Yes, if you mix sand with clay, it might turn to cement. Usually, the cap is 6-8" of sand placed on top of the clay sub surface. We normally roughen up the clay subsurface a bit to try to create an interface. In other cases, we rake it smooth, and add herringbone drains at about 100 ft. intervals, thinking the smoother surface promotes drainage.
I have done it. Hate it unless absolutely necessary.
First, you have to look at the water discharge rate curves of the proposed sand. With the wrong discharge and sand, it is possible to create pure soup in the top six inches of sand. Some sand needs 8-9 inches to drain properly. Some need on 4-5".
Then, as noted, tiles are almost a must to keep the water from running long ways just under the sand. You must also decide if you are cutting the sand in like a USGA Green dish, or feathering out the edges, like most sand caps on tees do. Feathering out takes a lot more sand, especially if you insist all of the fairway keeps the total sand depth, and the feathering must occur in the rough, going from 0 to 6-9" on a gradual, land conforming slope (you don't want an abrupt edge on every fairway)
Thirdly, you have to be cognizant that adding 6-9" of sand will disrupt some natural drainage patterns, and plan catch basins and the like on the high sides where necessary.
So, we only do it where truly required. We did it at the North course of La Costa and it has improved the turf dramatically, because of the low lying clays have accumulated salts. On the second course, just completed, we elected to use paspalum in the fw, and went without the sand cap because of its salt tolerance. We did it at Colbert Hills, but there was just no topsoil at all there. We have done it in Asia, where it seems to be common practice, even though I am not sure its required every single time its been used. Really don't know, though.