Solver Options

The following options are available on the Solver Options tab of the Project Settings dialog. The Project Settings dialog can be chosen from the toolbar, or the Analysis menu.

Analysis Type

There are two general types of models which can be created and analyzed in RS2:

Plane Strain

In most cases your RS2 model will represent a Plane Strain analysis. A Plane Strain model assumes that the excavations are of infinite length in the out-of-plane direction, and therefore the strain in the out-of-plane direction is zero.

Axisymmetric

An Axisymmetric analysis allows you to analyze a 3-dimensional model which is rotationally symmetric about an axis (for example, the end of a circular tunnel). Although the input is 2-dimensional, the analysis results apply to the 3-dimensional problem.

For more information, see the Plane Strain and Axisymmetric Analysis topics.

Consolidation Option

The consolidation settings can be modified. By default, the consolidation option is off (Consolidation Option = None). When it is on, solid-fluid coupling will be accounted for, and both coupled (Biot) and uncoupled consolidation analysis can be chosen from. This refers to the interaction between fluid pore pressure and solid stresses and deformation.

Consolidation Option: None

The default analysis is None (i.e. changes in pore pressure do not affect deformation and changes in loading or deformation do not affect pore pressure). This is the case for a total stress analysis (i.e. deformation is a result of changes in total stress). Note that in RS2 2019, Consolidation Option: None is equivalent to Consolidation Option: Uncoupled from RS2 9.0.

Coupled Analysis (Biot)

For a fully Coupled analysis based on Biot theory, you must first enable Transient Groundwater in Project Settings. Then you can choose the Coupled (Biot) analysis option. This is used for time-dependent consolidation analysis problems, in which changes in pore pressure can affect deformation and changes in loading or deformation can affect pore pressure.

See the Coupled Consolidation document in the theory section for more information.

Note: Coupled analysis cannot be used in conjunction with Dynamic nor Thermal analysis.

Uncoupled Analysis

This analysis type improves the stability and the speed of consolidation analyses. First, at the beginning of the stage, an undrained simulation will be conducted to compute the excess pore pressure. Next, a transient groundwater analysis will be carried out to obtain pore pressure dissipation. Finally, another solid analysis will be carried out to calculate the change in the stress caused by the changes in pore pressure.

If the Use Effective Stress Analysis check box is selected (on the Stress Analysis page of the Project Settings dialog), then deformation is a result of changes in effective stress. This is a partially coupled analysis where changes in pore pressure, and thus effective stress, affect deformation but changes in deformation or loading do not affect pore pressure.

Note: For uncoupled consolidation analysis, the Groundwater method must be set to Transient FEA. Uncoupled analysis cannot be used in conjunction with Dynamic nor Thermal analysis.

Drainage Behaviour

When consolidation analysis is enabled (Coupled (Biot) or Uncoupled), you can set the drainage behaviour for each stage under the Stages page in Project Settings. Available choices are Undrained, Drained, or Fully Drained. Note that the first stage is set as Fully Drained by default.

Apply Viscous Properties

The Apply Viscous Properties option allows you to apply viscous properties to materials. When the Apply Viscous Properties option is checked, the Visco-Elastic model is available for the user to select, which is under the Stiffness tab of Material Properties dialog.

Since viscous behavior is time-dependent, the Stage Time needs to be specified when the Apply Viscous Properties option is checked. Under the Stages tab in Project Setting dialog, the Time column will be available. All stages except for first stage should have time. The Define by Date button will also be available on the Stages page, to define the stage time using the calendar, and times will automatically be calculated according to the date.

Solver Type

The Solver Type determines how Compute solves the matrix representing the system of equations defined by your model. Three methods are available:

• Gaussian Elimination
• Conjugate Gradient Iteration
• Pre-Conditioned Conjugate Gradient Iteration

The default method is Gaussian Elimination, but note:

• For solving large problems, you may have to use one of the Conjugate Gradient solution techniques, since disk swapping (swapping between disk and memory during the analysis) is enabled with the Conjugate Gradient methods. Disk swapping is NOT available with Gaussian Elimination.
• If all your materials are Elastic, the solution time will be faster with the Conjugate Gradient techniques
  

Use XFEM

XFEM (Extended Finite Element Method) is a mesh independent method for modelling joints. Mesh will not be conforming with joint elements, and joint elements will be analyzed implicitly. By default, the Use XFEM option is OFF. For more information about XFEM, see the RS2 Theory Manual: XFEM, and the RS2 Verification Manual: XFEM.

NOTE: When the Use XFEM option is enabled, the dynamic and thermal analysis will be turned off.