Stabilizing a Suburban Stream Bank
The Hobson Creek Corridor Restoration Project
Uses a Unique Coir Block System
to Protect a Stream Channel in Naperville, Illinois
By Greg Northcutt in an exclusive interview with
Ted Gray, a professional engineer and certified professional
in erosion and sediment control (CPESC)
Before (top), during (center) and after (bottom) installation of the coir block and fabric system used to stabilize the erosion of a stream channel running through a 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-inch diameter) provided structural support for the toe of the slopes (center). Coir block was then installed above the rolls, from two to five blocks high. The layers were stepped back to produce a wall face with about a 2.5:1 slope (bottom).
Specifying An Effective, Efficient Coir Block and Fabric System
he 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 project, the vegetated walls specified were not only equal or less costly to construct than conventional fabric-wrapped soil lift walls, but they are easier to construct, stronger and more durable.
The funding programs available for this project encourage the use of environmentally sound stream stabilization techniques and discourage the use of hard armor practices to stabilize the streambanks, reports Ted Gray, CPESC, PE. For instance, past hard armor practices such as retaining walls, concrete channels, or even extensive use of rock rip-rap in some cases can worsen erosion in downstream unprotected areas since they efficiently deflect stream energy.
In contrast, the use of practices in this project, such as Bio-D Block (made by Rolanka International), a-jacks, fiber roll, natural rock riffle grade control and natural channel cross-section design along with native plantings can dissipate stream energy, improve on-site and downstream bank stability, enhance aesthetic and recreational value and provide wildlife habitat. Moreover, adjacent residential property values can increase where natural stream channel design occurs.
Gray’s calculations of shear stress forces in the stream showed that soil bioengineering techniques would likely stabilize many areas of the eroding stream bank over the long term. “Stream bank soil bioengineering stabilization combines permanent or biodegradable structures to stabilize the toe of slope along with the deep root structure of native plant materials to provide deep-seated stability in bank slopes,” says Gray. Instead of soil-filled burlap bags covered with erosion control blankets to build soil lifts or re-shaping bank slopes to the point where residents would lose their backyards, Gray specified the BioD-Block coir block and fabric system.
Coir block is a biodegradable structure of densely-packed coir for stabilizing streambanks, and is an alternative to creating soil lifts via geotextiles and soil-filled burlap bags covered with erosion control blankets.
“This coir system is an easier, more efficient way,” Gray says. “Rather than relying on combining several different materials on-site to maintain the shape of the soil lifts, the coir block system provides an all-in-one structure to shape and construct a stabilized and vegetated bank,” says Gray.
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. “It represents the next generation of this soil bioengineering technique,” says Lanka Santha, P.E., who developed the system.
Comparing Lift-Building Techniques
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 stream banks from erosion. This approach restores stream banks 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. The 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.
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. An Alaska Department of Transportation (DOT) study, published in 2003, evaluated eleven stream bank restoration sites where 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.
Above and Below: 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. The inner layer is made of nonwoven coir or burlap. After geotextile fabrics are installed, the soil is 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.
At another project (a creek restoration), flooding completely destroyed fabric-wrapped soil lifts. Gravel and soil was removed along with as much as 20 feet of the stream bank 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. According to the Alaska DOT report, “Improvements to the methods and materials used in fabric encapsulated soil lifts should be considered. Outer fabrics with greater tensile strength, abrasion resistance or other techniques should be evaluated for use on streams where ice damage may occur.”
A Better Way
The BioD-Block system consists of a coir fiber block made of tightly compressed, long coir fibers and measuring 10-feet long, nine-inches wide and 16-inches 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. Depending on application, the blocks are available in a choice of three fabric lengths–-to match site conditions. The fabric extends back 16 to 48 inches from the top and from 28 to 75 inches from the bottom. The wrapped woven coir fabric has a machine direction dry tensile strength of 1,740-pounds per foot and cross direction dry strength of 1,176-pounds per foot.
These features offer several advantages over conventional fabric-wrapped soil lifts, according to Santha:
A Sturdier, More Durable Structure
Part of this reflects the nature of thick coir block that provides better support and protection for the soil behind it than fabrics alone. What’s more, 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 systems higher performance. “Because the tensile strength in the machine direction contributes to the structural support of the build soil lifts, it’s about 40 percent stronger than the cross direction tensile strength of typical coir fabrics used to build soil lifts,” Santha says. 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.”
The coir blocks provide a fixed height for the soil layers, greatly reducing the time and effort required to make the soil layers with a more 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.
The coir block system can be used in a number of different ways to restore stream banks, depending on individual site conditions. For example, coir block can be adapted to projects with minimal cut and fill requirements as well as 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 an existing slope surface and planting vegetation in between the rows to eliminate and fill behind the blocks.
Three separate projects illustrate the success 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. The phase one, which was completed last October, involved a 750-foot reach 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. Construction services were awarded to Landscape Resources, Inc. Rock riffle grade control structures were installed to prevent further down-cutting of the stream channel. After reshaping the eroded stream banks to a 3:1 (H:V) slope or flatter, 12-inch diameter, densely packed BioD-Roll coir rolls were installed in a six-inch trench and secured with two-inch diameter. wood sakes fastened with one-eigth-inch chord to provide structural support for the toe of the slopes. In other areas where more severe erosion occurred, one to three rows of concrete “a-jacks” were used to stabilize the toe of slope. The 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 existing stream channel suffered from water erosion (center) and needed long-term stabilizing to prepare for a townhouse development. Stabilization began by preparing a base for a vegetated retaining wall by grading it to a depth of 16-inches below final grade. This left room for layers of coir block and fabric to be installed (above) with setbacks to form a finished 2:1 slope at the desired height. Willow cuttings were used between the coir blocks, and eight grass varieties (millet, bermudagrass, fescue and rye, et al.) seeded the backfill (bottom).
The coir fabric (attached to the block) was staked in place behind the blocks. The contractor devised a strategy to tie the top of the blocks and anchor them to the slope with wood stakes. “We added these tie backs as extra insurance to prevent any of the blocks from overturning until the establishment of native plantings at the site,” Gray says. The coir wall system was then backfilled and planted with a native plant seed mixture. Various types of native plantings were specified for each site depending on the level of sunlight exposure and the anticipated erosive forces. For instance, in the less-eroded sites that received more sunlight, herbaceous two-inch diameter plugs such as switchgrass, fox sedge or cord grass were installed. In the more highly eroded areas, shrubs were installed including dogwood, willow and viburnum along with native plant plugs along the top of the bank.
As of August, the re-constructed slope remained fully intact. “So far, it’s performing very well,” Gray says. “Our intent was to design 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.”
Because of the initial success of phase one of the project, phase two will attempt to stabilize another 850-foot section of the stream banks with the coir block and fabric system.
Dealing With the Wrong Soils
This past winter, Bill Stinnett, superintendent for Massana Construction, Marrietta, Ga., supervised a Clayton Country Water Authority stream bank restoration project in Morrow, Ga. It involved five areas of East Jesters Creek, each about 50 or 100 feet long and slopes of about three to five feet high. Original plans included wrapping soil in one-foot-high lifts, wrapped in a coir fabric, to build a retaining wall for stabilizing the stream banks. However, Stinnett reports that the soil wasn’t suitable for this. “We needed some clay to make it work, but the soil was mostly sand and silt and would have washed out,” he says. The revised plan featured the use of the BioD-Block system.
“The use of practices in this project such as Bio-D Block, a-jacks, fiber roll, natural rock riffle grade control, natural channel cross section design and native plantings can dissipate stream energy, improve on-site and downstream bank stability, enhance aesthetic and recreational value and provide wildlife habitat.”—Ted Gray, PE, CESPC
Thirty-five rock-cross vane structures were built to stabilize the stream channel. The base for the retaining wall was prepared by grading it to a depth of 16 inches below final grade. Layers of the block and fabric were then installed, with setbacks, to form the finished 2:1 slope to the desired height.
The backfilled structure was seeded with eight varieties of grasses include millet, Bermuda grass, fescue and rye. Willow cuttings were used between coir blocks. The project was completed in mid-April. “Looking back, I wouldn’t have done anything differently,” says Stinnett. “The grass came up beautifully and the wall looks great.”
A Wetland Application
This coir block and fabric system was also used successfully by the Massachusetts Municipal Wholesale Electric Company (MMWEC), Ludlow, Mass., to restore the banks of six streams following installation of a 20-inch gas pipeline in wetland areas during the summer of 2002. “The regulatory agencies wouldn’t allow the use of conventional streambank stabilization materials, like riprap,” says Mike DiMauro, environmental engineer with the company.
“The site lent itself very well to the biodegradable coir product.” After re-grading the stream banks, single layers of BioD-Block were installed on either side of the steam in appropriate lengths, following the manufacturer’s recommendations. “The product is very flexible and conformed to the contours,” he says. “It was very easy to install.” Following installation of the system, a wetland seed mix was planted over the backfilled area. “The product has performed very well. Plants are growing up through the fabric and the blocks are slowing degrading as they fill naturally with sediment carried by the stream.”
More about the author: During the fall of 2003, Ted Gray and Associates, Inc., a stream and lake management and restoration firm in Oakbrook Terrace, IL, become 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 stream banks. For more information, contact Gray at www.TedGray@msn.com
The Project: The Hobson Creek Corridor Restoration Project in Naperville, Ill.
Challenge: Stabilizing a steep stream slope from experiencing runoff from continuing site development.
Site Details: Invasive plant species had shaded out native ground cover leaving already compromised stream banks even more vulnerable to erosion that threatened utilities and building foundations.
Funding: Primarily provided by DuPage County and an Illinois Environmental Protection Agency Section 319 grant program that encourages the use of environmentally-sound construction practices.
The International Erosion Control Association (IECA)
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Popular topics include: The Dynamics of Soil; New Practices and Methods for the 21st Century; Designing for Sediment & Erosion Control on Construction Sites; Environmentally Sensitive Streambank Stabilization; How to Select, Establish and Use Plants for Erosion Control; Low Impact Development: Saving Soil by Design; Soil-Loss Estimation for Construction Lands Using RUSLE 2.0
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