Construction Details for Retaining Wall
Seems to be a million ways to do a retaining wall and I don’t want to pick the wrong one. I believe this drawing makes a lot of sense. Anyone have a better tried and true method?
specifically what I noticed about this one is that the fabric separates the drainage rock backfill from the rest of the soil. A lot of details don’t have any fabric. But this one makes sense to me because wouldn’t you want to, just like in a French drain, keep the silt and fines away from the drain pipe, the drain rock, and the pores of the wall itself (I’ve read you want to keep the backside of the wall clean for water to seep through here if needed… but I am not at all sure of all that..)
Another question I’d have is if this is too much gravel backfill? – or do you want to backfill nearly all the way up to sod level?
gravel under first row looks good, and depth of first row looks good too. I would use the trapezium-shaped stone in my wall. Wall is about 2’ or just shy of that
any thoughts?
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Hi Kevin.
I've built and repaired a bunch of retaining walls with timber, field stone, quarried stone, and modular blocks. The drawing is an excellent example of how to do it right. Of course the need for drainage and drainage requirements depend on the soil. But a little extra money and effort here is cheap insurance for a good wall.
The only thing I would do differently (with field stone), is to make the wall a bit deeper at the base and taper it for the first few feet. That's how I was taught to do it and when I have fixed walls with a solid base, it has meant less of the wall had to be torn down before I could start rebuilding.
Thanks for the response. Just to clarify: By deeper you mean the first row of blocks set deeper into the ground? I saw a lot of guys doing their first row set on top of packed paved base also which makes a lot of sense. Also, by taper do you mean you would like to see the wall ‘leaning back’ row after row, if you will. The blocks I will use have that lip on the back that will give it this effect if that’s what you mean.
Sorry for the confusion Kevin. By deeper, I meant front to back, and tapered up to the width you want the wall.
Kevin, I agree with Brian that your image shows a mostly-good detail. (My first business was Sticks & Stones, mostly building dry-laid stone walls and patios.) The thing I don't like about the image is the sloped joints between stones--if anything, they should be sloped back into the hill, not away where the stones will slide forward over time. But the drainage detail looks good to me.
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Not shown is that the surface layer should be impervious (keeping surface water on the surface and directing it over the wall). Maybe even a sheet of gravel covered plastic instead of grass in the upper left?
Clean gravel lower than a drain creates a water retention pond - making the soil below it weaker (ie, mud). The different color for the base gravel is indicating something less porous/retentive (eg crushed limestone with fines)?
Hi Kevin,
This article from Fine Homebuilding shows materials and methods for four different types of retaining walls: https://www.finehomebuilding.com/membership/pdf/17431/021173068.pdf
Side note--in Ontario 2' is where you need to start worrying about handrails, etc. You may want to check your local code for requirements in regards to that.
I agree with Mike's point about the slope of the joints.
The 'lean' of the wall is called the batter. Often expressed as a rise:run ratio. 6:1 being pretty common where I am. Preferences vary by region, materials, and builder. To my sensibilities, that wall looks a bit steep, but that seems pretty common in residential (my background is with trail-based stone work—dry set natural stone stairs cases, retaining walls, etc.)
Regardless of the specific batter, the top of each successive course should be sloped back towards the hill being retained such that a consistent batter can be maintained and so gravity sends the rocks into the hill rather than away from. Cast blocks makes this less of a fuss and less of an art, and with a 'lip' on the block, perhaps they can be set closer to level. (There's surely manufacturer specs available for each block type)
I don't see much wrong with the drainage details depicted in that image. Maybe lower the drain a bit. If you are using a compactable gravel base as shown, slope it into the hill, same as the stones, then place the drain pipe back at the bottom of that slope, putting the pipe elevation closer to the bottom course of stone.
A quality non-woven geo-textile fabric would be preferable to cheap landscaping fabric or woven materials.
One other note:
The 'tie-backs' in the shown detail are obviously specific to natural stone. If using block, there are other systems of tying-back that you may consider if deemed necessary— such as plastic grid— which makes the mass of the backfill part of the overall structure. There may be ways to do this with the geotex fabric as well.
At only 2' tall, I can't say how necessary something like that would be. Depends on how much water you're dealing with, soil type, etc. or how much insurance you want to build into the wall. Local builders may have valuable insights on this.
I hate to state the obvious here but your wall is 2' high. You need none of this. Go out and get some allan blocks, stack 'em 2' high or thereabouts and you're done. Unless you are holding back a creek, there is not enough hydrostatic pressure to worry about. Any water will come through the seams.
This is all especially true if land behind the wall is level and does not significantly slope upwards. In that scenario, you can easily calculate the load of the two vertical foot.
Maybe. 'None of this' is a pretty broad statement.
2 ft walls can fail just fine given the right conditions and time. Or inattention to basics.
Does Kevin need an engineers stamp on this? Probably not.
Should he use basic and proper wall building techniques? I can't possibly think why not.
Build it with properly prepared and placed base course, proper batter for the material, broken joints, etc. The basics.
The minimum after that, for me, would be to backfill with crush. It's too easy and too robust an addition to skip because 'you don't need it' so is said. An exception might be exceptionally well-draining soils.
Things like fabric, drain pipe, and reinforcements are arguably less critical, but it of course depends on conditions and goals—and requires judgement. Expansive soils with a steep back grade and large watershed is certainly a different animal than well-draining soil, level grade off the top, and little water shed. Pipe might be nice if Kevin wants to move the water someplace besides the other side of the wall.
It's a case of internet advice being given without knowing all the details, so 'err on the side of caution' is a prudent principle.
FWIW, I built a 2' retaining wall with no gravel, impervious layer, or fabric. It's fine.
But, in my opinion, responses should try to be considerably more generic than the specific question asked. Then the Q&A serves as a future reference, avoiding endless variations of people asking almost the same question. Ie, there is "what should Kevin do" and then there is closer to "all about building retaining walls". The former is quickly worthless and the latter is a valuable resource.