THE RESPONSE OF LOW AND HIGH SWELLING SMECTITES TO SODIC CONDITIONS

Swelling and dispersion of clays are the primary processes responsible for the degradation of soil hydraulic properties in the presence of e xchangeable Na. The relative importance of these processes was evaluat ed by studying the response of low and high swelling smectites to sodi c conditions. Smectite from Ariake bay sediments in Japan represented the low swelling smectite, and clay from the Kamenose landslide in Jap an represented the high swelling smectite. Changes in hydraulic conduc tivity (HC) and clay dispersivity of sediment-sand mixtures (10, 20, a nd 30 g of sediment mixed with 90, 80, and 70 g of sand, respectively) as a function of total electrolyte concentration (TEC) (0.05 and 0.01 M Cl- and distilled water, (DW)) and sodium adsorption ratio ((SAR) o f 0, 10, and 20) of the percolating solutions were measured. In the lo w swelling smectite, no changes in HC were measured in the electrolyte solutions (TEC greater than or equal to 0.01 M Cl-) at the three SAR values. When the low swelling smectites were leached with DW,the HC of the Ca-smectite increased, whereas the HC of the SAR 10 and 20 treatm ents decreased. Clay dispersion and migration out of the 10% soil colu mn was substantial. The increase in HC in Ca-mixture of low swelling s mectite leached with dilute solutions was attributable to the collapse of the open microstructure that prevails in electrolyte solutions. In low swelling smectite, clay dispersion was the main process responsib le for HC deterioration under sodic conditions, and smectite dispersio n was prevented when TEC exceeded the flocculation value of the clay. Clay dispersion increased with an increase in exchangeable sodium perc entage (ESP), and it affected the HC of the porous media only when the pores were fine and the dispersed clay plugged the conducting pores. In the high swelling smectite mixtures, a gradual decrease in the HC w as measured as the TEC decreased and the SAR increased. In these mixtu res, swelling was the main process responsible for HC deterioration in electrolyte solutions with TEC greater than or equal to 0.01M Cl-. Sw elling increased with an increase in clay percentage, ESP, and decreas ing TEC.