Installation Guideline: Riprap

Geotextile Placement

  1. Place the geotextile without wrinkles or folds on a smooth graded surface approved by the project engineer.
  2. Orient the geotextile with the machine direction parallel to the direction of water flow. This is normally parallel to the slope for erosion control runoff and wave action and parallel to the stream or channel for stream bank and channel protection.
  3. Use key trenches or aprons at the crest and toe of the slope to anchor the ends of the geotextile. 18" anchoring pins may be an acceptable option to expedite construction.

Joining Adjacent Sheets

  1. Successive sheets of the geotextile can be overlapped upstream over downstream and/or over down slope in a "shingle effect."
  2. Overlap adjacent rolls a minimum of 12" in all instances except when placed under water. Overlaps under water should be a minimum of 3'.
  3. In cases where wave action or multidirectional flow is anticipated, all adjoining sheets perpendicular to the direction of flow should be sewn.
  4. Soil CBR will determine if overlapping or sewing is the correct option. AASHTO offers these general guidelines for sewing versus overlapping:
    Soil CBR > 3    Minimum overlaps of 0.3 - 0.45 meters
    Soil CBR 1-3    Minimum overlaps of 0.6 - 1.00 meters
    Soil CBR < 0.5 Must be sewn

Riprap Placement

  1. Begin the riprap placement at the toe and proceed up the slope.
  2. Avoid stretching and tearing the geotextile. Do not drop heavy riprap from a height of more than 12". Do not drop smaller sizes of riprap from a height exceeding 3' unless it can be demonstrated that the placement procedures will not damage the geotextile. Do not allow riprap with a mass of more than 225 lbs to roll down the slope.
  3. Field monitoring should be performed to verify that the riprap placement does not damage the geotextile.
  4. Backfill all voids in the riprap with smaller stone to ensure full coverage.
  5. For underwater applications, place the geotextile and backfill material on the same day.
  6. After placement of the riprap, avoid any grading above the geotextile that results in movement of the riprap.

Repair

  1. In lieu of specific project guidelines, overlap the damaged geotextile by a minimum of 36" in all directions with the replacement geotextile.

Storage

  1. Geotextile rolls are wrapped in a UV protective cover.
  2. Contractor should insure rolls are adequately protected from moisture.
  3. If stored outdoors, the geotextile should be elevated from the ground and covered with a tarpaulin or opaque plastic.