10-01-04 | News

Coir Block Brings New Dimension
to Streambank Stabilization

Coir block (BioD-Block) is a biodegradable structure of densely-packed coir for stabilizing streambanks, an alternative to creating soil lifts via geotextiles and soil-filled burlap bags covered with erosion control blankets.

Last fall, Ted Gray and Associates, Inc., a stream and lake management and restoration firm in Oakbrook Terrace, Illinois, became one of the first in the Midwest to use a recently-developed biodegradable system of densely-packed coir blocks and woven coir matting to build an environmentally-friendly vegetated wall for stabilizing streambanks.

The use of geotextiles to confine soil in lifts between layers of live plants has become an increasingly popular soil bioengineering technique for creating vegetated retaining walls. However, in this case, the vegetated walls were not only easier and less costly to build than conventional fabric-wrapped soil lift walls, but they are stronger and more durable.

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. Invasive plant species had shaded out native groundcover had left 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. This program encourages the use of environmentally-sound construction practices.

A partial failure of a fabric-wrapped soil lift is not an unusual occurrence with geogrid, according to an Alaska Department of Transportation study. Such soil lifts are generally built by placing soil on geotextile fabrics. An outer synthetic layer or biodegradable fabric is woven into a mesh to reinforce the soil, then an inner layer of nonwoven coir or burlap. The soil is then compacted and the excess fabric is wrapped over the front and top of the soil and staked. The lifts are stacked to create a geotextile retaining wall.

 

The funding requirements limited the use of hard armor to stabilize the streambanks, reports Ted Gray. A professional engineer and certified professional in erosion and sediment control (CPESC) with a master?EUR??,,????'???s degree in aquatic ecology, his 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.

?EUR??,,????'??Streambank soil bioengineering stabilization combines permanent or biodegradable structures to stabilize the toe of slopes along with the deep root structure of native plant materials to provide deep-seated stability in bank slopes,?EUR??,,????'?? he explains. Instead of soil-filled burlap bags covered with erosion control blankets to build soil lifts for this project, Gray specified the coir block and fabric system, BioD-Block, made by Rolanka International, Inc. ?EUR??,,????'??This coir system is an easier, more efficient way,?EUR??,,????'?? he says. ?EUR??,,????'??Rather than relying on other materials to maintain the shape of the soil lifts, the coir block system itself provides all the needed structure to shape and form the vegetated bank,?EUR??,,????'?? he adds.

Other stream restoration projects among the first to use this coir block system include one in Massachusetts and another in Georgia. In each case, this unique approach provided a cost-effective way to improve the performance of fabric-wrapped soil lifts. ?EUR??,,????'??It represents the next generation of this soil bioengineering technique,?EUR??,,????'?? says Lanka Santha, P.E., who developed the system.

Building the Lifts

In many situations, fabric-wrapped soil lifts offer a much more natural alternative to hard armor practices, like concrete, gunnite or rock rip rap, to protect streambanks from erosion. This approach restores streambanks in a way that blends in with the site and improves habitat for fish and wildlife.

Typically, the soil lifts are constructed 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 coir fabric of twisted coconut fibers woven into a strong mesh, provides high tensile strength to reinforce the soil. An inner layer of nonwoven 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.

Before, during and after installation of the coir block and fabric system to stabilize the erosion of a stream channel through this townhouse development. A coir block and fabric system was used rather than relying on synthetic geogrids to maintain the shape of the soil lifts. Rock-riffle grade control structures were installed to prevent further erosion of the stream channel, then the streambank was reshaped. Densely-packed coir rolls (12-in. diameter) provided structural support for the toe of the slopes. Coir block was then installed above the rolls, from 2-5 blocks high. The layers were stepped back to produce a wall face with about a 2.5:1 slope.

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 becomes 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 fibrous root systems of the willows 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. The Alaska Department of Transportation study, published in 2003, which evaluated 11 streambank restoration sites in which a geogrid was combined with an inner burlap filter to build fabric-wrapped soil lifts. At one river site, 20 feet or more of the soil lifts had partially collapsed. 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 one creek restoration project, flooding completely destroyed fabric-wrapped soil lifts. Gravel and soil was removed along as much as 20 ft. of the streambank from holes in the burlap fabric in the face of the lifts. Meanwhile, much of the geogrid material was trailing out from the remaining soil lifts. ?EUR??,,????'??Improvements to the methods and materials used in fabric encapsulated soil lifts should be considered,?EUR??,,????'?? the reports states. ?EUR??,,????'??Outer fabrics with greater tensile strength and abrasion resistance, or other techniques, should be evaluated for use on streams where ice damage may occur.?EUR??,,????'??

A Better Way

The BioD-Block system consists of a coir fiber block made of tightly compressed, long coir fibers and measuring 10 ft. long, 9 in. wide and 16 in. high, and a woven coir fabric. This fabric is wrapped around one side and the top and bottom of the block, leaving two free ends. As with conventional soil lifts, soil is place on the bottom fabric and covered with other piece of fabric extending back from the top of the block. Unlike, conventional soil lifts, however, the coir block forms the face of the soil lift. The blocks are available in three fabric lengths to help match site conditions. The fabric extends back 16 in. to 48 in. from the top from 28 in. to 75 in. from the bottom. The wrapped-woven coir fabric has a machine direction dry tensile strength of 1,740 lbs. per foot and cross direction dry strength of 1,176 lbs. per foot.

According to Santha, the coir fabric has a number of advantages over conventional fabric-wrapped soil lifts:

Sturdier and More Durable

The roots of willows and other vegetation grow into the block and embed it to the soil, creating a solid, natural protection for the soil mass. Santha explained that the manufacturing process also contributes to the structural support of the build soil lifts, making it ?EUR??,,????'??about 40 percent stronger than the cross direction tensile strength of typical coir fabrics used to build soil lifts. The male-female ends of the block produce, strong continuous sections while maintaining structural integrity. The result of all this is stronger, more stable structure.?EUR??,,????'??

Faster, Easier Construction

The coir blocks provides a fixed height for the soil layers, which creates a uniform height and reduces the time and effort required to make the soil layers.

Lower Construction Costs

In most situations the coir block system eliminates the need for an inner fabric.

Versatility

The coir block system can be used in a number of different ways to restore streambanks, depending on the site conditions. For example, it can be adapted to projects with minimal cut and fill requirements and those involving much more reshaping and filling. The blocks can be place in a vertical or angled position and in single or multiple layers. Other possibilities include placing rows of blocks directly on a slope surface and planting vegetation in between the rows to eliminate and fill behind the blocks.

Field Reports

Three projects illustrate successful use of the coir block system in different types of applications.

Stabilizing an Urban Stream

This system played a key role in the Hobson Creek project. Phase 1, completed October 2003, involved a 750-ft. 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-in. diameter densely packed coir rolls, (BioD-Roll made by Rolanka) were installed to provide structural support for the toe of the slopes. Bio-D Block was then installed in layers directly above these rolls. Averaging two to three blocks high, but ranging up to five blocks high 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. ?EUR??,,????'??We added these tie backs as extra insurance to prevent any of the blocks from overturning,?EUR??,,????'?? Gray says. 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 some two dozen herbaceous plants like switchgrass and fox sedge.