Overlapping zones and interfaces

FLAC3D
1

Hi again,

I’m getting illegal geometry errors because my interfaces aren’t working as intended, and I don’t know why.

I’m basically modelling longwall panels, in a complex geology. The mesh was generated by Griddle, there are interfaces which represent geological faults and so on, but the problem doesn’t rely there.

Cut outside
Cut outside1111×725 188 KB

General view of the model

Since I’m excavating longwall panels, I’m using Large-Strain mode on in this stage, the model seems converging after deleting some zones, however the interfaces I set on the roofs of the longwall panels aren’t detected and overlapping occurs, giving thus implausible displacements in the longwall panels (higher than their actual thickness or height). Here you can see the situation.

Overlap inside
Overlap inside1068×550 138 KB

Overlap in the panels,

Overlap outside cut
Overlap outside cut1267×782 51.4 KB

Panels totally excavated and adjacent zones get overlapped.

The code for interface creation looks more or less like this;

zone interface name ‘roofs’ create by-face separate new-side-group ‘Rocks_middle’ merge-nodes false range group ‘roofs’
zone interface name ‘roofs’ node prop stiffness-normal 4e10 stiffness-shear 2e10 friction 35
zone interface name ‘roofs’ tolerance-contact 1e-2
zone interface name ‘roofs’ node initialize-stresses.

The ‘Rock-middle’ group is the group in which the panels are excavated, I suppose that by doing this you impose the ‘master’ group, otherwise the interfaces get deleted as you excavate. Am I right about this?

The ‘roofs’ face group was generated from a geometry import from the exact surface mesh used in Griddle.

Now when I look at the interfaces I have in my model, only the ones of the geological faults are there, so it seems that the ‘zone relax excavate’ command is also deleting my roof interfaces.

What can I do to prevent interface deletion when excavating?

Thanks so much in advance, any ideas will be greatly appreciated.

Felipe

2

Hi Felipe. I am might be incorrect here, but I believe that if interfaces are properly set up, they should not allow inter-penetration of zones even in the large strain mode. Would you be able to extract or set up a simpler model (maybe a subset of your model) in a way that you still see the same issue and send it to flac3dsupport@itascacg.com? With this general and rather complex description that you provided it is hard to give a more specific advise.

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3

Hi @Felipe, I don’t know how to prevent the deletion in this case, but I sometimes find the order in which commands are given for interfaces can prevent this.

On the interpenetration issue, an “appropriate” stiffness is often needed and it’s related to your zone size. I can’t find the exact reference apart from the “Choice of Material Properties” in the FLAC3D documentation.

All Properties Have Physical Significance

“… There may also be problems with interpenetration if the normal stiffness is very low. A rough estimate of the joint normal displacement that would result from the application of typical stresses in the system should be made. This displacement should be small compared to a typical zone size. If it is greater than, for example, 10% of an adjacent zone size, then there is either an error in one of the numbers, or the stiffness should be increased if calculations are to be done in large-strain mode.”

What I do is run a FISH function to determine the minimum vertical dimension and maximum stiffness of the zones in contact with the interface and then apply 10 times the P-wave modulus devided by the minimum zone size as the normal stiffness of the interface. Note that this is for a case where I am only interested in the shear on the interface in FLAC2D so may not apply to your case.

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