Filter requirements to control piping

A protective filter is a layer of graded granular material placed between the base soil (the soil being protected) and a drain or coarse material, to allow water to escape without letting soil particles wash out.

Basic requirements

1. Retention (piping) criterion: the filter pores must be small enough to hold back the particles of the base soil.
2. Permeability criterion: the filter must be much more permeable than the base soil, so that no pore pressure builds up and the water escapes freely.
3. The filter must be well graded, with no fines that could migrate, and be stable (no segregation) and durable.
4. It must be thick enough, and not clog; in multiple layers, each layer must satisfy the criteria with respect to the one next to it.

Terzaghi's filter rules for controlling piping

$$\frac{D_{15}(\text{filter})}{D_{85}(\text{base soil})} < 4 \text{ to } 5 \quad (\text{piping})$$ $$\frac{D_{15}(\text{filter})}{D_{15}(\text{base soil})} > 4 \text{ to } 5 \quad (\text{permeability})$$

Additional (U.S. Bureau / Corps of Engineers) conditions: $D_{50}(\text{filter})/D_{50}(\text{soil}) < 25$ and the grain-size curve of the filter is roughly parallel to that of the soil. For slotted pipes, $D_{85}(\text{filter}) >$ slot width (or 1.2 times slot width).