Washout of Grains from Filtered Sand and Gravel Materials

Some presently used filter criteria are illogical. Besides satisfying $D_{15}/d_{85}\le 5$ and $D_{15}/d_{15}\ge 4$, both the base and filter materials must possess grading stability when exposed to seepage. There are two reasons for grading instability; scantiness of intermediate grain sizes, which interrupts an internal filter formation process; and loose grains, which move through the void space between fixed grains. A mathematical model for washout of grains and internal filter formation (self‐filtration) in cohesionless materials is developed on the basis of grain and pore geometry considerations and found to agree with results from laboratory tests on sand and gravel mixtures. The model discloses both scantiness and loose grains and predicts the extent of grain washout and the volume decrease caused by the washout. For soils with a large content of fine grains, the hydrodynamic number is shown to have considerable influence on the extent of the washout of grains.