The necessity of the use of subsurface drainage envelopes (envelopes) is one of the major concerns which are brought up in the first stages of design and construction of a drainage project. Clay percentage of soil is the first index to predict this requirement. In this study, in order for the calculation of gradient ratio (GR) and the assessment of clogging potential and soil particles movement into the drainpipe, the permeameter test was carried out on three samples with clay and clay loam textures. 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, discharge variation, system permeability, gradient ratio and exit gradient were investigated. The results showed that the values of gradient ratio in the systems without envelope in most cases were greater than one which indicates high particle movement potentials. Nevertheless, soil particles movement happened when the values of this index exceeded 3. The ratio of outflow from the systems with mineral and synthetic envelopes to the ones without envelope ranged 2.0-3.5 and 1.4-1.8, respectively. As hydraulic gradient was increased, system hydraulic conductivity decreased in a way that the greater decrease happened in the system without envelope. Furthermore, by the calculation of hydraulic failure gradient and its comparison to exit gradient at different hydraulic gradient values, the resistance of soil particles against flow pressure was analyzed. The results indicated that the system without envelope had the least and the most performance in samples No. 2 and 3, respectively.