From time to time, most builders encounter the need to hold back soil. Whether you have to terrace a sloped yard, support a parking lot, or level an area for a pond and waterfall, a retaining wall is the tool for the job. Even to simply level a building site, you may have to cut fill out and support the slope that remains, or fill in an area and hold the fill in place.
In the past, the builder's choices were a timber wall, a dry-laid stone wall, or an engineered concrete retaining wall (see "Retaining Wall Choices," below). But these days, many contractors prefer to use segmental block retaining walls. Every material has its advantages and disadvantages, of course. But for many jobs, segmental block walls combine most of the advantages of a timber or stone retaining wall, without most of the drawbacks.
Retaining Wall ChoicesDry-laid stone will always be my favorite material for retaining walls. I love the beauty of it, and you can't beat it for durability: Properly built, stone walls can last forever. We build our stone walls 18 to 24 inches thick with hand-cut stone, and stack them carefully to be self-supporting. Stone is very heavy, which helps it hold back the weight of soil. But it does have structural limitations; I've built a few stone walls 8 feet high, but we typically don't go higher than 4 feet. And stone's cost is a drawback. The stone itself is quite economical: Anywhere in the country you can find a stone yard or quarry that has native stone for a few dollars per ton. I pay $10 a ton in my area for local native limestone; special stone that we truck in from Pennsylvania runs us about $100 a ton. A ton gives us 20 face square feet, so stone walls cost 50¢ to $5 per face square foot just for the material. But the hand labor of cutting and stacking the heavy stone pushes the cost up to $40 a square foot and higher. Preservative-treated timber walls are much more economical. And wood is much lighter than stone or block, which is especially helpful on sites with difficult access. If you have to walk up a backyard slope to your wall location, you can carry timbers in on your shoulder: An 8-foot pressure-treated 6x6 weighs less than one concrete retaining wall block. Timber walls are also good if you want a certain look. On houses with vertical cedar siding, we sometimes build vertical timber retaining walls and face them with vertical cedar for a matching architectural appearance. The problem with timbers is durability. I've built literally miles of timber retaining walls. Some have lasted 20 years or more; others have begun to rot after 3 or 4 years. For whatever reason, the treatment effectiveness seems to vary. It's a good product for a customer who wants something economical and doesn't necessarily want a lifetime solution. Timber walls also have some structural limits. I've built them up to 10 feet high, but that was back in the days before segmental block. For slopes that high, you really have to ask whether you want to use a material you can't be sure will last five years. These days, I put in only a couple of timber jobs a year, typically for a low wall or planter box. Segmental block wall systems are quite durable, and they definitely take the prize for structural capability. They're designed to automatically "batter" back into the hillside as we stack them up, and successive courses either get pinned together or have shapes that interlock. Backed up by loads of compacted granular fill layered with reinforcing geogrid, these systems can do the work of a concrete and rebar structure at lower cost. Engineers are speccing segmental wall systems for bridge abutments, highway overpasses, and slopes 40 feet high and higher. It's hard to imagine a residential requirement that these systems couldn't satisfy. And while concrete walls need footings that rest below the frost line, segmental walls can flex to handle moderate amounts of freeze-thaw soil movement without damage, so we can set them in just a 1-foot or 2-foot trench. Cost for segmental block varies depending on location and manufacturer. When all the backfill, geogrid, and labor are included, a segmental block wall typically costs a little less than stone but quite a bit more than timber. Appearance can be a limitation — block walls aren't suited to every architectural need. But as new tumbled, profiled blocks are coming out every year, our ability to be creative continues to expand, and segmental systems have become quite popular with customers. The systems are versatile and allow a lot of flexibility in structural design. For the money, you don't get the natural beauty of stone, but you do get a variety of choices in style along with reliable engineered strength. Sizing Up the SiteAs with any retaining wall, geogrid and block systems have to be individually designed for every site. That means carefully assessing the ground the wall will rest on, the soil it has to hold back, the wall's necessary height, and any unusual loads before making any design decisions (see Figure 1). Once we know all these factors, we can make decisions about block type, base preparation, fill material, and reinforcement.
Figure 1.Block suppliers routinely provide engineering for large walls, but the engineering is only as good as the soil information. Critical factors include compaction (subsoils, gravel base, and reinforced fill must all be well compacted); geogrid choice and placement (grid of the specified strength and length must be placed at the specified locations); and drainage (surface runoff should be led away from the wall mass with an impermeable clay cap, and groundwater must be drained away with drain tile protected by filter fabric). If any doubt exists, the soils engineer responsible for the site should supervise the wall design. Soil conditions. Whether it's a new home or an existing house, I'm always careful to determine the site soil conditions in advance (Figure 2). I visit the site several times before my crew starts work, and if it's a new building site, I ask the site contractor not to fill the area where we'll be putting our wall or steps. Taking control of that location, and responsibility for it, improves my odds of success.
Figure 2.The author always checks site soils carefully to identify uncompacted fill or problem soils. The upper left photo shows fill on the site of an existing 40-year-old home. The material is a silty sand that drains well. The author excavated down to original virgin soil for his wall base and placed filter fabric over the bank to keep fines out of the drain system (top right). The photo on the bottom shows a nearly vertical cut bank of native clay soils where contractor Jason Sweeney excavated the hillside to make room for a pool. Drainage is particularly important behind this wall, because the clay becomes soft and heavy when wet.
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