To make certain that the backfill material is more pervious than the soil to be drained, the relationship used is

(D_{15})_{f i l t e r} \leq 5 (D_{85})_{p r o t e c t e d s o i l}

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(D_{15})_{f i l t e r} \geq 5 (D_{85})_{p r o t e c t e d s o i l}

No worries! We‘ve got your back. Try BYJU‘S free classes today!

(D_{15})_{f i l t e r} \leq 5 (D_{15})_{p r o t e c t e d s o i l}

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(D_{15})_{f i l t e r} \geq 5 (D_{15})_{p r o t e c t e d s o i l}

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Solution

The correct option is D $(D_{15})_{f i l t e r} \geq 5 (D_{15})_{p r o t e c t e d s o i l}$
The filter material should be coarse enough to allow easy passage for percolating water which can move easily without any build up of water pressure in the filter. For the filter to provide full drainage,
$k_{f i l t e r} \geq 25 k_{p r o t e c t e d s o i l}$
Where k is the permeability and it is proportional
to square of particle size. So,
$D_{15 f i l t e r} \geq 5 D_{15 p r o t e c t e d s o i l}$

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