Perhaps the cheapest effective retaining wall solution is using steel universal column posts with treated timber sleepers placed between the steel posts. Such a wall is strong and if the sleepers to warp or rot, they can be easily replaced.
That said, such a wall, if constructed poorly, can be a horrible eye-sore. To avoid this, below are some tips on making sure your retaining wall is both effective and attractive.
Base and Height of Wall
First thing to consider is determining your base height and top of wall height. If your wall is near to your house, then you’ll want the base of the wall to be at least 1 inch below the eventual path level beside your house, such that your path or paved area can slope away from your house. Once this point is determined, the wall height can be estimated. It’s most convenient to determine wall heights in 200mm (approx 8 inch) levels, as this is the typical sleeper height.
Distance Between Posts (Centers)
Next determine the end points of the wall. Let’s say it’s 13.6 meters from one corner to the other. Use some math to work our how many bays would be required to get a post separation between 1.1 and 1.5 meters. In this case, 10 bays at 1.36m each would be a good solution. The wider the bay, the more warping of sleepers will be apparent in the future. Anything beyond 1.8m is risky as it can require replacing warped sleepers within a few months.
Hole Depth & Diameter
Next is hole depth and diameter. The hole should be at least as deep as the height of the steel that is above ground level. Hence, a 600mm high wall needs holes at least 600mm deep. The steel column should go down into the concrete about 90% of the depth of the hole. When the height is only 200mm, it’s best to make the hole at least 350mm. In weak sandy soils, you’ll need to go beyond these recommendations, while in hard stable ground, such as drilled rock, you can get away with lower depths. With diameter, 350mm holes should suffice for most walls under 1 meter if the ground is quite firm. Increase the diameter to 450mm and beyond in soft soils.
Getting a Straight Wall
The trick to this is using string-lines between star posts that are hammered into the ground at each end of the wall. One string line will mark the top front of the wall. This should be level in most cases. If you don’t have professional leveling tools, a good trick is to place an eye behind the string line, and look through it to the brickwork of a house or a level roof gutter line and adjust the string line until it is parallel to that line. If this isn’t available, get a small string hanging level and hand it in the middle of the line. You can measure from the house outside wall to each end of the string line to make sure your wall is parallel to the house too.
The top string line needs to be very tight to avoid sagging. Another string line runs about 6 inches above the base of the post. A common practice is to make the lower string line about 10 mm out from the top string line for each meter of height. This means the wall will lean back slightly into the soil it will retain. Nothing looks worse than a wall than leans forward, so this helps to prevent that happening should there be a little movement, or should you get one of your steel posts in at a slightly different angle to the others.
Dropping the Steel Posts into the Concrete
This is the real art of making a nice retaining wall and I’m about to share a trade secret with you that can save a lot of hard effort, bashing and stress which results when a larger steel post is sitting in the wrong position while the concrete is hardening.
The first tip is to place the steel post in the hole before the concrete is poured (for 800mm and above heights), make sure it is sitting in a position which lines up to the two string lines. As the concrete is put into the hole, have someone push down on the steel post, to make sure the base doesn’t shift position too much.
Once the concrete fills the hole, lift the post up to a height about 6 inches above the top string line. If the concrete is viscous enough, a post should usually sit without sinking under its own weight when 6 inches above its finishing height. While the post is in this position, observe its angle relative to the 2 string lines. If the base is in too far, then push the top of the steel post away from the string line at an angle and then wiggle it to make it lower into the concrete at an angle. This moves the base inward compared to where it was. After lowering it on an angle, pull the post back toward the string line to see if the base and top are now in line with the post. If the base is now too close to the lower string line compared to the top string line, you need to do the opposite. Life the post above the string line, then pull it toward the string line a few inches and wiggle it down at this angle. If the concrete is quite firm, you may need to use a hammer and a wood block to protect the steel.
With a little practice, you’ll get the hang of moving the base in and out so that the post angle is within a mm of the bottom string line while exactly on the top string line. All this can be done without clamps or support rods. With smaller posts, just drop them into the concrete and use the same method of angling the post to move the base to the right position relative to the string lines.
You can see some of the walls I’ve built using this technique at my website. I hope these tips will help some of you to build a professional looking retaining wall, without the stress of trying to shift the base of your posts with crow bars or continual trial and error of replacing the entire post in the concrete for each try.