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.
Concrete block retaining wall
Manufacturers have created decorative concrete blocks that are specifically designed for dry-stacked retaining walls — that is, you don’t need mortar. Modular in dimension, they’re easier to work with than stone. Sometimes they are called Allan blocks, referring to a specific manufacturer.
Most blocks are designed to be battered, and many have predrilled holes for inserting stabilizing pins or rebar to lock them together. When you follow the manufacturer’s directions to the letter, this project is straightforward!
Stacked-stone retaining wall
A stacked-stone retaining wall should have a backward slant (batter) of 2 to 3 inches (5 to 7.6 cm) for dry-set stones and 1 to 2 inches (2.5 to 5 cm) for mortared stones. This slant helps your wall counteract gravity. If you prefer to use mortar, see Chapter 5 for full details.
Building a 2- or 3-foot (.6 to .9 m) high wall of dry-stacked (no mortar) stones requires considerable lifting and adjusting of stones, but it’s enjoyable work if you aren’t in a hurry. It pays off with a handsome wall. Follow these steps, and consult Figure 4-4:
1 Dig into the hill, about 15 to 18 inches (38.1 to 45.7 cm).
2 Dig an ample base.We recommend a trench 2 feet (.6 m) deep and wide enough for the broadest stones plus a 4-inch (10.1 cm) drainpipe.
3 Place gravel in the trench and compact it, 4 inches (10.1 cm) deep.Lay the drainpipe along the back corner, sloping it toward the outlet end. Then add another 12 inches (30.5 cm) of stones to the trench.
4 Using the largest stones, set the first course in place.Tilt the stones back slightly, toward the bank. Check frequently that the top of the wall is level. When it isn’t level, make adjustments.
5 Drape landscape or geotextile fabric against the inside back wall, and backfill with gravel or drain rock as you lay each course.
6 Build up the wall with stones.As you lay each course, lean the wall into the hill at a rate of 2 to 3 inches per foot (5 to 7.6 cm per .3 m) of wall height. A 3-foot (.9 m) wall, for example, must lean 6 to 9 inches (15.2 to 22.8 cm) into the hill.
7 Within 12 inches (30.5 cm) of the top of the wall, fold the filter fabric over the stones and backfill the rest of the wall with topsoil.If you want, install a capstone.
© John Wiley & Sons, Inc.
FIGURE 4-4: Building a stacked concrete or stone wall, without mortar, requires careful placement of the stones.
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