How to build a Sleeper Retaining Wall.
First there a few different designs of timber walls and depending on the height and the theme you want to go with the rest of your landscape then your choice will differ as to which one to build.
Timber walls can be split up into two main types, closed faced or open faced. This simply means what it says, the closed face wall will be the likes of logs or sleepers both fixed in by posts while on the other hand the Open Face Walls have gaps in the face basically for drainage. These walls are the tie back and crib, generally used when the wall has to be higher than 0.9m (3ft) although not limited to. This page deals with the procedure of the Closed Face where the most common style is to have Vertical Posts holding back horizontal rails.
Some walls will require a batter on the face (leaning back in) then this is built with a slight variation in the procedure.
Specifications for various wall heights design specs and materials to be used are on their own page so if you need to take a further look.
So with the construction of a closed face Wooden Retaining Wall, whether you are using old Railway Sleepers, Treated Sleepers or Logs or even just Hardwood Sleepers, the process is basically the same, it’s just your material calculations will be different.
Setting the Line.
For the alignment of the posts you must first establish where the base of the wall be, now depending on what material you are using the thickness will vary. Generally if you are building a wall higher than 600mm (2ft) then you will need drains and if you are working with Sleepers 150mm (6″) x 75mm (3″) then the total thickness to allow for the base will be 100mm (4″) for the drain, plus 75mm (3″) thick for the rails (horizontal timber) and then the Posts, which are usually thicker than the rails so I would use either 150mm (6″) x 100mm (4″) thick or 200mm (8″) x 100mm (4″) thick, depending on the height. See Retaining Wall Specifications.
So adding up the thicknesses equals 275mm (11″) minimum, out from the face of the fill or the cut, the material you need to retain. So you can set your string line up keeping this distance away from the fill to be retained.
Place the peg past each end of where the wall will be as you need to dig and set the end posts first, you will then be using these posts to string off to set all the others. So here is the procedure to follow, it will be the same for any closed face wall.
Step 1. Setting the Starter Posts.
Once you have determined where the wall will be (see above) your end posts have to be set in solid concrete. The depth of the holes will vary according to the height of the wall, See Retaining Wall Specifications.
Whether the post are set vertical or with a batter, makes no difference, only set these the first day. If the line is a long one then you might want to think about also setting a post in the middle somewhere, this will ensure that the string line won’t sag too much for the next stage. You can use either a standard concrete mix which is a ratio of 3 x gravel, I generally use 10-12mm (½”) gravel – 2 x sand – 1 x general purpose cement, mixed with enough water to just make it moist not runny.
This will allow you to compact the concrete in the hole around your post, I generally just beat it down with a mattock handle or the knob end of a crow bar. Or you can use rapid set concrete, just follow the instructions.
Step 2. Setting the rest of the Posts
The next day you can place two string lines from these posts to each other separated apart as much as possible then measure along the line multiples of your post centres. See Retaining Wall Specifications. All the other posts can be set to these lines and a spirit level use just for the sideways vertical. Remember to keep the post just off the line so as not to distort it in anyway.
It is of course much easier to set these all the posts with two people, one holds the post in the right place and the other places in the concrete, it can be done on your own but is very difficult. It is also a good idea to leave these posts to set for at least a couple of days before you start fixing in the horizontal rails.
Step 3. Fixing the Rails
Once you are satisfied that the posts are set then you can start with the bottom rails, fixing them to the back of the wall and starting from the lower ones first if you are on a slope.
These should be fixed level, although in some cases you can follow the slope of the ground, just be careful to make the panels all the same slope between the posts, otherwise it will not look good.
Always use Gal fixings especially if you are using Treated Timber. You can either bolt through the post are what I do is use 150mm (6?) Gal flat head nails fixed through the rail straight into the post. Remember it has a lot of fill behind it so it’s not going anywhere.
Step 4. Then the Drains
The drain agg pipe can be place in before you get too high just for ease sake, I would stack and fix about three high and then line the back with plastic, fixed to the back of the wall with staples or tack nails.
Place the specified agg line in at the bottom, laying it on the ground to the end See Retaining Wall Drainage Guidelines, back filling with gravel up to just below where the plastic is fixed, this will allow an overlap when you continue.
Then it’s just a matter of repeating the process until you get to the top of the wall. Remember when you place the next layer of plastic behind the wall make sure you overlap it at the bottom. When you get to the top you only need to place the plastic over the join of the rail below. Let the fill come to the top, you don’t really want to see any plastic.
When the finished height of the wall is critical.
If you are building a wall where the finished height is critical then some time it is better to start fixing from the top and work your way down fixing each rail underneath the previous one. You can measure but with timber there can be a tolerance of about 3-4% so this can make a big difference to your finished height. Doing it this way of course you will have to do the drain when you finish all the rails.
Treated Log walls.
I will always Like the sleeper wall use a larger post size to the rails and I tend to use Winged splits for the rails, these give a better bonding surface area when they are stacked on top of each other than the logs do.
You just have to be accurate with cutting your rails as the round log post does not have a large contact surface area that the sleepers do, fixing the rails, I use the same nails but just angle them to engage the post where the rail makes contact. This way you will not see it from the front. If you use a larger diameter log this will help this problem.
Other than these differences the procedure is the same.
Here is another closed face wall but it is only to be used without any anchors up to 600mm (2ft) high and must have a drainage system.
Here the Sleepers are laid flat on their faces and stacked on top of each other, with each one stepping in 25mm (1″) giving the wall face a 25% batter. each sleeper is fixed to the one beneath it and the small wall is pretty well free standing, relying on the weight of the sleepers to keep it in place. Gravel must be place directly in behind the sleepers up to the level of the base of the top one, because there is nothing fixing it to the ground you cannot risk an moisture build up and putting any pressure on the wall.