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I would never bury couch in a trench and I wouldn't use chemicals either as I'm organic. However, I've not found it any great problem where you're growing annual stuff but, if you've got it around where you plant rhubarb, fruit bushes, or any perennial plants then it will always be a pain. I'm growing on fertile but very weedy soil and it's a constant battle.

I wouldn't bury anything as creeping grass and bindweed will still find its way to the surface (I know from experience). We have a bank here made from all the clods and stuff removed from our veg patch the first year we weeded and dug it over - and it had already been a veg patch!! but the amount of clumps of paddock grass were awful to try to remove.

CC - double digging is basically digging the soil to double the depth of a fork/spade; there is loads of info in gardening books so no doubt on the internet as well. You remove one spit (that's one spade/fork's depth) and bung it temporarily elsewhere, then dig down again, then you backfill that deep hole with compost and the first spit from the next section, and so on. That probably doesn't help you one bit - have a google for a drawing or photos!

No, MMM, that really is useful, thank you. It hadn't even crossed my mind that it might mean double the depth. (I don't really know what I thought it meant but it wasn't that anyway.) Makes total sense!!

Sorry, I've only just seen your question.

Yup, although we don't do it quite like that it's the same end result. We took the turf off (equivalent to one spade deep) then dug out another layer and put it in the wheelbarrow. Then put the turf upside down in the base,cover with a layer of cardboard - it takes quite a while to decompose so helps keep the grass at bay. Then tip the soil back in, mixing it with compost if required. We were filling raised beds and didn't need double spade's depth because we were raising the level up (if that makes sense )