• Filter drains should be design in such a way that all the seeping water through the dam is effectively drained off.
  • The filter consists of more than one layer.
  • The filter layer which comes in contact with the seeping water first is of fine material.
  • Subsequent layers of filter are made of sand of increased coarseness.
  • The last layer from which water comes out of the filter is made of gravel.
  • The filters of filter drains are also know reverse or invert filter.
  • The soil of earth dam and foundation surrounding the filter is know the base material.
  • According to earth manual of US Bureau of Reclamation Washington 1960 following four main requirements should be satisfied.
  • Filter material should be fine and poorly grad so that the voids in the filter are small and thus prevent base material from entering the filter.
  • The filter material should be coarse and pervious in relation to base material.
  • This aspect facilitates rapid removal of seeping water without building up any seepage force within the filter.
  • The openings or perforation in the pipe drain should be adequate to admit all the seeping water safely.
  • The thickness of filter material should be sufficient to provide a good distribution of all particle sizes, throughout the filter.
  • The thickness should also be adequate to provide safety against piping.
  • The D15 size of filter material must not be more than 4 to 5 times D85 size of the base material.
  • This prevents the foundation material from carrying through the pores of the filter material.
  • This keeps seepage forces within the filter to permissible limit.

\[\frac{D_{15}-of-filter}{D_{85}-of-base-material}< 4to5< \frac{D_{15}-of-filter}{D_{15}-of-base-material}\]

  • The requirements cited above should be satisfied.

\[(i)\frac{D_{15}-of -filter}{D_{15}-of -base- material}= 5-to-40,provided-that- the -filter -does- not -contain -more- than -5%- of -material -finer -than -0.075- mm.\]

\[(ii)\frac{D_{15}-of- filter }{D_{85}-of- base -material}= 5-or-less.\]

\[(iii)\frac{D_{85}-of- filter}{Maximum- opening -of -perforation -of- the- pipe}= 2-or-more\]

(iv) The grain size curve of the filter material should be about parallel to the curve of the base material.