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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.

Cutoff walls represent an interesting solution for the containment of the pollution of superficial groundwater. For polluted sites, the purpose of a cement-bentonite cutoff wall is to minimize contaminant transport and the primary design requirement for such materials is the low hydraulic conductivity. Despite these barriers are often cast in place as provisional tools, recently their wide use imparted the need for a better understanding of cement-bentonite walls also in the long-term. This certainly implies not only the need to study the time evolution of the cement-bentonite hydro-mechanical properties in a contaminated environment, but also the necessity of a continuous monitoring of the efficiency of the system. To this aim, the use of dedicated devices cast in place inside the wall when the mixture is still fluid proved to be particularly suitable to intercept and analyse the fluids flowing through the barrier. In this paper, the results of a numerical study are presented, with the goal of suggesting criteria about the optimum spacing and geometry of these devices.