Mohr Diagram
The Mohr diagram plots the planes you have selected against a stress tensor. Each plane is a point at its effective normal stress and shear stress. The diagram also draws the three Mohr circles of the tensor and the Coulomb failure envelope, so you can see which planes are critically stressed. It can add each set's joint-strength envelope and the Hoek-Brown envelope of a geotechnical domain beside them.
It is drawn in the Charts pane, next to the 3D view.
The Mohr diagram needs the Geomechanics module on your licence. Without it, a message says so; your stress tensor is unaffected and every stress analysis still uses it.
Opening it
- Right-click the stereonet in the Charts pane and choose Mohr Diagram.
It plots the planes selected in the tree, as the Charts-pane stereonet plots them: orientation measurements, faults and picks. Lineations are left out. Select something else in the tree and the diagram follows.
What it shows
| Element | Meaning |
|---|---|
| Mohr circles | The σ1–σ3 circle (heavier line) and the σ1–σ2 and σ2–σ3 circles, with σ1, σ2 and σ3 marked on the axis. |
| Points | One per plane, in its stereonet colour. A plane on or above the failure envelope is ringed red (critically stressed). A selected plane is drawn larger, with a dark halo. |
| Failure envelope | The tensor's Coulomb line, τ = C + μσn′, from its Cohesion and Friction Coefficient. |
| Set envelopes | For each Set of the plotted planes that has a joint strength of its own, its envelope τ(σn) as a solid line in the set's colour, labelled J1 (Mohr-Coulomb) or J1 (Barton-Bandis). |
| Domain envelopes | For each geotechnical domain with Show on Mohr Diagram ticked, its Hoek-Brown envelope at its GSI, dashed, labelled Domain (Hoek-Brown, GSI 55). The domain's colours cycle through six fixed colours. |
Stresses are effective: the tensor's Pore Pressure is taken off the normal stress, and the x axis then reads Effective normal stress σn′ (MPa). With no pore pressure it reads Normal stress σn (MPa). The y axis is Shear stress τ (MPa).
A tensor that varies with depth is taken at the centroid of the plotted planes, and the subtitle gives that depth; when no plotted plane has a position, or the centroid is off the ground surface, it is taken at the tensor's reference depth, and the subtitle says so.
The title names the tensor, e.g. Mohr diagram: stress tensor 'Regional' (active). The subtitle counts the planes plotted. The legend gives the envelope, the pore pressure and how many planes are critically stressed. The envelope names are listed at the top right in their colours; the dashed ones say so.
Selecting on it
- Click a point to select its plane. Drag a box to select several. Ctrl adds to the selection; a click on empty space clears it.
- Double-click a point to fly the 3D view to that plane.
The selection is shared: what you pick here is selected on every stereonet, in the tree and in the 3D view, and the other way round. Picking on the diagram does not change which planes it plots, which follow the tree.
The right-click menu
| Command | Effect |
|---|---|
| Go To in 3D View / Go To Selected in 3D View | Fly the 3D view to the plane right-clicked, or to the selected planes. Shown when there is a plane to go to. |
| Stress Tensor | Choose the tensor to plot against: Active Tensor (name), or any tensor by name. Shown when the project has more than one tensor. |
| Show Stereonet | Switch the Charts pane back to the stereonet. |
| SVG Document... | Save the diagram as an SVG file. |
| Planes Table (CSV)... | Save the stresses on every plane of the diagram as a CSV table (below). |
| Copy to Clipboard | Copy the diagram as an image. |
| Reset View (Zoom & Pan) | Undo any zoom and pan. |
The planes table
Planes Table (CSV)... writes one row per plane on the diagram, for the tensor it is drawn against, with these columns:
| Column | Meaning |
|---|---|
name | The plane's name. |
set | Its Set, if it has one. |
dip, dip_direction | Its orientation, degrees. Blank for planes that are not orientation measurements. |
sigma_n_eff_mpa | The effective normal stress σn′ on the plane, MPa. |
tau_mpa | The shear stress τ on the plane, MPa. |
slip_tendency | τ / σn′. |
coulomb_factor | (C + μ σn′) / τ, with the tensor's cohesion C and friction coefficient μ: the shear strength over the shear stress. Below 1 the plane plots beyond the failure line. |
critically_stressed | yes for a plane ringed red on the diagram, otherwise no. |
The messages panel says how many planes were written and where.
In a report
An analysis report can take the Mohr diagram as a figure: choose Mohr Diagram from the report editor's figure list. It uses the planes and the tensor of the Charts pane, so select the planes first.
Reading it
- A point above the Coulomb line is critically stressed under the tensor's failure parameters. A set envelope above it, as Barton-Bandis is at low stress, shows that the set's own strength would hold.
- The Hoek-Brown envelope is the strength of the rock mass, not of a joint. A plane that plots under its joint envelope but over the domain's Hoek-Brown curve is held by its joint strength, not by the intact rock between joints.
- All of it is for one stress state: the tensor at the centroid of the plotted planes when it varies with depth (or at its reference depth, with no positions). Planes at different depths are not each given their own stress.
See also
- Stress Tensors — the tensors and their failure parameters.
- Sets — joint strength per set.
- Rock Mass Classification — the Hoek-Brown envelope of a domain.
- Stereonet User Guide — the stress attributes field and stress axes on the net.
- Geomechanics — how the pieces fit together.