A simple method to consider water density changes in the calculation of the degree of saturation of swelling clays

When dealing with numerical computations in geotechnical engineering, the use of adequate values of concerned parameters is an aspect of mayor interest. Water transfers play an important role in the behavior of engineering clay barriers for deep underground waste disposal facilities. When absorbed by clays, water properties may considerably differ from the free water state. Water density ρ_w is a main parameter in the calculation of volumetric water content θ_w and thus the degree of saturation S_r, a main state parameter in unsaturated soils mechanics. Many authors have pointed out an important increase of water density in swelling clays (see Winterkorn 1936, Martin 1962, Villar 2000, Marcial 2003, among others). This increase of ρ_w may lead to considerable errors if one assumes that ρ_w keeps equal to 1 Mg/m in such materials. This work presents a simple method to consider possible increases of ρ_w when calculating S_r values. A ρ_w–e curve is indirectly obtained from saturated high plasticity clay samples, and then some hypotheses based on experimental works (Prost 1975, Sposito & Prost 1982, Tessier 1984, Marcial 2003) permit to estimate p_w changes in the unsaturated state. The ρ_w–e curve is obtained for a MX80 clay in saturated conditions and values ranging from 1,02 to 1,57 Mg/m are obtained when the void ratio decreases from 0,957 to 0,358. The proposed method is applied in a real case to illustrate the procedure. The water retention curve in constant volume condition is represented in terms of S_r for ρ_w = 1 Mg/m and considering ρ_w changes. In the first case S_r values up to 118% are obtained for a suction of 100 kPa; in the second one, S_r reduces to 86% for the same suction value. The presented method is a simple tool to deal with the increase of ρ_w in swelling clays and keep the degree of saturation within the defined range of 0 to 100%.