Retaining wall foundations and french drain beside each other?

[Minyarra]

Hi folks,

I need some help. We live on acreage and recently dug out along a bank near our home to create a courtyard and utility zone. 

I need to put up a retaining wall of about 800mm high, which I am using the lintels with 75mm thick sleepers. I need to dig in some foundations which will be at least 400mm deep for the posts, but I also need drainage near here. That last storm turned the zone into a big muddy pond. 

I will get an excavator in to tidy it up and ensure it is sloping away from the home, which is toward the retaining wall.

My question is; how close can I put a french drain to the foundations  of the retaining wall and not undermine them?

[JacobZ]

Hi @Minyarra,

A warm welcome to the Bunnings Workshop community. It is fabulous to have you with us/

You can sit a french drain right against the back of the retaining wall before backfilling. You just need to ensure it sits above the top of the concrete footings so as not to undermine the soil around them.

This diagram in the brochure for Adbri Masonry's Versawall shows a good example. 

The french drain should sit lower than the finished ground level, with the agi pipe above the height of the concrete footings, or in this example the "450 x 130mm compacted roadbase or concrete pad".

When you build the drain, make sure you line the trench with geotextile fabric to stop dirt from settling between the gravel and blocking things up.

If I have misinterpreted and you are planning to build the french drain on the outside of the retaining wall, I would leave it at least 300mm away from the face of the wall. This will give enough space that the water can be captured without causing subsidence against the concrete foundations. Once again, just make sure you wrap the drain in geotextile fabric to prevent the soil from settling between the gravel.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob

[Dave-1]

Good morning @Minyarra 

Love foundation walls Tho you mention at 400mm deep is a bit shallow even with concrete footings. For an 800mm wall height (and looking at what you are wanting to hold back) I wouldnt go for anything less then 600mm, maybe even 700mm as a mininmun. 

You could always put in a grill drain about 400mm from the wall, it needs minimal depth and will catch any water on this sid eof the water.

Dave

[Minyarra]

Thanks @JacobZ 

I will be running a drain behind but also need a drain in front. 

Thanks also @Dave-1 

I will take your suggestion and run some of those posts deeper. Several can't because I have hit boulders; going to drill into the boulders and then use some chemset screws and then surround with a wide concrete bed. In several others where I can't get beyond 600, I was thinking of driving several star pickets deeper within the holes and then concrete. I think that will strengthen it up.

Im thinking that of sticking with a French drain because we will have gravel over the clay. I don't think a surface grill drain will work in that instance; it will be 500mm away from the wall.

[Minyarra]

Thanks @Dave-1 

Really appreciate you taking the time to respond. I will run holes deeper as you suggest. 

[Dave-1]

Good Evening @Minyarra 

Sounds like a plan. I dont think its worth it with star postsas they will be too narrow to really retain the weight of the soil. Like you say a wider concrete base would be an Idea. Think like a back to front L, so the short side is out onto the driveway(below the surface to make it tidy), same rule applies, 1/3 out the front to 2/3rds the height. 

Take a bunch of photos I think it will be a great project to post up here. Never hurts to have too many photos.

Dave