Abstract

Abstract Necessity to use envelope (filter) in subsurface drainage systems is one of the important instances that could be taken into consideration in the beginning of a drainage project. The first indicator of the need for a drain envelope is the percentage of clay in the soil. In this research, permeameter test was used to determine gradient ratio and evaluate the clogging potential and probability of soil particle passage through drain plate openings in three clay and clay loam soils (amount of clay between 25 – 40 percentage) with different chemical qualities (SAR between 17 - 83 and EC between 10 – 100ds/m). Treatments in this experiment were drainage systems with and w/o envelopes. In system with envelope two types of envelopes (granular and fiber) were used. Through conducting this experiment, outflow variation, system permeability, gradient ratio and exit gradient were investigated. Results showed that most of the time the values of gradient ratio in the system w/o envelope was more than one which indicates clogging potential. Nonetheless, soil downfall was occurred when this Index was measured greater than three. The ratio of outflow from the systems with granular and fiber envelopes to systems w/o envelopes ranged 2.0-3.5 and 1.4-1.8, respectively. As hydraulic gradient was increased, system hydraulic conductivity decreased with the greater decrease occurring in the system w/o envelope. Also, with determination of hydraulic gradient failure and comparison of that with exit gradient value in each gradient head, soil particle resistance  against pressure head was studied. Results indicated that the system w/o envelope in soil samples of 2 and 3 had the minimum and maximum performance receptively.         Keywords: Chemical Quality, Clay Percentage, Envelope, Failure Gradient, Permeameter Test