Grainsize effect on the crushing behavior of unsaturated granular solids

Failure and strain-localization in granular soils and porous rocks subjected to high pressures involves considerable alteration of the microstructure. Such processes are affected by the grainsize characteristics (e.g., grain size distribution and mean grain size), as well as by environmental variables such as moisture content and capillary pressure. In this contribution, we use a reformulation of the Breakage Mechanics theory to model comminution in wet granular assemblies. By using an extensive dataset for sands, we quantify the relation between a geometric descriptor of the assembly (i.e., the mean grainsize) and the constants that control the suction air-entry value and the stress threshold at the onset of crushing. Such relations are used to define contrasting scenarios for the coupling between degree of saturation and yielding. In the first scenario, the suction air-entry value scales inversely with the mean grainsize (as suggested by the capillary theory), whereas the energy input for comminution is assumed to be independent of the size of the grains. The outcome of this assumption is that changes in degree of saturation play a more intense role in finer gradings. In contrast, if we assume that the energy input for grain breakage scales inversely with the size of the particles (e.g., in accordance with scaling laws inspired by linear elastic fracture mechanics), the effect of the degree of saturation is predicted to be stronger in coarser assemblies. These results provide a theoretical basis to infer the relation between continuum-scale properties of soils and rocks (e.g., the yielding stress at the onset of crushing) and basic attributes widely used for soil/rock characterization (e.g., the grain size distribution)

Mathematical capture of failure processes in elastoplastic geomaterials

Abstract This paper discusses a strategy to identify failure conditions in geomaterials simulated by elastoplastic constitutive laws. The main objective is to express different forms of failure through the same formalism. For this purpose, we use a set of material instability indices combining the concepts of loss of controllability and critical hardening modulus with a simple, but versatile, elastoplastic model for soils and soft rocks. This choice has allowed us to (i) compute the instability indices in analytical form, (ii) capture the implications of non-normality and prior deposition/lithification history and (iii) inspect a broad range of failure modes (e.g., brittle and ductile failure, static liquefaction and compaction banding). It is shown that, although each mode of failure has its own specific features, they can all be encapsulated in a unified mathematical representation. To obtain these results, the instability moduli must reflect the static/kinematic constraints that generate the failure process at stake. Thus, the instability indices are expressed as functions of both the hardening modulus and additional terms of kinematic origin, with the latter terms reflecting a control-dependence of the plastic response. Such results describe a procedure for achieving a unified definition of failure in elastoplastic geomaterials, which is closely linked to the theory of controllability and encompasses the intuitive notions of 'hardening' and 'softening' as particular cases

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

This 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 undrained kinematic constraints and material failure is fundamental in predicting potential instabilities arising from changes in drainage conditions. The examples shed light on the mechanics of static liquefaction and set a framework for applying the principles of material stability to the triggering analysis of flow slides induced by undrained shear perturbations

Constitutive modelling approach for evaluating the triggering of flow slides

The paper presents a methodology to evaluate flow slide susceptibility in potentially liquefiable sandy slopes. The proposed approach accounts for both contractive and dilative volumetric behaviour during shearing using the MIT-S1 constitutive model. As a result, it is possible to distinguish among different types of undrained response induced by a rapid shear perturbation. The first part of the paper describes the general methodology for infinite slopes, providing an index of stability for incipient static liquefaction in shallow deposits. The methodology accounts for the anisotropy due to the initial stress state and uses simple shear simulations to assess instability conditions as a function of slope angle, stress state, and density of the soil. The resulting stability charts define the margin of safety against static liquefaction and the depths likely to be affected by the propagation of an instability. The second part of the paper applies the methodology to the well-known series of flow failures in a berm at the Nerlerk site. The MIT-S1 model is calibrated using published data on Nerlerk sands and in situ cone penetration test (CPT) data. The analyses show that in situ slope angles α = 10°–13° are less than the critical slope angle needed for incipient instability. Liquefaction and flow failures were therefore promoted by small perturbations in shear stresses that could be generated by rapid deposition of hydraulic fill

Modelling suction instabilities in soils at varying degrees of saturation

Wetting paths imparted by the natural environment and/or human activities affect the state of soils in the near-surface, promoting transitions across different regimes of saturation. This paper discusses a set of techniques aimed at quantifying the role of hydrologic processes on the hydro-mechanical stability of soil specimens subjected to saturation events. Emphasis is given to the mechanical conditions leading to coupled flow/deformation instabilities. For this purpose, energy balance arguments for three-phase systems are used to derive second-order work expressions applicable to various regimes of saturation. Controllability analyses are then performed to relate such work input with constitutive singularities that reflect the loss of strength under coupled and/or uncoupled hydro-mechanical forcing. A suction-dependent plastic model is finally used to track the evolution of stability conditions in samples subjected to wetting, thus quantifying the growth of the potential for coupled failure modes upon increasing degree of saturation. These findings are eventually linked with the properties of the field equations that govern pore pressure transients, thus disclosing a conceptual link between the onset of coupled hydro-mechanical failures and the evolution of suction with time. Such results point out that mathematical instabilities caused by a non-linear suction dependent behaviour play an important role in the advanced constitutive and/or numerical tools that are commonly used for the analysis of geomechanical problems in the unsaturated zone, and further stress that the relation between suction transients and soil deformations is a key factor for the interpretation of runaway failures caused by intense saturation events

Anisotropic breakage mechanics: From stored energy to yielding in transversely isotropic granular rocks

The microstructure of geological solids is affected by their deposition history, which generates complex systems of grain contacts and damage patterns able to induce mechanical anisotropy. This paper aims to disclose the connection between energy storage and the yield surface of cross-anisotropic granular rocks, with the goal to simplify the representation of their mechanical anisotropy. For this purpose, a reformulation of Continuum Breakage Mechanics (CBM) is proposed to introduce material symmetries associated with energy storage processes. It is shown that, due to the thermodynamic consistency of the selected approach, the energy release resulting from comminution is influenced by the anisotropic characteristics of the elastic energy potential. As a result, the model is able to capture naturally and without additional fitting parameters the dependence of the yielding envelope on the relative orientation between bedding planes and loading direction. The performance of the new CBM model has been tested by performing parametric analyses which elucidate the role of cross-anisotropic elastic properties on the yield surface of a granular rock. Furthermore, its accuracy has been assessed against laboratory results available for two sandstones exhibiting dependence of the yield stress on the orientation of the bedding planes. Despite the simplicity of the selected model, the results emphasize that the proposed approach captures the salient features of the deformation response of anisotropic granular rocks, thereby disclosing an intimate connection between grain-scale energy release and cataclastic yielding which greatly simplifies the mathematical description of intrinsic inelastic anisotropy

Book of Abstracts Report from: International Workshop on Education of Future Geotechnical Engineers in Response to Emerging Multi-scale Soil-Environment Problems

Grant number: CMMI-1443990Book of abstracts compiled by over 40 participants, as part of a workshop organized by Principal Investigators Chloé Arson and Giuseppe Buscarnera, under NSF Grant CMMI-1443990. The workshop addressed the new skill set needed by Geotechnical Engineers to solve the multi-scale, multi-physics problems faced by modern technology. New research areas such as solid/fluid transition from the grain to the landslide; geological waste storage from micro-cracks to fractured reservoirs; bio-engineered geomerials from natural bacteria to designed structures; energy piles from soil properties to geotechnical performance; geotechnical earthquake engineering: from ground motion to structural safety all require a broader range of knowledge than previously provided in the typical Geotechnical Engineering course of study. This International Workshop aims to identify key challenges for the education of new generations of geotechnical engineers, focusing on undergraduate education. This book of abstracts was one of the outcomes of this workshop, for articles to be submitted for a special issue in the journal ASCE Journal of Professional Issues in Engineering Education and Practice.National Science Foundation (NSF