and
D50 (Filter) < 2 mm
The properties of the filter to be placed adjacent to the riprap are as follows:
D50 (Riprap)
< 40
(1)
D50 (Filter )
200
D50 (Filter ) >
= 5 mm
so
40
D15 (Riprap)
>5
(2)
D15 (Filter )
100
D15 (Filter ) >
= 20 mm
so
5
D15 (Riprap)
< 40
(3)
D15 (Filter )
100
D15 (Filter ) >
= 2.5 mm
so
40
D15 (Riprap)
<5
(4)
D85 (Filter )
100
D85 (Filter ) >
= 20 mm
so
5
Therefore, with respect to the riprap, the filter must satisfy these requirements
2.5mm < D15 (Filter) < 20 mm
D50 (Filter)
> 5 mm
D85 (Filter)
> 20 mm
These riprap filter requirements along with those for the base material are shown in Figure
6.39. Any filter having sizes represented by the double cross-hatched area is satisfactory. For
example, a good filter could have these sizes:
D85 = 40 mm
D50 = 10 mm
D15 = 4 mm
6.73
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