Surface Armor for Erosion Protection
Initial movement of stones within the mattress; and
A condition where the mattress shape has been deformed by stone movement,
but the mattress is still functional.
The tests indicated that ungrouted mattress thicknesses of nine inches (23 centimeters)
or less could withstand significantly higher velocities than previously believed. Grouting
would increase the allowable velocities even more. It was noted, however, that wire mesh
strength may be a major factor controlling mattress stability.
Filler stone sizes must be more uniform than for typical riprap. The smallest size must
be larger than the mesh openings, but the largest size must be small enough to eliminate large
voids between stones in the filled mattress. Streambed cobbles are sometimes used to reduce
cost where they are locally available.
Corrosion, abrasion, and vandalism can be minimized by grouting the gabions with a
sand-asphalt mastic or concrete. However, the accompanying loss of permeability may
require that special provision for hydrostatic pressure relief be provided. Also aesthetic and
environmental aspects of the project may suffer.
For corrosive or abrasive situations, previous users or manufacturer's representatives
should be consulted for information on measures used to ensure successful application under
similar conditions. Even in areas of good water quality, water chemistry may be such that
galvanized wire will corrode. For this reason, a polyvinyl chloride coating on the wires is
often specified.
Care in handling and filling is necessary to avoid damage to protective coatings,
especially with plastic coated gabions in cold weather. For work above water, filling in place
is preferable to filling before placing. Otherwise, extreme care must be taken during handling
to avoid deforming or damaging filled gabions.
Construction must be carefully supervised. Some crucial points, such as care during
filling, and complete lacing of the mattress components, are costly to a contractor's operation
and present a temptation for short cuts. Some handwork is usually necessary for proper
filling, and this in particular may be resisted by a contractor unless it is clearly specified.
On steep slopes, keying-in or anchoring the mattress at the top of the slope is
recommended.
For large jobs, a manufacturer may offer custom-sized gabions for optimum design
and construction efficiency.
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