The Brew Cabin

Well done to all concerned on a very useful and usable site.
I am doing a major DIY renovation which includes replacing and extending up to a 4,75m high retaining wall.
The wall will be waterproofed externaly, but in addition I will be installing a land drain to remove the hydrostatic pressure.
The intention is to go the clean stone and geo-textile route, but I had thought to use a perforated plastic pipe before reading the advice about its limitations with regard to the depth of backfill.
I would appreciate any thoughts on the best solution in this situation.
Regards - Terence Post

Assuming you mean 4.75 metre high, that is serious construction, and in the UK would require Building Regulation Approval and submission of a structural engineers calculations and design philosophy. The great advantage of this is if the wall fails you are covered by professional indemnity insurance.

Thanks for the comments. Yes it is major construction, unfortunately dictated to a certain degree by local planning issues, but mainly by the existing situation.
I do have engineers drawings and building regs approval, but the question of drainage has only been dealt with, to my mind, in a limited way.
On the assumption that the wall is designed for a worst case scenario it should be able to withstand the hydrostatic pressures.
However, I do not intend to rely on any one factor, whether that be designed strength or indeed waterproofing claims of being able to withstand x meters of hydrostatic head.
I think it is common sense to do everything possible to reduce the hydrostatic pressure, which brings me back to my original querie - what suitable types of perforated pipe exist that can withstand back filling up to say 5m?

For backfilling to that sort of depth, I would only consider a rigid pipe, either clayware or concrete, but it's well worth taking a look at what drainage composites might be available.

If you can 'line' the rear orf the retainer wall with a one-way composite sandwich, then you've no need to worry about using a perforated pipe at such a depth, and you can be assured that there's no risk of a build-up of hydrostatic pressure. The situation in the building trade is now such that it's often cheaper to use a composite than rely on the old 'perf pipe and clean backfill' option - many of the designs I now see are opting for composites wherever possible, as they are quicker, cheaper and simpler to install.

It's worth talking this over with your designers/engineers and see what they feel. As long as you can effect some form ouf suitable outfall for the collected water, even if it's just an attenuation tank at some distance from the wall, then a composite offers a simple and straightforward solution.

Thanks for the comments Tony.
I have looked into the geo-composite side a bit more and can see the benefits; especially in the light of the fact that the ends of the wall (which is an "L" shaped structure with the point stabbed into the bank) are easily accesible due to the falling ground levels as you approach the ends making dispersal easy.