-

Prediction of the rate of flow through the natural soils, sediments, or compacted soils is a common practical problem. It is even more serious when permeability changes with time due to consolidation and /or solubilization of soil materials. Therefore, a reliable method for determination of hydraulic .characteristics and Their changes during leaching is necessary. First, the relevant information about properties of gypsiferous soils, particularly their hydraulic conductivity were collected. Next, three disturbed samples were taken each from a different province of Iran with gypsiferous soil, namely, Vafraghan plains in Saveh NckooAbad and Abshar plain in Isfahan, and Ghir-Karzin Plain in Fars Province. The samples contained (16-22), (21-32) and (6-10) percent gypsum, respectively. The samples were analysed for physical (size distribution, Atterberg limits, specific gravity and compaction), Hydraulic (permeability) and chemical (gypsum content of soil and leachates, soluble salts in soil) properties. To investigate the process of solution and fixation of gypsum in soils, a physical model was devised and tested under the following conditions:
- leaching by distilled water under 75 cm. hydraulic head for a period of 90 days.
-leaching by distilled water under 135 cm. hydraulic head for a period of 90 days.
- leaching by distilled water under 215 cm. hydraulic head for a period of 90 days.
- leaching the sodium oxalate stabilized soil samples, by distilled water under 215cm. hydraulic head for a period of 90 days.
- leaching by magnesium chloride solute (5% normal) in order to increase the soulbility of gypsum, under 215 cm. hydraulic head for a period of 90 days. The results showed that the soil samples from Isfahan, Saveh and Shiraz were
classified as SM, SC and CL according to UNIFIED soil classification system, respeCtively. Degree of leaching for three samples were (13.0-43.1), (4.3-30.1)



and (2.9-9.4) percent, respectivelyAn inverse relationship was observed between hydraulic conductivity and gypsum concentration in leachate.
Permeability test showed that in gypiferous soils, this parameter is a function of soil texture and gypsum crystal forms, where it may however decrease, increase or remain constant during leaching. Due to the movement of soil particles and solubilization of gyppm at the beginning, the range of permeability coefficient was relatively high, while gradually decreased to a constant value. By increasing hydraulic head, range in variation decreased.