A laboratory study was conducted in the soil lab of Agricultural Faculty of Tehran University to evaluate S.C.S and VSBR methods for designing granular drain envelopes. A one- dimentional downward flow permeameter was used to test the subsurface drain envelopes.In the first step, blocking of the granular envelopes was evaluated measuring outflow during Ts period and the existing problem distinguished. In the second step, an upward flow permeameter apparatus was constructed to eliminate the pervious apparatus
failures. All tests were repeated the outflow (Qa) and hydraulic head (H) being measured at piezometric locations. Hydraulic gradients were then measured in the soil, envelope materials, and complete system (soil and envelope) to estimate the hydraulic conductivity (K). This study revealed that the envelope materials designed by V.S.B.R method, for which the KdKs ratio was always greater than one (KdKs> 1), exhibit better performance as compared with S.C.S envelopes. In addition V.S.B.R envelopes exhibited better performance in terms of secondary blocking, Iells ratio, outflow and hydraulic gradient in the cover zone.
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