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How to add drainage for retaining wall on clay base?

With an upcoming shed install, I will be installing a 600mm high retaining wall using sleepers to retain the upslope earth from the cut required.

On a few test digs, I have noticed solid clay for the bottom 300mm that is holding water for days with no drainage through the adjacent earth. Due to this, will I need to consider multiple layers of ag pipe drainage, one at the base and the other midway up at soil level, or will the base ag pipe do the job?

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Comments

JacobZ

Hi @biggles20,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

That's an interesting question that I can't say I've considered before.

My thinking is that if you have a layer of drainage gravel, at least 300mm wide behind the retaining wall, that runs from above the clay level, down to the base of the wall below the clay level, then naturally, water will end up in the agi pipe at the base.

I think you would get away with just 1 x agi pipe at the base of the wall as long as there isn't clay directly above it, but it couldn't really hurt to have a second one above the clay layer.

Allow me to tag @Dave-1, @AlanM52 and @Jewelleryrescue for their thoughts.

Jacob

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/54870#Comment_54870

Good Evening @biggles20

The rear base of the retaining wall with agi pipe (100mm) with gravel above it (not clay) will help with any water traveling subsoil wise down the slope, hits your wall and then drops down to the base of the wall. I would have maybe a foot backs worth of gravel plus pipebefore the footings of your posts.

As a side note, when you build your retaining wall you will change the subsurface flow of water. I had this happen when I had the sewage pipes replaced and when I installed stormwater pipes, water takes the easiest path. The lawn drys out easier now then before and also the lawn dosnt stay as soggy.

Dave

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