Define Tension Crack

If your model includes a Tension Crack boundary, then the water level in the Tension Crack zone can be specified with the Define Tension Crack option in the toolbar or the Properties menu.

The water level in a Tension Crack is very important to the stability analysis, because the additional hydrostatic force which can be exerted by water in a tension crack, can significantly lower the factor of safety of slip surfaces which intersect the Tension Crack zone. The unit weight of the water in the Tension Crack zone is used to calculate the hydrostatic force applied by water in the Tension Crack. This unit weight is determined from the Pore Fluid Unit Weight entered in the Project Settings dialog.

A Tension Crack boundary can be created in two ways:

• Manually with the Add Tension Crack option in the Boundaries menu.
• Automatically, by selecting the Create Tension Crack if failure surface ... checkbox in the Tension Crack Properties dialog (see below for details).

Tension Crack Water Level

There are several different options for defining the water level in a Tension Crack.

Dry

A completely dry Tension Crack zone. No additional hydrostatic forces will act on the slip surface.

Filled

A completely water-filled Tension Crack zone. This is the worst-case scenario and will exert the maximum hydrostatic force on the slip surface. When you initially add a Tension Crack boundary to a model, the Filled option is the default setting which will automatically be in effect.

Percent Filled

A percentage from 0 to 100 can be specified. This percentage defines the height of water in the Tension Crack, along any vertical line from the Tension Crack boundary to the ground surface. Note that 0 percent is equivalent to the Dry option, and 100 percent is equivalent to the Filled option.

Filled Below Elevation

This option allows you to enter a value of Elevation defining the water level in the Tension Crack zone. NOTE that the Elevation is the absolute value (i.e. y-coordinate) of the water level. It is NOT the relative depth from the bottom of the Tension Crack boundary. Use the Filled to Depth option if you want to specify the water level as a relative value.

The value of Elevation should be somewhere between the minimum y-coordinate of the Tension Crack boundary, and the maximum y-coordinate of the slope above the Tension Crack zone. The Elevation defines a constant, horizontal water level within the Tension Crack zone.

Filled to Depth

This option allows you to enter a value of Depth. This defines the level of water in the Tension Crack zone, relative to the position of the Tension Crack boundary.

Use Water Table / Use Piezometric Line

If your model includes a Water Table or Piezometric Lines, then the water level in the Tension Crack zone can be automatically defined by the location of the Water Table, or a Piezometric Line. The Tension Crack zone will be assumed to be full of water, between the Tension Crack boundary and the Water Table or Piezometric Line, if the Water Table or Piezometric Line is drawn through the Tension Crack zone.

Display of Water Level

When you specify a water level in a Tension Crack zone, the actual level of water will be indicated by the blue/gray shading of the vertical line pattern which is used to highlight the Tension Crack zone on the model.

• When the Tension Crack zone is Dry, the vertical line pattern will be entirely gray.
• When the Tension Crack zone is Filled, the vertical line pattern will be entirely blue.
• When an intermediate water level is specified, the actual water level will be indicated by the height of the blue lines of the vertical line pattern. Gray lines will be shown above the water level.

Create Automatically

This option will automatically create a tension crack if a failure surface is near vertical. If the checkbox is selected, a user-defined angle from vertical can be specified. If the base angle of a failure surface exceeds this limit, then a tension crack will automatically be created for the slip surface at that location. Note that this option applies to individual slip surfaces, so the tension crack location may be different for each slip surface.