# Forum Home Renovation Retaining Walls  Aggregate gravel options for drainage behind rock retaining wall

## Artie Fufkin

I've gone overboard with drainage behind our new rock retaining walls. Given they cost $15k (5 separate walls totalling approx 50 metres in length and between 200 and 1000mm in height), I figure it's worth the investment. 
I've dug pits behind, the walls, layed geotextile (terrastop), topped by approx 100mm of 20mm river gravel, socked ag pipe (running to drainage points along the wall approx 3-4 metres apart), then another 300mm plus of 20mm river gravel, give or take (depending on the height of the wall).  
Problem is that, with one section of wall to go, I've run out  of 20mm river gravel and the only supplier in Canberra no longer supplies. 
So what are my options?  
Some websites have said blue metal, but the wall builder has said don't use it (I understand that it can compact and prevent water from getting through). 
This website suggests crushed blue gravel or shale:  ORNAMENTAL GRAVELS 
2 more questions: 
Is (crushed) blue gravel the same as (crushed) blue metal? 
Isn't shale just hardened clay, which will similarly compact and block drainage when wet? 
Any other suggestions, particularly for any local (Canberra experience)? 
Thanks in advance. 
Artie

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## barney118

What did you do for waterproofing? Did you run plastic up against the wall before you filled it?
I would have also run core flute ontop of plastic for waterproof/abrasive resistance.
Ideally you should put coarse aggregate on the bottom, 50 mm rail ballast then your 20 mm. 
You can use crushed up bricks and concrete, any aggregate, maybe the blue metal they refer to is metal dust and this will compact, but 20  mm blue metal is not a problem.  
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## Kwikee

20mm blue metal should be fine, provided there are no fines in it. Another option is coarse scoria, the red-coloured porous rock.

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## shauck

Scoria gets my vote

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## Artie Fufkin

Thanks all for the advice. 
Re installing waterproofing - not sure why I would do that? I want the water to flow out wherever possible, especially for the dry stack walls, rather than being stuck behind anything. I've just lined the bottom of the trench and the dirt-side of the trench with geomesh to keep the dirt out of the aggregate and away from the ag pipe.

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## shauck

You've got it right. Plastic would create a wall holding water with great pressure. Not good.

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## adzza

Most people put plastic behind the sleepers so they don't get water marks on the sleepers over time. I agree however this is silly. I don't care what the wall looks like as long as it works efficiently and let's the water out!  
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## Marc

Recycled concrete 20/40

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## Bloss

mm - 20mm not available? or just temporarily so? From Canberra Sand & Gravel  ORNAMENTAL GRAVELS or do you mean Corkhills Gravels . 
Recycled brick & concrete at different grades ACT RECYCLING AUSTRALIA

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