Critical filter is one of the principal parts in an embankment dam which is preventing the dam
against erosion. There are important technical and economical problems for engineers to select the
filter characteristics which are acceptable from view points of necessity and economical aspects.
Since, experimental methods may cover different parameters of the soil-filter system; experimental
modeling is characterized as the most appropriate method to investigate soil-filter system. No
Erosion Filter (NEF) test has a high capability to evaluate the soil-filter system. Most design criteria
and many researches for grading curve of filter only pay attention to the size of D15 of filter and
there are fewer attentions to smaller size and fine content of filter. In this research using two groups
of filters which have same grading curve in larger size of D15 but are different in smaller size, 24
NEF tests were performed. Contrary to existing criteria, the obtained results revealed considerable
effects of fine content and the particles which are smaller than D15 and these factors can cause
improvement or weakening filter performance to control soil erosion.