Answer:

Relationship between backfill material and soil to be drained

To ensure effective drainage of soil, it is important to use a backfill material that is more pervious than the soil to be drained. The relationship between the particle size distribution of the backfill material and the soil to be protected is defined by the D15 and D85 values.

What are D15 and D85 values?

- D15: It is the particle size diameter below which 15% of the soil or backfill material particles are finer.
- D85: It is the particle size diameter below which 85% of the soil or backfill material particles are finer.

Relationship between D15 and D85 values

- If (D15)Filter ≤ 5 (D85)Protected soil: This relationship indicates that the backfill material has a smaller particle size than the soil to be protected. This can lead to clogging of the drainage system due to the fine particles in the backfill material being transported into the soil to be protected.
- If (D15)Filter ≥ 5 (D85)Protected soil: This relationship indicates that the backfill material has a larger particle size than the soil to be protected. This can lead to the formation of voids between the backfill material and the soil, which can reduce the effectiveness of the drainage system.
- If (D15)Filter ≥ 5 (D15)Protected soil: This relationship indicates that the backfill material has a larger particle size than the soil to be protected. This can ensure that the backfill material remains in place and does not migrate into the soil to be protected. Additionally, it allows for the formation of a permeable interface between the backfill material and the soil to be protected, which promotes effective drainage.
- If (D15)Filter ≤ 5 (D15)Protected soil: This relationship indicates that the backfill material has a smaller particle size than the soil to be protected. This can lead to clogging of the drainage system due to the fine particles in the backfill material being transported into the soil to be protected.

Conclusion:

Therefore, the correct relationship between the D15 and D85 values of the backfill material and the soil to be drained is (D15)Filter ≥ 5 (D15)Protected soil to ensure effective drainage.