Singular Matrix Error

[lindaatik]

Hi,

I am modeling 2 retaining structures with soil in between them. If I make the soil mesh in between the walls uniform (i.e. same element size for all elements), the program runs fine. If I make the mesh such that soil element size increases going from the wall to the center line of the soil, I get the following error: " UMD2FA: matrix is singular".
Can anyone tell me what's happening?

Thanks a lot,
Linda.

[fmk]

the matrix is singular means one of 3 things:
1) you don't have enough constraints.
2) your elements have been defined incorrectly resulting in 0 tangent somewhere.
3) your materials have been defindef incorrectly, 0 tangent somewhere.