Foreword

As rightly suggested by Prof. Serge Leroueil in the preface to a previous edition of the present version of Prof. Eduardo Rojas’ book, a thorough elucidation of the essential features of the hydro-mechanical behavior of unsaturated soils, still a rather elusive endeavor, has been closely linked with efforts to isolate the relevant, effective stress fields governing their mechanical response. This book is a commendable attempt at demonstrating the suitability of the effective stress principle in defining a unified theoretical framework within which the most essential features of unsaturated soil behavior can be explained, and thus experimentally demonstrated, when the key constitutive relationships are presented as an extension of classical saturated soil mechanics, particularly in its three traditional categories: permeability and seepage, shear strength, and volume change.

In the present edition of the book, further elaboration on some of the contents of the original 12 chapters of the book is presented, including additional experimental evidence substantiating the appropriateness of a network porous-solid model postulated by Prof. Rojas that considers both micropores and macropores, and their inherent interconnections, for either dry, partially saturated, or saturated soils. The network model presents several outstanding features, including its apparent ability to reasonably reproduce the structure of the test soil based on grain and pore-size distributions, simulate soil-water retention curves and assess Bishop’s effective stress parameter. Furthermore, a probabilistic solid-porous model is introduced to considerably reduce the memory storage requirements during the explicit integration of all constitutive equations via computational drivers. Finally, a unified elastoplastic framework for expansive, collapsible, and compacted soil materials is introduced.

Four additional chapters have been added to the present edition of the book, including a simulation of soil-water retention curves via the porous-solid model as the test soil deforms (Chapter 13); simulation of undrained triaxial testing on unsaturated soils via a fully coupled hydromechanical constitutive model (Chapter 14); simulation of volumetric behavior of compacted soils under different stress paths via a coupled model (Chapter 15); and, finally, application of the probabilistic porous network model to establish an analytical equation for the relative hydraulic conductivity (Chapter 16). Results from well-thought-out experimental efforts reported by fellow scholars in the literature have demonstrated the potential of these frameworks in capturing, to a very promising extent, the hydromechanical response of unsaturated soils.

In his preface to the previous edition, Prof. Leroueil asked himself whether it was necessary to put all this information together into one single eBook, and his answer was a grammatically resounding “yes,” highlighting the subtle continuity and congruence of topics in one single document, from schematically thorough physical models to the equivalent effective stress equation and its practical applications. I could not agree more, and hence would like to emphasize another critical dimension to Prof. Eduardo Rojas’ scholarly work: its manifest potential as invaluable reference material in our quest to overcome the persistent challenge to change in the advancement of unsaturated soil mechanics in undergraduate education and civil engineering practice.