Field Stress

The Field Stress option allows the user to define the in-situ stress conditions prior to excavation. Two options are available for defining field stress in Phase2 - Constant or Gravity field stress. To define Field Stress:

Select Field Stress from the toolbar or the Loading menu.

TIP: you can also right-click on the Stress Block / Seismic Load icon and select Field Stress from the popup menu.

In the Field Stress dialog, select the Field Stress Type (Constant or Gravity), and enter the field stress parameters. Select OK.

Constant Field Stress

The Constant field stress option is used to define an in-situ stress field (prior to excavation) which does not vary with position or depth. See the Constant Field Stress topic for details.

Gravity Field Stress

The Gravity field stress option is used to define an in-situ stress field which varies linearly with depth. Gravity field stress is typically used for surface or near surface excavations.

The depth can be measured from either:

A user-specified Ground Surface Elevation.
The actual ground surface of your model.

See the Gravity Field Stress topic for details.

Custom Field Stress

For either Constant or Gravity field stress, the field stress for individual materials may be customized, if necessary, by selecting the Advanced button in the Field Stress dialog, and specifying the field stress for any material(s) in the model. See the Custom Field Stress topic for details.

Stress Block

The relative magnitude and direction of the In Plane principal field stresses, are indicated by the Stress Block, displayed in the upper right corner of the view. If the Stress Block looks wrong then make sure you have correctly entered field stress parameters (i.e. Sigma 1, Sigma 3 and Angle, for a Constant field stress, or the Horizontal / Vertical In Plane Stress Ratio, for Gravity field stress). The display of the Stress Block can be toggled on or off with the Stress Block option in the View menu.

Load Splitting

If your model is staged (i.e. the Number of Stages in the Project Settings dialog is greater than 1), then by default, 100% of the field stress induced load is applied to the model in the FIRST stage. However, for staged models, the user has the option of "splitting" the field stress induced load, between any desired stages of the model (rather than applying the entire load in the first stage). This is done with the Load Split option in the Loading menu, see the Load Split topic for details.

Also note:

In Phase2 it is assumed that the stress field is aligned with the analysis section – i.e. one of the three in-situ principal stresses must be aligned with the longitudinal axis of the excavation. A fully 3-dimensional, arbitrarily oriented stress field cannot be entered in Phase2.
Since the yield functions used in the Compute engine are three-dimensional (i.e. they depend on all components of the stress tensor) it is important that the user enters a stress (or stress ratio) in the out-of-plane direction.
The convention for stress is POSITIVE COMPRESSION, so in most cases you will be entering positive values for stresses.