Appendix A: Design Procedure for Riprap Armor
No stone shall have a/c exceeding 3.5.
To determine stone dimensions "a" and "c," consider that the stone has a long axis,
an intermediate axis and a short axis each being perpendicular to the other. Dimension "a"
is the maximum length of the stone which defines the long axis of the stone. The intermediate
axis is defined by the maximum width of the stone. The remaining axis is the short axis.
Dimension "c" is the maximum dimension parallel to the short axis. These limitations apply
only to the stone within the required riprap gradation and not to quarry spalls and waste that
may be allowed.
A.2.2 RELATION BETWEEN STONE SIZE AND WEIGHT
The ability of riprap revetment to resist erosion is related to the size and weight of
stones. Design guidance is often expressed in terms of the stone size D%, where % denotes
the percentage of the total weight of the graded material, including quarry wastes and spalls,
that contains stones of less weight. The relation between size and weight of stone is
described herein using a spherical shape by the following equation:
1/3
6W%
D% '
(A.1)
s
where
D% = equivalent-volume spherical stone diameter (ft),
W% = weight of individual stone having diameter of D%, (lbs), and
= saturated surface dry (SSD) specific weight of stone (lbs per cu. ft).
Design procedures for determining the stone size required to resist the erosive forces of
channel flow are presented in Section A.4.
A.2.3 UNIT WEIGHT
Unit weight of stone s generally varies from 150 to 170 pounds per cubic foot.
Riprap sizing relations are relatively sensitive to unit weight of stone, and s should be
determined as accurately as possible. In many cases, the unit weight of stone is not accurately
known because the quarry is selected from a list of approved riprap sources after the
construction contract is awarded. Under these circumstances, a unit weight of stone close
to the minimum of the available riprap sources should be used in design. Contract options
covering specific weight ranges of 5 or 10 pounds per cubic foot should be offered when
sufficient savings warrant.
A-5
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