Drained / Undrained Settings

In RS2, the drained or undrained behaviour can be specified. It can be defined by material, or (and) by stage if consolidation analysis is enabled. See the Materials and Stages sections below for details.

Materials

In general, the drained or undrained behaviour can be specified by material. In the Define Hydraulic Properties dialog, the user can define the Material Behaviour. Two choices are available:

• Drained
• Undrained

Stages

The drained or undrained behaviour can also be specified by stages, only if the consolidation analysis is enabled. To enable the consolidation analysis, select the Consolidation Option as Coupled (Biot) or Uncoupled in the Solver Options page of the Project Settings dialog. Then, in the Stages page of Project Settings dialog, an extra column is provided to set drainage behaviour for each stage. Three choices are available:

• Drained
• Undrained
• Fully drained

When modelling undrained behaviour, depending on the consolidation option, the compatibility between by material and by stage is different, as stated below.

a. Consolidation = Uncoupled, undrained

When modelling undrained behaviour, if Consolidation Option = Uncoupled, both stage and material behaviours can work together. They can be used either separately or jointly. It means:

1. To set any stage to undrained: select corresponding stage behaviour = undrained, material behaviour = drained.
2. To set any material to undrained: select corresponding stage behaviour = drained/fully frained, material behaviour = undrained.
3. For any material, when its stage and material behaviours are both set to undrained, it will behave undrained throughout the model.

b. Consolidation = Coupled (Biot), undrained

When modelling undrained behaviour, if Consolidation Option = Coupled (Biot), the stage and material behaviours cannot work together. Specifically, if stage behaviour = undrained, then the associated materials must have material behaviour = drained. It means:

1. To set any stage to undrained: select corresponding stage behaviour = undrained, material behaviour = drained.
2. To set any material to undrained: select corresponding stage behaviour = drained/fully drained, material behaviour = undrained.
3. For any material, the stage and material behaviours cannot be both set to undrained.

NOTE:

1. The Fully Drained option is only available in stage behaviour.
2. By default, both material and stage behaviours are set to drained.
3. For an undrained model, the calculations to account for undrained behaviour can either be involving an undrained effective stress analysis, or engaging a global matrix, depending on the undrained settings. See the Undrained Behaviour of Soil – Undrained Calculations section for details.
4. For comprehensive undrained behaviour modelling, the drainage behaviour setting is only one aspect, the strength and stiffness parameter inputs should also be considered. See the Undrained Behaviour of Soil topic for details.