15 March 2001
If dispersive clays are encountered, the designer must obtain the services of an expert that has
had previous experience with detecting and determining the best way to handle these clays.
Dispersive clays normally deflocculate when exposed to water of low salt content, the opposite of
aggregated clays that remain flocculated in the same soil-water systems. Generally, dispersive
clays are highly erosive, are subject to high, shrink-swell potential, and have lower permeability
rates than aggregated clays.
188.8.131.52. The filter material must be much more permeable than the material being drained. The
grain size curve of the filter should be smooth (no gap grading) and parallel to or flatter than that
of the protected material. The following conditions should be met to permit water movement.
d15 percent passing size of filter material
d15 percent passing size material being drained
To prevent clogging of perforated pipe or screens, the following criteria should be met:
d85 percent passing size of filter material
> 1.2 slot width or 1.0 hole diameter
slot or hole diameter
To prevent clogging of the openings in porous pipe, the following criteria should be met: