Landscaping IDEAS Retaining Wall
How soil “pushes” (and how to build a wall that pushes back)
When you contemplate the retaining wall you’re about to build, you may imagine how firm and solid it’ll appear from the front, or how great the new garden will look above it. But unless you give serious thought to what goes on behind and below the wall, it may not look good for long. A poorly built wall can lean, separate, even topple—and it’s out there in plain sight where all your neighbors can point and snicker. You don’t want that!
Lots of people think a retaining wall needs to hold back all 6 gazillion tons of soil in the yard behind it. It doesn’t. It only needs to retain a wedge of soil, or elongated wedge of soil, similar to that shown in Fig. A. In simple terms (our apologies to all you soil engineers out there): Undisturbed soil—soil that has lain untouched and naturally compacted for thousands of years—has a maximum slope beyond which it won’t “hang together” on its own. This slope is called the failure plane. If left alone, the soil behind the failure plane will stay put on its own. But the soil in front of the failure plane—the natural soil or the fill you’re going to add—wants to slide down the failure plane.
Gravity, along with the slope, directs most of the weight and pressure of the fill toward the lower part of the retaining wall. Since soil weighs a beefy 100-plus lbs. per cu. ft., you need some pretty heavy material—large retaining wall blocks, boulders, timbers or poured concrete—to counteract the pressure. Just as important, it needs to be installed the right way. Here are three key principles in building any solid retaining wall:
- Bury the bottom course, or courses, of the retaining wall one tenth the height of the wall to prevent the soil behind from pushing the bottom out (Fig. B).
- Step back the blocks, rocks or timbers to get gravity working in your favor (Fig. B). This lets the walls lean and push against the fill. Walls built perfectly vertical (Fig. C) get gravity working against them the second they start leaning outward even just a bit. Most concrete retaining wall block systems have some kind of built-in lip (Fig. D) or pin system (Fig. F) that automatically creates the step back as you build.
- Install a base of solidly compacted material (Fig. B) so your wall stays flat. A level wall provides modular blocks, stone and timbers with more surface contact with the courses above and below them. They fit together more tightly. The more contact, the more friction and the stronger the wall. Apply these three rules, and you’ll create a strong wall. But even a well built wall won’t survive unless you take care of two troublemakers: water and uncompacted soil.
Figure A: What a retaining wall retains
A retaining wall needs to retain all the material that fills the space between itself and the failure plane—the steepest angle at which existing soil can hold itself together before caving in.
A retaining wall only needs to hold back a wedge of
soil, not everything behind the wall.