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Some
erosion control solutions are meant to last.
By
Roberta Baxter
Retaining
walls are everywhere you look, and more are installed
every day. In the past, most were cast-in-place concrete
or timber walls, but in recent years, the segmental
retaining walls (SRWs) seem to dominate the market.
Applications can range from huge highway overpasses
to a short wall in a backyard.
Advantages
of the SRWs over concrete are cheaper cost, the plethora
of choices for color and texture, ease of installation,
shorter installation time, and unsurpassed strength.
Innovations are continuing in the SRW market in both
the wall block and the supporting geogrid.
Components
of an SRW are the blocks, connectors, and reinforcing
geogrid. The use of these components depends on the
height and design load of the wall. Shorter walls might
not require geogrid reinforcement, and some block designs
do not need connectors. Blocks are made of concrete,
either in a dry process or a wet cast. Design standards
for SRWs have been published by the National Concrete
and Masonry Association. Many companies also provide
design and engineering services.
Most dealers
of SRWs around the United States and Canada are licensees
of the block technology. Having local dealers cuts down
on shipping costs and time.
California
In-Place and Green
In California,
land with an ocean view can sell for $250-$500 or more
a square foot. Installing a retaining wall often can
make an otherwise unusable portion of a lot suitable
for building for much less than that.
Much of the
California construction is residential. Vinci Pacific
of Del Mar, CA, is a grading and excavation contractor.
Sean Christiansen, estimator/project manager, describes
a project in which a canyon was filled in and walled
to provide land for six new houses. The block used was
a buff-colored Verdura 40 from Soil Retention Products,
with 30 ft. of geogrid placed every 3 ft. "They are
time- and cost-effective," he says. Another advantage
is the ability to build wall and fill at the same time.
Handplacing the blocks and backfilling took about two
months, and Christiansen notes that the construction
was a fairly smooth process. Houses now are being built
on the site, and some are occupied.
Although
zoning ordinances in many municipalities allow retaining
walls to be no more than 6 ft. high, variances sometimes
are granted if a wall is plantable. In this case the
Verdura walls were planted by another contractor. A
plantable wall can be completely vegetated within a
year, adding to the aesthetic appeal of the wall.
"Retaining
walls are making money for builders," remarks Dean Sandri
of Soil Retention Products in Carlsbad, CA. "It's not
a bad investment." Sister company Soil Retention Systems
has installed more than 3 million ft.2 of
retaining walls since 1987; according to Sandri, Verdura
blocks are sold to both do-it-yourself markets, such
as Home Depot, and large construction and development
companies.
Christiansen
recalls a multiwall system for a landowner. The valley
property was plagued by a water table at about 10 ft.
deep and an insignificant view. Vinci Pacific contractors
built a hill for the home with about 30,000 yd.3
of dirt fill. The lowest retaining wall, built with
Verdura 30 and geogrid reinforcement, was 30 ft. high;
two other walls, 10 ft. and 5 ft. tall, were built at
higher elevations. The total retaining wall space was
15,000 ft.2 The landowner now has a view
of a golf course.
"The reality
is that most of the land left is hillside and requires
retaining walls," Christiansen states. The 2-in. lip
on the Verdura block does require a finished slope of
about 1:4.
The blocks,
which come in four basic colors and custom colors, are
molded and compacted by vibration and then are allowed
to set for a day. The interlock is a 4,000-lb. shear
lip. Because of the California location, Soil Retention
products have been tested for up to 30-ft./sec. water
flow and seismic events. The highest single walls are
50 ft., with tiered walls reaching up to 80 ft. tall.
Overflow
in Kansas
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| Craig
Crossing Park in Lenexa, KS |
Craig Crossing
Park in Lenexa, KS, contains a retention pond that experiences
periodic overflow from the creek. Mowing to the edge
of the pond was difficult, and the city wanted a natural-looking
wall built to stabilize the pond area. The area is a
favorite recreational site with hiking and biking trails
nearby.
The original
design designated natural stacked limestone. Ben Manthei,
managing partner of Redi-Rock headquartered in Charlevoix,
MI, states that one reason Redi-Rock walls ultimately
were chosen for the project was their design and coloration.
Courses of block were laid, with the final course set
back about a foot, allowing animals and people to easily
step out of the pond. By staining the block a cream
color and applying a texture to the face and top of
the blocks, the Redi-Rock was made to resemble natural
limestone.
"The city
liked it because it stacked straighter and looked like
real stone," explains Gerald Heckman of St. Joe Concrete
Products in St. Joseph, MO, the supplier of the blocks.
The cost of Redi-Rock was comparable to natural stone.
The blocks
are produced with a wet-cast process and stain is applied
in the color chosen by the customer. The texture came
from special forms. Each block weighs 2,200 lb., making
them among the largest blocks in the industry. Manthei
says, "The integrity of a wall system is based on the
size of the rock." The size of the blocks also decreases
the amount of site preparation. If the foundation is
good compacted soil, the site is ready for block placement.
For Craig Crossing Park, the area was marshy, so a layer
of 1-in. stone was spread under the blocks. Because
geogrid is not required, less property is required for
tiebacks and walls can be set close to property lines.
The Craig
Crossing Park project covered 2,400 ft.2
and incorporated 417 blocks. The area has been flooded
and drained several times with no change to the integrity
of the wall. Ron Tebbenkamp of Dennis Johnson Construction,
contractor for the project, recalls, "It went exceptionally
well
Heckman notes
a growing interest in this type of wall construction,
citing recent inquiries from cities and counties. One
advantage is the lack of setbacks for a retaining wall.
One project was a retaining wall next to a cemetery,
where land could not be disturbed. Other inquiries have
come from agencies needing to reline ditches and wetland
areas.
Blending
In
When homeowner
Mary Coleman bought an old estate perched in the hills
near San Jose, CA, she had the house that was built
into the hillside mostly torn down and rebuilt into
a 7,700-ft.2 residence. Although the yard
had been nicely landscaped, including retaining walls,
when the original house first was built 30 years ago,
crews used the tennis court for parking during reconstruction,
and because the fence was down, deer ate much of the
old landscaping. Coleman hired Lisa Costello of Two
Hands Unlimited in Watsonville, CA, to rebuild her landscape,
including adding new drainage areas and a new retaining
wall.
Costello,
who holds both engineering and landscaping licenses,
chose AB Stone by Allan Block of Edina, MN. Costello
was certified as a Master Wallbuilder by Allan Block
through a course she attended. She notes that the company
also provides free engineering services, and many clients
like that despite her engineering expertise: "It gives
them a second opinion."
For Coleman's
yard, Costello needed to replace a retaining wall against
a vertical cut. The footing was 24 x 24 in. filled with
0.75-in. drain stone. A geogrid was placed up about
3 ft. Both measures were "overkill," according to Costello,
because the blocks would have withstood the load alone.
The block used was the AB Stone, handplaced with a 12°
setback. The finished wall is about 5 ft. high x 100
ft. long.
"I'm quite
happy with it," Coleman says of the look of the finished
wall, noting that the color and texture of the block
blend well with her house and the area. "No issues at
all."
Tall in
Atlanta, GA
A recent
project in the Atlanta area produced the tallest SRW
in North America. David Pitre, engineering representative
for Dallas, TX - based Pavestone Company, explains that
two land bridges were built to provide access from a
church to a parking lot. One of the structures required
a retaining wall 62 ft. high.
The area
bordered a wetlands area, so disturbance and fill had
to be kept to a minimum. The product chosen was Anchor
Landmark, an SRW design with a positive connection system.
A polymeric locking bar and the channel and locking
flange provide the load capability.
Pitre notes
that a common use for SRWs in the Atlanta area is covering
old timber retaining walls. Many of these timber walls
are at least 10-15 years old and deteriorating; SRWs
can be placed over the timber with minimal work and
time.
Commercial
development in Atlanta and Denver has made these cities
prime sites for SRWs, according to Pitre. "It's a huge
and growing market."
Biking
in Wisconsin
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| An
SRW used in the town of Mequon, WI |
The town
of Mequon, WI, wanted a 6-mi. section of bike trail,
including the five bridges that were necessary in some
segments of it, to reflect the community's rural heritage.
Great Lakes Marine Contracting usually installs poured-in-place
concrete bridges, but for aesthetic purposes and at
the request of a citizen committee, the company decided
to use SRWs as bridge abutments for this project - in
this case, Mesa Retaining Wall Systems from Atlanta-based
Tensar Earth Technologies. "The main point for us here
was the connection strength between the blocks and the
geogrid," explains Aron Jarr, product manager for Mesa
Retaining Walls at Van Der Vaart Inc., the supplier.
The block
chosen was the natural-color Mesa standard unit, 8 in.
high x 18 in. wide x 11 in. deep nominally, and Mesa
3 and Mesa 4 geogrid. Materials were required to meet
Wisconsin Department of Transportation standards. The
blocks were handplaced by a two-man crew from Great
Lakes Marine Contracting, and five rustic-looking wooden
bridges were built on top of them. The walls are 10
ft. high, and about 3,600 ft.2 of block were
used. "The application was pretty straightforward,"
Jarr recalls. "No problems." John Huck, City of Mequon
engineering supervisor and project manager, remarks,
"The bridges look fantastic!"
Historic
Minnesota
A historic
project in Minnesota required special handling. The
Minneapolis Park and Recreation Board had acquired Mill
Ruins Park, formerly the site of a flour mill. The site
is on the National Register for Historical Places, and
special care was needed for the restoration. A tailrace
from the old mill required a retaining wall, and the
board wanted it to have the look of old stone to fit
the historical atmosphere. Versa-Lok Weathered Standard
Retaining Wall System was chosen because of the look
of natural stone.
The contractor,
Martin Lake Contracting Inc. of Stacy, MN, faced some
challenges with this project. After starting a leveling
pad of crushed gravel and sand, the crew realized that
the nearby Mississippi River and adjacent lock and dam
were causing flooding in the work area. Riprap was placed
in the tailrace bed to prevent further erosion. A soil-separation
fabric was installed with extra gravel to avoid washout
and provide a good base for the first course of block.
Subsequent courses were built up and interlocked with
nylon/fiberglass pins. "The pinning system gives us
more flexibility in keeping the wall straight," explains
Brad Koecher of Martin Lake Contracting. Versa-Lok's
solid blocks also mean there is no need to fill blocks,
saving time.
Other Versa-Lok
walls were constructed in the area, include a parking-lot
wall and a river inlet. The look of the block has added
to the historical feel of the project.
Canada
Stones
A project
for Chrysler Canada in Brampton, ON, required curving,
high-visibility walls. A new access road was required
to replace a slope and berm. Because the site would
be visible to the public, Chrysler wanted an attractive
wall. In addition, a stormwater management area at the
foot of the wall had to remain in place.
Bob Morawczynski
of Regent Landscaping, the contractor on the project,
used Risi Stone Systems's Pisa 2 stone, together with
Mirafi 2 geogrid, to form the curved walls; for similar
projects Morawczynski uses Stratagrid with Pisa 2 stone
to provide additional support when connecting curving
and straight walls. A total of 6,000 ft.2
of retaining wall was installed. A crew of two compacted
2,000 yd.3 of dirt behind the wall and added
a drainage zone to prevent the buildup of hydrostatic
pressure. The entire project took the crew nine days
to complete in September 1994. "It was quite an interesting
wall because of the curves," comments Morawczynski.
"Beautiful!" He adds that, after almost 10 years' time,
the wall still is in perfect condition.
Allison Uher
of Risi Stone Systems in Thornhill, ON, explains that
the solid block doesn't have to be filled, making the
system stronger and speeding up installation. The integrated
tongue-and-groove design provides self-sloping placement
and strength. Risi also provides design services.
Risi Stone
Systems are available in a variety of sizes from 18
lb. to more than 1,700 lb. "Good for a large range of
applications," relates Uher.
Frequent
contributor Roberta Baxter specializes in science and
technology topics.
EC
- November/December 2003
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