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Appendix A: Design Procedure for Riprap Armor
A.2.4 GRADATION
(1) The gradation of stones in riprap revetment affects the riprap's resistance to
erosion. The stone should be reasonably well graded throughout the in-place
layer thickness. Specifications should provide for two limiting gradation curves,
and any stone gradation as determined from quarry process, stockpile, and in-
place field test samples that lies within these limits should be acceptable. Riprap
sizes and weights are frequently used as D30(min), D100(max), W50(min), etc. The
D or W refers to size or weight, respectively. The number is the percent finer by
weight. The (max) or (min) refers to the upper or lower gradation curves,
respectively. A standard form for plotting riprap gradation curves is provided
as Figure A.1. The gradation limits should not be so restrictive that production
costs would be excessive. The choice of limits also depends on the underlying
bank soils and filter requirements if a graded stone filter is used.
(2) Standardized gradations having a relatively narrow range in sizes (D85/D15 = 1.4 -
2.2) are shown in Table A.1. Other gradations can be used and often have a
wider range of allowable sizes than those given in Table A.1. One example is the
Lower Mississippi River standardized gradations (see EM 1110-2-1601) that are
identical to the Table A.1 gradations except that the W50(max) and W15(max)
weights are larger, making them easier to produce. Most graded ripraps have
ratios of D85/D15 less than 3. Uniform riprap (D85/D15 < 1.4) has been used at
sites on the Missouri River basin for reasons of economy and quality control of
sizes and placement.
(3) Rather than using a relatively expensive graded riprap, a greater thickness (1-1/2
to 2 times that of graded stone as indicated in paragraph e) of a quarry-run stone
may be considered. Some designers consider the quarry stone to have another
advantage: the gravel and sand-size rock present in the quarry stone provide a
rudimentary filter. This concept has resulted in considerable cost savings on
large projects such as the Arkansas and Red River navigation projects in the
United States. Not all quarry-run stone can be used as riprap; stone that is gap-
graded (some sizes missing from gradation) or has a large range in maximum to
minimum size is probably unsuitable. Quarry-run stone for riprap should be
limited to D85/D15 # 7.
(4) Determining optimum gradations is also an economics problem that includes the
following factors:
Rock quality (durability under service conditions);
Cost per ton at the quarry (including capability of quarry to produce
a particular size);
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