by Rick Stryker
“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.
Soil Load
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.
Water Load
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.
Building Options
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.
Cast-in-place Concrete
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.
Originally published in the 2004 November/December
issue of Camping Magazine.
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