Ok, thanks for your answer.

I've selected my vertex and welded them one by one.

It did stick some of my splines together. Now I ended up with only 3 splines that refuse to stick together.

Here is a picture to show my situation that I believed to be simple.

my 3 splines to be stuck are the outer circle and the shapes on its right and on its left.

How would you join those 3 shapes ?

thanks