California Department of Transportation
 

Fiber Reinforced Earth Fill (FREF) With Brush Layering

Brush Layering on a Fil lSlope

What is This Treatment?

Brush layering work includes harvesting green cuttings from existing alder, cottonwood or willow stands and embedding these cuttings in horizontal layers perpendicular to the slope face. The cuttings extend back into the slope similar to geotextile fabric, acting as reinforcement. Unlike conventional reinforcement, brush layers root along their lengths and act as horizontal slope drains. This drainage function can greatly improve mass slope stability.

 

When to Use This Treatment?

Best used on fill slopes, revegetation, and stream bank stabilization projects, but can also be used on fill slopes. This treatment is used to provide erosion protection, increase vegetative cover, and enhance habitat.

Benefits:

  • The strongest biotechnical slope stabilization technique that does not incorporate hard engineering structures.
  • Provides immediate slope reinforcement from unrooted brush cuttings and horizontal geotextiles. As roots develop, improves slope stability and
    shear resistance by creating a rooting matrix with geogrids throughout structural lifts.
  • Creates slope breaks that shorten slope length and reduce runoff velocities.
  • Creates vegetative filters for debris.
  • Increases infiltration rates on dry sites.
  • Acts as a horizontal drain on wet sites.
  • Provides for vegetation establishment, cover, and natural recruitment.
  • Creates immediate water quality improvements.
  • Can be modified for use in small drainages for nickpoint/ headcut stabilization by hand crews.

Limitations:

  • Nonstandard practice - requires design and approval by a Civil (Geotechnical) Engineer based on site calculations.
  • Requires accessible, local stand of alders, cottonwoods or willows from which to harvest cuttings.
  • Unsuitable for rocky slopes, slopes over 1:1, or slopes with limited equiptment access.
  • Must have solid footing.
  • May require reconstructed rock footing in stream environments.
  • Limited overland flow dependent on fabric type.
  • Rooting and establishment of brush may be less successful if installed during dry season or without irrigation.
  • Temp. irrigation may be required during establishment period, particularly during first dry season and on south-facing slopes.

Technical Design Tips:

  • Should be designed under the direction of and approved by a registered professional civil engineer based on site calculations.

  • May require weeding and supplemental irrigation during the establishment period.

  • Maximum slope gradient of 0.5:1 (H:V) has been used in extreme cases for slope tie-ins to natural grades.
  • Geotechnical and hydrologic conditions must be evaluated.
  • Subsurface drains may be needed on dense soil types.
  • Fabric tensile strength and coarseness need to match application
    conditions. Use of natural, biodegradeable fabrics vs. long-lasting
    geotextiles should be weighed.
  • Typical encapsulated lifts range from 1 to 3 feet thick Topsoiling may be used between lifts.
  • Must have solid footing.

Specifications

Details:

Cost:

Updated 04-15-2014