Waterfront
areas are treasured for their beauty, diversity of wildlife and plant
species, and abundance of both commercial and recreational opportunities.
The downside, of course, is the potential for erosion, flooding and
storm damage, which often requires man-made protection to shield fragile
environments and ensure personal safety and property protection. Articulated
concrete block systems (ACBs) are emerging as a vital part of this
protection-providing an economical, durable, environmentally sound
soil stabilization system.
ACBs
are a flexible revetment system that provide effective erosion control
and can also include plantings to maintain a natural appearance. Articulated
concrete blocks are effective and economical for a wide range of erosion
problems, and are easily installed above or below the water line,
as either cabled or noncabled systems.
Love's
Park Creek
In Love's Park, Illinois, city officials and residents sought to control
high water flows and sediment deposits in Love's Park Creek. The open
channel drainage and storm water conveyance system sees everything
from a trickle to high velocity deep flows. The highly erosive creek
meanders through industrial and residential areas, threatening residential
property and municipal playgrounds. Flows carry large amounts of sediment,
which deposit in the creek bed and diminishes the channel's water
capacity, causing flooding.
Previous
solutions included periodic sediment excavation, which was hampered
by the lack of hard surface floor to support the skidsters and small
end loaders used to excavate. In addition, portions of the bank were
stabilized using poured-in-place concrete. Although this helped the
excavation, residents were unhappy with the aesthetics, and long-term
performance of the concrete was compromised by slab heaving and cracking
from freeze-thaw exposure and water pressure from within the banks.
A new approach was required to control erosion, provide natural beauty
and resist freeze-thaw damage. Articulated concrete blocks met the
challenge.
For
Love's Park Creek, the U.S. Army Corp of Engineers (USACE) chose a
noncabled ACB system with cores to support vegetation. According to
Paul A. Kowalczyk, P.E., Assistant Chief of the Design Branch for
the USACE, "Other systems considered were riprap (not very compatable
with a residential area), concrete (not as aesthetically appealing),
and grass lined channel (not durable enough). The riprap was not as
attractive to the city because of the appearance, difficulty of maintaining
it, and safety issues because of the project's location within residential
areas." ACBs supply a hard stable surface to support excavation
equipment, facilitating stream maintenance. ACB revetments are also
resistant to freeze-thaw damage and allow release of water pressure
within the stream banks.
Unfortunately,
potential liability must also enter into the decision-making process
for most municipalities. Public accessibility of waterfront areas
demands a stable and smooth revetment. For the children of Love's
Park, who inevitably play and climb along the creek banks, the ACBs
offer a surface that will not shift and whose smooth surface is less
likely to catch or twist small ankles.
Over
274,000 ft2 (25,455 m2) of the interlocking units were placed at installation
rates up to 75 ft2 (7 m2) per man hour. Overall, choosing ACBs over
other conventional systems saved the City of Love's Park thousands
of dollars. Homeowners along Love's Park Creek appreciate the natural
beauty and increased safety. The vegetation, which eventually covered
the stream banks, visually softens the banks while also helping support
the local habitat. According to Kowalczyk, "in general, our sponsor
(the City of Love's Park) has been pleased with the aesthetics (green
slopes), performance (high flows have not damaged the channel) and
maintainability of the project."
Shoreline Protection
in the Florida Keys
The Florida Keys are an environmentally sensitive area, renowned for
natural beauty and pristine waters. Unfortunately, an abandoned landfill
on Fleming Key, part of the Key West Naval Air Station, was subject
to extensive shoreline erosion, exposing unsightly steel and concrete
debris. In addition, the area was overgrown with non-native vegetation,
which was choking out the native species. For remediation, the U.
S. Navy required an effective shoreline repair and protection system
with a minimum life of fifty years. The initial baseline for the project
was a granite riprap revetment, estimated to cost approximately $2.1
million. After evaluating submitted bids, the Navy chose a cabled
articulated concrete block system, installed on a newly constructed
3:1 slope along the entire 1800-foot (549-m) length of shoreline.
ACBs
were chosen for performance, economy and aesthetics. Seven-and-a-half-inch-thick
(191-mm) ACBs with a 12- to 18-inch-thick (30- to 46-mm) bedding layer
were able to provide long-lasting erosion protection under conditions
that would have required a 4.5-foot (1.4-m) depth of riprap with a
minimum boulder size of 3 feet (0.9 m). Although thickness requirements
are project specific, depending on embankment slope and flow conditions,
a 4-inch-thick (102 mm) ACB revetment can very often replace 18 to
36 inches (0.5 to 1 m) of riprap. Part of this thickness difference
is the level of engineering involved in designing the two systems.
ACBs are produced under controlled conditions that provide consistent
and predictable strengths and allow precise placement and interlock.
Riprap stability, on the other hand, varies with stone shape, size,
weight and durability, which cannot be precisely controlled.
The
ACBs were placed as cabled mats, 8 feet wide and 41.3 feet long (2.4
by 12.6 m), which were assembled on-site using the concrete blocks,
high strength polyester cables and aluminum sleeves. The cables facilitate
placement, particularly on steep slopes or in underwater applications.
The cabled mats were lifted by crane and set into place, quickly covering
the slope.
The
performance specification also required that the structure's appearance
harmonize with the surrounding environment. ACBs provide a uniform
finish on the slope, which were subsequently planted with native species.
To reintroduce native vegetation, over three hundred red mangrove
trees were planted along the most sheltered area of the shoreline
revetment. The open cells of the ACBs provide protection for the mangrove
seedlings until they mature and allow mangrove propagation via their
unique "prop root and riser" root system. In addition to
the mangroves, sea purslane was planted on the upper four feet (1.2
m) of the slope, also in the ACB cores. This native grass-like plant
will spread to cover the slope, providing a natural appearance and
help prevent the re-introduction of non-native plants.
The
final cost of the ACBs, including all design, permitting, construction
and replanting was $1.1 million, or approximately $11 per square foot
($119 per square meter)-almost half of the riprap protection concept.
The Navy expects maintenance to be limited to removing non-native
plant species, which may become re-introduced, and to addressing the
flank end riprap treatment, which may migrate down slope. In short,
ACBs provide a permanent, economical solution and perform as a natural
looking shoreline embankment in this highly sensitive environment.
In
project after project, articulated concrete block revetment systems
are providing a host of benefits to waterfront communities. Whether
chosen primarily for environmental benefits, economy, pedestrian safety,
aesthetics or long-term durability and maintainability, ACBs are providing
this entire range of benefits. Their unmatched versatility and performance
make ACBs a favorable solution for shorelines, creeks, boat ramps,
spillways, overflow or drainage channels, bridge abutments or levees.
Building
Applications
|
Interlocking Concrete Paving | Segmental
Retaining Walls | Articulated
Concrete Block Paving | Concrete
Masonry Units
