Hello Julca,
I downloaded your “ground” file and imported it into UE4. I was able to see the same wavy lines that you have posted in your screenshot. I then viewed your UV’s inside of the details panel of your object. I see that you have the holes heavily triangulated from the corner of your object. I believe this is where the issue comes from.
Although this link deals primarily with light mapping it also touches on many of the principles and theories of UV unwrapping.
Have a look at the documentation and see what you can come up with.