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If you're thinking about building a landscape retaining
wall, there's something new out there that can make your installation
more reliable. It's generically called ''geogrid'' -- a synthetic textile
mesh that stabilizes soil, boosting the success rate of all but the smallest
retaining walls. Although you won't yet find geogrid products at retail-level
outlets, it's worth pursuing from industrial suppliers. Even if you're
hiring someone else to build your retaining wall, don't assume that geogrids
will always be installed where they could provide benefit. If you run
into a landscape contractor who doesn't know what geogrids are, pick up
the phone and find another.
Traditionally, the only thing that kept retaining walls
standing was the mass of the wall itself, and the degree that the wall
sloped into the soil being retained. Although these two factors can sometimes
be enough to endow a retaining wall with a long and non-eventful life,
they're often not sufficient. That's why it's so common to see retaining
walls losing the battle of the bulge; leaning, breaking and flopping over
as a result. Why risk this outcome when you don't have to?
In the early 1980s, geogrid technology made its
way across the Atlantic from England. The photo of the installation-in-progress
shows how it's incorporated into the backfill held behind a retaining
wall, and woven into the masonry units making up the structure. This endows
the soil mass with enough internal strength to resist the forces of gravity
pulling it downhill. Geogrid does for soil what steel reinforcing rod
does for concrete. It adds tensile strength to a material that would otherwise
only have significant strength in compression. It's been used to support
retaining walls and steep slopes along highways and on numerous rail embankments.
Depending on the situation, geogrid is installed in horizontal
layers, with varying spaces and embedment lengths, depending on conditions.
It's presence takes sideways pressure off the wall by transforming the
soil into a structural part of the system, not just a load to be held
back. In fact, it's even possible to build a masonry-free retaining wall,
using only geogrids to stabilize the soil, while vegetation-covered landscape
fabric
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forms the visible face. Imagine a flat, green, steeply
sloping face and you'll get the picture.
Regardless of design, geogrids are easy to apply, invisible
in the finished installation, and an inexpensive way to ensure long-lasting,
trouble-free retaining wall performance. Cost is about 0.50 cents per
square foot for residential-grade versions.
Geogrids look enough like plastic snow fence that some
people try to save money by substituting, though that's bad idea. All
brands of geogrid are engineered products, both in terms of manufacture
and installation. Take a look at the details of the weave shown in the
close-up and you'll get a sense of what's going on. Most types are designed
to minimize the stretch-factor of the product along its bearing axis (the
woven strands), but with little lateral strength because none is required.
This is called uniaxial geogrid. The slightly curved fibers that extend
from left to right are just for connecting to the wider, woven strips
-- the ones that actually do the work. Stiff geogrids that are strong
in all directions -- called biaxial -- can be used to strengthen road
and driveway surfaces, too, preventing tire track rutting. Permanent roads
have even been built over peat bogs using stiff, biaxial geogrids
You can learn more about geogrids by typing the word into
any Internet search engine or by contacting these companies. All offer
products along with the design advice you'll need to apply them correctly.
Nilex Inc.
http://www.nilex.com
Risi Stone
http://www.risistone.com
Strata Systems Inc.
http://www.geogrid.com
Tensar Earth Technologies, Inc
http://www.tensarcorp.com
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