Ground Prep problem?

I have been fighting drought with half my problem being my soil set up. My garden is on a hill. I have 10 inches of sandy loam topsoil sitting on top 48 inches of sand, which sits on red clay. I nearly have to have water running all the time.
I am thinking of backhoeing a hole beneath small garden plots down to the clay to get rid of all the sand. Then line the hole with 6mil. black plastic and refill the hole with the sandy loam soil and every thing else organic I can get my hands on. I'm thinking this would conserve the water I use by slowing down the lateral leaching away and give the roots much more organic matter to work with.
Yes, I am also preparing to do raised bed gardening as well.

Input please.

I definitely wouldn't line the hole with plastic, I really don't understand what that would accomplish and since it won't allow drainage through it, as you plant trees or shrubs that have roots that will go down that far, they'll end up sitting in water that can't drain through into the clay below. It also sounds like an awful lot of work to dig up 4 feet of sand, you can probably just dig up part of that and mix in a bunch of organic material and get the same effect. If you want to get rid of all that sand it's certainly not going to hurt but it seems like an awful lot of work. But the more organic material the better!

When I had to grow stuff in sand (lived near the beach then) I just chucked heaps of old cow poo ...sheep poo....lucerne chaff....really anything that will hold moisture....If you do have access to clay soil...mix some up with water and throw it all over the sand (it helps to bind the particles together.) Good luck that sounds very frustrating.

I would agree with the above, your sandy soil is much better to work than your clay, you just need to add as much manure/compost as you can to help hold the sand in place AND it will help retain moisture, pollythene will just act like a gutter under the soil and the water will shed from under the sand if you are working on a slope, you will be able to build up a more organic soil over time by digging in the manure/compost as you get it avaliable, I know this is disheartening, but unless you have a millionares bank book, you just have to be more patient till the soil improves as you add to it, removing all this amount of soil wont help, you dont say how steep the hill is, it may well be that if it is a real severe slope, all you can grow is plants thay will stablise the slope/soil till you get it more friable for other uses. hope you dont have to go to your exteme, but be a bit more patient and just improve a bit at a time till you get used to how the ground can work for you not how you can work the ground, hope this helps and you get even a small patch sorted out. good luck, Weenel.

i've never heard of a combination like that before. clay with 48" of sand and then 10" of
sandy loam. did a hurricane drop all that sand from the beach on top of the clay?

Have you considered terracing the hill?