Editorial: Failure in multiphase geomaterials

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 contr