Giuseppe Buscarnera

Model Prediction of Static Liquefaction: Influence of the Initial State on Potential Instabilities

AbstractThis paper examines the influence of the initial state of sands on the potential for undrained instability. The main goal is to illustrate how advanced constitutive modeling of sand behavior can be used to evaluate the susceptibility for static liquefaction. The methodology is based on the concept of latent instability, in which the potential for collapse is contingent on particular boundary conditions. A generalized effective stress soil model, MIT-S1, is used to support the analysis and is combined with a theoretical approach for identifying loss of control owing to undrained shear perturbations. The theory is evaluated using experimental evidence available for Toyoura sand to point out the key role of void ratio and consolidation history and to provide experimental validation for the theory. Model predictions are then used to disclose the subtle role of drained preloading paths in promoting liquefaction instabilities. The ability of the constitutive model to reproduce the interplay between undr...

A particle-water based model for water retention hysteresis

A particle–water discrete element based approach to describe water movement in partially saturated granular media is presented and tested. Water potential is governed by both capillary bridges, dominant at low saturations, and the pressure of entrapped air, dominant at high saturations. The approach captures the hysteresis of water retention during wetting and drainage by introducing the local evolution of liquid–solid contact angles at the level of pores and grains. Extensive comparisons against experimental data are presented. While these are made without the involvement of any fitting parameters, the method demonstrates relative high success by achieving a correlation coefficient of at least 82%, and mostly above 90%. For the tested materials with relatively mono-disperse grain size, the hysteresis of water retention during cycles of wetting and drainage has been shown to arise from the dynamics of solid–liquid contact angles as a function of local liquid volume changes.

Path-dependence of the potential for compaction banding: theoretical predictions based on a plasticity model for porous rocks

The paper presents a theoretical investigation of compaction banding based on a plasticity model for high-porosity rocks. The selected model is featured by a nonassociated flow rule and two internal variables simulating the competition between softening and hardening in the brittle-ductile transition. The parameters have been calibrated for two extensively studied rocks that exhibited localized compaction under laboratory conditions. In particular, the constants that control the compaction banding domain are defined by matching the stresses at which localized compaction was found in the experiments. The resulting parameters are used to simulate the stress-strain response for two loading modes (i.e., triaxial compression and radially constrained deformation), thus exploring the role of stress paths and kinematic constraints on the evolution of the compaction banding potential. The analyses suggest that the loading paths able to mobilize the plastic resources of the rock alter the potential for compaction banding through irreversible effects. A notable example is oedometric compression, for which the potential to generate localized compaction tends to vanish during the initial stages of inelastic loading. These predictions suggest that constraints to the mode of deformation can hinder the occurrence of compaction bands in the field. Moreover, they suggest that the interplay between kinematic constraints and evolution of the compaction banding potential can be used for experimental characterization purposes. As a consequence, these results emphasize the importance of complementing stress-controlled experiments with other tests able to explore different stress paths, thus providing additional data for an accurate characterization of rheological properties and strain-localization characteristics.

Is Wetting Collapse an Unstable Compaction Process

Abstract This paper provides an interpretation of the phenomenon of wetting collapse in fine-grained soils based on principles of material stability. For this purpose, classic experiments displaying the accumulation of irreversible compaction have been reinterpreted through a plasticity model for unsaturated soils. The resulting simulations have been inspected mathematically, with the goal of detecting a possible loss of control of the wetting process. The reanalysis of the experiments suggests that, if the wetting-induced deformations are interpreted as homogeneous, the considered compaction events do not tend to be affected by a loss of material stability. As a result, such well-known phenomena have been explained as controllable modes of plastic deformation. This feature has been found to be valid regardless of the mode of saturation (i.e., it is valid for both controlled suction removal and water volume injection). Indeed, parametric analyses have shown that the compaction process tends to become unst...

Controllability criteria for soils saturated by a compressible fluid

AbstractThe liquefaction of sediments with gas bubbles occluded in the pore fluid can be studied neither through classical methods for saturated soils nor through concepts valid for unsaturated soils with continuous fluid phases. To fill this gap, this paper proposes to adapt to gas-charged soils a set of techniques that until now has been used only for fully saturated soils or unsaturated media with a continuous gas phase. In particular, the pore fluid has been modeled as a homogenized liquid-gas mixture of nonnegligible compressibility, and an expression of second-order work for porous media filled by such a compressible fluid has been derived. The use of incremental constitutive relations for the solid-fluid mixture has thus enabled the definition of controllability criteria specific for quasi-saturated soils subjected to drained and/or undrained loading paths. Such criteria have been computed through an elastoplastic model for loose granular soils and have been used to reinterpret evidence from labora...

Evolution of the Water Retention Characteristics of Granular Materials Subjected to Grain Crushing

AbstractThis paper reports a series of experiments aimed at studying the effect of grain crushing on the water retention capacity of granular soils. Specimens of granular materials have been subjected to oedometric compression at high pressures, revealing that crushing causes significant alterations of both grain-size distribution (GSD) and soil water retention curve (SWRC). In particular, the experiments have shown that the suction air-entry value (sAEV) changes considerably during crushing, thus controlling the shape of the SWRC in proximity of saturated conditions. Such evidence has been interpreted through a number of GSD-dependent retention models available in the literature. In particular, the results have been used to verify the hypotheses of a recently proposed hydromechanical model based on the breakage mechanics framework, which enables the prediction of simultaneous variations in void ratio, GSD, and SWRC through constitutive relations linking the sAEV to the predicted degree of particle breaka...

Shale Fracturing for Energy Recovery: Current Issues and Review of Available Analytical and Computational Models

The paper reviews some of the major scientific challenges faced in the field of non-conventional energy production. Particular attention is given to shale rocks, in which the production of hydrocarbons involves hydraulic fracturing, fracture connectivity, multiphase flow, and reactive-transport. In regard to these problems, the paper describes a series of analytical and computational models to study fracturing in quasi-brittle solids and capture failure events in multi-phase geomaterials subjected to simultaneous changes in stress state, fluid pressure, moisture content, thermal and chemical conditions. Advantages and limitations of the different techniques are discussed, setting a vision for a new generation of multi-scale/multi-physical models for shale that can be used to support the optimization of the extraction processes, as well as to address industrial needs and mitigate the risks of environmental hazards, groundwater contamination and induced seismicity.

Loss of controllability in unsaturated soils

ABSTRACT A study on the triggering of instability modes during soaking of unsaturated soil specimens is presented. A constitutive model for unsaturated soils is linked in fact with the loss-of-controllability theory. The model is based on the concept of energy conjugated generalised stress variables. A convenient expression for effective stresses for partially saturated soils is adopted. It is shown that the saturation index acts as an extensive variable, the corresponding intensive variable being the modified suction. The controllability of generalised loading tests is then considered, with attention to tests at varying water content. It is shown that instability phenomena, such as collapse at constant axial loading and suction, can be described.

Solute Mixing Regulates Heterogeneity of Mineral Precipitation in Porous Media

Synchrotron X-ray microtomography was used to track the spatiotemporal evolution of mineral precipitation and the consequent alteration of the pore structure. Column experiments were conducted by injecting CaCl2 and NaHCO3 solutions into granular porous media either as a premixed supersaturated solution (external mixing) or as separate solutions that mixed within the specimen (internal mixing). The two mixing modes produced distinct mineral growth patterns. While internal mixing promoted transverse heterogeneity with precipitation at the mixing zone, external mixing favored relatively homogeneous precipitation along the flow direction. The impact of precipitation on porewater flow and permeability was assessed via 3D flow simulations, which indicated anisotropic permeability evolution for both mixing modes. Under both mixing modes, precipitation decreased the median pore size and increased the skewness of the pore size distribution. Such similar pore-scale evolution patterns suggest that the clogging of individual pores depends primarily on local supersaturation state and pore geometry

Loss of Controllability in Partially Saturated Soils

A study on saturation induced soil instability is presented. A constitutive model for unsaturated soils is linked to a theoretical approach able to deal with mechanical instability of fully saturated geomaterials. The theoretical approach is therefore extended to the more general case of partially saturated soils. The controllability of generalised loading tests is then considered, focusing on oedometric tests at varying water content. It is shown that instability phenomena, such as collapse at constant axial loading, can be described. In addition, it is discussed how oedometric instability modes can be interpreted as possible compaction band bifurcations. Finally, similar results are reported for the simpler case of isotropic state of stress, defining the most relevant material parameters in determining such instability modes.