How to Build Retaining Walls Stronger: The Family Handyman

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 toppleand 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 soilsoil that has lain untouched and naturally compacted for thousands of yearshas 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 planethe natural soil or the fill you're going to addwants 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 materiallarge retaining wall blocks, boulders, timbers or poured concreteto 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:

A retaining wall only needs to hold back a wedge of soil, not everything behind the wall.

A retaining wall needs to retain all the material that fills the space between itself and the failure planethe steepest angle at which existing soil can hold itself together before caving in.

Water can weaken retaining walls by washing out the base material that supports the wall (Fig. E). But far more frequently, it causes problems by building up behind the wall, saturating the soil and applying incredible pressure. That's when walls start leaning, bulging and toppling. Well built walls are constructed and graded to prevent water from getting behind the wall and to provide a speedy exit route for water that inevitably weasels its way in.

Take a look at the well-drained wall in Fig. D. The sod and topsoil are almost even with the top block, so surface water flows over the top rather than puddling behind. Just below that is 8 to 12 in. of packed impervious soil to help prevent water from seeping behind the wall. The gravel below that soil gives water that does enter a fast route to the drain tile. And the perforated drain tile collects the water and directs it away from the base of the wall, escorting it out through its open ends. There's nothing to prevent water from seeping out between the faces of the blocks, either; that helps with the drainage too. The wall even has porous filter fabric to prevent soil from clogging up the gravel. What you're looking at is a well-drained wall that will last a long time.

Now look at the poorly drained wall in Fig. E. There's a dip in the lawn that collects water near the top of the wall. There's no impervious soil, so the water heads south, slowly waterlogging and increasing the weight of the soil packed behind the wall. The homeowner put plastic against the back of the wall to prevent soil from oozing out between the cracksbut it's also holding water in. Yikes! There's no drain tile at the bottomthe trapped water can soak, soften and erode the base material. Not only that, an excavated trench that extends below the base lets water soak into the base material and weaken it. You've got a retaining wall that has to hold back tons and tons of water and saturated soiland when that water freezes and expands in the winter, matters get even worse.

A well built wall starts with a solid base.

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How to Build Retaining Walls Stronger: The Family Handyman