic Properties dialog, then you can enter an Ru value for the material.

If one soil type has regions of differing Ru values, then a different material will have to be defined for each different Ru value. Appropriate Material Boundaries will have to be added to the model, in order to define the different soil regions.

Use Grid Values

If the Groundwater Method in Project Settings is set to one of the 3 water pressure grid options – Grid (Total Head), Grid (Pressure Head) or Grid (Pore Pressure), then the Use Grid Values option will be enabled in the Define Hydraulic Properties dialog.

The Use Grid Values option simply allows the user to toggle the effect of a Water Pressure Grid, ON or OFF for a given material.

If the Use Grid Values checkbox is selected, then pore pressure will be calculated from the Water Pressure Grid, using the Interpolation Method specified in Project Settings.
If the Use Grid Values checkbox is NOT selected, the Water Pressure Grid will have no effect for that material. Pore pressure for that material will be set to ZERO throughout the analysis, (unless an Ru value is specified).
If the Use Grid Values checkbox is NOT selected, you can specify an Ru value to calculate pore pressure, see above.

NOTE: currently in Phase2 it is NOT possible to stage Water Pressure Grids, in the same manner as Piezometric Lines. Only ONE Water Pressure Grid may be defined for a model, and used at all stages. This may be improved in a future Phase2 upgrade, to allow multiple water pressure grids to be read into a model, and applied at different stages.

Copy To

The Copy To option allows you to define the groundwater/hydraulic information for one material, and then automatically copy the same information, to any other materials in the model. This is useful when all or most of the materials in a model, have the same groundwater conditions (for example, they all use the same water table).

The Copy To option is particularly useful if you are using the Stage Piezo Lines option, and your model has multiple materials and piezo lines. The staging information can then be defined only once, and automatically copied to all materials. For example:

Define your Piezometric Line staging information for ONE material.
Select Copy To. In the Copy Material Properties dialog, select the materials to which you would like to copy the information, and select OK.
The Copy To option also copies the Hu value for the current material, to the selected materials.