ise its imperviousness. Taking Chengdu clay as an example, four treatment schemes were tried. Na2C03 could reduce conspicuously the permeability of the saturated clay, but could not limit the pe leability rise in the altemate wetting and drying process. NaOH had a similar effect to Na2C03. NaCI could not only ecrease the saturated hydraulic conductivity, but could also effectively contain the permeability rise caused by evaporation. CH3COONa had a similar effect to NaCI. The mechanism of Na2C03, NaOH, NaCI and CH3 COONa decreasing the saturated hydraulic conductivity of the clay is that Na+ transformed Ca-montmorillonites in the original clay into Na-montmorillonites and the transformation reduces the sizes of effeective pores and the eff．eective porosity of the clay.