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RECP Flap With Brush Layering (Nonstandard)
What is This Treatment?
This treatment involves using locally obtained green cuttings, coir netting, a Rolled Erosion Control Product such as coir netting, structural backfill material, and local topsoil to construct embankment (fill) slopes. More specifically, this treatment consists of placing layers of:
- Geosynthetic reinforcement - typically placed 2' on center vertically.
- Backfill - typically structural material.
- Rolled Erosion Control Product - coir/coconut blankets placed every other geosynthetic layer or 4' on center vertically. The blankets are placed to "flap over" and protect the slope face.
- Local topsoil or compost to provide a rooting media for cuttings.
- Green locally harvested cuttings of cottonwood or willow.
When to Use This Treatment?
- Use for fill or reconstructed cut slopes between 2:1 and 1.5:1 (H:V). Most commonly used to construct 1.5:1 (H:V) fill slopes.
- For slopes steeper than 2:1 (H:V) a Geotechnical Design Report should be prepared by Caltrans Division of Engineering Services (DES) Office of Geotechnical Design. In addition, a preliminary evaluation may be required.
- Coordinate the use of this technique with Caltrans Division of Engineering Services (DES) Office of Geotechnical Design.
Consider Using With:
How is This Treatment Constructed?
- Structural backfill is placed in 8-inch lifts, and "keyed-in" to the adjacent existing slope at least 6.5 feet horizontally. Lifts of backfill are brought to 90% compaction by tractors (track-mounted crawlers).
- A geosynthetic reinforcement layer is placed horizontally between structural backfill lifts, typically every 2' on-center vertically.
- Following placement of the first geosynthetic reinforcement layer (and at every other primary geosynthetic reinforcement layer thereafter) coir netting is placed and fastened longitudinally with fabric anchors.
- A layer of local topsoil and compost is placed on top of the coir netting.
- A layer of locally harvested cuttings is placed on top of the local topsoil and compost growth media.
- Placement of structural backfill and geosynthetic reinforcement continues in layers, like a cake. At every other geosynthetic reinforcement layer, typically every 4' on-center, the coir netting is draped or flapped over the slope face, and a layer of harvested cuttings, topsoil, and compost is put in place.
Note: This typical section is schematic only and can not be used in a contract document. The scale, key dimensions, and critical details have purposely been omitted.
Photos of Installation
Photographs of installation sequence for coir flap with brush layering.
- 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.
- Cuttings create slope breaks that shorten slope length and reduce runoff velocities.
- Promotes vegetation establishment, cover, and natural recruitment.
- Since this treatment does not require wrapping the backfill, it is less labor intensive to construct than the Coir Confinement.
- Slopes greater than 2:1 (H:V) are too steep to be compacted by trackwalking. Coir confinement provides the resistive force necessary to hold the soil firmly in place in lieu of trackwalking.
- The slope face is stabilized and protected as the embankment is constructed. In the event of a sudden storm event exposed soil surfaces are protected. Construction can resume rapidly following a storm event.
- Yields a stable and visually aesthetically pleasing slope compatible with its natural surroundings.
- Promotes vegetation establishment and natural succession.
- Unsuitable for embankment (fill) slopes steeper than 1.5:1 (H:V), or slopes with limited access.
- Requires accessible, local stand of cottonwoods or willows from which to harvest cuttings.
- May require supplemental irrigation during establishment period, particularly during first dry season and on south-facing slopes.
Technical Design Tips:
- Always protect the face of embankment (fill) slopes steeper than 2:1 (H:V) to prevent the slumping of soil from between horizontal geotextile layers.
- Strongly consider covering (flapping) the face of 2:1 (H:V) embankment (fill) slope faces, based upon evaluation of constraints to compaction, angle of repose, backfill material, and rainfall intensities.
- Backfill lifts typically range from 2 - 4 feet thick. Consider specifying Local Topsoil in the outer face of structural backfill lifts.
- Fill slopes steeper than 2:1 (H:V) will typically require reinforcement by a geosynthetic fabric such as a geogrid. Geosynthetic reinforcement strength needs to be sufficient to meet slope engineering requirements and should be specified by a geotechnical engineer.
- Consider the benefits and liabilities of natural geosynthetic products versus longer lasting inorganic (plastic) products.
- Specifications and details under development.
- RECP Flap with Brush Layering prices not currently available..
- David W. Yam, " Slope Face Stabilization For Critical Slope Surfaces", State of California, Department of Transportation, District 04, 2008.
- Bowers H.D. "Erosion Control on California State Highways", State of California Division of Highways, 1949.