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The second criterion focuses on permeability, which measures how easily water can flow through the soil. The filter's D15 should be greater than five times the D15 of the core material to ensure that water drains adequately without creating high pore pressures. If D15 is 0.2 mm for the core, the filter must meet a minimum of 1 mm for appropriate permeability.
Gradation Control
Gradation control ensures that the filter maintains its effectiveness over time without significant erosion or clogging. The filter's D15 must be less than 25 times the D50 of the core's particles. Knowing the D50 of the core allows engineers to determine optimal sizes for the filter material effectively.