A buddy of mine was telling me about seeing a guy installing a flagstone walkway using a technique I've never used. It is an interesting technique, but I'm a little suspicious of how it would hold up. This guy excavated and formed up the area to be done in flagstone. He tamped in 1.5" of gravel fines (8910) and then tamped in a 1.5"-2.0" mixture of 32 scoops of sand and a 94# bag of portland. He cleaned his flagstone and then wet the back of the stone as he sat it on top of the dry sand/portland mix. After getting the stone layed down and level, he came back with a conventional mortar mix to fill in the joints. I'm not sure if he did any wetting of the stone (with the sand/portland mix underneath it) prior to grouting, but I'm assuming he didn't. He told my buddy that he's done many this way. What do you all think about this technique?