D15 (Filter )

5<

< 40

(6.25)

D15 (Base)

D15 (Filter )

<5

(6.26)

D85 (Base)

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.

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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