Drained-Undrained Strength

The Drained-Undrained strength option allows you to define a soil strength envelope which considers both drained and undrained Mohr-Coulomb strength parameters. The shear strength is defined in terms of effective stress parameters c’ and phi’, up to a maximum value of shear strength defined by the undrained cohesion Cu.

If c’, phi’ and Cu are all constant, then you can simply enter these parameters directly in the Define Material Properties dialog.  In this case, the shear strength envelope is defined by constant values of:

• Cu (undrained cohesion)
• c’/Cu ratio (drained cohesion c’ is defined as a fraction of Cu)
• drained phi’

If the cohesion is variable with depth:

1. Select the "Cohesion varies with depth" checkbox in the Define Material Properties dialog.
2. This will enable the Define button. Select the Define button, and you will see another dialog (the Drained-Undrained Strength Properties dialog), in which you can define the drained and/or undrained cohesion as a function of depth.

By default, the (Undrained) Cohesion Type = F(Depth) and the (Drained) Cohesion Type = F(undrained Cu). However, you may define the Drained and Undrained cohesion using any combination of the options described below.

Undrained Strength Parameters

In the Drained-Undrained Strength Properties dialog, the Undrained cohesion can be defined as:

• Constant
• F(Depth from Top of Layer)
• F(Depth from Horizontal Datum)
• F(Distance to Slope)

by selecting from the Cohesion Type drop-list.

• Cu (Top) is the undrained Cohesion at the top of the material layer (local to the slice).
• Cu Change is the rate of change of undrained Cohesion with depth.
• If you wish to specify a maximum undrained soil strength, select the Cutoff checkbox and enter a maximum value for Cohesion. If the rate of Cu Change is negative, then the Cutoff value represents the minimum soil strength.
• Cu (Datum) is the undrained Cohesion at the Datum elevation.
• Cu Change is the rate of change of undrained Cohesion with distance (ydatum-y) from the Datum.
• If you wish to specify a maximum undrained soil strength, select the Cutoff checkbox and enter a maximum value for Cohesion. If the rate of Cu Change is negative, then the Cutoff value represents the minimum soil strength.
• Datum is the datum elevation (y-coordinate) which is constant elevation for material properties.
• Cu (Slope) is the undrained Cohesion at the top of the slope elevation.
• Cu Change is the rate of change of undrained Cohesion with depth.
• If you wish to specify a maximum undrained soil strength, select the Cutoff checkbox and enter a maximum value for Cohesion. If the rate of Cu Change is negative, then the Cutoff value represents the minimum soil strength.

Drained Strength Parameters

The Drained cohesion can be defined as:

• Constant
• F(Depth from Top of Layer)
• F(Depth from Horizontal Datum)
• F(Distance to Slope)
• F(Undrained Cohesion)

by selecting from the Cohesion Type drop-list.

• c’ (Top) is the drained Cohesion at the top of the material layer (local to the slice).
• c’ Change is the rate of change of drained Cohesion with depth.
• c’ (Datum) is the drained Cohesion at the Datum elevation.
• c’ Change is the rate of change of drained Cohesion with distance (ydatum-y) from the Datum.

• Datum is the datum elevation (y-coordinate).

  A Cutoff value cannot be defined for the Drained cohesion.
• c’ (Slope) is the drained Cohesion at the top of the slope of the model.
• c’ Change is the rate of change of drained Cohesion with depth.