Retaining Wall Advice

peaceone23January 9, 2014

Hi Everyone

First of all, thanks for all the great reading (long time reader, first time poster)

I would really appreciate some advice on how to proceed with my retaining wall - in particular my messed up corner post.

Basically I am building a retaining wall in an L shape, which will then be filled, in order to extend our lawn on our sloping section. The wall is approx 9m x 10m ranging in height from starts 1m - 1.4m (at the corner) - 0.2m on the opposite end. Purpose is purely for making our usable flat lawn bigger and does not support a driveway, structures etc.

I have just had all the holes drilled by a small digger, but after sitting back and calculating the wood I require I realise I have made a mistake in marking out the holes in particular the corner post which makes up the L shape (and which is also the lowest point /will be the highest point from the existing ground level once filled)

I know that I should have started with the corner post and worked back from there, meaning post 1 would be much closer to the corner post (marked 2)

Materials being used - 200 diameter H5 rounds to be set in 400 diameter holes filled with 20mPa concrete & 150x50 H4 tongue & groove retaining boards fixed by 150mm galv nails to the inside of the posts. The inside will then be filled, (pushing the boards against the posts) & compacted (driven over by digger) with a mixture of discarded concrete pieces, clay & dirt -and finally a layer of topsoil

Drawings attached (hope they make sense) of the current situation and a few solutions as to how to proceed, but wanting any other ideas or recommendations on the proper way to go about it.
Or am I just stressing & worrying and it should be ok considering I have tried to over spec the wall in the first place? (e.g. holes are the height of poles out of the ground + 200mm)

110mm perforated drainage pipe + scoria + geotextile fabric will be used along both lengths of wall, joining at the corner discussed and will then carry on further past the wall to meet up with existing drainage pipe

Plan is to then build a 1m high fence on top of this (not sure if I should bold the fence posts to the retaining wall or set fence posts in concrete in/on the newly filled material?

Current situation -
Alternate Solution -


Here is a link that might be useful:

This post was edited by peaceone23 on Thu, Jan 9, 14 at 22:19

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the explanation leaves me with too many unanswered questions. It seems you're saying the true corner post hole, #2 (according to your diagram) doesn't exist in its proper location. But you're proposing attaching boards to it as an alternate solution....?Your links to pictures do not work. It would be better to embed them into the thread. Maybe you could deal with hole location first, and later the elevation issues, in order to keep it simple.

    Bookmark   January 9, 2014 at 8:26AM
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I have updated the first post to attach an image of the current situation - and the link at the bottom shows proposed solutions.

The true corner post #2 does exist - but because of it's location I can either only attach one walls boards to it - unless I alternate or notch the boards?

Or place another post @ proposed location #3 - But you will be able to tell I stuffed up then!

    Bookmark   January 9, 2014 at 4:06PM
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I understand better now. I don't know if this will help, but the only quick solution I can come up with is cutting of the corner of the retained area and spanning the two end posts with diagonal horizontal members, attached in some suitable way on the back side .... with possibly another post centered on that span. (I don't use wood retaining walls so I'm not commenting on the durability or suitability of your scheme. You be the judge.) I think you'll understand my sketch.

    Bookmark   January 9, 2014 at 11:08PM
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Thanks for your reply Yardvaark, that solution could work but means I sort of have the same problem at each end of that new piece of timber, with no posts supporting each end of it.

Another solution which I think could work is attached. Essentially attaching a 150x75 square post through both the retaining boards of one wall and post #2 with some m16 350mm coach bolts.

What do you think?

    Bookmark   January 10, 2014 at 6:20AM
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What do I think? I think that you need to remember that the forces behind the wall will be trying to push the wall forward and over. In your scenario the only thing keeping that from happening are your posts, and I'm not seeing where you mention how deep they're set. That depth is going to be determined by a number of factors, including local soils and the grading above and below the wall. Also keep in mind that by having your fenceposts extend behind the wall, the fence will act on the wall during windy periods.

So I think a local engineer is your best bet for building a wall that will last, not the internet. No one around here builds walls like what you're showing without an engineer because the longevity of the wall depends on a few posts resisting all those forces and that's it.

    Bookmark   January 10, 2014 at 10:10AM
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I thought I saw that the posts were going to be buried and set in concrete to a depth greater than their exposed height above grade. The terminology of parts used is foreign to me and since I don't do wood walls, I'm not offering a professional opinion of how sound I think the wall will be. My gut says the posts will be sturdy enough, but that the horizontal wood members are too thin and will bulge after not too much time. Double that thickness, at least, seems more appropriate.

Insofar as having "no posts supporting each end of it" [per my suggestion] I don't think that's the case as there is a post supporting the end of each primary wall. That support carries through from the primary wall to the diagonal piece, which is "keyed in" as well as being attached by a spike and held in place by earth pressure. However, if the posts eased out of position over time (which, of course, would never happen!) then the corner intersection of the wall could split apart. As I said, however, I think the horizontal pieces (which we would basically call 2 x 4's -- I think -- are too thin and weak for building a retaining wall. I've seen such used before and they bulge. Using them requires a much tighter post spacing. Being thin, they are also likely to succumb to rot much earlier than a stout member.)

Regarding the latest proposal, I think appearance would be the problem. The rectangular milled lumber profile introduced where all other vertical members are round posts ... well, it would look like a band-aid from the start. I think doing the same thing with just another post like you are already using would look much better. I don't even think people would say you messed up. If you are concerned about symmetry, you could even add a third post (on the other side of the corner post, placed around corner at 90*) in order to have is seem like a planned design detail. It wouldn't seem strange, I don't think, to have it look like you are beefing up the corner.

    Bookmark   January 10, 2014 at 1:16PM
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You might look at changing the corner post to a square one with angle brackets on the inside corner to support the retaining boards. Rip a second post to add a decorative hide to the metal brackets.
See detail below ....

    Bookmark   January 10, 2014 at 5:49PM
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Thanks everyone - given me plenty to think about and decide upon which is the best solution to the problem.

Just to mention - posts will be in the ground (with concrete) deeper than they are exposed +8 inches.

Much appreciated.

    Bookmark   January 10, 2014 at 10:43PM
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