Drainage Engineering and Salt Management

To ensure adequate hydraulic capacity without clogging, the permeability criterion for a granular drain envelope requires $\frac{D_{15 \text{ envelope}}}{d_{15 \text{ base}}} \ge 4$. If base soil has $d_{15} = 0.08\text{ mm}$, the minimum allowable envelope $D_{15}$ in mm is $4 \times 0.08$, which equals: