The word “retain” comes from two Latin words, “re” (back) and “tinere” (to hold) which just goes to show that even Julius Caesar had trouble with his garden! He needed to hold it back, and probably you do too! Here’s how:
Soil pushes down. Imagine you’re a soil particle at the top of a slope and the bottom of it looks oh-so-inviting. You need to keep that soil particle and all its pals at the top where it belongs, so you need a wall, but not just any old wall. It’s no good having a gorgeous one if it’s going to fall over. That nasty force called GRAVITY is pushing your soil downward, and soil ain’t no featherweight! It’s a hefty 100 pounds per cubic foot, and its sole aim in life is to flatten your wall, so it needs something equally robust to push back against it.
Dig the lowest courses of the wall to 1/10 the height of the wall to stop the force of the ground from pushing it out. Make the wall in backward leaning steps. Straight building allows the wall to fall as soon as it bulges. (Retaining wall blocks frequently have specially shaped lips so that they fit together neatly.) Keep the structure even; make a very sturdy base with solidly compacted material.
A retaining wall has two enemies: first, water. Water can damage a wall in two ways: first, it can wash out supporting material, or it can stubbornly accumulate behind your lovingly built wall and put so much force on it that finally it gives way. There is little you can do about water sneaking in – it’s a force of nature – but you can escort it out.
Have the top of your wall nearly even with the top of the soil. This way it flows harmlessly over. There is about a foot of compacted material underneath that, and gravel underneath that: (add filter material in case of clogging), and this allows water to run down to the perforated drain tile, which shows it speedily off elsewhere! If it can drain through the blocks themselves, so much the better. Don’t forget that water freezes and EXPANDS in winter. A badly built wall won’t last long then.
Poorly compacted material retains more water, and presses harder on your wall, so it’s important to do the following: rent a heavy duty plate tamper and compact the soil 4 inches (10 centimetres) at a time as many times as it takes to get the job done. Your neighbours may not thank you, but trust me, your garden will.
Depending on the height of your wall and the amount of soil, you may have to add a reinforcing grid. The higher the wall, the more the likelihood is that you’ll need it. Your gardening supplier can help you with this.
Backfill only with sand or gravel, not with topsoil. It loves water, and anyway, it’s better for growing flowers in!
Hope that helps! Have fun!