Cement-bentonite cut-off walls are commonly employed in geoenvironmental applications to limit ground water flow and pollutant transport. The wide diffusion of this artificial material in the current practice is not only due to its low permeability, but also to its simplicity of use. In this paper, experimental evidences about the role of curing on the hydro-mechanical behaviour of cement-bentonite mixtures are presented. Different curing times and curing conditions (representative for either water saturated or hydrocarbon polluted soils) have been considered, and their effects on both hydraulic conductivity and mechanical response in oedometer and triaxial conditions have been assessed. A unified hydro-mechanical framework, accounting for the changes of material fabric occurring with curing time and environment, is formulated. The hydraulic conductivity is very well predicted by a Kozeny-Carman like equation, whereas the mechanical behaviour is finely reproduced via an enhanced elastic–plastic constitutive model.

@en

Submarine buried pipelines interact with shallow soil layers that are often loose and prone to fluidization/liquefaction. Such occurrence is a possible consequence of pore pressure build-up induced by hydrodynamic loading, earthquakes, and/or structural vibrations. When liquefaction is triggered in sand, the soil tends to behave as a viscous solid–fluid mixture of negligible shear strength, possibly unable to constrain pipeline movements. Therefore, pipelines may experience excessive displacement, for instance, in the form of vertical flotation or sinking. To date, there are no well-established methods to predict pipe displacement in the event of liquefaction. To fill such a gap, this work proposes a computational fluid dynamics (CFD) framework enriched with soil mechanics principles. It is shown that the interaction between pipe and liquefied sand can be successfully analyzed via one-phase Bingham fluid modeling of the soil. Postliquefaction enhancement of rheological properties, viscosity, and yield stress can also be accounted for by linking soil–pipe CFD simulations to a separate analysis of the pore pressure dissipation. The proposed approach is thoroughly validated against the results of small-scale pipe flotation and pipe dragging tests from the literature.@en

This paper concerns the rheological characterisation of liquefied sands as non-Newtonian Bingham fluids. For this purpose, dam-breaking laboratory tests are often executed and interpreted, offering a viable option to identify the properties of fluidised water-soil mixtures. However, limited attention has been devoted so far to clarify what variables and measurements would allow unambiguous calibration of Bingham parameters, namely, the viscosity η and the yield stress τ _y . The numerical results of parametric studies based on the particle finite element method (PFEM) are critically inspected to gain deeper insight into the problem. First, it is confirmed that multiple η − τ _y pairs may reproduce the same experimental evidence when formed by only one measurement—usually, the post–dam-breaking displacement of the bottom toe (tip) of the liquefied mass. Then, two alternative procedures are proposed for unambiguous identification of both η and τ _y : one is based on monitoring the evolving aspect ratio of the fluid mass during free, gravity-driven flow; the other relies on a slightly different dam-breaking test, also including impact against a rigid obstacle. In particular, the latter approach reduces the relevant duration of the test, reducing the possible influence of reconsolidation effects on the calibration of rheological parameters.

@en

Submarine buried pipelines may often interact with shallow layers of loose sand that are particularly prone to liquefaction. When liquefaction is triggered by environmental loading and/or mechanical vibrations, the soil tends to behave as a viscous solid-fluid mixture and the pipeline to undergo either flotation or sinking. While a few indications can be found in the literature on the triggering condition for flotation/sinking, no conclusive methods are available to estimate the displacement of the pipe when liquefaction cannot be avoided. This preliminary work shows that combining in numerical simulations fluid and soil mechanics may successfully serve such a goal. The proposed modelling approach is compared to the results of small-scale pipe flotation tests, with emphasis on existing knowledge gaps and indications for future research on the subject.@en

Desiccation cracks are usually thought to start from the surface of an evaporating soil layer, and the available simplified models for crack initiation and propagation are based on this hypothesis. On the contrary, experimental results on a Dutch river clay showed that cracks in an evaporating soil layer may start and propagate below the surface, confirming earlier findings by other researchers. A simple one-dimensional model was set up to analyse the consequences of different hypotheses about the material behaviour on the crack onset in a homogenous soil layer undergoing surface drying. The results of the model show that dependence of the material behaviour on the rate of water content change is a necessary requirement for cracks to initiate below the surface. The conclusion suggests that, to properly understand cracking in an evaporating soil layer, an intrinsic time scale for the mechanical response must be accounted for, among all the other factors which were previously highlighted by other researchers. The key factor to predict crack onset below the surface is the dependence of the drying branch of the water retention curve of the compressible soil on the rate of drying, which would be justified by a rate dependent fabric evolution.@en

Modelling the water retention behaviour requires proper understanding of all the processes which affect the amount of water stored in the pore network, depending on the soil state and the soil history. Traditionally, in many applications a single water content - suction curve is used. This approach limits the applicability of the retention data to practical cases, especially when fine grain soils are dealt with, when the deformability and activity of the clay fraction significantly affect the interaction with water. On the other side, water retention is being recognised more and more as a fundamental information in the description of the mechanical response of the soil, as it provides the key connection to the partial volumetric strains in a deformation process. With reference to the work performed at the Politecnico di Milano in the last years, a contribution on the understanding and modelling the coupled water retention-mechanical response in deformable soils is presented. The contribution aims to: (i) summarise the mechanisms which contribute to water retention; (ii) point out the role played by an evolving fabric and the fluid properties on water retention; and (iii) provide an overview on some of the consequences of evolving water retention properties on the mechanical behaviour.@en