THE CHALLENGE
Two major slopes had to be disturbed Slope #1 faces southeast with a steep upper portion inclined at approximately 1.2:1. The middle and lower portions are inclined at approximately 1.5:1. The slope ranges in elevation from 855 feet above mean sea level (msl) near the toe to 920 msl at the top. Slope #2 faces south, inclined at approximately 1.5:1 near the top and approximately 2:1 in the lower portion of the slope. It ranges in elevation from 835 msl at the bottom of the slope to 920 msl at the top.
A team of experts were organized to plan and implement a solution for this very sensitive challenge. Ninyo & Moore, geotechnical consultants, performed subsurface exploration, laboratory testing of the soils, and slope stability analysis.
The general findings were undocumented fill, Quaternary colluvium, and bedrock of the Tertiary Topanga Formation. They determined global stability concerns were satisfied, but recommended that additional local (or surficial) stability was required. Two alternatives were offered for the treatment of the slopes.
First, to increase the surficial factor of safety of the slope to greater than 1.5 by removing the surface soils and rebuilding the slope with a geogrid. Second, to improve the surficial stability of the slope by re-grading and installing a turf reinforcement mat (TRM). In both alternatives, the slope was to have a reestablished vegetation cover.
THE ENKA SOLUTION
The project manager selected the second alternative, which increased the stability of the slope surface by increasing the erosion resistance. A variety of TRM’s and their corresponding features and benefits were considered. EnkaMat was chosen after the project manager decided on the types of vegetation needed for this particular locale. The potential for sheet (overland) flow during precipitation was also a consideration. EnkaMat R45 was selected for Slope #1 based on the steep angle of the slope (approximately 1.2:1 to 1.5:1.) EnkaMat 7020 was chosen for Slope #2 (based on angles of 1.5:1 to 2.1).
EnkaMat 7020 is a root reinforcement matrix made of a monolithic three-dimensional nylon matrix. It withstands shear forces of 10 psf in channels and has a superior holding capacity for soil, root mass, and vegetation. EnkaMat R45 is a unique product that incorporates the EnkaMat matrix (heavy nylon filaments entangled and fused together) with a highstrength polyester geogrid. It has a high tensile resistance of 3,000 lbs/ft, and was developed to be used on very steep slopes.
The slopes were re-graded slightly, dressed and compacted with a sheepsfoot mounted to a winch that was attached to the dozer at the top of the slope. Any existing tree root systems were pruned back but left intact during the re-grading to provide added stability to the slope surface.
INSTALLATION BENEFITS & RESULTS
The EnkaMat was installed into an anchor trench (doubled as a utility trench) at the top of the slope and deployed downhill. It was backfilled and a soil berm was then installed at the top to eliminate runoff from flowing over onto the slope. The EnkaMat was intentionally cut and fashioned around any tree or root penetrations and fastened securely around these penetrations. This technique is possible with EnkaMat because unlike the typical chopped-fiber TRM, it does not unravel or lose its structural integrity when cut in the field.
The anchoring devices used to hold the EnkaMat on the slopes included 10 inch U-shape staples and 18 inch J-hooks made from #3 re-bar.
After the EnkaMat was installed, straw wattles were installed 2/3 of the way down the slope to slow down any runoff water. 7,000 square yards of EnkaMat was installed on the two slopes. An irrigation system was installed on top of the EnkaMat. Iron Horse Construction installed sprigs of rosemary into the EnkaMat 18 inches on center for the vegetation cover. The contractor and the project manager were very pleased with how well the EnkaMat conformed to the contours of the terrain providing intimate contact with the soil. The construction crew also commented on the improved traction and workability once the EnkaMat was installed.