Drainage behind retaining wall - Do weepholes really work?

[janinemanning]

I have a garden that slopes up away from the house which had a small (600mm high) retaining wall just 600mm from the back of the house. It had the usual weep holes but I never had any water escape from them. Wanting to create a patio directly outside my back door, I have excavated from the back of the house a further 6 metres (all by hand and 16 skips), and now need a retaining wall of 1m-1.25m. My problem is that the amount of water that is currently seeping out of the bank and into a hastily built trench amounts to several gallons every single day, which I drain off manually 2 or 3 times a day. To complicate matters, my excavation revealed an old clay land drain which is very active. All this water must need diverting before the retaining wall is built and I am not convinced that weep holes are the answer. What would you suggest? I intend to dig footings that are 600mm wide by 500mm deep and the retaing wall will be built in natural stone to give a width of 450mm. One positive aspect of my excavation is that my cottage, which has flagstone floors directly laid on earth, did not suffer from the usual damp musty feel this winter.

[danensis]

Remember that if the wall is higher than 1.2m you will need building regulations approval.

[Tony McC]

You need an Collector/Interceptor drain behind the wall whiich will need to be directed to a suitable outfall point, perhaps a nearby SW syatem, a MH or a soakaway.

The Interceptor draiun could be a standard perforated pipe sort of construction, or you might use a drainage composite,(aka a waffle drain or fin drain), depending on the site conditions.

Weepholes DO work, when they are unobstruted and when there is somewhere for the water to go, but all too often, weepholes are left in brickwork with no regard for what happens to the groundwater before it gets to the weephole, and the walls are usually backfilled with clay and all sorts of crap. The result is a non-functioning weephole and a damp wall.

Danensis is right - if your planned retainer is more than 1200mm high it MUST be designed by a structural engineer and you'd be very well advised to seek BCO approval.

[janinemanning]

Thank you both for this. I think I'll be making the wall just 1.2 m high. (You don't know my building inspector.)

One further question if I may. Should the concrete foundations of the wall also act as a base for the land drain, so that if my original width of footing was 600m, should they now be 600mm+415mm. Or, having dug footings 415mm away from the bank to be retained, and had concrete foundations installed, should I excavate behind the foundation to install the drain on bare soil? The fin drain system seems to be on the foundations but is this absolutely necessary? (I've got the cost of the extra concrete in mind but if it's necessary then so be it.)

Many Thanks

[Tony McC]

If it's not too late - it doesn't matter whether the collector drain is sat atop an extended footing or over bare earth. Do whichever is easiest for you.