D15 (Filter )
5<
< 40
(6.25)
D15 (Base)
D15 (Filter )
<5
(6.26)
D85 (Base)
6.13.1 PROBLEM 1 Filter Design
The properties of the riprap and base material are given in Table 6.6. Determine if filter is
needed between the riprap and the base material. Note: Generally, sediment sizes are given
in mm for the English system.
Table 6.6. Sizes of Materials.
Base Material (Sand)
Riprap (Gravel)
D85 = 1.50 mm
D85 = 24 mm
D50 = 0.75 mm
D50 = 12 mm
D15 = 0.38 mm
D15 = 6 mm
In accordance with the recommended sizes for filters:
D50 (Riprap)
12
=
= 16
D50 (Base)
0.75
which satisfies requirement 6.24
D15 (Riprap)
6
=
= 16
D15 (Base)
0.38
which satisfies the requirement 6.25
D15 (Riprap)
6
=
=4
D85 (Base) 1.5
which satisfies the requirement 6.26.
The riprap itself satisfies the requirements for the filter so no filter is needed.
6.13.2 PROBLEM 2 Filter Design
The following filter design is taken from Anderson et al. (1968). The properties of the base
material and the riprap are given in Table 6.7. Determine if a filter is needed.
6.71
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