# Boundary conditions in a small deformations problem

Dear users,
I’m new to OGS 6 and I’m trying to solve a 2D small deformations problem, which looks like the following: say, a section of the earth’s crust is given with Young’s modulus, Poisson coeff. and materials density written in the vtu-file. Then, only gravity force is being applied to the model, like in the picture below.

Unfortunately, I’m not sure how to specify the boundary conditions in this case (for that reason, in my opinion, the linear solver diverges).
Will be grateful for any help with the question.

Since I’m not able to upload any files yet, here’s the link to the project file and meshes.

Hi @SimonZakharov, I guess this might be related to the orientation of your slice.
In 2D, I usually use the xy plane. I’m not sure whether OGS accepts also yz and xz planes. But, your plane lies somewhere in 3D.

For xz there is an MR open (Draft: Use x-z coodinates in 2D problems (!3670) · Merge requests · ogs / ogs).
So, it is also not working yet.

I see; thank you for the tip.
So, if I get it right, both in 2D and in 3D cases the faces (or boundaries) of the mesh must be strictly parallel to the coordinate axes?

This is only required for 2D. (In 3D this is only relevant for the BC components you specify, as they are with respect to the cartesian coordinate system, not the boundary normals) This is because vtk/vtu always contains 3d coordinate points. So for 2D, ogs has to reduce the data somehow. If this involves a rotation, the resulting plane/slice would not be uniquely defined (Its not clear what is up what is down. If there is a g for example, how does it correspond to the resulting plane etc.).