Discussion: Use of liquid limit state to generalize water retention properties of fine-grained soils

Abstract

The need to predict hydraulic properties of unsaturated soils has long been recognized in both soil science and geotechnical engineering disciplines. Apart from the obvious need to predict water availability for plant growth and for other agricultural purposes, methods to predict hydraulic properties of unsaturated soils are important when assessing the fate and transport of contaminants in environmental engineering. Hydraulic conductivity of unsaturated soils is mostly governed by their ability to retain moisture at various suctions. The relationship between moisture contents and corresponding suctions is often displayed in what is known as a moisture characteristic curve. Semi-empirical equations, based on capillarity theory, are commonly used to trace moisture characteristic curves (e.g. Hillel, 1982; Jury et al., 1991). The equations express the relationship between volumetric water contents of soils and suction pressures in terms of porosity (maximum volumetric water content); residual (irreducible) water content; suction at saturation, otherwise known as air-entry suction; and empirical constants indicative of pore size distribution. These parameters are highly variable in the case of fine-grained soils; this often leads to erroneous estimation of water retention properties. Several studies have indicated that, at liquid limit state, fine-grained soils possess unique values of shearing resistance (of the order of 1.7-2.8 kPa) and suction pressure (about 6 kPa) (Russell and Mickle, 1970; Wroth and Wood, 1978; Whyte, 1982). Nagaraj et al. (1991) showed that the saturated hydraulic conductivities of several clays (with liquid limits ranging from 60 to 330) at their liquid limit states are almost constant at about 2.5 ] 10 -6 mm/s. More recent studies reported by Nagaraj et al. (1993, 1994a) concluded that it was feasible to generalize stress-state - permeability relations for normally consolidated and overconsolidated clays using their liquid limit as the reference state. For instance, Nagaraj et al. (1993) proposed that for normally consolidated clays, e/e LL = a - blogp' (1) e/e LL = c + dlogk (2) where e is the void ratio, e LL is the void ratio corresponding to the liquid limit water content, p' is the effective consolidation stress, k is the coefficient of permeability, and a, b, c, and d are empirical constants whose values depends on the units used for P' and k. Griffiths and Joshi (1989) documented data which supported a linear relationship between e/e LL and log p'. These studies suggest that, at void ratios corresponding to a liquid limit state, the pore size distribution patterns are nearly the same. On the basis of this discussion, one can hypothesize that the liquid limit state may serve as a reference state with respect to which the water-holding capacity of fine-grained soils at other states may be referred; in other words, the moisture characteristic curve at a given void ratio is uniquely related to that for the liquid limit state. The purpose of this technical note is to examine this hypothesis using experimental observations made on a number of soil mixtures by the authors as well as those reported by others.