sooby said:... Where I am now there is shallow sandy loam overlying heavy clay, and that can make drainage quite an issue in some spots.
I have a similar situation, but my top layer is very shallow and entirely made-by-hand.
I can manage drainage by visualizing the clay layer as an impervious "floor" under my shallow soil layer and raised/sunken beds. The clay "floor" has to slope uniformly DOWN from any area that I want plants to grow in.
If your overlayer is too thick to shovel or hoe aside while you "shape" the clay floor under it, my method won't help. But I can "slope" the "floor" under a bed to carry water away downslope, or cut a slit trench deeper than the "floor" of a bed, to carry water down and away from that bed.
I think of it like gardening on top of a concrete slab. Anywhere the slab has an un-drained low spot, I can expect water to pool and roots to drown. So I look for the shortest path from that un-drained low spot to some even lower spot, and cut a slit trench to connect the two.
