Journal article

Failure in multiphase geomaterias

The contributions assembled in the present volume proceed from the lectures of the 2009 ALERT Geomaterials School devoted to the Failure in the multiphase geomaterials. The multiphase behaviour of geomaterials used to be mainly considered from the point of view of the serviceability of the structures, or in other terms, of deformation analysis (REGC, Vol. 9, 2005). On the other hand, our understanding of the failure mechanisms in geomaterials exposed to thermal, chemical, and atmospheric fields with all their multiphysical coupling has reached today a fairly mature stage. Just the variety of failure modes and variables involved has expanded significantly. The editors of this volume believe that focusing on failure of geomaterials in the presence of multiphysical phenomena will provide an excellent tool for discussion, learning and outlining future developments in this fascinating and critically important area. The discussed topics aim at providing theoretical, experimental and numerical tools that allow to deal with a large number of applications like underground structures (storage, civil engineering), surface structures (earth and concrete dams, embankments), natural sites (slopes, cliffs) as well as the use of the geosphere (petroleum and gas extraction, mines and quarries, both underground and surface). When dealing with the behaviour of multiphase geomaterials, instances of complexity and interaction are numerous, mainly because of the coexistence of several constituents and phases, their physical and mechanical interactions, their reactivity, and their often non-linear behaviour. Flow involves fluids and transport of chemical species. Exothermic reactions can occur within geomaterials (degradation of the waste, chemical reactions) and the emitted heat is conducted by soil, rock or concrete. Geomaterial deformation depends not only on the classically understood effective stress, but also on suction and temperature, as well as on the chemical history of material. Coupled transient analyses are in fact a characteristic feature in this field. This volume contains nine chapters. Emphasis is given to the presentation of the fundamentals and new concepts that help understanding failure mechanisms in multiphase materials. A special attention is given to the potential of the presented tools for analysing practical multiphysical geomechanical problems. As an introduction to the School, the conventional and advanced concepts of failure for geomaterials are introduced by R. Nova. The conditions for shear band occurrence (non-homogeneous bifurcation) are given. It is shown that homogeneous bifurcations can occur in the hardening regime (e.g. the so-called static liquefaction) even before the occurrence of failure by shear banding. Other instability conditions of this kind are discussed and connected with the nullity of the second order work. It is also shown that the familiar concept of undrained strength can be better understood as a condition of controllability loss. Chapter Two by R. Charlier is an introduction to the constitutive multiphysical processes with an emphasis on the modelling and coupling aspects. A number of different couplings are discussed, with respect to fluid flow (saturated and unsaturated) and the thermal transfers in deformable porous media. Some aspects of the numerical modelling with the finite element method are also discussed. Failure conditions in soils occurring during or as a result of heating or by altering their chemical environment appear to be strongly dependent on the history of the application of stress and temperature and or the chemical change. In Chapter Three, T. Hueckel discusses several cases of such history leading to various modes of failure. He identifies and interprets them in terms of Thermal and Chemical Cam-Clay models. Particular attention is given to the influence of thermal variability of the coefficient of the critical state, M, or the angle of internal friction. A detailed analysis of the material history offers an explanation of an apparent confusion about whether the soil strength is decreased or increased by temperature. Cases of dependence of failure conditions of clay on the ion concentration in pore water and on its acidity in the context of a landslide triggering mechanism are also discussed. The scope of Chapter Four by L. Laloui is to present global mechanisms of soil desiccation, including drying shrinkage and cracking. The results of an experimental study of desiccation are presented, in which strains, suction, water content, degree of saturation and crack geometry are investigated. Those results show that cracking initiates close to the onset of de-saturation. Insights into the micro-scale are proposed to explain this observation. A scenario for the processes leading to crack initiation is further established in terms of the macroscopic variables: an assessment of the stress building up is proposed, until a critical point at which the tensile strength is met. Desiccation crack pattern formation is finally discussed. The physical processes which occur during an earthquake exhibit several coupled phenomena as large variations of stress, pore pressure and temperature take place in the slip zone. The multiphysics aspects of the seismic slip are presented by J. Sulem in Chapter Five. It is shown how the thermo-poro-mechanical couplings due to shear heating can be associated with phase transition such as vaporization of the pore fluid, melting of fault gouge and to chemical effects such as dehydration of minerals. The different competing effects that may influence dynamic slip and affect the weakening of the shear stress are discussed. Coupled phenomena are of paramount importance for a correct understanding and a reliable description of the mechanical response of geomaterials to external actions. Given the variability of the conditions influencing the response of geomaterials, laboratory investigations often require advanced multiphysical testing techniques capable to approach the real conditions in terms of intensive state variables. The contribution by V. De Gennaro in Chapter Six presents some results obtained using advanced laboratory testing to characterise failure in chalk addressing their intrinsically multiphysical nature. Underground nuclear waste storage constitutes one of the major application fields for the multiphysics geomechanics. A rational design of repositories requires a good understanding of the interacting thermo-hydro-mechanical phenomena that occur in the engineered barrier and adjacent rock. A. Gens presents in Chapter Seven multiphysical and numerical tools that are applied to the simulation of two large scale tests: a mine-by test involving the excavation of a shaft in an argillaceous rock and a large-scale high- temperature heating test in fractured rock. The serviceability of concrete structures is a coupled problem in which fracture and damage are coupled with several environmental attacks. G. Pijaudier-Cabot deals in Chapter Eight with the mechanical damage, chemical damage and permeability in quasi-brittle cementitious materials. A description of chemo-mechanical damage and the case of calcium leaching, which is relevant to waste containment vessels is made. The second example of implementation of continuum damage models discussed deals with coupled damage-permeability effects. In the case of diffuse damage, the material permeability is controlled by the decrease of average stiffness due to micro-cracking. A relationship between permeability and damage, consistent with the two above asymptotic cases, is defined. A critical issue in the analysis of the various presented multiphysical problems is the development of numerical methods for post-failure behaviour. This is the topic presented in Chapter Nine by F. Collin. Rupture in geomaterials is often preceded by a localization of the deformations within thin bands. The strain localization is thus an important process. To deal with interactions occurring between the different phases of porous media, a regularization technique based on the second gradient model is extended to multi-physics couplings. We believe that this volume provides to the postgraduate students, researchers and practitioners, a valuable introduction and a sound basis for further progress in the challenging field of coupled multiphysical processes in geomechanics in general, and in failure mechanics problems in particular.


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