I am wondering if the sand I found under my back concrete patio is ok to be left where it is. I will be adding the 12" of compacted gravel on top of the sand, then the thin sand to set the pavers in? Is this ok? Is virgin clay the only thing you wouldnt want to have underneath a paver patio? I mean topsoil is ok as long as the proper 12" of compacted gravel goes ont op of it right?