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About This Project
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!
How Soil ‘Pushes’ (And How To
Build A Wall That Pushes Back)
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.
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:
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A strong wall features well-compacted base material, compacted material in front of the wall to prevent kick-out, and stepped-back materials.
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A wall that has an uneven base, no compacted material in front of it and no step-back to the materials will eventually fail.
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Install a base of solidly compacted material 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.
Uncontrolled Water Weakens
Walls
Water can weaken retaining walls by washing out the base
material that supports the wall.
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.
Poor Compaction Adds Pressure to
Walls
Even if you have only a small wedge of soil to retain, compaction is important. If your failure
plane is farther back so your wall needs to retain more fill, weight and
pressure, then compaction and a reinforcing grid become critical. These two
things help increase internal friction and direct the pressure of the fill you
add downward, rather than at an angle pushing against
the wall.
Good compaction doesn’t mean dumping a couple of feet of fill behind the wall, then jumping up and down on it in your workboots. Nope, good compaction means adding 3 or 4 in. of material, compacting it with a heavy, noisy vibrating plate tamper from your friendly neighborhood rental yard, then repeating these steps over and over. Your landscape supplier or block manufacturer (if you’re using modular blocks) can tell you whether you need to install reinforcing grid, and at what intervals. The taller the wall, the more likely you’ll need reinforcing grid.
Never backfill with, or compact, topsoil; it will break down and settle, creating a water-welcoming trench behind your wall. Use sandy or gravelly materials, which compact much better. And always make certain you don’t become overzealous and compact your wall outward.
Timber Walls, Tall Walls, Building Codes and
Other Stuff
By themselves, landscape timbers and railroad ties lack
the weight to hold back soil. To make these walls strong, you need to add
“deadmen,” anchors that lock the wall into the soil behind them. The same pressure that’s pushing
against the wall pushes down on the deadmen to keep them (and therefore the
wall) in place. The principles of stepping back, installing good drainage and
compacting also apply to timber walls.
Walls of any material that are taller than 4 ft. play by the same rules—it’s just that the wedge of soil is too big and heavy to be held in place by the weight of the materials alone. Some communities now require building permits and construction details for walls exceeding 4 ft. in height. We think that’s a good idea too. Many modular block manufacturers can supply printed sheets of structural information. For tall slopes, a series of tiered walls is a good substitute for a single tall wall. But an upper tier can apply pressure to a lower tier unless it’s spaced the proper distance—you know, behind the failure plane. The rule of thumb is to set back the upper wall twice the height of the lower wall.
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