Hard Armor Solutions
Thousands of years ago, Egyptian society depended on the Nile River, the foundation of their civilization.
People sailed on it, fished from it, and even bathed in it. Most importantly, they planted crops in the rich soil next to the river.
Using clever techniques and hydraulic technology well ahead of its time, Egyptians then irrigated those crops with water from the Nile. Living in harmony with one of the most magnificent rivers known to man, this agrarian society flourished into kingdoms and empires that will live on forever in the history books of scholars and the imaginations of school children alike.
Life on the Nile was not without its challenges. In his ancient writings, the Greek historian Herodotus described the summertime floods in which the distance from one bank of the river to the other was two days' journey in length. This could be very detrimental to the crop fields, either by drowning the plants or eroding the topsoil. In the winter, he wrote, the water level would drop low again and stay that way until the next summer.
As just mentioned, the Egyptians used many techniques to work with the river's mercurial nature. In the case of flooding and soil erosion, they did not simply pack up shop and search for higher ground. Besides damming and building canals, they also employed some methods of erosion control.
They constructed box-like containers using hemp rope and filled them with rocks. Placing these contraptions along the banks of the river, they could reduce the amount and strength of the water hitting the soil. The rocks would absorb the impact, and the hemp boxes would keep those rocks in place. The water level wouldn't rise as much, and the Egyptians, in turn, would have a chance of holding onto that exceptionally rich topsoil. All thanks to hemp rope and rocks.
Today, we would recognize these units as rudimentary gabions. Deriving their name from the Italian word for “big cage,” modern gabions are just that: large, wire cages filled with rocks and then placed atop soil vulnerable to erosive forces. They are one of the most common types of hard armor, a powerful tool for erosion control.
Hard armor is just what it sounds like: heavy-duty infrastructure. According to Richie Prejs, northeast area manager for Williamsport, Maryland-based Maccaferri Inc., “When soil erodes off the side of a river, that soil then accumulates downstream and forms a kind of natural dam. Then the water level behind it rises and you get flooding. When hard armor is preventing erosion, that damming phenomenon doesn’t take place.”
“It all starts with the engineers. They’re the ones who are going to sit down and design the projects,” continues Prejs. “They’ll know the soil parameters; they know the area that’s eroding and where your hard armor solution is going to go,” he said. “If it’s a sandy material, you might do a layer of geotextiles first, then place your hard armor, whether it be gabions, articulated concrete block, or plain old riprap.” Engineers usually design for the 100-year event.
You can use hard armor in any situation where there is risk of soil erosion. Placed over bare soil areas, it is useful along steep slopes, channels, riverbanks, vegetated waterways, arid areas and other locations where tough erosion control is required.
For this article, we’re going to focus our attention on applications in rivers and streambanks, since this area is one of the most common—and one of the oldest— hotspots for heavy erosion. While rivers are constantly changing and affecting the topography abutting their banks, people need these bodies of water for their survival.
The ancient Egyptians built their civilization close to the Nile because it was their lifeblood. They needed the water, and the nutrient-rich soil that lined its banks. In many ways, our modern relationships to rivers are no different.
What has changed is the technology available to us in our efforts to stabilize the riverbanks. “The key benefit of these products is their permeability and flexibility, allowing groundwater to flow through them or allowing these products to be installed along shorelines,” said John Slupecki, CPESC, of Motz Enterprises, Inc., based in Cincinnati, Ohio.
There are three main types of hard armor: the aforementioned gabions, riprap and good old concrete. Gabions are made by putting rocks or broken concrete chunks inside individual wire mesh baskets. These baskets are usually made of galvanized or polyvinyl chloride-coated steel. Once the rocks are put into place inside the gabion, the lid is closed.
Each cage is attached to the next one, and the next, and so forth, creating a gabion wall. Together, a series of interlocking stone-filled baskets can form a flexible, permeable structure that can be used for a large variety of applications, such as bridge abutments, channel linings, streambank stabilization projects and many others.
They’re best for environments with high water velocities, where you’ll need structures that can take heavy beatings and not only stay intact, but also hold the soil underneath it in place.
Zach Titus, national technical sales manager for Terra Aqua Inc. in Fort Smith, Arkansas, sees a lot of projects that use gabions in conjunction with soft armor. “On a big creek channel, where a creek bends or curves, you’ll have lots of water pressure, so the project will call for a gabion system there. Then where the creek is straight, it will use soft armament.”
The more intense the conditions, the harder the armor that’s needed. Gabions are a versatile solution. Their wire caging affords both flexibility and stability. They can be stacked and arranged in different formations to meet site-specific needs.
Say a project doesn’t call for so much structure. Maybe you’re only working with a small area, or the water velocity isn’t so intense as to require caged reinforcement. Riprap, also called rubble shot rock and rock armor, is the most basic, and usually the least expensive means of erosion control available.
Simply speaking, to build a riprap wall, rocks are stacked next to a riverbank so that they naturally interlock, creating a structure.
To a layperson, riprap may just look like a pile of stones that have been dumped someplace. However, when built correctly, a riprap barrier is actually a carefully engineered system.
The key to building a successful riprap barrier is to vary the layers and sizes of rock. You want them to fit together so they absorb, rather than deflect, the impact of water. Sharp-angled rocks are better suited for interlocking than smooth, rounded rocks.
Rock that’s native to an area is less expensive and better-suited to the local environment. It also looks more natural against the existing landscape, and should be aesthetically pleasing.
Riprap works well to protect rivers, streambanks and coastlines from erosion caused by water and ice. Like gabions, it too can be used to shore up bridge abutments, pilings and other structures against damage and overflow, particularly in flood-prone streams, rivers and channels.
For added reinforcement, riprap can be stacked atop geotextile fabric or mats. This helps maintain separation between the rocks and the underlying soil, so the rocks won’t eventually sink in.
Depending on the terrain, rock placement can be done with a skid loader or crane. Sometimes a steel plate is used to tamp the rocks into place. You can self-adjust riprap to small amounts of earth movement. It’s relatively easy to install, and can be repaired simply by placing additional rock where needed.
Certain climates call for different kinds of rock. In the freeze/thaw cycles of the northeast, for instance, granite is preferred because it can withstand those harsh conditions.
Riprap structures are effective only when constructed correctly. When they fail, it’s often due to what’s called flanking. In this scenario, water overtops or undermines the structure, defeating its purpose. If improperly-sized stones are used, currents or large waves can easily displace them. That’s why the Environmental Protection Agency does not recommend riprap for use on slopes steeper than 2:1. Riprap can actually increase erosion when used improperly.
It also isn’t the ideal choice for high-velocity water flows; the swift-moving water can just wash rocks away. For those kinds of applications, harder-armored solutions, such as gabion baskets or articulated concrete walls, work better.
If gabions are not a good solution for your particular project, your final option is to bring in the really hard stuff. These solutions include mechanically-stabilized earth (MSE), poured concrete, and articulated concrete blocks, along with some others.
When flexibility combined with even harder armoring is desired, articulated concrete block (ACB) systems may be the answer. Generally, these are individual concrete blocks, interconnected by cables, ropes, geotextiles or geogrids. However, some systems don’t use cables; instead, they hook together using male and female tabs.
A lot of hard armor, such as poured concrete, isn’t flexible; it will crack or break if there is any earth movement. Gabions are flexible, but aren’t always right for every application. A fast-moving stream or the continual pounding of the ocean waves is going to need something sturdier that will stay in place, to stop erosion.
Many concrete mat systems also offer open cells that can be filled with growth medium, so that individual blocks can be seeded and vegetated. In this sense, ACB is a hard-armored solution that is also green.
Remember, all revetment systems have their advantages and disadvantages. Each type of hard armor has its place. In some instances, riprap can be as effective as gabions or concrete. For example, Slupecki points out that rock riprap is good for large wave or stormwater outlet energy, and requires a low degree of design engineering. Equally as important, it is easy for the contractor to use.
Some of the disadvantages are that it can dislodge or move in high flows of water; it can be unsightly, and can’t be vegetated. It is also costly to ship, should rock not be locally available.
Gabions, on the other hand, offer high strength, make good surface contact, and allow the use of various types and sizes of rock. They are also good for structural engineering. However, it is labor intensive to build the baskets and requires a high degree of engineering.
Articulated concrete block has its advantages when used appropriately in specific circumstances. ACBs are typically arranged by a tied or interlock system that links each individual block, forming a very flexible, yet very sturdy whole. Compared to loose rock, this setup has superior structural engineering and higher strength. These facets can be important, or even necessary, in certain erosion conditions.
On the other hand, one disadvantage to concrete is that it is very heavy, which adds to shipping costs. It also requires specialized heavy equipment to install, and is more labor-intensive overall. Sometimes, the high degree of structure afforded by ACBs can be a problem, too.
It requires a high degree of design and engineering, and we all know that in any job, the more complicated it is, the more ways it can go wrong.
With all these options, the question remains: Exactly where and when do we need hard armor? There is, after all, such a thing as soft armor, which primarily makes use of vegetation and geosynthetics to reinforce an erosion site.
How do we know when to use hard or soft armor, and if we are using hard armor, what type to choose? The majority of the time, you’ll be off the hook. The engineers on the job will already have made these decisions. It will be your job to make the most of the chosen reinforcement in application. However, it is to your benefit to know as much as you can about hard armor, in the instance that an engineer’s on-paper assessment ends up conflicting with the reality of a jobsite. Learn all you can, and you’ll be prepared when the day comes to solve a soil-erosion problem with hard armor.