This standard provides design criteria for protective granular filters in hydraulic structures like dams and weirs. Its primary purpose is to prevent internal erosion (piping) of fine soil particles due to seepage, while simultaneously allowing water to drain freely to relieve pore pressure. The design is based on specific ratios between the particle sizes of the filter material and the base soil being protected.
Lays down criteria for the design of granular filters to protect hydraulic structures from piping and erosion.
Quick Reference Values
Piping (retention) ratio D15(filter)/D85(base)<= 5
Permeability ratio D15(filter)/D15(base)4 to 20
Uniformity ratio D50(filter)/D50(base)<= 25
Max fines (<75 micron) in filter material5%
Maximum particle size in filter75 mm
Filter opening size to prevent movement of uniform grain size base materialD85 (base)
Key Formulas
D15(F) / D85(B) ≤ 5 — Retention (Piping) Criterion: Prevents base material from entering the filter.
4 ≤ D15(F) / D15(B) ≤ 20 — Permeability Criterion: Ensures filter is permeable enough to allow drainage.
D50(F) / D50(B) ≤ 25 — Uniformity Criterion: Ensures filter and base gradations are not excessively different.
Practical Notes
The criteria are primarily for cohesionless base soils. Dispersive clay soils require special investigation and cannot be designed for using these criteria alone.
Ensure filter materials are clean, hard, durable, and free from organic matter. The particles should be rounded rather than angular.
For critical structures or non-uniform base soils, constructing a graded filter with multiple layers may be necessary to satisfy the criteria between each layer.