I'm building a retaining wall using 'H' blocks (see picture). Each block will have 4 vertical rebars and 2 horizontal rebars and will be filled with concrete. I live in Spain and my builder has never seen blocks like this before. He wants to lay them in the standard staggered fashion, however I think that they should be laid one on top of the other so as to ensure that a columns of concrete are formed. He also wants to use mortar between the blocks is this necessary?