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.
Dry-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.
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Sizing Up the Site
As 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.