Effects of Exchangeable Cations on Hydraulic Conductivity of a Marine Clay

-A laboratory study of the hydraulic conductivity (HC) of a marine clay with monovalent, divalent and trivalent cations revealed large differences in HC. The exchangeable cations employed in this study are Na, K, NH4, Mg, Ca and AI in order of increasing valency. An interpretation of the results derived from consolidation tests suggests that HC is significantly affected by the valency and size of the adsorbed cations. An increase in the valency of the adsorbed cations leads to quicker rates of consolidation and higher HC, while, for a constant valency an increase in the hydrated radius of the adsorbed cations results in a lower rate of consolidation and HC. The reduction in HC was related to the dispersion and deflocculation of clay. Lower valency and higher hydrated radii of the exchangeable cations enable the double layer repulsive forces to predominate, thereby increase dispersion and defloccalation. Key Words--Consolidation, Dispersion, Exchangeable cation, Hydraulic conductivity. I N T R O D U C T I O N Hydraulic conductivity (HC) is a measure of the ease with which a fluid passes through a material. It is one of the fundamental engineering properties of a soil and the knowledge of which is required in seepage, settlement and stability calculations. The HC for a given soil depends on size and shape of the particles, void ratio, arrangement of the pores and soil particles, properties of the pore fluid and the amount of undissolved gas in the pore water (Michaels and Lin 1954, Lambe 1954). In fine grained soils, HC under saturated conditions is controlled by the microstructure of the soil matrix which in turn depends on the type of the clay mineral present in the soil, the composition of the exchangeable cations and the electrolyte concentration in the pore water system. The ease with which flocculation, deflocculation and swelling of a clay mineral take place affects the structure of the pores and, hence, the HC (Quirk and Schofield 1955). There is a necessity felt by environmental geotechnical engineers to alter the HC characteristics of the soils using different chemicals offering various cationic systems and in view of this the study of cation adsorption and ion exchange has evinced a lot of interest in the field of HC of marine clays. In this experimental study an attempt has been made to evaluate the importance of exchangeable cations in regulating the HC of a marine clay. Adsorption of cations having different hydrated radius and valency by the fine grained soils are the two important factors to be considered in proposing any mechanisms dealing with differences in HC of fine grained soils. Wada and Beppu (1989) recommended treatment with AI to smectitic clay for its efficiency in increasing the HC and help in the decrease of exchangeable Ca, Mg and K. Shainberg et al (1987) Copyright 9 1995, The Clay Minerals Society observed that the effect of exchangeable potassium percentage (EPP) on the HC of smectites depended on the charge density of clays. The HC of smectites having low charge density changed markedly when leached with dilute solutions as the EPP of clay increased. It has also been observed that HC decreases with increasing exchangeable sodium percentage (ESP) and decreasing salt concentration. From the reported results it can be seen that there is no definite influence of potassium on HC and this may be due to possible variation in the soil mineralogy and sample preparation techniques. As per Ahmed et al (1969) and Quirk and Schofield (1955) HC is related to exchangeable cations in the following order Ca = Mg > K > Na. However, some researchers report larger aggregates of greater stability in potassium saturated soils than in soils saturated with divalent cations (Ceeconi et al 1963, Ravina 1973), suggesting that potassium in the exchange sites increases soil HC. Quirk and Schofield (1955) also suggested that the swelling of clay particles could result in total or partial blockage of the conducting pores. It is known that swelling in clayey soils increases with a rise in the percentage of monovalent exchangeable cations. Deflocculation, dispersion and clay movement into the conducting pores are also responsible for the plugging of the soil pores (Quirk and Schofield 1955). The importance of dispersion in soil HC has been recognized by several investigators (Frenkel et a11978, Pupisky and Shainberg 1979, Shainberg et al 1981). The objective of the present study is to investigate the effect of exchangeable cations on the HC of a marine clay. This work reports the results of one dimensional consolidation tests on a marine clay homoionized with a series of monovalent, divalent and trivalent cations. The mechanisms controlling the HC of clays are presented and discussed.