For a long period Soil Mechanics has remained at the semi-empirica stage, and only a few decades ago it has shown a tendency to become a fundamental science. However, this evolution is taking place slowly; in spite of the efforts of numerous research scientists, the very complex rheological laws of soils are still not well known. Even if these laws were elucidated, it would take a long time still to deduce simple rules from them for reliable and convenient use in current practical engineer ing. In the pursuit of these distant aims - and of others more imme diate - fundamental research and applied research are very active, both in Rheology and Soil Mechanics. The complexity of the problems to be solved should incite the laboratory researchers and the engineers to a continuous collaboration. Everyone acknowledges the advantage of these connections although aware of the difficulty of realizing this wish. However, contacts are being made little by little between the repre sentatives of the different branches of Rheology and Soil Mechanics, to the great benefit of science. The bureau of the International Union of Theoretical and Applied Mechanics (IUTAM), aware of the importance of these two associat ed fields of mechanics, considered it possible to accelerate the natural and necessary processus of their interpenetration by organizing in Grenoble, from 1st to 8th April 1964 an International Symposium on Rheology and Soil Mechanics.

Instabilities Modeling in Geomechanics describes complex mechanisms which are frequently met in earthquake nucleation, geothermal energy production, nuclear waste disposal and CO2 sequestration. These mechanisms involve systems of non-linear differential equations that express the evolution of the geosystem (e.g. strain localization, temperature runaway, pore pressure build-up, etc.) at different length and time scales. In order to study the evolution of a system and possible instabilities, it is essential to know the mathematical properties of the governing equations. Therefore, questions of the existence, uniqueness and stability of solutions naturally arise. This book particularly explores bifurcation theory and stability analysis, which are robust and rigorous mathematical tools that allow us to study the behavior of complex geosystems, without even explicitly solving the governing equations. The contents are organized into 10 chapters which illustrate the application of these methods in various fields of geomechanics.

At the VIIth International Congress on Rheology, which was held in Goteborg in 1976, Proceedings were for the first time printed in advance and distributed to all participants at the time of the Congress. Although of course we Italians would be foolish to even try to emulate our Swedish friends as far as efficiency of organization is concerned, we decided at the very beginning that, as far as the Proceedings were concerned, the VIIIth International Congress on Rheology in Naples would follow the standards of time liness set by the Swedish Society of Rheology. This book is the result we have obtained. We wish to acknowledge the cooperation of Plenum Press in producing it within the very tight time schedule available. Every four years, the International Congress on Rheology represents the focal point where all rheologists meet, and the state of the art is brought up to date for everybody interested; the Proceedings represent the written record of these milestones of scientific progress in rheology. We have tried to make use of the traditions of having invited lectures, and of leaving to the organizing committee the freedom to choose the lecturers as they see fit, in order to collect a group of invited lectures which gives as broad as possible a landscape of the state of the art in every relevant area of rheology. The seventeen invited lectures are collected in the first volume of the proceedings.