Problem

Increasingly, geotechnical designers are seeking waysto control soil erosion in critical areas without the use ofconcrete or rock. This situation was recently encountered in the rapidly growing town of Issaquah,WA, located near I-90 about 15 miles east of Seattle.Construction of the Issaquah Highlands Communityhousing development required the clearing of an areaexceeding several hundred acres, which produced anexceptionally large stockpile of marginal soil classifiedas “weathered topsoil”. Due to the very loose, non-cohesive, erodible soil composition, engineersdetermined that it could not be used as a base for building construction. Standard slope construction methods would also not be adequate since therewas significant risk of slope failure due to the soil’s composition and structural capabilities. Afterestimating the expense of hauling the weathered topsoil and disposing of it at an off-site location, thesite’s owner decided to use it in an area that would eventually become a series of athletic fields.Utilization of this marginal material for this purpose would satisfy the objectives of the project,although an innovative approach would be needed in engineering the slopes around the athletic field.

Solution

Engineers at Icicle Creek Engineering in Washington state proposed a geotextile reinforced 1:1 (H:V) slope design to supportthe perimeter of elevated athletic fields. The primary considerationwas to achieve a balance between the economics of constructingand stabilizing the steep fill slopes and maintaining level athleticfields-a value engineered approach. The owner sought to make theslopes as steep as possible so they would not consume land thatcould otherwise be used to build houses upon. He liked the engineering proposal but requested that the slopes be vegetated foraesthetic purposes. The engineer’s biotechnical design ultimatelydetailed a system that would be easier to maintain than alternativessuch as rock or concrete.

In response to the demands of the design, ACF West Inc.(Woodinville, WA) worked with the engineers to produce the finaldesign involving North American Green’s high performance C350Composite Turf Reinforcement Mat (C-TRM) for a soil wrapped,vegetated slope face. Ben Nelson at ACF West provided assistance

on the project. Nelson recalls, “We became involved when the contractor, Summit General, asked usfor a materials estimate. We then began talking with the engineer to determine their primary designconcerns and see if they would consider alternate materials.” Mr. Nelson discussed with the engineers what properties were desired in an erosion control material that might be used in such adesign. “High on the list was the face stability of each lift of soil, they regarded this soil as ‘highly

CASESTUDY

Volume 7

Number 1

Nove m b e r 2003

14649 Highway 41 NorthEvansville, Indiana 47725

812-867-6632 1-800-772-2040FAX 812-867-0247www.nagreen.com

North American Green C350: Innovative Design for Steep

Embankments

Installation invo l ved anchoring the C350 to a subsurface geogrid system. The wall eventually becomesve g e t a t e d .

This value engineered design eliminatesthe need for hard armor such as rockand concrete.

erodible’ and did not want to see any sluffing, rilling, or eroding”, notes Nelson. Effective soil retention properties, desirable moisture retaining mulch benefits, and long-term vegetation reinforcement, and high tensile strength were otherconsiderations that made the C350 C-TRM a perfect match to the project’s needs.

Installation

Steep slopes and loose soils are typically a combination for disaster in caseswhere the soil may be left exposed and unprotected or covered with a simpleshort-term temporary erosion control blanket. For this project it was necessary tonot only control surface erosion, but to also ensure the structural integrity of theslope. Both of these concerns were addressed by applying the C350 C-TRM following a procedure of layering the slope with geogrid reinforcement between2.00 foot layers of soil wrapped by the C350. The C350 acting as the wrap onthe face ensured that the soils were in direct contact with the matting. A sectionof the matting was tied back into the geogrid on the top and bottom of the soilwrap to retain slope stability. However, in order to perform this part of the design,the matting had to perform a function similar to a geogrid, with high tensilestrength to support the soil lift. With a tensile strength of 658 x 910 lbs/ft, the

engineer determined C350 possessed the necessary strength, which also allowed the contractor to use less of the primaryreinforcement geogrid material.

The geogrid and C350 system was secured with 12 inch pins near the face of the slope. The face of the slope washydroseeded after installation of the C350, even though North American Green generally recommends seeding beneath thematting. In this case, it was impossible to hydroseed before the matting installation because of the construction sequence.The C350 had to be installed at the same time as the geogrid in order to compact the soils at the wall face prior to removing the forming system. Seeding could only take place after the wooden forming system was removed. With thisdesign, the slope is structurally stabilized by the layers of geogrid that extend backinto the slope. The C-TRM is anchored to the grids and filled with soil, progressively building the slope upwards at a 45 degree angle. C350’s permanentnet structure holds the face of the slope in place while reinforcing vegetation,whereas the dense coconut fiber matrix will retain small soil particles and act as amulch to accelerate vegetation growth.

Performance

As a result of the importance the owner placed onestablishing a vegetated slope face, the designerselected the North American Green C350. Unlikesome T R M ’s requiring soil in-filling to enable maximum performance, the C350’s combination ofdense coconut fiber and a three-dimensional netstructure effectively retained soil on the steep unvegetated slope while also retaining adequate soilmoisture for quick seed germination. Even afterheavy rain caused flooding in the area that impacted the newly constructed wall, the C350 retained soil and encouragedvegetative growth. After nearly a year after installation, vegetation had become well established and the permanent three-dimensional net structure of the C350 will remain indefinitely to reinforce the base of the vegetation and preventfuture erosion.

Use of the C350 in this way is widely considered an environmentally friendly “biotechnical” or “bioengineered” application-terms used to describe some engineering practices involving the mutual use of man-made materials and natural vegetation to control soil erosion in an aesthetic manner.

Vegetation becomes progressive l ymore dense and is reinforced by theC350 C-TRM to provide permanentprotection against soil erosion.