Abstract
Cement bentonite mixtures are often used to build slurry walls for the containment of both aqueous and non aqueous pollutants, due to their quite low hydraulic conductivity and relatively high ductility and strength. Although their hydro-mechanical behaviour in saturated conditions has been studied in the past, a part of the slurry wall is expected to rest above the groundwater level. The hydraulic characterization in unsaturated conditions is then particularly relevant to evaluate the performance of the barrier, especially when it is aimed at containing non aqueous pollutant liquids which are lighter than water (LNAPL). These non wetting fluids rest above the water table and their penetration is possible just if the barrier is unsaturated. This paper presents some preliminary results of a laboratory characterization of the water retention behaviour of three different cement bentonite mixtures. The mixtures, prepared at cement – bentonite mass ratios ranging from 4:1 to 6:1, were immersed in water and cured for 28 days. Their water retention behaviour was then determined along drying and wetting paths through different techniques, namely axis translation, filter paper and vapour equilibrium. In the high suction range, the water content – suction relationship was found to be independent of cement-bentonite ratio. In the low suction range, the water content at a given suction was found to decrease for increasing cement bentonite ratios.
Highlights
Slurry trench cut-off walls are widely used for the containment of pollutants
The hydraulic characterization in unsaturated conditions is relevant to evaluate the performance of the barrier, especially when it is aimed at containing non aqueous pollutant liquids which are lighter than water (LNAPL)
This paper presents some preliminary results of a laboratory characterization of the water retention behaviour of three different cement bentonite mixtures
Summary
Slurry trench cut-off walls are widely used for the containment of pollutants. The hardening process conveys the material both relatively high strength and low hydraulic conductivity. According to most design codes, after one month of curing, laboratory samples of the hardened slurry should have a hydraulic conductivity K lower than 10-9 m/s and a uniaxial strength higher than 200 kPa [3]. The hydro-mechanical behaviour of cementbentonite mixtures varies with time, achieving better hydro-mechanical properties, since hydraulic conductivity decreases and mechanical strength increases [3]. Their constituents are very sensitive to the chemical composition of the pore fluid, which affects the hydration reactions of the cement and the electrostatic forces acting between the bentonite particles. This paper shows some experimental results about the water retention properties of different cement-bentonite mixtures after 28 days of curing
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