Cristina Jommi

An insight into the role of hydraulic history on the volume changes of anisotropic clayey soils

[1]xa0Volume changes of natural and compacted soils induced by changes in their water content have many practical implications in the service life of earth dams, river and canal embankments, and waste disposal facilities. An insight into the overall strain response of a clayey soil upon gradual wetting and drying is provided here. Experimental data coming from oedometer and isotropic tests under suction and net stress control are presented for a compacted clay with an initial anisotropic fabric, highlighting the relevant role played by the hydraulic path on collapse, swelling, and shrinkage strains. Irreversible strains could be observed after wetting-drying paths and the subsequent drying-wetting cycle. Both stress and hydraulic histories play a role in the evolution of the directional fabric of clayey soils. The experimental data could be reproduced with a rather simple elastic-plastic constitutive model with a mixed isotropic-rotational hardening, previously conceived for saturated soils. The model is extended to unsaturated conditions by substituting the saturated effective stress with a measure of the average stress acting on the soil skeleton and by introducing generalized hardening rules governed by both plastic strains and degree of saturation. Coupling between the mechanical and the hydraulic behavior is provided by the water retention curve, in which degree of saturation is adopted as a useful measure of the soil water content.

A model for coupled electro-hydro-mechanical processes in fine grained soils accounting for gas generation and transport

A theoretical and numerical model is developed for the quantitative analysis of coupled processes taking place in active waste containment systems, such as electrokinetic barriers or fences, in which a low intensity DC current is circulated across the clay barrier to move polar and non-polar contaminants. A novel feature of the proposed approach is the allowance for the presence of air in the pore space. Under unsaturated conditions, all transport coefficients involved in the electrokinetic process are strongly dependent on the degree of saturation of pore liquid. In order to assess the predictive capability of the proposed theory and to appreciate the impact of gas production at the electrodes, a series of numerical simulations of simple one-dimensional electrokinetic tests have been performed. The results of the simulations compare reasonably well with data obtained from laboratory experiments performed on an illitic clayey silt. The numerical results indicate that the impact of gas production at the electrodes can be significant, even in low-intensity and short-duration treatments.

On the use of infinite elements in the analysis of two-dimensional layered elastic systems via discretized integral equations

Abstract A boundary integral equation approach is presented for the two-dimensional plane strain analysis of horizontally layered elastic systems resting on a rough and rigid base. The validity of Somigliana identity for unbounded layers is discussed and an Infinite Boundary Element is proposed in order to take into account the lateral unboundedness of the physical problem in the discretized equations. This element is implemented in a computer code based on a successive stiffness solution procedure. A series of numerical tests concerning single- and multi-layered problems is illustrated, and the performance of the proposed solution method is compared with that of more traditional boundary element and finite element techniques.

Modelling the Water Retention Domain of a Compacted Scaly Clay

A body of experimental data on the water retention behaviour of compacted samples of an Italian scaly clay is analysed. The mechanical and the hydraulic behaviour of compacted samples of this clay are governed by multi-scale arrangement of scales and aggregates, which characterises the compacted clay fabric. At least two different pore networks, namely intra-aggregate and inter-aggregate, may be identified, which interact one with the other along coupled hydro-mechanical paths. The retention behaviour is interpreted in the framework of a comprehensive multi-scale modelling approach recently proposed for compacted clays of low and medium activity, which proves to be able to account for the interaction between the two main micro-structure levels. Dependence of the hysteretic retention domain on void ratio for the active soil is discussed.

Enhancement of a Commercial Pressure Plate Apparatus for Soil Water Retention Curves

Pressure plates are widely used to measure soil water retention properties of soils in the laboratory with axis-translation technique. Imperfect sealing of the pressure chamber, exchange of water vapour between the soil and the air in the chamber, lack of soil-plate contact, part of the porous plate directly in contact with air in the pressure chamber, and evaporation of water from the ballast tube may lead to incorrect estimation of the water volumes exchanged by the soil. An improved version of a commercial pressure plate apparatus is presented, which proved able to reduce the intrinsic drawbacks of the original design. The system was also equipped with an image-processing based data acquisition system for continuous measurement of exchanged water.

Water retention properties of a compacted clayey silt including void ratio dependency and microstructural features

An experimental study focused on the water retention properties of a compacted clayey silt with dominant bimodal pore size distribution is reported. A water retention model accounting for soil microstructural features and void ratio is used to track the observed hydraulic behaviour. The model parameters were calibrated on the basis of drying and wetting data at a given dry density and the model predictions are verified along drying and wetting paths at different void ratios.

Modified Grading Curve – SWCC Relations

In the ongoing research a topological interpolation method was elaborated over the N -1 dimensional space of the grading curves – represented by a N -1 dimensional simplex - using c N 2 data. The surface is selected on the basis of the grading entropy concept and the interpolated function is extended to the whole simplex by the grading entropy map. In this paper the method is further developed. The extended method is illustrated using the examples of the dry density and the parameters of the van Genuchten soil water retention function.

MULTIPHYSICS MODELING OF ELECTROKINETIC PHENOMENA IN UNSATURATED FINE-GRAINED SOILS

A multiphysics model for the analysis of coupled deformation and reactive flow of pore fluids, chemical species and electric current has been developed to assess the influence of the geochemical reactions taking place within the soil mass on the effectiveness of electrokinetic remediation treatments. The results of FE simulations have shown that the presence of carbonates in the solid phase may give rise to gas production, significant changes in the degree of saturation and buffering effects with respect to the pore water pH. Under such conditions, the efficiency of the electrokinetic treatment can be significantly reduced.