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Project Difficulty:
Moderate
Estimated
Project Time: 2 days for a 20-foot-long wall
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Stones
Gravel
Sand
Mason's twine
Shovel
Heavy work gloves
Wood stakes
Landscape fabric
Brick hammer
Point and pitching chisels
4' spirit level
Line level
Plantings (optional)
Mortar (optional)
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A dry-stacked stone retaining wall relies on gravity, friction, and the interlock of the stones to resist the overturning motion of an earth embankment. It consists of two "wythes" or vertical stacks.
A well-built, dry-stacked retaining wall is superior to a mortared retaining wall because dry stacking leaves spaces for ground water to drain through. However, the stones may require considerable cutting and shaping to create the interlocking fits that are key to a good dry-stacked wall. Easily workable stone like sandstone or limestone rubble will usually be best.
Dry-stacked stone retaining walls up to 3 feet high do not require a concrete footing. They may be laid directly onto the soil in a 6-inch-deep trench. For stability, the inside of the wall must tilt toward the embankment at least 2 inches for every foot of wall height. If you don't mind looking at a sloped outer face, you can simply tilt the entire wall back. If you prefer a plumb outer face, make the wall gradually thicker as you build it up. Use gravel as a backfill behind the wall and use longer, bond stones as anchors for the wall. (Fig. A)
Rather than build a retaining wall more than 3 feet high, build sections of shorter, terraced walls. In addition to requiring a concrete footing, retaining walls more than 3 feet high get tricky to build, and in many municipalities you'll need a building permit if the wall is more than 4 feet tall. Often, a better alternative to a single high wall is to build two shorter walls stepped back against the slope. At its base, a 3-foot-high wall should be 18 inches thick. (Fig. B)
Building the Wall
While handling the stones, wear heavy work gloves to protect your hands. If you need to cut or shape the stones, wear safety goggles to protect your eyes.
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1.
Excavate the Trench
Cut the angle of the embankment back 30 to 45 degrees from plumb to give you room to place the stones and the gravel for drainage behind the stones. Loose or sandy soils tend to collapse, so cut the angle more sharply. In soil that drains well, excavate a 6-inch-deep by 18-inch-wide trench along the length of the wall. Remove all grass, sod, roots, and large rocks. The first course of stones can be laid directly in the excavation. (Fig. 1)
In dense or clay soils or in areas that do not drain well, excavate 10 or 12 inches deep and add a 4- to 6-inch-deep gravel or crushed-stone drainage bed. Level the drainage bed with a rake and tamp the gravel or stone to compact it. Then lay down a layer of landscape fabric, which will allow moisture and air to penetrate but will prevent the soil from settling into the gravel. Overlap adjoining sections of fabric by 4 inches. Place a 2-inch bed of sand in the trench for leveling the stones.
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2.
Set a String Line
A level base course is important to the stability of the wall. Drive wood stakes just beyond the ends of the excavation near the front face of the wall. Tie mason's twine between the stakes. Hang a line level on the twine and adjust the twine until it is level. (Fig. 2)
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3.
Lay the First Course of Stones
Starting at one end of the wall, carefully fit each stone, seating it firmly in the soil or sand bed and following the twine for level. Use your largest stones for the first course, not only to create a good base but to avoid having to lift and adjust these heavy pieces at higher levels. Dig out under the stones or fill in spaces with soil, if necessary, to get the stones to sit firmly without wobbling. Pack the spaces between stones in this first course with soil to give the wall a stable base. (Fig. 3)
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4.
Set the Next Courses
Set the next few courses of stone on top of the first, keeping in mind that you want the inside of the wall to slant back at least 2 inches for every foot of height. As mentioned, the front of the wall can be plumb or it can follow the slope.
Lay stones in successive courses so that they overlap the stones above and below. Avoid creating continuous, straight vertical joints. The overlapping pattern will produce a stronger wall, giving it the stability it needs to resist pressure from the soil.
Install long stones that stick back into the backfill about every 4 feet horizontally in each course. These are called bond stones or deadmen, and they help tie the wall into the hillside. Offset these bond stones in each course. (Fig. 4)
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5.
Add Drainage Backfill
After the first few courses of stone are laid, begin adding gravel as drainage backfill behind the wall. Try to keep the gravel layer at least 6 inches thick. (Fig. 5)
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6.
Add the Remaining Courses
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Set the remaining courses of stone in the same manner. Remember to let each course of the inside wythe jut an inch or so farther toward the embankment so that the inside face will slant about 2 inches per foot.
When building up the wall, carefully select each stone for the best fit and check its fit as you lay it. If a stone wobbles on a point or sharp corner, use a brick hammer and a point or pitching chisel to shape it so that it will sit more securely. You also can use small pieces of stone as shims to make the stones fit more tightly together; although for the best-looking wall, you'll want to minimize shims. If you do use shims, insert them from the outside of the wall (Fig. 6).
All the stones should be slightly inclined toward the soil embankment so that the weight of the wall leans into the hill. While stones should fit snugly together in a retaining wall, don't worry about filling every little void between stones. You want to leave plenty of natural "weepholes" to allow groundwater to pass through.
Check your work periodically using a 4-foot-long spirit level to keep each course approximately level. Continue backfilling with gravel every two or three courses.)
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7.
Cap the Top of the Wall
You have some design options here. One approach is to stop building 6 or 8 inches before you get to the top of the slope. Pack dirt between the top course of stones, and then cover the wall with soil to bring it up to the top grade. Then you can plant a ground cover above the wall. The plant's root network will help prevent erosion and help hold the wall together. This approach looks great in a casual country garden. (Fig. 7)
Another approach is to leave the top of the wall exposed as you would for any freestanding stone wall. You can mortar the stones of the top course in place or cap the top course with mortar.
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