Terracing slopes with timber retaining walls not only solves erosion problems but can also add to the visual appeal of yards and gardens. Because earth and water create tremendous pressures behind a retaining wall, you must make the structure strong and provide for adequate drainage.
The design described on the following pages meets these requirements; moreover, it is easy to build, presents a trim face unmarked by nails or fasteners, and in most localities requires no building permit. This design, however, is not suitable for walls that are more than 3 feet high; they call for both a permit and the services of a structural engineer. If you have a long, steep slope, consider terracing it at intervals with two or more 3-foot walls.
Wooden retaining walls can be made of any timbers that have been treated to resist rot and termites. Railroad ties were once the material of choice, but they have fallen out of favor because the wood is treated with the preservative creosote, which is poisonous to many plants.
Pressure-treated 6-by 6-inch timbers of poplar or pine, either rough- or smooth-sawed, are excellent alternatives. They are treated with environmentally safe preservatives and come in convenient 8-foot lengths, which can be cut as needed with a chain saw (box, next page).
If you build the wall near the bottom of the slope, you will need to add fill dirt behind it. Alternatively, you can excavate higher up, erect the wall against the slope’s face, and cart away the leftover dirt. The first option increases your level yard space above the wall; the second, below it.
Before excavating, establish the locations of possible underground obstacles such as dry wells, septic tanks, and cesspools, and electric, water, and sewer lines.
Be sure the cutting teeth are sharp and the chain is at the proper tension: You should never be able to pull it more than ⅛ inch away from the bar. Steady the timbers on solid supports for sawing, and chalk cutting lines on the timbers as guides. Wear goggles to protect your eyes from flying woodchips. Brace the saw firmly on the ground before starting it, and hold the saw with both hands when cutting. Because pressure-treated lumber contains pesticides, wear a dust mask when sawing it and wash your hands thoroughly afterward.
Anatomy of a retaining wall
The bottom course in this 3-foot retaining wall is set in a trench and anchored by 42-inch-long bars of ⅜-inch reinforcing steel, or rebar. Successive courses are secured with 12-inch galvanized spikes. Several features help the wall withstand the pressure of earth and water behind it. Each course is staggered ½ inch toward the slope. Reinforcing timbers—deadmen run 8 feet back into the hillside and rest on 1-foot-long timber crossplates anchored with 42-inch spikes. Sidewalls are built up on the corner deadmen and connected to the wall by interlocked corners. Four-inch perforated drain tile buried in gravel and 1-inch gaps between adjacent timbers in the second course provide escape routes for the water behind the wall.
Marking the wall trench
Drive 5-foot stakes at the points you have chosen for the corners of the wall. Tie a line between the stakes, and level it with a line level. Measure to find the point where the line is farthest from ground level (left). This is the lowest grade point; mark it with a stake. Drop a plumb line every 4 feet along the line, and drive stakes at these points to mark the outer edge of the wall. Transfer the line from the 5-foot stakes to the lower stakes.
Digging the trench
Starting at the lowest grade point and working out to the corner stakes, dig a level-bottomed trench that is 1 foot deep at the lowest grade point and 1 foot wide along its full length. Spread a 6-inch layer of gravel in the trench and tamp it down. Check the base of the trench with a carpenter’s level. Remove the stakes. Lay the timbers for the first course in the trench. Their tops should be even with ground level at the lowest grade point. To lay out deadman trenches, start at one corner and stretch an 8-foot line at right angles to the wall; drive a stake at the end of the line. Repeat at the other corner and at 6-foot intervals in between.
Securing the first two courses
At the center of each timber and 6 inches from each end, drill vertical holes completely through, using the ⅜-inch bit. Then drive 42-inch spikes through the holes and into the ground with a sledgehammer (right). Lay the second course so that the joints between timbers do not coincide with those of the first. Set the second course ½ inch closer to the hillside, and leave 1-inch gaps between timbers to serve as drainage holes. Drill three holes through each timber, and drive 12-inch spikes through the holes to pin the first two courses together.
Bracing the wall
For the deadmen, dig trenches—their bottoms level with the top of the second course—back to the stakes. Across the ends of the deadman trenches, dig crossplate trenches 3 feet long and 6 inches deeper than the deadman trenches at that point. Lay the crossplates in place, then set the deadmen on top of them so their other ends rest on the second course, ½ inch back from the front face. Drill pilot holes and drive 42-inch spikes through the deadmen and the cross-plates and into the ground. Drive 12-inch spikes through the deadmen into the second course. For the third course of the wall, cut timbers to fit between the deadmen—making sure that the joints don’t align with those of the second course—and secure them with spikes.
Laying a drainage run
On the back of the wall, nail pieces of galvanized screen over the drainage gaps in the second course of timbers. Shovel a bed of gravel behind the wall, leaving enough space to run a length of 4-inch perforated drain tile along the top of the bed and under the deadmen (above). Then add another 6 inches of gravel.
Interlocking the corners
After completing the fourth course of timbers, lay a sidewall timber at each end of the wall and secure it to the corner deadman with 12-inch spikes. Lay another sidewall timber atop the first so that its end is set back ½ inch from the face of the fourth course. Secure it with spikes. Fit the timbers for the fifth course between the sidewall timbers, fastening them with spikes. Continue laying the front and sidewall courses in this manner, making sure to offset the timber joints between courses and stagger each course ½ inch closer to the hillside. Then drill horizontal holes through the corner deadmen and those sidewall courses that extend to the front of the wall, and drive 12-inch spikes to secure the corners (left). Spread a 4-inch layer of soil behind the wall and tamp it with a hand- or gas-powered tamper. Spread and tamp additional 4-inch layers until the fill is level with the top of the wall.