Advice needed on digging out and then refilling soil from massive raised planter

[Wiifitmama]

We have a huge raised planter (two railways sleepers high) in the middle of our garden. We use it for growing vegetables. We had it built 8 years ago when we moved in and had the garden redone. Lovely for a number of years, but over the last few years, nothing has grown well and the water just runs right out the bottom.

I realise that the soil has degraded inside as we have never dug it over or done anything to it. We have decided this month to have the whole thing dug out and the top half completely removed (though dh is suggesting we need to remove everything?). The we will refill. But we need to refill with more than just soil. I feel like it needs multiple layers below the top soil to help deal with drainage/water retention etc.

Any advice on what to do? I was thinking a sand layer of some kind. But I have no idea what else!

Hopeful bump!

last year was very dry so nothing grew anywhere.

maybe add lots of mulch? lots and lots.

if you want to retain water, don't add sand. that makes water drain away quicker.

Ah ok! See, I know nothing! So what keeps the water from draining away so quickly?

If that was me, and i often do shit like this, i'd dig out the old soil, sieve out the roots, fill the bed half with old soil and and mix in fresh compost and well rotted manure.

No sand.

Then every autumn, top up with your home made compost.