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Building Principles: Holding Up the World
“Appearances can be deceiving,” it’s been said. Nowhere is this truer than in the public works infrastructure where there are few, if any, visible moving parts. Recently, we’ve had a spate of calls concerning retaining walls that are not, well, retaining any more. Most often, those failures can be traced to one of a few errors or omissions in the layout or construction. This issue, we’ll look at the forces that are acting on the wall, how they are resisted or lessened, as well as some common materials used in retaining wall construction.
Since our first experience with blocks in the playpen, we’ve all come to understand that things can only be stacked so high before the free-standing stack becomes unstable, wobbles, and collapses. A slightly shorter, but still stable stack tumbles with just a small push at the top. The shorter the stack, the more force it takes to tip the stack over. As the stack gets progressively shorter, the stack no longer tends to fall over, but the pushed block slides over the one beneath it.
In the same way, retaining walls are intended to resist the forces behind them and retain the mass behind them. The forces pushing on the back side of the wall are not as obvious as they may seem. In this simplest case, the mass of the soil being held back attempts to push its way through the wall. If the soil slopes up from the top of the wall, that, too will add load — the steeper the slope, the higher the load. If you add the push of water that collects behind the wall and any structures near the top of the wall (like a building or a road and its vehicles), a large pushing force is generated that seeks to topple or slide the wall apart. Whether the wall lasts a day, a year, or a decade will largely depend on the wall’s height, materials, and construction method.
You can see how quickly things get very complex in this system with no moving parts. This is why most building codes require an engineer’s seal on the design and construction of any retaining walls taller than four feet (some codes limit un-engineered walls to three feet and less). But the principles and logic of good retaining wall planning are the same whether the wall is three feet or forty feet. So now that we’ve looked at the forces working against the wall, let’s consider how we counteract them.
The tendency of the wall to topple (or “the overturning moment”) has to be resisted by a force that tries to push the wall in the opposite direction. In the simplest case (and often in emergencies), the wall is braced from the unloaded side, in an effort to push the soil and the wall back. On some very large retaining wall projects, these braces are cast into the concrete with the rest of the wall, and it’s referred to as “buttressed.” Architecturally, buttress designs can be very attractive, but they need to be built exactly according to the plan in order for everything to work as it should. So it’s not likely that they’ll be part of your next in-house retaining wall project.
Another way to resist this pushing, overturning force is called a “dead man.” These are anchors buried in the retained area of the wall that use the weight of the soil behind the wall to help resist its own load. It operates much like a simple metal bookend where the book itself prevents the end from sliding or flipping. Part of the difficulty comes in knowing how many, how long, where, and how deep the anchors should be to provide stability to the whole wall, as well as how to fasten this to the wall and spread its resistive force across the face. Across the face of wooden walls (treated lumber or railroad ties), you may have seen what looked like the ends of the posts intermingled with the long sides. These were the other ends of the anchors or “dead men,” which were resisting the tendency to overturn. That’s a sign that the designer and/or builder had a good idea of what needed to happen to prevent the wall from tipping.
Probably the single most common cause for simple retaining wall failure is the force exerted by water that collects on the soil side. If you think about it, a dam is just a special case of a retaining wall designed to hold back water instead of soil. To put specific numbers behind this idea, understand that for each foot of depth, the load on the wall increases thirty-two pounds per linear foot. At the base of our 4-foot wall, then, there are an additional 128 pounds of load per foot of wall from water load. If the wall is relatively thin or uninsulated and the water freezes behind the wall, the expansion of the ice will add even more load. Although we could design a structure that would resist these forces, the most economical answer here is to ensure that the water doesn’t collect there in the first place. That’s accomplished by installing freely draining material (normally uniformly sized stone) with a perforated or slotted drain pipe (typically four inches) running the entire length of the wall. The stone should extend not less than eight inches behind the wall, and a filter fabric should separate the stone from the native soil. As an additional safeguard, the pipe can be inside a special fabric sleeve to help prevent fine soil particles from entering and clogging the pipe. The pipe needs to be sloped to drain, or should extend through the wall periodically to provide a place for the water to go. Of course, you can also add dead men or buttresses to resist this water load; however, the economics of this particular work is such that typically, drainage is a more economical solution.
There are several options available from which retaining walls can be successfully built. Some, like a fieldstone masonry structure, require considerable skill to design and build. Others may require some simple tools (powered ones are nice, but not a requirement) and strong help. If we’re still considering in-house projects, we’ll skip the fieldstone option since the camp with a stonemason on staff is rare.
Timbers, Railroad Ties, Wood Products
Often, we see walls made from used railroad ties, because someone got them cheap, and they were square (so they stack nicely!). A little food for thought — the railroad paid big money to pick them up. Either the ties were rotten and wouldn’t support the track (not a good material choice for a structural wall), or they’re soaked in creosote and were leaching this nasty stuff into the groundwater — and the EPA made them remove them (also probably not a good choice). Yes. They’re dark and they look rustic. But they’re messy, probably not sound, and someone else’s environmental mess. Don’t go this way.
Rock-Filled Wire Baskets
Modular Wall Solutions
The modular solution is not a retaining wall panacea, however. The foundation for the first layer (or “course”) of block must be extremely well compacted and carefully leveled. Installation has to proceed one layer at a time, filling the block (to add weight), adding stone as backfill, and brushing the top of each stone clear of sand, soil, and stone, before adding the next course. If the wall is being installed along a slope (say, alongside a road), attaining a level foundation becomes more complex, then the foundation has to be stepped to keep the block courses level.
As a final thought on modular systems, beware of the similar looking, but smaller landscaping block that you may see at the local garden center. These stack much like many modular systems, but rely only on a concrete lip on the bottom (normally in the back) of each block which hooks onto the row beneath it. The horizontal soil loads will shear the lip from the block with only a few feet of soil retained, and the wall will fail. Use these systems only to contain landscaping beds, and not as retaining walls.
Keeping slopes in place can be difficult, complex, and expensive. Certainly no organization has the manpower, time, or money to do things more than once. When considering the stabilization of steep slopes with a retaining wall, carefully consider all the options, the available funds and time, and choose a me