Technical Notes


The indiscriminate dropping of large stone rip-rap on an exposed geotextile will cause severe damage in the form of holes and loss of tensile strength. If drop heights cannot be controlled, either high survivability geotextile properties must be specified or a bedding layer must be placed above the geotextile.

Reverse filters commonly associated with erosion control construction specifications reflect the need to control the flow of water over the surface of the geotextile. As such, specific construction criteria should include the following:

  • Slopes steeper than 2.5 to 1 should not be used without laboratory testing to confirm stable soil-geotextile friction angles.

  • Slopes should be graded to provide a smooth, fairly level surface. The geotextile should be laid with the machine direction of the geotextile placed parallel to the slope. Folds and wrinkles of the geotextile should be avoided.

  • Adjacent rolls should be shingle overlapped in down slope or down stream direction at least 12 inches or should be seamed. Overlapped seams may be secured using metal pins as necessary.

    The Corps of Engineers specify steel securing pins, nominally 3/16 in. in diameter, 18 in. long, pointed at one end and fitted with 1.5-in.-diameter washers at the other end for use in securing geotextiles in firm soils. The pin spacing is a function of the slope with the following spacing is recommended:

    Slope Pin Spacing in All Directions
    3:1 2 ft
    3:1 to 4:1 3 ft
    4:1 5 ft
    >4:1 6 ft

  • The placement of stone cover should begin at the base of the slope and at the center of the geotextile-covered zone. The placement of the cover material may result in the tensioning of the underlying geotextile.
  • For geotextiles having properties exceeding that required for "protected" applications but without a protection layer, the height of drop for stones less than 250lb should be placed without free-fall.

  • For geotextiles with a 6 inch sand protection layer and having properties exceeding that required for "protected" applications, or geotextiles having properties exceeding that for "unprotected" applications, the height of drop for stones less than 250 lb should be placed with less than 3 feet of free-fall.

  • Stone weighing in excess of 100 lb should not be allowed to roll along the surface of the geotextile.

  • Contouring of the stones should be achieved during their initial placement, with no grading of the stones after placement allowed.

The handling of the termination of the geotextile at the top and bottom of the slope or channel is important. If done poorly, erosion can occur thereby undermining the slope and losing stability of the overlying system. Thrace-LINQ suggests you key the geotextile into the insitu soil at both the top and bottom at your slope. Do not key the geotextile at the top of slope until the armor layer is in place to the top of the slope.