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Retaining wall foundations and french drain beside each other?

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?

courtyard 1.png courtyard 2.png

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Comments

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

https://www.workshop.bunnings.com.au/discussion/comment/54151#Comment_54151

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

https://www.workshop.bunnings.com.au/discussion/comment/54151#Comment_54151

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

https://www.workshop.bunnings.com.au/discussion/comment/89296#Comment_89296

Thanks @Dave-1

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

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/110438#Comment_110438

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

Minyarra

https://www.workshop.bunnings.com.au/discussion/comment/122588#Comment_122588

hi @Dave-1 and @JacobZ

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?posts in placeone of many rocks the step beforestarting to load in the sleepersjust complete - looking pretty good3 months later "we're all individuals"

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/131335#Comment_131335

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

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/131335#Comment_131335

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

Minyarra

https://www.workshop.bunnings.com.au/discussion/comment/135854#Comment_135854

Thanks for your comments @Dave-1

The posts have been rechecked and are vertical; sleepers all have 'wiggle' room but taking them out and re-enforcing them all seems to be necessary

Minyarra

https://www.workshop.bunnings.com.au/discussion/comment/135855#Comment_135855

thanks @MitchellMc , I will send photos to the store for comment. maybe the 're-enforcement' comment could be added to a A5 on how to install retaining walls along with @Dave-1 comment about ensuring wiggle room, unless those comments are there already and I just didnt read them 😞

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/138950#Comment_138950

Good Morning @Minyarra

This is the difference between never having installed a retaining wall and having done some, to a few, to lots 🙂

You are now on the path 🙂 The spacing of the posts I do think is in the manufactures specs, the warping ive seen smaller instances but not as bannanaish as yours 😕

My "spacing rule" is something I was taught in reference to my Grandfather 🙂 After he had passed my Grandma would always remind us that the fence rail spans were too long as it saved him and his mates from digging extra holes, they shot her down (imagines 1950's stirring) and then afterwards all the rails sagged 🙂 By here telling me that story when I was ten (I am 50++ now) it really set in my head what happens to "spans" You now have the equivilent 🙂

The re-enforcing sounds like a good inbetween option. And even switching the bannanas around so teh top row only bows in across the run could work. I would definently re-enforce with a vertical piece of timbe across each span (in the middle) tho.

Dave

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/138950#Comment_138950

Hi @Minyarra,

As @Dave-1 mentioned, it's likely that the supplier has documentation covering the spans of their sleepers. In this guide on How to build a retaining wall, even a low non-structural wall requires "Every 2.4m or 3m sleeper must have posts at either end and one in the centre.".

Also, note that a retaining wall 1300 mm high would likely require engineering in most areas and council approval. The engineer would have picked up the missing middle post. With larger walls, there are a whole heap of different requirements, such as construction, footer size, drainage, etc., that need to be taken into consideration. That's why DIY walls typically stop at below the 1000mm mark.

With the addition of a brace timber behind the wall and long landscape screws, you should hopefully be able to pull those sleepers back into shape.

Mitchell

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