Drainage around foundations

[davejura]

Hi folks. We have a sloping site and our concrete foundations are about 1.4m deep on the upper side. The brickie has started the blockwork and wants me to put the drainage and backfill in after 2 courses to make life easier for him (and me!) The wall is classed as retaining and is 3 blocks thick. There is water seeping through the upper face of the trench and the soil is classed as boulder clay. So I need to drain the water away from the rear wall. I was thinking 100mm perforated twinwall surrounded by 50mm max gravel (read that is the best size for clay soils). Should I also use a non woven membrane? And what's the best way to get the water to flow as obviously the foundations are level? Was thinking sitting a brick centrally and running the pipe both ways. Foundations are stepped, so once we turn the corner it shouldn't be a problem. Any advice much appreciated!

 

[tonyshouse]

I like easy to maintain so if it was mine I would rigid perforated pipes 1:60 fall 1:40 better, yes membrane, pipe bedded on aggregate 

 

 

[markc]

Perforated drainage pipe (single or twin wall) geotextile laid in bottom of excavation and up the side,  clean ballast/pebbles etc. (No fines) under and covering the pipe, textile over the ballast and then backfill with anything as long as it’s not heavy clay.

Edited August 31, 2021 by markc

[Stewpot]

There seem to be no end of theories about how to do this, the most contentious of which is whether to use a semi-perforated pipe with the perfs. up or down. My logic says perfs. up, otherwise the water will just drain out again at the first opportunity, moving the problem down the line a bit, rather than properly draining it away to a soakaway or ditch, for example.

 

However, they all seem to agree that non-woven geotextile is the thing to use. The woven stuff is tougher, but I think it is more susceptible to fines and possibly roots getting through, causing blockages in a few years time. Otherwise, I'd go pretty much with what others have said. The slope you use may depend upon how long the run is, and how deep you can go. You should be able to get away with 1:100 if you have to, but try to avoid sags in the pipe.

Edited August 31, 2021 by Stewpot

[davejura]

Thanks folks. I've ordered a roll of non woven geo textile and have 100mm perforated twinwall. Onwards and upwards!

 

 

Edited September 1, 2021 by davejura