Laboratory Study of Irrigation Water SAR/Salinity Influence on Envelope Performance in Subsurface Drainage System

Necessity to use envelope is one of the more importance requirements in the design of subsurface drainage system. Sodium adsorption ratio (SAR) and salinity of irrigation water (ECi) are affecting parameters on soil particles dispersion.This potentially causes the sedimentation in drain pipes where no envelope applied. In current research, permeameter test were done to determine gradient ratio and exit gradient in order to evaluate the clogging potential and the probability of soil particle movement into drain pipes using three water samples with different amount of sodium adsorption ratio and salinity. Permeameter tests were accomplished on three drainage systems including drainage system without envelope and drainage system with granular and synthetic (PLM) envelopes. Through this experiment, the values of outflow variation, system permeability, gradient ratio and exit gradient were investigated. Results showed that the values of gradient ratio in the system without envelope were between 1.34 and 2.74 which indicates considerable clogging potential in flow entrance location into drain. Nonetheless, soil particles movement into drain pipe was not observed. whereas in systems with envelope, values of these indicators often were less than one. With calculation of hydraulic failure gradient and comparing this gradient with exit gradient value in each gradient value, soil particle resistance against pressure head was studied. Results indicated that the difference between filtration function of systems without and with envelope were negligible whereas hydraulics function difference between them using the first and the second water samples were significant.