Interface around circular tunnel may take material from wrong side

Problem description

When having one or more circular tunnel shapes inside a larger circular tunnel with interfaces, PLAXIS 2D AE can have difficulties in finding what the correct side for the interface is.
This can happen when e.g.,:

• modelling a circular tunnel with an outside interface and another tunnel inside it
• modelling a circular tunnel with a so-called thick lining made in Subsegments in the Tunnel Designer wizard using this button:

Figure 1. Tunnel geometry with possible incorrect interface

This issue can have the following consequences:

• when the material inside the circular arc is different from the material outside the tunnel, and the interface material assignment is set to From adjacent soil, the calculation may take the wrong material dataset for the interface behaviour;
• If the material inside is set to non-porous, the interface may not contain any pore pressures. For instance steady state pore pressures in submerged conditions and excess pore pressures in undrained conditions.
• Node connectivity may be wrong, resulting in wrong stresses in the interface, and so also possible different behaviour in the soil around the tunnel.

It is not always easy to check if this is happening in the model. If the material assignment in the interface is set to take the material from the adjacent cluster and the materials inside the tunnel and outside the tunnel are different, you can check in your model if this is happening: open the table with interface results, and look at the material dataset reference in the first column. This should refer to the defined material dataset, and not to the material dataset used in e.g. the inside of this arc.

This issue only happens for fully circular tunnels that consist of just one segment.

Solution

This issue is now solved since PLAXIS 2D 2015.00: now a circular tunnel will by default consist of two segments in stead of just one. This prevents this issue to occur. Note: for full circular shapes the above mentioned problem may still occur.

For PLAXIS 2D AE, this issue can be solved if instead of one segment the circular tunnel is designed with two segments. For existing models, a simple workaround would be to draw a line over the tunnel such that the entire tunnel will be split into two parts. When designing a new circular tunnel, define two arcs of 180 degrees each:

1. inside the tunnel designer using the “Free” shape type option in the “General” window of the “Cross section” tab.
2. In the Segments tab, add one arc of 180° with the correct radius
3. Then press the symmetric close button to make the other symmetric half
4. Then in the “Properties” tab the positive interface can be assigned to both segments, along with other user features (e.g. plates)
5. Do not forget to Regenerate the tunnel
6. And generate the mesh.

Now the model will have correct interfaces defined.

Figure 2. Workaround step 1: set the shape to Free

Figure 3. Workaround step 2 and 3:
add a segment of 180 degrees and symmetrically close the tunnel

Figure 4. Workaround step 4: Add Properties to the segments