I have a customer who has a drainage ditch or ravine in her front yard. She has a cement sidewalk across it with two 12 inch pipes at the bottom of the ravine for water to pass under and thru the cement. the ravine is approximately 4 foot deep. She would like to have the face of the cement wall faced with limestone blocks that are roughly 4"x4"x2'. The question I have is what method should be used to attach the blocks to the wall? On a typical retaining wall that we would build we would start with cinder blocks and place metal strapping in between the blocks and then insert the metal strapping between the facing material to bind the two together. How do I attach the stone to the cement face? Thanks for your help.