Laboratory Study of Influence of Clay Content on the Need for Subsurface Drainage System Envelope

Necessity to use envelope (filter) in subsurface drainage systems is one of the important aspect that should be taken into consideration of 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 the drain plate openings in three clay and clay loam soils with 20-40 percent clay SAR 17 – 83، and EC 10 – 100dS/m. Treatments in this experiment were drainage systems with and w/o envelopes. In envelope d system two types of envelopes (granular and fiber) were used. Outflow variation، system permeability، gradient ratio and exit gradient were investigated. The results. showed that the gradient ratio in the system without envelope were greater than one implying a high clogging potential. Nonetheless، soil downfall was occurred when this index was bexceeded. 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. Resistance against pressure head was also studied with determining hydraulic gradient failure compared to exit gradient value at each gradient head. Results indicated that the without envelope system in soil no 2 and 3 had، receptively the minimum and maximum performance.