Part 5/8:
Piping is a phenomenon that occurs when water from an upstream source erodes soil particles, creating a "pipe" in the ground, which can lead to substantial structural failure. To prevent this, the first criterion states that the diameter at which 15% of the filter's material passes (D15) must be less than five times the diameter at which 85% of the core material passes (D85).
For instance, if D85 is measured at 3 mm, then the filter's D15 must be less than 15 mm to meet the criterion.