Water Pressure Grid

Groundwater conditions are most accurately modeled by a grid of water pressure data, from which the pore pressure at any point in the soil can be determined. The grid could be based on field measurements, or could be the output from a groundwater seepage analysis program, for example.

The Water Pressure Grid option is only enabled if the Groundwater Method in Project Settings is set to one of the three Water Pressure Grid options available in SLIDE:

• Grid (Total Head)

• Grid (Pressure Head)

• Grid (Pore Pressure)

To define a Water Pressure Grid:

1. Set the desired Water Pressure Grid type in the Project Settings dialog.

2. Select Water Pressure Grid from the Boundaries menu.

3. Grids can be imported (see below for details), or entered manually as described in steps 4 to 5.

4. Each point is defined by x and y coordinates, and a value, corresponding to the Water Pressure Grid type (total head, pressure head, or pore pressure) selected in Project Settings.

5. Enter the data in the grid. When finished select OK, and the location of each grid point will be marked by a blue triangle on the model.

NOTE: the Water Pressure Grid values can be displayed at each grid point with the Display Options dialog. By default, they are NOT displayed.

Importing Water Pressure Grid Files

The Import button in the Water Pressure Grid dialog allows the user to import an existing Water Pressure Grid file, into the current SLIDE model. The following file formats can be imported:

• .PWP files (Pore Water Pressure file) – a .PWP file is a simple text file, where each row of the file has the format (x, y, value). The type of the value must correspond to the Groundwater Method in Project Settings (total head, pressure head or pore pressure)

• .DXF files – Water Pressure Grids can also be imported into SLIDE from a .DXF file (this is useful if for example, a flow net has been digitized from a drawing using AutoCAD).

• .SLW files (Slide Groundwater Analysis file) – if you have performed a finite element groundwater analysis with SLIDE, then the results can be imported back into SLIDE as a Water Pressure Grid file, if desired. The data at each node of the finite element mesh will be imported, in the form of either Total Head, Pressure Head or Pore Pressure, according to the selection of the Groundwater Method in Project Settings.

• SEEP/W Total Heads – SEEP/W is a groundwater analysis program produced by GEO-SLOPE Inc. To import a grid of Total Head data from a SEEP/W file, use the .SEP file option. Note that this is only enabled if the Groundwater Method in SLIDE is set to Grid (Total Head) in Project Settings.

Exporting Water Pressure Grid Files

The Export button in the Water Pressure Grid dialog allows the user to export the current data in the Water Pressure Grid dialog, to a .PWP file (see above for the definition of a .PWP file). This is useful if you wish to later import the same Water Pressure Grid into another SLIDE model.

Using a Water Table with Pore Pressure Grids

If a Water Pressure Grid is being used for pore pressure calculations, a Water Table can still be defined, for the following purpose:

• A Water Table can be defined in combination with a Water Pressure Grid, if the user wants to ensure that the region above the Water Table is assigned zero pore pressure. This will ensure that even if a non-zero (positive) pore pressure value is calculated from the Water Pressure Grid at some point above the Water Table, it will automatically be set to zero.

This may be useful at times, for example, if a Water Pressure Grid is defined by an insufficient number of points, the user can add a Water Table to ensure that zero pore pressure is always used ABOVE the Water Table, although the Water Pressure Grid interpolation procedure will still be used to calculate pore pressure at all points BELOW the Water Table.

Note that negative pore pressures interpolated from Water Pressure Grids are automatically set to zero, whether or not a Water Table is defined, so it is not necessary for the user to account for this.