Surface Armor for Erosion Protection
the placement of larger blocks may not be practical, and their in-place orientation may depend
more on chance than on design specifications.
When rubble contains large amounts of fines and/or oversize blocks, the layer
thickness should be increased generously over the theoretical riprap thickness that would be
required for the same site conditions.
A granular or fabric filter can be used to improve performance, but at the sacrifice of
economy. Some risk in performance is inherent in rubble, and the additional risk of using it
without a filter is usually accepted.
Slag is a granular material which is a by-product of steel-making. It is most
commonly known for its use as railroad track ballast. Advantages
Slag may be relatively inexpensive when available locally, and its use recycles material
that might otherwise be wasted. It is dense, durable, and angular, and is often available in a
range of sizes, which gives it the same basic properties as stone riprap. Disadvantages
Leachates from slag may affect water quality, and some displacement of slag by
persons searching for scrap steel has been reported. At one site on the Ohio River, some
spalling from weathering and subsequent erosion of the fines has been observed, but this has
not occurred at other sites. Typical Applications
Slag would be a suitable choice where it is the least costly effective armor material,
and where site conditions and chemical tests of the slag indicate that there would be no
detrimental effects on water quality.
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