Evelina Fratalocchi

Water permeability of unsaturated compacted kaolin

The paper presents an experimental investigation into the permeability of compacted kaolin. Tests were performed in a suction-controlled oedometer using the air overpressure technique: ambient air pressure (raised above the atmospheric pressure) was kept constant and pore water pressure was modified. Permeability was measured by monitoring water inflow in response to a suction decrease and interpolating experimental data using a simplified solution of Richards’ equation (unsteady-state method). Corrections were made to account for the impedance of the high air-entry ceramic disc, the water lost by evaporation into the air pressure line and the air diffusing through the ceramic. The experimental results have shown that axis-translation technique may be problematic at high degrees of saturation and that permeability can be significantly underestimated.

Consolidation of Dredged Sediments in a Confined Disposal Facility: Hydraulic Conductivity Constitutive Relations

When large strains are expected, such as for dredged sediments in confined disposal facilities, models that take into account constitutive relations for compressibility and hydraulic conductivity are required to study consolidation processes. A study has been carried out to design the filling and consolidation phases of a confined disposal facility for dredged marine sediments. The possibility to determine consolidation constitutive laws has been investigated using common incremental loading oedometer tests instead of more suitable but timeconsuming tests. The void ratio-hydraulic conductivity domain determined by oedometer test results has been compared with direct measurements at different void ratios. The results show a low trustworthy of this approach.

Hydraulic Conductivity and Sorption Capacity of Special Barrier Materials in Inorganic Solutions

The hydraulic performance and migration parameters of cementitious mixtures are currently under investigation by an experimental study. The main aim of the research is to investigate the diffusion coefficient and sorption capacity of the mixtures and their interaction with inorganic sulphate solutions at different concentrations, as a function of their composition and curing time. The paper shows and discusses the results related to the hydraulic performance and the sorption capacity as a function of the mixtures composition. Simulations of solute migration through cut-offs made by the different mixtures investigated show the importance of taking into account their sorption capacity to optimize the design.

Modeling Solute Transport Through Geosynthetic Clay Liners Permeated with Inorganic Solutions

GCLs are employed in containment applications, where both advection and diffusion need to be evaluated as possible migration mechanisms of contaminants. Permeation column tests were carried out on a GCL using two synthetic multispecies inorganic solutions of different ionic strength and containing equimolar concentration of heavy metals (Pb, Zn, Cu). The hydraulic conductivity (k) of the GCL to inorganic solutions increased by one order of magnitude relative to permeation with water. The solute breakthrough curves were interpreted using the software Pollute®, which allows modeling solute transport in case of variable transport parameters and nonlinear equilibrium sorption. The experimental data were best fitted by assuming effective diffusion coefficients of solute species increasing with time.

About the state parameters of active clays

The common use of active clays, such as bentonites, as hydraulic and contaminant barriers for landfill and soil remediation applications, including the final disposal of nuclear waste, needs to be supported by adequate theoretical models that take into account the physical, chemical and mechanical coupled phenomena. The elementary particle (platelet) of a bentonite is characterized by a high total specific surface (S) and a permanent or fixed negative electric surface charge (sigma) that interacts with the ions in solution at different concentration (cs) in the pore water. This interaction is the primary factor that determines the final fabric of the solid skeleton and, consequently, most of the bentonite properties in terms of both mechanical behavior and solute transport. The tactoid formation or flocculation is the primary phenomenon that influences bentonite behavior and can be quantified with appropriate state parameters, such as the effective specific surface, Seff, the average number of platelets per tactoid, Nl,AV, and the electric fixed-charge concentration of the solid skeleton, csk,0. On the basis of the aforementioned state parameters, a theoretical hydro-chemico-mechanical framework has been developed. The validity of this framework has been tested, within the present paper, by comparison of its predictions with some of the available experimental results on bentonites with more than 70% montmorillonite content, in terms of hydraulic conductivity, swelling pressure and osmotic efficiency versus void ratio and ion concentrations of the pore solution.

Modeling contaminant leaching and transport to groundwater in Tier 2 risk assessment procedures of contaminated sites

Abstract The current approach to the management of contaminated sites includes a risk assessment procedure as a crucial step. In some jurisdictions (e.g., Italy), groundwater must be considered as a target of contamination. In other cases (e.g., USA), risks from exposure to contaminated groundwater (e.g., through the ingestion of contaminated water) must be taken into account. If the contaminant source is located in the vadose zone, the risk assessment requires modeling of contaminants leaching and subsequent transport to groundwater. In Tier 2 risk assessments, analytical transport models are used for this purpose. This paper compares steady state and a transient approach for modeling the leaching and migration of contaminants to groundwater. The steady-state model that was used considers soil–water partitioning, soil attenuation during transport and dilution with groundwater in a defined mixing zone. The transient model that was used considers source depletion due to leaching and volatilization, one-dimensional solute transport and a novel approach to the modeling of dilution with groundwater. Comparative simulations were performed with regard to a given site geometry and to index substances representing different contaminant classes: aromatic solvents, chlorinated solvents, and heavy metals. The results of the study indicate that, although steady-state models are simple and easy-to-use, taking into account the time variable is essential for a reasonable simulation of contaminant transport to groundwater, particularly for contaminants with high partition coefficients migrating through fine-grained soils.

Hydraulic and mechanical behaviour of cement-bentonite mixtures containing HYPER clay: impact of sulfate attack

Cemented clays are regularly employed as cut-off walls to isolate polluted soils or in ground improvement technologies. The objective of this research was to evaluate the performance of a polymer-treated bentonite (HYPER clay) and its impact on hydraulic and mechanical properties of cement-bentonite (CB) mixtures in contact with sulfates. In this research, bender elements were installed in a hydraulic conductivity cell, to simultaneously monitor the small-strain shear modulus (G(0)) and the hydraulic conductivity (k) of cement-bentonite mixtures. Initially, during permeation with deionized water, an increase of G0 with time was observed, due to cement hydration (hardening). Conversely, after prolonged permeation with sulfates, a decrease of G0 and a gradual increase of k are expected due to degradation of the cemented material. Preliminary results show that k of mixtures containing untreated bentonite increases due to contact with sulfates; conversely, k of mixtures containing HYPER clay remains unaffected. The stiffness modulus G0 was not significantly affected after a sulfate contact period of about 100 days in none of the cement-bentonite mixtures. More extensive monitoring is required for evaluating long-term mechanical degradation. These observations suggest that the use of polymer-treated bentonites could improve the resistance of cement-bentonite mixtures to sulfate attack maintaining a low hydraulic conductivity.