Assigning interface between two groups internal contact

I am trying to assign an interface to the internal contact between two groups (a DSM grid and confined soil) by using the below commands but I keep getting the below error. Is there a good way to resolve that?

zone interface ‘joint1’ create by-face separate range group ‘DSM_Fill’ group ‘Fill’

FLAC3D error

Hello @Milads,
I am not familiar with this error. I would double check the zone geometry at the two zones mentioned in the error message.
Additionally, you can send this question along with the necessary data files to

This error often appears when you have a nearly degenerate zone (usually a very flat or extremely thin one) somewhere near the (zone,face) indicated in the error message. The error comes from zone separation logic (not from interface creation) and is typically attributed to the fact that zone separation logic has certain internal tolerance which is used in different ways to detect zones connected to the separation surface and other neighboring zones. If some zone edges are smaller than this tolerance, it may cause this error. This may happen when users create somewhat poor quality meshes in Griddle and import them to FLAC3D.

So one way to fix this is to carefully investigate the separation surface (mesh), attempt to identify bad elements/faces in it, and fix the grid (or maybe simply delete bad zones). If you’re deleting bad zones in the model (it would be ok if the model is huge and there is a small number of bad zones), you could write a small script that iterates over all zones in the model, check each zone quality and if it is below certain value, assigns some group to such zone. Then you can simply visualize and if necessary, delete such zones.

There are some other reasons for this error to happen. For example, a complex intersection between multiple separation surfaces.

Note that interface logic was improved in FLAC3D 9 and, as far as I remember, in the latter updates of FLAC3D 7, so at least make sure to update to the latest v7. Also, FLAC3D 9 has new zone joints (which are more robust version of interfaces). They should work better in such situations.