Thank you, Wolfgang, for your suggestion. I've read step-11 tutorial but I still can not figure out what kind of constraints I can impose in my case. Here is a picture of a void with enriched nodes. Blue triangls are enriched with Heaviside function, red rectangles are zero degrees of freedom. When there are no whole cells inside a void (only cells intersected by the boundary are inside a void) I have a regular solution. If at least one cell is inside a void entirly (no intersection with void's boundary) I get a singular matrix. Correct me if I'm wrong, I think I could put one (or some) of enriched nodes inside a void to be zero and constrain red rectangle nodes to be equal to some neighbor nodes with enrichment. Or I'm missing something and there is a "right" way to treat this case?
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