Building Blocks: Gabions & Riprap
Confining rocks to use as a stabilization system is hardly a new idea. In fact, the earliest forms of today’s gabion technology date back to the early Egyptians with archaeological evidence indicating that they used rudimentary baskets of woven reeds 7,000 years ago as a way to protect the banks of the sacred Nile River.
The Romans fortified their battlements and buildings in this manner. Such technology was commonly used in Victorian England, and can also be seen in the construction of forts in the United States dating back to the mid-1800s. Gabions will also be used extensively in both functional and decorative manners at the 2012 Summer Olympic Games throughout the Olympic Park in Stratford, East London.
The word gabion is derived from the Old Italian, gabbione, and the Latin, cavea, both meaning cage. Adopted and adapted throughout civilization, gabion technology has never grown old; rather, it is timeless, versatile and highly effective. The ancients, though, would hardly recognize the high-tech gabions of today, manufactured from wire mesh and bearing little resemblance to the relatively unchanged procedures that remained in place well into the 20th century.
Gabion containers are interconnected and filled with stone at the construction site. They are permeable, inherently flexible and monolithic, the latter feature the result of connecting the cages before they are filled with rocks or rubble. Since gabions are selfdraining, interstitial spaces in the rock help dissipate energy generated by flood currents or wave action— characteristics that allow gabions to provide protection unlike rigid or semi-rigid systems that are suspect to collapse when even the slightest changes occur in their foundation.
“We know the Egyptians were holding rocks in place along the banks of the Nile with ropes and branches and whatever they had available. The difference today is the technology we have developed within the past 20 years,” said George Ragazzo, general manager of Modular Gabion Systems of Mobile, Alabama, a division of C.E. Shepherd Company in Houston, Texas. They have been installing gabions since 1957.
Maccaferri, Inc., of Williamsport, Maryland, is another company employing evolving gabion technology. Maccaferri built
its first gabion-retaining structure along the banks of the River Reno
in Casaleccio, Italy, in 1893. According to Marketing Director Ghislain
Brunet, a portion of that structure still stands today.
Applications and benefits
Gabions are most often utilized in areas where ground conditions are inherently unstable and prone to shifting and sliding. Gabions provide a needed alternative to concrete walls that would crack or buckle under such pressure.
“The most common use is for retaining walls for stabilization, followed by channel linings. But, we’ve also had a tremendous amount of interest in recent years from architects using gabions as columns or for exterior cladding as a thin layer to give walls protection,” said Ragazzo.
Among many uses, gabions are regularly used for headwall and culvert outlets, drop structures, levees and weirs, gravity walls, retaining walls, mechanically stabilized earth walls, architectural walls, soil stabilization, rockfall mitigation, freestanding walls, noise and environmental barriers, and decorative architectural cladding for walls and buildings.
“The big advantage of using gabions is free drainage.
Ground water will drain freely and there is no pres sure build-up in the back of the wall,” said Brunet.
“Also, they are very cost-effective, because you can use the local materials and local suppliers. And, they don’t require highly-skilled labor. With everyone talking about job creation, this is a good way of creating jobs in the industry. When more than 50 percent of the cost of a job is labor, not having to hire specialized workers is a good way to keep your costs down.”
While most instances calling for the use of gabions are relatively straightforward, others can be both challenging and exceptionally unusual.
For example, as part of one construction project, Modular Gabion Systems was called upon to build 2.2 kilometers of protective walls to control hillside runoff alongside the outflow channel, and also fashioned a 50-ton capacity bridge entirely from gabions.
“For that bridge, they first laid corrugated metal pipes in the creek bed to allow the water to pass freely, then built the gabions on top of that. Vehicles could drive on it right away. It’s the most unusual job I think we’ve ever been involved with,” said Ragazzo.
Maccaferri was involved with design/build reconstruction of the I-35W Mississippi River Bridge in Minneapolis that killed 13 and injured 145 when it collapsed in August of 2007. The replacement I-35W Saint Anthony Falls Bridge was constructed on an accelerated schedule for $234 million and reopened 13 months later in September of 2008—100 days ahead of schedule.
“That was a very challenging project. The architects decided to have a wire-type gabion wall and we were selected for the design/ build project. On most jobs, there are two levels of review; on this project, we went through seven levels. It was very intense and there were numerous modifications incorporated after our initial design. We were on the job for approximately eight months and are very proud of our work,” said Brunet.
For Spring Creek drainage improvements in Mobile, Alabama, Modular Gabion Systems constructed an approximately 7,600 cubic yard gabion wall and scour apron to stabilize the bank and to prevent undercutting. Unnecessary joints were eliminated, yielding a structure of uniform strength.
If not for the availability of gabion systems, Modular Gabion’s construction of a 48-foot high mechanically stabilized earth wall in Birmingham, Alabama, and Maccaferri’s construction of a 76foot high wall at a mine in Nevada would have been far more difficult projects to tackle. As Brunet said, “You just don’t see many walls that are higher than 70 feet in the United States.”
Ragazzo said improved welding techniques have allowed for more precise measurements that yield a stronger wire mesh assembly. Baskets connected with a pre-formed spiral that more closely resembles a corkscrew result in the strongest connection available.
The type of wire used in any assembly, ranging from basic galvanized wire to stainless steel and newly-developed alloys such as galfan (95 percent zinc/5 percent aluminum), depends on the environment to which it is exposed. “What we use all depends on the environment and how corrosive it is.
Under normal conditions, wire can last from 20 to 30 years, perhaps a bit longer. Harsh environments such as salt water and wave action reduce the life expectancy,” said Ragazzo.
The use of fusion-bonded PVC coating can double, or even triple, the lifespan of a wire which, in a normal setting, is 75 years. “In harsher environments, such as stream restoration or where there is exposure to wave action, it’s closer to 50 years. And, when PVCcoated wire is used in a dry wall setting, such as a parking lot retaining wall, the lifespan is perhaps 40 to 50 years,” said Brunet.
Although the initial cost of PVCcoated product is higher, the longterm savings can’t be ignored.
The expanding global use of stainless steel gabions—popular in Europe for a number of years— raises the bar even higher. Testing has shown an anticipated life expectancy of 80 years or more, regardless of the environment into which stainless steel baskets or cages are introduced. In addition to their cost effectiveness, stainless steel gabions provide the best defense against corrosion and prevent bleeding through to an external surface, such as concrete or plaster.
Modular Gabion Systems used stainless steel gabion mattresses for its reconstruction of a four-mile stretch of US-98 in Fort Walton Beach, Florida, that was destroyed by Hurricane Ivan in 2004. Sixteen miles of buried mattresses were placed under the shoulders and on both sides of the median strips. The company employed the same technique when rebuilding Bay View Boulevard in Pensacola, Florida.
Regardless of the material or the system, all gabions provide flexibility, strength and permeability.
Economy is derived through the use of unskilled labor, usually available local materials and the ability to build many gabions without the need of mechanized equipment. Well-designed and well-constructed systems are virtually maintenance-free once in place. Perhaps the most attractive feature of gabions is the fact they are able to accept a full load immediately, thereby eliminating the curing process normally associated with concrete.
What about riprap?
not utilized nearly as often as gabions, riprap remains a cheaper
alternative and cost-effective hard armor solution most commonly used to
protect river banks, stream banks and coast lines from
both scour and naturally occurring erosion caused by water and ice. Also referred to as rubble, shot rock and rock armor, riprap is also commonly used to shore up bridge abutments, pilings and other structures as an effective deterrent against undermining and impact damage by submerged debris or ice floes.
Brunet said that while Maccaferri provides riprap services, “that work accounts for no more than five percent of our business at this time.” MGS, as its name implies, works solely with gabions and does not engage in riprap work at all.
Riprap revetments are an especially effective mitigation tool in high-velocity water flow streams, rivers and channels that are susceptible to flooding. Varying layers and sizes of sharply-angled rocks are used in a revetment because they fit together in such a manner as to protect eroding banks while absorbing, rather than deflecting, the impact of rapidly flowing water. Smooth, rounded rocks are ill-suited for this purpose, since they don’t interlock and resistance to movement is decreased.
Riprap materials consist of a variety of rock types that are most often commercially mined. Climate is an all-important consideration in determining choice of rock.
For example, in the heavy freezethaw cycles of the northern and northeastern United States, durable stones such as granite are favored because of their ability to withstand even the harshest conditions. Broken limestone is another common choice, and rebar-free concrete rubble from building and paving demolition can also be used in certain conditions.
While functionality certainly outstrips aesthetic needs, it is important that contractors consider the surrounding environment and utilize materials native to the region whenever possible. Riprap installations should blend seamlessly into the existing landscape and not stand out as an ugly pile of rocks.