Holding up the world: building principles.
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
Far and away, this is the most common arrangement that we see, and equally often it's the type that is newest and in failure. With wood, there is the additional consideration of warping with changes in the moisture content of the wood. As discussed in previous articles, wood treated with copper salts does the best job at resisting absorption of water from the environment, and generally speaking, the higher the copper content, the higher its resistance to absorption. However, the more copper, the more green-blue it looks and the more it costs, two factors which may technically make a suitable product, but the least appealing from an owner's point of view. Even well-constructed timber walls do not last as long as other materials.
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
These are also called "gabions." After a leveling and anchoring layer is prepared as a foundation, the baskets are opened up, filled with uniformly sized rock, and then wired shut. The next layer is wired to the layer below it, and the process is repeated. Installation is pretty quick, relatively speaking, and although you need to put a filter fabric behind the wall to prevent soil from working into the stone baskets, a drain isn't necessary since there is plenty of space to let water through. This may be a super solution for many applications, and their use is highly subjective. We've had clients who loved them--and others who rejected the idea altogether. Trumpet vine, honeysuckle, (and poison ivy) all can grow on the face without damaging the structure, and this goes a long way to hide the fence material that makes the basket. This arrangement also has some limitations that should be considered. The galvanized wire basket can corrode if your native soils or runoff has a low pH ("acid"). Obviously, if the wires rust through, the rocks will spill out, and the wall has failed. Also, the delivered cost of uniformly sized rock to fill the baskets, and the machinery and labor to fill, wire, and stack the baskets may make this solution more expensive than others, particularly for low, short walls.
We've discussed in earlier articles some of the considerations required when working with Portland cement concrete. (See the January/February 2004 Camping Magazine, "Building Principles.") We know that steel has to be placed in certain locations within the concrete matrix to compensate for concrete's low strength in tension. Combine that with our new understanding of the complex loads which seek to demolish our wall, and you can see that a designer needs to reconcile the loads with the size and placement of the steel to ensure a lasting project. The contractor might tell you that he's done lots "like this," and he may well have. A properly designed and constructed concrete retaining wall should last many years, and his standard warranty is likely only for one. However, be sure that you get an extended warranty (like ten years) on the work. After you ask for that, he'll probably hedge, and you'll see the merit in complete design and construction assistance from your engineer.
Modular Wall Solutions
You may have seen several different combinations of these systems. They go by trade names of Keystone, Redi-Rock, Pavestone, and Versa-Lock among many, many others. Let's use the Keystone product for our discussion. The components are designed to resist the forces that we were discussing previously, providing the compressive strength of concrete, in sizes that can be handled by manual labor crews, without masons to work concrete or ironworkers to set steel. The individual blocks resist sliding by a combination of friction between layers, super-high-strength fiberglass pegs, and--for higher walls--by dead men in the form of buried grids. They are not held together by mortar but by gravity alone, so water seeps through the joints, and hydrostatic pressure does not build. They can be placed in straight lines or broad curves, and can be set almost vertically, or with an offset to reduce the visual impact. Stairs can be built in to the design using similar methods and matched materials. With only minimal equipment and the simple instruction manual, most can be constructed with a cadre of dedicated (but strong) folk.
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 method that will serve camp best over the long haul.
Diagrams courtesy of Keystone Retaining Wall Systems, Inc.
Rick Stryker is a professional engineer with Camp Facilities Consulting, providing study, design, permitting, and construction consultation services to the camp and conference center community. Camp personnel may contact him at 570-296-2765 or by e-mail at email@example.com.
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|Title Annotation:||retaining walls|
|Date:||Sep 1, 2004|
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