Constant Field Stress

The Constant Field Stress option assumes that the in situ principal stresses are constant in magnitude and orientation. For excavations which are relatively deep below the ground surface, this is a common and reasonable assumption for numerical modeling purposes. To define a Constant field stress:

  1. Select Field Stress Type = Constant in the Project Settings dialog, or click on the Field Stress button in the Sidebar.

  2. Enter the Field Stress parameters in the Sidebar. The required input parameters depend on the Analysis Type - Plane Strain or Complete Plane Strain, as summarized below.

NOTE: the convention for stress is POSITIVE COMPRESSION, so in most cases you will be entering positive values for stresses.

Constant Field Stress + Plane Strain

If the Analysis Type = Plane Strain, the principal stresses are defined in terms of the IN-PLANE major and minor principal stress, and the OUT-OF-PLANE stress. The following parameters are required.

Sigma 1 - the MAJOR IN-PLANE principal field stress

Sigma 3 - the MINOR IN-PLANE principal field stress

Sigma Z - the OUT-OF-PLANE field stress (i.e. normal to the analysis plane). Note: Sigma Z is not necessarily the intermediate principal stress, when considering all three dimensions. Sigma Z could be the overall major, intermediate or minor principal stress, depending on the magnitude of the in-plane principal stresses. Also, since the strength factor calculations in Examine2D are three-dimensional (i.e. they depend on all components of the stress tensor) it is important that you define a value for Sigma Z.

Angle - the Angle defines the orientation of the IN-PLANE principal stresses, and is measured between the positive x-axis and the direction of Sigma 1. The Angle is measured in degrees, and the counter-clockwise direction is defined as positive.

NOTE: for Plane Strain, the Stress Block icon indicates the relative magnitude and direction of the In-Plane principal field stresses. If the Stress Block looks wrong then check the values of Sigma 1 and Sigma 3, keeping in mind the definition of Angle.

Constant Field Stress + Complete Plane Strain

If the Analysis Type = Complete Plane Strain, the in situ principal stresses can be arbitrarily oriented with respect to the excavation axis. The following parameters are required.

Sigma 1 (Magnitude, Trend, Plunge) - the true 3-dimensional MAJOR principal stress (orientation is defined in terms of Trend and Plunge)

Sigma 2 (Magnitude) - the true 3-dimensional INTERMEDIATE principal stress (orientation is automatically orthogonal to the Sigma1/Sigma3 plane).

Sigma 3 (Magnitude, Trend, Plunge) - the true 3-dimensional MINOR principal stress (orientation is defined in terms of Trend and Plunge)

Tunnel Orientation (Trend, Plunge) - for a Complete Plane Strain analysis, because the in situ principal stresses can be arbitrarily oriented with respect to the excavation axis, it is necessary to define the Tunnel Orientation (i.e. Trend and Plunge of the Excavation axis).