Please note that, in finite element theory, only continuity of displacement is guaranteed at nodes, not equilibrium of stress. Stress values are only approximated through shape functions. This may explain why your stress values are not exactly equal

It would be good to see input and output data.

Thanks for your answers, but I need a practical approach to proceed with my model.

I will attach some photos, so you can understand easily what this problem is about.

There's an excavation inside a box (this excavation can have any shape, not symmetrical necessarily), and in each face of the box, I need to set a sigma 1, sigma 2 and sigma 3 stresses which are the in situ pressures. (see theoretical_problem.gif attached).

For a case like this, we need to set fixed boundary conditions for the entire box, and proceed adding the respective explained stresses.

As I said before, if you add a s1, s2 and s3 in ANSYS, they cancel each other out, because they are applied in opposite faces, and have same magnitude. THIS DOES NOT OCCUR IN ABAQUS. And the results you get in this case in Abaqus, are the logical ones, i.e; the excavation tends to converge due to the action of tectonic stress.

So the question is basically, Is there a way to set boundary conditions, or pressures in ANSYS to avoid this issue (stresses cancellation) so I can get the results like I am getting in Abaqus?

An engineer before suggested me the following procedure:

" The good setup is to take a cube, and on three faces, apply a displacement constraint with zero displacement in X on the face normal to the X axis, a zero displacement in Y on the face normal to the Y axis, and a zero displacement in Z on the face normal to the Z axis.  Now you have a cube that isn't going anywhere.

On the opposite face to the one that had the displacement constraint, apply a pressure. The reaction force on the face with the displacement constraint will have an equal and opposite force to the face with the pressure applied."

Sadly, by doing so in Ansys you get a result like this (see bad_result.PNG attached).

PS: I know this stope modeled is kind of symmetrical, and I could divide it and in that case I would not have any problem with stresses, but I am just experimenting to know if there is some way to simulate tectonic stress so I can have an excavation with any shape...

Kind regards, and I hope everything is clear now.

Hi Felipe Gutiérrez,

Thank you for illustrations and clear explanation. The engineer’s suggestion is perfectly right, but the results in bad_result.PNG look as if material was not deleted from the excavation. Please check your model. I suggest to play with the simplest model (cube with 3x3x3=27 elements under action of hydrostatic pressure) and compare results for continuous cube and the cube without central element. The difference will be seen. Also note that you can model the excavation by assigning a very small value of Young's modulus for the material inside excavation domain.

Good luck.

I thank everyone who discussed here, because we have just made a great topic to learn from.

Yesterday I was able to see my mistake, and as Victor J. Аdlucky says, the engineer's suggestion that was given before was completely right. I did not apply the BC's properly, and that was the reason of the bad results shown.

And I want to particularly give a huge acknowledge to you, Claudio Pedrazzi

Now I will show the good results, which were gotten by following the engineer's suggestion. And it can be seen clearly that because of the excavation, induced stresses were triggered around this mining stope ,and the section of it, shrunk. (convergence).

The controversy about the displacements that I previously showed as bad results, is that this is not the kind of behavior you would expect in reality. In fact, which is expectable is that the cross section of the excavation loses area, which is convergence. Anyway all of this is an issue related to plot options, deformation in X, Y and Z axis.

Best regards, to you all. please never stop posting on these forums.

Yes, I am truly aware that this model could be easily reduced to a 2D one, but this was just a training... to know and learn how to face complex excavations or situations in future. The idea was just to know how to generate in situ stress.

I am definitely going to try it out with weak springs, I do not like how deformation is presented at first, as you mention.

Thank you again! perhaps I will add some pictures with the weak springs to see and analyze the differences.

Best regards!,

Felipe