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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?
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
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
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
sorry for the delay as i have now moved onto a pergola and decking.
Below are some photos. The project went well except that the sleepers aren't very good. They are the Bunnings Ironwood (more expensive) 2400x200x75.
I needed to cut into the bank further so that I had more clearance behind the tanks. Thankfully the Bunnings return process is pretty efficient because I went back twice to increase the height of the Galintels. I ended up with 2100h Galintels for a 1300mm wall. The holes were hand-dug; augers dont work well here because of the amount and size of the rocks (the biggest one pulled out was 44kg; the ground was super muddy which just made it all a bit more difficult)
On day one, the retaining wall looked good. All sleepers were straight and fitted nicely. We stained it in merbau finish to match other timber.
Unfortunately now, even though the ground hasn't yet been pushed into place, a bunch a bowing in different directions and I am fearful what happens when the ground is loaded behind them.
Please advise your thoughts. Is there a way to straighten them back up or do they need replacing? is this a quality/warranty issue or expected?
Good Afternoon @Minyarra
😕 thats a shame about the sleepers. I was concerned the posts were leaning but its just the sleepers you have that are being "indervidual"
So the question that comes to mind, how much clearance did those sleepers have when you slotted them into place?
Where they hammered in or just dropped in with a few mm eiether side for expansion?
If they have been hammered in tightly, I would be tempted to take them out, trim the lengths so there is a 5mm gap either end and reverse the bend so its facing into the hill. Thoses sleepers generally shouldnt bend with a force behind them.
The secondary question I have is still along the same lines. The "good" sleepers, do theyhave wiggle room (as in when they were installed where they short by 5mm or more either end?
Dave
I'm sorry to see that, @Minyarra. What's the spacing between posts? For anything over 1500mm, I would always recommend either another post or, at a minimum, a vertical brace section in the middle, which prevents this type of warping. At 2400mm, those sleepers will really do whatever they want over the distance, including bowing like bananas.
I’d recommend contacting the store where you purchased the sleepers and discussing the issue with the store manager. They will likely pass your photos on to the manufacturer, and the manufacturer will work with the store to determine the cause and, if necessary, find a resolution. At this stage, it’s difficult to say exactly why the warping has occurred, so getting the ball rolling with the store is the best way to have the supplier inspect the sleepers and determine what’s happened. I’m sorry for the inconvenience this has caused.
Mitchell
