Appendix A: Design Procedure for Riprap Armor
estimates of these parameters are used in the analysis, the added safety
factor should not be used. The safety factor should be based on the
anticipated error in the values used. It should not be relied on to correct
inaccurate or assumed stone sizing parameters. The depth-averaged
velocity over the riprap is the parameter to which the rock size is the most
sensitive. A 10 percent change in the velocity will result in a nearly 100
percent change in the weight limits of the riprap gradation (based on a
sphere) and about a 30 percent change in the riprap thickness. The riprap
size is also quite sensitive to the unit weight of the rock to be used: A 10
percent change in the unit weight will result in a 70 percent change in the
weight limits of the riprap gradation (based on a sphere) and about a 20
percent change in the riprap thickness. The unit weight of the rock can be
determined only from tests of samples from the quarry and is not exactly
representative of all the rock that the quarry will produce. The rock size is
not nearly as sensitive to the depth parameter as to the other two
parameters, but depth should not be neglected. Reliable estimates for all
three parameters should be made before the inclusion of a safety factor.
(c) Vandalism and/or theft of the stones is a serious problem in urban areas
where small riprap has been placed. A W50(min) of 80 pounds will reduce
the effects of theft and vandalism. Sometimes grouted stone is used around
vandalism-prone areas to prevent loss of riprap.
(d) The completed revetment will contain some pockets of undersized rocks, no
matter how much effort is devoted to obtaining a well-mixed gradation
throughout the revetment. This placement problem can be assumed to
occur on any riprap job to some degree but probably more frequently on
jobs that require stockpiling or additional handling, and where improper
placement methods are used. Therefore a larger safety factor should be
used when stockpiling, additional hauling, or poor placement methods will
be used.
(e) The safety factor should be increased where severe freeze-thaw is
anticipated.
The safety factor based on each of the above considerations should be
considered separately and then the largest of these values should be used in
Equation (A.3).
A-20
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