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I am dealing with a specification that reads as follows: "Coarse filter shall have D15/d85 </= 4, where D15 is the particle size corresponding to 15% finer by weight of the coarse filter and d85 is the particle size corresponding to 85% finer by weight of the fine filter." What would be the best way to determine this calculation? The conventional way of determining this value does not seem to apply since 85% finer by weight normally applies to coarse material and 15% finer by weight normally applies to fine material.

RE: Gradation Requirements

"The D85 (root zone) is defined as the particle diameter below which 85% of the soil particles (by weight) are smaller. The D15 (gravel) is defined as the particle diameter below which 15% of the gravel particles (by weight) are smaller. For bridging to occur, the D15 (gravel) must be less than or equal to eight times the D85 (root zone)."

Does the above answer your question? https://plantscience.psu.edu/research/centers/ssrc....

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