The main purpose of a filter in an earthen dam is to

Allow water to pass while preventing soil particles from being carried away, preventing piping

Increase the phreatic line height

Which criterion makes the filter much more pervious than the base?

D15(filter)/D15(base) > 5 (and < 40)

Filter must have 20% passing No. 200 sieve

When is a multi-layer filter used?

When one layer cannot satisfy both retention and permeability rules, each layer designed for the previous one

2072 Kartik · 4 marks

Filter material in earthen dam

Short answer

A filter prevents piping by retaining base soil while allowing free drainage to lower the phreatic line and protect the toe. It is designed with Terzaghi’s criteria: D₁₅(filter)/D₈₅(base) < 5 for retention, D₁₅(filter)/D₁₅(base) > 5 for permeability, maximum 5% fines, and thickness 0.3–1.0 m.

Write the purpose of use of filter material in an earthen dam. Explain its design principle.

A filter is a graded layer of sand-gravel placed between the fine soil of an earthen dam (the base soil) and a drain or a coarse zone. It lets seepage water pass but stops soil particles from being carried away.

Purposes of the filter

Prevents piping and internal erosion by holding back the fine particles of the dam body or foundation.
Allows seepage to leave the dam freely, so the phreatic line is lowered and the pore pressure at the d/s toe is relieved.
Prevents the drain or rockfill from being clogged by soil.
Provides a transition between the fine core and the coarse shell, so that core material cannot be washed into the shell and any cracks in the core are sealed.
Protects the d/s face and toe from sloughing and boiling.

Design principle

A filter must satisfy two opposite requirements: its voids must be small enough to retain the base soil, and it must be much more pervious than the base soil. The Terzaghi-Bertram (USBR) criteria are used:

Requirement	Criterion
Piping (retention)	$\dfrac{D_{15}(\text{filter})}{D_{85}(\text{base})} < 5$
Permeability (drainage)	$\dfrac{D_{15}(\text{filter})}{D_{15}(\text{base})} > 5$ (and $<40$ )
Parallel gradation	$\dfrac{D_{50}(\text{filter})}{D_{50}(\text{base})} < 25$
Fines	Not more than 5% of the filter passes the No. 200 sieve; the filter must not be gap-graded

where $D_x$ is the particle size at which $x$ % of the soil is finer.

Other points: the grading curve of the filter should be roughly parallel to that of the base soil. If one layer cannot satisfy both rules, a multi-layer filter is used, each layer designed for the one before it. Typical thickness is 0.3-0.6 m for a horizontal blanket and 1.0 m or more for a chimney drain. The filter should be compacted lightly to avoid segregation and crushing of the grains.

✓ Answer checked by Claude Opus 5.5 (high effort)

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