length, measured from the highest point to the lowest — the horizontal distance)This basic information will determine how many terraced beds you’re going to need.After doing that, it’s just math. Decide how long each of your beds will be, say, for example, 10 feet (3 m). Divide by the total run (say, 50 feet, 15.2 m), so you need five beds: 50 divided by 10 = 5 (or 15.2 divided by 3 = 5).When the rise is greater than your height, measure the hillside in increments.Start work from the bottom, trenching and building as you work your way up the hill. The back of the last bed should be as high as the front wall of the first bed.If you plan to stabilize your wall with deadmen (horizontal braces; refer to the nearby sidebar about jargon), you’ll need additionally to create trenches every 4 feet (1.2 m), perpendicular to the wall, to accommodate them. One item that makes the work easier and faster is a tamping machine that you can rent. A tamping machine smooths out and compacts the soil surface so it’s as solid and level as possible, without ruts or air pockets; a minimum of two or three passes over a project area is recommended. Follow all instructions about operation and safety.
Get cleared for takeoff. Make sure the project area doesn’t have any underground pipes or buried cables. If there’s any chance there’s natural-gas lines, call your utility company so they can come out and check, and mark spots to avoid — a free service. To make sure, call 811, a nationwide service that checks for utility lines.
Water can ruin your wall. You must install provisions for drainage in order to prevent water from saturating the soil behind your retaining wall and exerting pressure on it, which can destabilize it or move it or parts of it out of position — a snafu you want to avoid.To do so, either create weep holes (holes or gaps that allow drainage) or install a perforated drainpipe (perforated, to facilitate absorption and drainage). Working and backfilling with stones will also help stabilize the project. (You know the neighbor’s wall that’s falling down? We wager that if you look for weep holes, you won’t find them.)
Positioning a retaining wall
Properly positioning a retaining wall is important because you want it to be stable. Here are the two main ways to do this:
Set the retaining wall well away (say, several feet, or a meter, or more) from the original hillside, and then fill in behind the wall. When you install the wall, you still create a base and backfill with some gravel, but the rest of the gap between the original hillside and your wall will require extra soil to be brought in.
Set the retaining wall close to the top of the original hillside. In this case, you need far less extra soil. In fact, you may be able to simply reuse the soil removed from the base of the hill, shoveling it in near the top of the wall.
Installing a retaining wall
To orient you to this project, the following steps generally apply to installing retaining walls of any kind:
1 Clean up the designated area by removing plants, grass, and any loose debris.
2 To create a base, start at the bottom of the hill or slope and dig a trench with a shovel.Dig an inch (2.5 cm) for every 8 inches (20.3 cm) of wall height. For example, if your wall will be 3 feet, or 36 inches, tall, do the math: 36 inches divided by 8 inches = dig 4½ inches deep (or, in metric: dig 11.4 cm deep).The length and width depends on the materials you’re using.GETTING ACQUAINTED WITH SOME JARGONHere are definitions for some technical terms and materials that may be unfamiliar to you when dealing with retaining walls.:#1 stone: Replaces “drain rock” of old; basically, it’s compactable gravel.Batter/battered: In this context, it refers to a particular type of angle, for any kind of wall that has been built with an intentional slope.Deadman/deadmen: Horizontal braces.Geotextile fabric: Synthetic material that is permeable. It’s used to aid in drainage, provide erosion control, and increase soil stability. It’s basically better, more expensive landscape fabric.Rebar: Means reinforced bar; it’s a tension device made of galvanized steel, used to reinforce and strengthen structures (such as concrete or masonry).Torx drive screws: These have a six-lobed, star-patterned screw drive. Recommended because they’re more secure than Phillips-head or slot-head screws; the design allows for a higher torque to be exerted without damage to the screw itself or the tool.
3 Tamp down the soil in your 4½-inch (11.4 cm) deep trench.
4 Install something to relieve the pressure of water building up behind your new wall.Provide either weep holes along the base of the wall or a perforated drainpipe behind the wall.
5 Add several inches (cm) of stones (#1 stones are best).If you’re using a perforated drain pipe, surround/bury it. Tamp down the stones. Check that the stone base is level, adding or shifting stones as necessary until it is.
6 Line the bottom and perhaps also the sides with landscape or geotextile fabric.Use plenty of extra extending beyond the boundaries; you can always trim after your timbers or stones are in place, but you can’t extend too-short pieces. Try to use one continuous piece, but if that isn’t possible, use overlapping pieces. (Doing so not only helps hold the soil down, but it will also prevent weeds from encroaching.) Don’t trim it yet; wait until you’re done. You don’t want soil to come through and cause clogs. Use washed stone because unwashed stone is loaded with silt.
7 Begin to install the wall material.When putting in more than one tier, two things are important:Batter them so each succeeding layer leans back into the hill (in other words, it won’t be perpendicular; this slight slant helps your wall counteract gravity).Backfill each tier with gravel as you go to strengthen it and aid drainage.
There’s more than one way to make an effective retaining wall. Other construction materials to consider include timber, concrete blocks, and stacked stones. Here’s some details about each, including how to erect them properly.
Timber retaining wall
To build a retaining wall out of landscape timbers (pressure-treated 6 x 6 boards specified for ground contact) or composite timbers, follow these instructions (see Figure 4-3):
1 Start by creating the outer wall.Lay timbers horizontally on the tamped-stone base (the first one and part or all of the second may be below ground level, embedded in your base). Stagger the end joints; this forms stronger corners. Stagger joints at least 3 feet (.9 m). Don’t use old railroad ties! They’ve been soaked in the toxic and messy preservative creosote. When it leaches into the soil, it contaminates groundwater and can also kill or harm plants. Railroad ties should never be used in the vicinity of edible plants.
2 Add successive rows, but batter them atop one another ever-so-slightly back into the hill about a half inch (1.3 cm).Do so until you reach the desired height (typically 3 feet, .9 m, tall; generally speaking, taller ones require additional stability; construction of taller ones is best left to the pros).
3 Anchor the ends of each timber to the underlying timbers.Every 2 feet (.6 m) or so, drill pilot holes and insert long spikes of rebar. Drive them deep enough so that the tops align just below the top of the finished wall.
4 Midway up the wall, perpendicular to it, dig level trenches back into the hillside every 4 feet (1.2 m), back far enough to accommodate a 4-foot (1.2 m) deadmen (horizontal lengths of timber).Attach the deadmen flush with the outside of your wall, using 12-inch (30.5 cm) spikes or Torx drive screws. (If you lined the back of the excavation with landscape or geotextile fabric, you’ll have to cut holes to accommodate the deadmen.) After they’re placed, anchor them with 12-inch (30.5 cm) spikes or Torx drive screws. You’ll need additionally to create trenches every 4 feet (1.2 m) for deadmen perpendicular to the wall, to accommodate them.© John Wiley & Sons, Inc.FIGURE 4-3: Give a low retaining wall deadmen for stability.
5 Install the remaining rows of timbers, again securing with rebar.Finish backfilling behind the wall, including the space between the deadmen.
6 Drill weepholes, about 4 feet (1.2 m) apart.A good location is in the second row of timbers above ground level. Angle the weepholes upward.