Question 1

What's the purpose of a filter in an embankment dam?

Accepted Answer

A filter prevents internal erosion (piping). It must be (1) coarse enough to drain seepage water freely (no buildup of pore pressure), (2) fine enough to retain the protected base soil's particles, and (3) graded uniformly enough that filter particles don't migrate themselves. Modern dams typically have a chimney drain (vertical filter) plus a horizontal toe drain. The filter is the most critical safety element in an earthfill dam — most failures trace to inadequate or omitted filters.

Question 2

What is the piping ratio rule?

Accepted Answer

D15(filter) ≤ 5·D85(base). This Terzaghi-Sherard piping ratio ensures filter pores are small enough that the largest 15% of base particles bridge the openings and stop migration. For dispersive clay base soils Sherard 1989 tightened this to D15(filter) ≤ 9·D85(base) only if base D85 < 0.075 mm. NRCS NEH-633 (Soil Mechanics) and USACE EM 1110-2-1901 both adopt these ratios.

Question 3

What is the permeability ratio rule?

Accepted Answer

D15(filter) ≥ 4·D15(base) (some sources use 5). This ensures the filter is much more permeable than the base, so it actually drains water rather than itself becoming a barrier. Without this rule, water could pond inside the embankment and build pore pressure faster than seepage carries it away.

Question 4

What is uniformity coefficient Cu and why does it matter?

Accepted Answer

Cu = D60/D10. Sherard recommends Cu ≤ 6 for filter materials, with Cu ≤ 4 preferred. A wide gradation (high Cu) lets fines migrate through coarser pores within the filter itself, defeating its purpose. Uniformly graded sand-and-gravel is what you want; well-graded crushed stone with too much fines is what you don't.

Question 5

What's the maximum particle size for filter sand?

Accepted Answer

NRCS NEH-633 limits D100 ≤ 76 mm (3 inch), D90 ≤ 38 mm (1.5 inch). Larger particles are excluded because they don't contribute to filtering and risk segregation during placement. Filter sand should also have less than 5% passing the #200 sieve (no more than 5% finer than 0.075 mm) — fines clog the filter.

Question 6

What if my base soil is gap-graded or dispersive?

Accepted Answer

For gap-graded base soils, replace the natural D85 with D85 of the smaller end of the gap (the 'controlling' fraction). For dispersive clays (high SAR / sodium-rich), Sherard's tightened rule applies — and add a no-erosion filter test (NEF) per NRCS NEH-633 Appendix 26B. These cases warrant lab-verified design rather than relying solely on the screening ratios above.

Rule	Criterion	Reason
Piping (Terzaghi-Sherard)	D_15F / D_85B ≤ 5	Filter pores small enough to retain base D₈₅
Piping for fine clay base (D_85B < 0.075 mm)	D_15F ≤ 9·D_85B	Sherard's relaxed rule for very fine cohesive base
Permeability	D_15F / D_15B ≥ 4	Filter at least 4× more permeable than base
Uniformity	C_u = D_60F/D_10F ≤ 6	Avoid internal instability (filter eroding itself)
Maximum size	D_100F ≤ 76 mm (3 in)	Limit segregation during placement
Fines content	% passing #200 ≤ 5%	Avoid plugging the filter pores

Filter Criteria Calculator

Base soil (the protected material) — gradation

Filter material — gradation

Detailed criteria

Worked example

Example — filter for silty-clay core

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