Gravitational Field Stress

The Gravitational Field Stress option is used to define an in situ stress field which varies linearly with depth from the ground surface. This is typically used for surface or near surface excavations. The depth can be measured from either:

a Ground Surface Elevation (ground profile is a single horizontal elevation).
a Ground Surface Boundary (ground profile is defined by a Ground Surface Boundary).

The principal in situ stresses are oriented in the vertical and horizontal directions. The vertical stress at a given point is equal to the depth multiplied by the unit weight of overlying material. The horizontal stress is equal to the vertical stress multiplied by a Horizontal/Vertical Stress Ratio as given by the following equation.

To define a Gravitational field stress:

Select Field Stress Type = Gravitational in the Project Settings dialog, or click on the Field Stress button in the Sidebar.
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.
Define the Ground Surface by entering the Ground Surface Elevation in the Sidebar, or by defining a Ground Surface Boundary with the Add Ground Surface option.

If you define a Ground Surface Elevation, the elevation is the vertical (y) coordinate of the ground surface with respect to your coordinate system. It is NOT the depth below ground surface of your excavations! A horizontal dotted line marking the Ground Surface Elevation will automatically be displayed on the model. The display of this line can be toggled on or off in the Display Options dialog.
If you define a Ground Surface Boundary, by default it is assumed that depth is measured from the Ground Surface Boundary, in order to calculate the vertical in situ stress. However, if you wish to define a Ground Surface Boundary but measure the depth from a specified Elevation, then clear the Use ground surface boundary checkbox in the Project Settings dialog, beside the Gravitational Field Stress option, and enter the Elevation in the Sidebar.

Gravitational Field Stress + Plane Strain

If the Analysis Type = Plane Strain, the horizontal principal stresses are defined by in-plane and out-of-plane horizontal/vertical stress ratios. The following input is required.

Ground Surface Elevation or Ground Surface Boundary - (vertical distance from ground surface) = depth

Overburden Unit Weight - (unit weight) x (depth) = vertical stress

Horizontal Stress Ratio - (horizontal stress ratio) x (vertical stress) = IN-PLANE horizontal stress

Out of Plane Stress Ratio - (out of plane stress ratio) x (vertical stress) = OUT-OF-PLANE horizontal stress

NOTE: the Stress Block icon indicates the relative magnitude and direction of the In-Plane principal field stresses. For a Gravitational field stress, the Stress Block is always oriented with the vertical and horizontal axes. If the Stress Block looks wrong then check the value of the In-Plane Horizontal Stress Ratio.

Gravitational Field Stress + Complete Plane Strain

If the Analysis Type = Complete Plane Strain, the horizontal principal stresses can be arbitrarily oriented with respect to the excavation axis. The following input is required.

Ground Surface Elevation or Ground Surface Boundary - (vertical distance from ground surface) = depth

Overburden Unit Weight - (unit weight) x (depth) = vertical stress

Trend of Major Horizontal Stress - the direction of the largest horizontal stress, measured clockwise from north, in degrees

Major Horizontal Stress Ratio - (major horizontal stress ratio) x (vertical stress) = major horizontal stress magnitude

Minor Horizontal Stress Ratio - (minor horizontal stress ratio) x (vertical stress) = minor horizontal stress magnitude

Tunnel Orientation - for a Complete Plane Strain analysis, it is necessary to define the Tunnel Orientation (i.e. Trend of the Excavation axis), because the in situ horizontal principal stresses can be arbitrarily oriented with respect to the excavation. Note: the Plunge is automatically set to zero, because the Gravitational field stress option assumes that the excavation axis is horizontal.