How soil pushes (and how to build a retaining wall that pushes back)

When you contemplatehow to build aretaining wall, you may imagine how firm and solid itll 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, the retaining wall design may not look good for long. A poor retaining wall design wall can lean, separate, even toppleand its out there in plain sight where all your neighbors can point and snicker. You dont 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 doesnt. 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 wont 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 youre 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 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. Thats 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. Theres 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 youre looking at is a well-drained wall that will last a long time.

Now look at the poorly drained wall in Fig. E. Theres a dip in the lawn that collects water near the top of the wall. Theres 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 its also holding water in. Yikes! Theres 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. Youve 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 strong retaining wall design features well-compacted base material, compacted material in front of the wall to prevent kick-out, and stepped-back materials.

A wall that has an uneven base, no compacted material in front of it and no step-back to the materials will eventually fail.

Even if you have only a small wedge of soil to retain like that shown in Fig. A, 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 (Fig. F), rather than at an angle pushing against the wall. Good compaction doesnt mean dumping a couple of feet of fill behind the wall, then jumping up and down on it in your work boots.

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 youre using modular blocks) can tell you whether you need to install reinforcing grid, and at what intervals. The taller the wall, the more likely youll need reinforcing grid.

When building a retaining wall, 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 dont become overzealous and compact your wall outward.

From top to bottom, a well-built wall either prevents water from getting behind the wall or ushers it away quickly when it does.

Water trapped behind a wall pushes against it and increases the weight of the soil, which also pushes against it.

By themselves, landscape timbers and a railroad tie retaining wall 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 (Fig. G). The same pressure thats 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 rulesits 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 thats 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 its spaced the proper distanceyou know, behind the failure plane. The rule of thumb is to set back the upper wall twice the height of the lower wall.

Compacting backfill in 3- to 4-in. layers and installing a reinforcement grid directs pressure downward, rather than against the wall.

A deadman helps anchor a timber wall in place when building a retaining wall. The same pressure thats pushing against the wall is pushing and holding the deadmanand therefore the wallin place.

Have the necessary tools for this DIY how to build a retaining wall project lined up before you startyoull save time and frustration.

Avoid last-minute shopping trips by having all your materials ready ahead of time for this how to build a retaining wall project. Heres a list.

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How to Build a Retaining Wall Stronger | The Family Handyman

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April 26, 2019 at 3:45 am by admin
Category: Retaining Wall