Appendix A: Design Procedure for Riprap Armor
Number of tons required;
Miles transported;
Cost of transportation per ton-mile;
Cost per ton for placement;
Quality control during construction (it is easier to ensure
even coverage with a narrow gradation than with a wide
gradation); and
Need for and cost of filter.
On large project involving many different design conditions, savings in total
cost can often be realized by using a few standard gradations, selecting the standard
gradation next up from the computed gradation.
A.2.5 LAYER THICKNESS
All stones should be contained reasonably well within the riprap layer
thickness to provide maximum resistance against erosive forces. Oversize stones,
even in isolated spots, may result in riprap failure by precluding mutual support and
interlock between individual stones, causing large voids that expose filter and bedding
materials, and creating excessive local turbulence that removes smaller size stone.
Small amounts of oversize stone should be removed individually and replaced with
proper size stones. When a quarry produces a large amount of oversized stone,
consideration should be given to changing the quarrying method, using a grizzly (sieve
for large rock) to remove the oversize stone, obtaining the stone from another source,
or increasing the riprap layer thickness to contain the larger stone. The following
criteria apply to the riprap layer thickness:
It should not be less than the spherical diameter of the upper limit W100 stone
or less than 1.5 times the spherical diameter of the upper limit W50 stone, whichever
results in the greater thickness. The thickness thus determined should be increased
by 50 percent when the riprap is placed underwater to provide for uncertainties
associated with this type of placement. At one location in the Missouri River basin
in the United States, divers and sonic sounders were used to reduce the underwater
thickness to 1.25 times the dry placement thickness.
A-9
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