Thanks woody and r896neo for your replies. And RonnyRaygun and theprinceofdarkness.
Yes, I agree that a facing wall can look unsightly if not protected from water in the soil, and also from water running onto it from a patio above it.
in that case a plastic drainage channel could be installed near to and parallel to the edge of the patio, as I have had to do after building a retaining wall holding back material on which I laid a patio.
I had to use an angle grinder to dig out a channel in the slabs for the drainage channel to be placed flush with or slightly lower than the slabs.
It's done the trick but I could have avoided all that extra work.
With this other wall, 6 feet high and 10 metres long, and not yet built, I'm learning as much as I can before doing any work.
I've been advised to call in a structural engineer, but the one I emailed has not got back to me, so I have to try to work it out for myself.
I told him all I wanted was step-by-step instructions, but I think the job was too small for him.
As regards the cavity between the retaining and facing wall, I'm going to place impermeable plastic sheet over the back of the retaining wall, but I'll do as you say and use wall ties. Is it really true,by the way, that with just concrete infill the facing wall would eventually pull away?
Here, in case you are interested, is my provisional design plan for the wall.
First, I'll make the foundation about a foot thick, the trench already being about 9 inches deep, the remaining 3 inches being above ground level.
I'll build the wall about 9 inches away from the clay cliff which is to be retained, the gap to be filled with gravel and a land drain, a perforated plastic pipe, at the bottom but above patio level in case I decide to leave weep holes in the wall.
The pipe will empty the water into a plastic drainage channel at the end of the wall and running alongside a low side wall at right angles to the main wall, then at right angles again, so that now it's parallel to the main wall and venting into the same drain as the gutter downpipe.
There'll be weed control fabric between the gravel behind the wall and the clay cliff, in order to keep the gravel clean.
The retaining wall will be made of 18 x 9 x 9 inch hollow blocks for strength, this to be achieved by tying horizontal rebars in the foundation concrete to upright rebars which go through the holes in the blocks.
This means that the blocks have to be lifted up and placed over the upright rebars and slid back down, the uprights being about three feet high for convenience.
When the wall is at the right height each rebar will have another tied to it in order to reach the 6 foot height.
As the wall is built, the holes in the blocks, going down to the footings, will be filled in with concrete.
The wall, I forgot to mention, will be sloping slightly back, for extra strength.
It is possible, for all I know, that this will be enough to make the wall secure, keeping in mind that the side walls will be tied in to the ends of the main wall, in the middle of which there will be steps down from the 6 feet level to the patio, these being very substantial.
But I can't be sure, so instead of paying several hundred pounds to a structural engineer I 've decided to build a second retaining wall up against the first and tied to it, with wall ties or reinforcing mesh.
It'll be made of ordinary concrete blocks, which used to be called breeze blocks, laid either on edge or flat, with wall ties for the facing wall.
In front of the facing wall I'll build a raised bed using breeze blocks laid flat for the retaining walls, and with wall ties for the facing wall.
It will be at sitting height with flat coping stones to finish it off.The earth in the raised bed will cover the part of the foundation 3 inches above ground level and protect it, I hope, from frost.
That, anyway, is the plan.
Thanks again for the replies.