he application of filters as a protective measure is a must
in the construction of closed drainage in fine sand, run-
ning ground or loose soil irrespective of the pipe mate-
rial used. Even with small gradients these soils penelrate into
the drain with the seepage stream. The staff of the Central
Asian Irrigation Research Institute have conducted laboratory
and field tests of natural sand, sand-gravel mixtures and pipe-
filters as drainage screens. This research has resulted in recom-
mendations which make it possible to design economically sub-
stantiated filter compounds comprising natural components
which ensure a required degree of stability and water intake
capacity of closed drainage for loose soil.
In order to ensure the seepage stability of the ground the
maximum diametre of the filter pores must be smaller than the
rated value as determined by the formula

лн И el em, (1)

J 4 cos ©
Неге f (Wn) is equivalent to the following:
at Wp <2 (\, ) = 0.025 $д.ст;
а! 2 «< М, < 7 {(Wn)= 0.1 sq. cm;
а! Wy > 7 К( №, ) =0.17 sq. cm.

Here J— maximum head gradient in the drainage zone,;
0 —angle between the seepage velocity direction and
gravity,
Wy — plasticity factor: