Project Case Study: Stable streams

December 2004 » Feature Articles
Use of geotextiles to confine soil in lifts between layers of live plants has become an increasingly popular, environmentally friendly technique for creating vegetated retaining walls that can stabilize streambanks.
Greg Northcut

Biodegradable block system controls streambank erosion

Use of geotextiles to confine soil in lifts between layers of live plants has become an increasingly popular, environmentally friendly technique for creating vegetated retaining walls that can stabilize streambanks.

In many situations, fabric-wrapped soil lifts offer a more natural alternative to hard-armor practices, such as concrete, gunnite, or rip rap. Further, this approach can restore a streambank in a way that blends in with a site and improves habitat for fish and wildlife.

Typically, workers construct soil lifts by placing soil on top of a portion of two horizontal geotextile fabrics. An outer layer of a synthetic geogrid or a suitable biodegradable fabric—such as a fabric of twisted coconut fibers (coir) woven into a strong mesh—provides high tensile strength to reinforce the soil. An inner layer of non-woven coir, burlap, or other matting prevents piping of soil fines through the coarser outer fabric. After the soil is compacted, the remaining fabrics are wrapped over the front and top of the soil mass and staked in place. These lifts are built one on top of another and set back to form a geotextile retaining wall.

Live plant cuttings, usually dormant willows, are placed between the layers, protruding from the face of the constructed bank. These branches reduce the shear stress on the face of the bank. The cuttings, plus the static weight of the wrapped soil lifts, produce a strong structure that is designed to withstand bank shear forces until the vegetation is established.

As the willows grow, their dense branches help protect the bank from the erosive forces of flowing streams. These branches also provide cover and shade for fish and wildlife. At the same time, the willows’ fibrous root systems bind the soil particles to anchor the lifts. By the time any natural fabric materials degrade, the willows should be well established and stabilizing the bank.

In some cases, however, this technique has failed to meet performance expectations. An Alaska Department of Transportation study published in 2003 (see Bioengineered banks, page 47), for instance, evaluated 11 streambank restoration sites where a geogrid was combined with an inner burlap filter to build fabric-wrapped soil lifts. At one site, 20 feet or more of the soil lifts had collapsed partially. It appeared that bank ice or spring ice floes had ripped the geogrid apart and soil material had disappeared where the burlap filter had deteriorated.

At another project site, flooding completely destroyed fabric-wrapped soil lifts. Gravel and soil along as much as 20 feet of the streambank were removed from holes in the burlap fabric in the face of the lifts.

Meanwhile, much of the geogrid material trailed out from the remaining soil lifts.

"Improvements to the methods and materials used in fabric-encapsulated soil lifts should be considered," according to the report. "Outer fabrics with greater tensile strength and abrasion resistance, or other techniques, should be evaluated for use on streams where ice damage may occur." A new development A recently developed, biodegradable block system that combines a densely packed block of coir fiber with a woven coir fabric is designed to make construction of these encapsulated soil lifts easier, while producing a stronger, longer-lasting structure at a lower cost.

The BioD-Block system, offered by Stockbridge, Ga.-based RoLanka, consists of a 10-foot-long by 9-inch-wide by 16-inch-high, coir fiber block made of tightly compressed, long coir fibers and a woven coir fabric. This fabric is wrapped around one side and the top and bottom of the block, leaving two free ends.

Similar to conventional soil lifts, soil is placed on the bottom fabric and covered with the other piece of fabric that extends back from the top of the block. Unlike conventional soil lifts, however, the coir block forms the face of the soil lift. Depending upon the application, blocks are available in a choice of three fabric lengths to match site conditions—ranging from 16 to 48 inches long on the top, and from 28 to 75 inches long on the bottom.

According to RoLanka, the advantages of coir blocks over conventional fabric-wrapped soil lifts include the following:

Sturdier, more durable structure. The thick coir block provides better support and protection for the soil behind it. Additionally, the roots of willows and other vegetation grow into the block, embedding it to the soil and creating a solid, natural protection for the soil mass. The way in which the woven coir fabric is manufactured also contributes to the system’s higher performance. The tensile strength in the machine direction (1,740 pounds per foot) contributes to the structural support of the soil lifts. It is about 40 percent stronger than the cross direction tensile strength of typical coir fabrics used to build soil lifts, according to RoLanka. Additionally, the male-female ends of the block produce strong, continuous sections while maintaining structural integrity.

Faster, easier construction. The coir blocks provide a fixed height for the soil layers, reducing the time and effort required to construct soil layers that have an attractive, uniform height.

Lower construction costs. In most situations, the coir block system eliminates the need for an inner fabric.

Also, the ease of construction cuts labor expenses.

Versatility. The coir block system can be used in a number of different ways to restore streambanks, depending upon site conditions.

A field application

The Hobson Creek Corridor Restoration Project was undertaken to stabilize a stream channel and banks where the stream passed through a townhouse development in Naperville, Ill. A relatively steep stream slope and more runoff from continuing site development in the urbanizing watershed had increased the erosive forces of the stream. Also, invasive plant species had shaded out native ground cover, leaving the highly erodible streambanks even more vulnerable to erosion. As a result, severe erosion was threatening utilities and building foundations. Much of the funding for the project was provided by DuPage County and an Illinois Environmental Protection Agency Section 319 Grant Program, which encourages the use of environmentally sound construction practices.

The funding requirements limited the use of hard armor to stabilize the streambanks, according to Ted Gray, P.E., CPESC, founder and director of Ted Gray & Associates, Oakbrook Terrace, Ill. Gray’s calculations of shear stress forces in the stream showed that soil bioengineering techniques would stabilize many areas of the eroding streambank over the long term and, therefore, play a key role in the Hobson Creek project. Phase 1 of the project, which was completed in October 2003, involved a 750-foot stretch of the stream. Patrick Engineering, Lisle, Ill., provided surveying and permitting assistance while Ted Gray & Associates designed the channel and streambank stabilization work and provided construction services.

Rock riffle grade-control structures were installed to prevent further downcutting of the stream channel. After reshaping the eroded streambanks, 12-inch-diameter, densely packed coir rolls (RoLanka’s BioD-Roll) were installed to provide structural support for the toe of the slopes. Bio-D Block then was installed in layers directly above these rolls. Averaging two to three blocks high—but as high as five blocks in some places—the layers were stepped back to produce a finished wall face with about a 2.5:1 slope.

The coir fabric was staked in place behind the blocks and the contractor devised a strategy to tie the top of the blocks and anchor them to the slope with wooden stakes. "We added these tie backs as extra insurance to prevent any of the blocks from overturning," Gray said. The coir wall system was then backfilled and planted with a native plant seed mixture and, depending on sun exposure and expected erosive forces, shrubs such as dogwood, willow, and viburnum, or plugs of herbaceous plants such as switch grass and fox sedge.

In March 2004, before the ground cover had emerged, the wall remained fully intact.

"So far, it’s performing very well," Gray said.

"We designed the slope so that in the future, when the materials biodegrade, the slope will be vegetated at a stable angle. The big test will be in about three years after the coir material degrades and the plant roots are stabilizing the slopes. Then, we’ll know exactly how the project performed. Based on results thus far, I think it will work out well." As a result of the initial success of Phase I, Phase 2 of the project is scheduled to be built this summer. Phase 2 involves stabilizing another 850-foot section of the stream’s banks with the coir block and fabric system.

Greg Northcutt is president of Northcutt Communications, Inc., an editorial services firm in Port Orchard, Wash., which has been covering environmental issues and the construction industry since 1988. He can be contacted at 1-360-895-1887, or via e-mail at gregnorthcutt@att.net.


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