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

- 1
  
  ${(D_{15})}_{f i l t e r} \leq 5 {(D_{85})}_{p r o t e c t e d s o i l}$
- 2
  
  ${(D_{15})}_{f i l t e r} \geq 5 {(D_{85})}_{p r o t e c t e d s o i l}$
- 3
  
  ${(D_{15})}_{f i l t e r} \leq 5 {(D_{15})}_{p r o t e c t e d s o i l}$
- 4
  
  ${(D_{15})}_{f i l t e r} \geq 5 {(D_{15})}_{p r o t e c t e d s o i l}$

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