Preface: In my final year as an undergraduate at Oxford University, I undertook a project on the warping of asymmetrical steel beams with Dr Edgar Lightfoot. I took no formal lectures on soil mechanics, although Dr Lightfoot also gave a few lectures on slip-lines and bearing capacity within an optional 'speciality' paper on civil engineering. He also gave me career advice along the lines that "there is this new theory called critical state soil mechanics, which seems to be worth investigating". I duly bought a copy of Schofield and Wroth's book on that subject, and so began my education in soil mechanics. I subsequently studied for my PhD with Professor Peter Wroth, and cut my teeth as a University Lecturer at Cambridge in the group then headed by Professor Andrew Schofield. It is therefore with humility, and a sense of the wheel having turned full circle, that I find myself writing a preface to this 'retrospective' new book by Andrew; indeed, I have a sense of being back under examination, wondering what grade my former professor will assign. Much of this book describes the developments leading to the original Cam Clay model, focusing on fundamentals of the shearing of soil. The aim is to lay the groundwork of understanding that should form the basis of geotechnical design, guiding engineers towards the class of behaviour to be expected under different combinations of effective stress and water content. There are a few equations, but simple ones; much greater challenge rests in the arguments put forward regarding soil behaviour and the intellectual effort needed to keep pace with the author. After the Special Lecture that he delivered at the 2001 ICSMGE in Istanbul, he commented that it was "heard without comprehension". The lack of comprehension was not to do with complexity of concepts or algebra, but with grasping the underlying message and appreciating the gap between the understanding that many experienced academic and practising engineers do indeed have, and the misleading language and teaching that pervades much education in soil mechanics. The book is divided into 6 chapters, which progress from the simple planar sliding of soil towards plastic design in geotechnical engineering. But Andrew Schofield is not constrained by sequence and rather than write a conventional text-book, he had in mind the sort of book that "engineers might read on a flight and leave on their office coffee tables". The 'coffee-table' image came from a reviewer of the proposed book, perhaps meant as disparaging, but is excellent advice here: the book invites reading at a single sitting, both because it is intensely interesting, and because of the author's global approach, with much cross-referencing - across the centuries as well as between chapters. After reading, it is a book to be left readily available for frequent dipping, both for the pleasure in the historical anecdotes spread across the last 400 years and to reinforce the fundamental understanding of soil behaviour conveyed in the book. The frontispiece illustration is the lynch-pin to the ideas the author wishes to convey, and is referred to throughout the book. Heroes (Coulomb, Hvorslev and Taylor) and villains (Terzaghi in particular) are identified in Chapter 1, with detailed discussion of the nature of friction, the role of interlocking and the misinterpretation of Hvorslev's empirical envelope of peak strengths as indicating true cohesion. The second chapter focuses on the critical state, correcting Casagrande's critical void ratio to allow for the effective stress level, and liquefaction, contrasting extreme forms related to ultra-high void ratio, or to near zero effective stress. Historical anecdotes replace the usual glossy pictures of a coffee-table book, and suitably leaven the technical arguments, and one of many rewards for those who read the book will be the connection described here between the latter form of liquefaction and the 17th century poet, Herrick. There are frequent (positive) quotations from Terzaghi's writings in the literature, but inevitably for someone so fond of dogma it is not difficult to find negative examples. His assertion of cohesive bonding between soil grains, and rejection of the usefulness of Rankine's limiting stress states, are two such examples that are discussed at some length in Chapters 3 and 4. In defence of his (c, f) strength model, Terzaghi did advocate that clay should be tested "under conditions of pressure and drainage similar to those under which the shear failure is likely to occur in the field". However, that caveat seems to have been overlooked and, even today, the c-f strength model is taught widely and used inappropriately, Current teaching is littered with calculations where the effective stress differs significantly from the conditions under which the strength measurements used to generate the c-f fit were derived. Modern teaching often applies such a model to bearing capacity analyses on sand, without adjustment for the resulting high stresses, or to the stability of slopes and cuts, where pore pressure dissipation would destroy any apparent c. Students who understand soil strength according to Andrew's approach, are wise to these dangers. A modest ambition for the present book might be to see the words cohesion and adhesion excised from our soil mechanics vocabulary, replacing them with shear strength (at a given water content and effective stress level) and, on the rather rare occasions where it is appropriate, cementation. The basis of the original Cam Clay model, including background in the theory of plasticity and the experimental evidence for the internal plastic work, is described in Chapter 5. Limitations of this simple model in terms of anisotropy, soil sensitivity and cyclic loading are readily acknowledged. As a basic framework for teaching, however, the model still has much to offer and it is refreshing to be taken through the careful experimental data on reconstituted clays on which it is based, and the (now classical) examination questions from the Cambridge Tripos of nearly 40 years ago. Once armed with the simple concept of wet and dry of the critical state line, students will understand whether a sample will wish to contract or dilate, whether pore pressures generated during undrained shearing will tend to the positive or negative, and conditions where ductile plastic deformation might change to brittleness and fracture. The ability of the model to quantify these states is immediately appealing to modern stude ts, rather than them having to digest purely qualitative explanations. Andrew Schofield deserves to be regarded as one of the geniuses of the latter half of 20th century soil mechanics. His Fellowship of the Royal Society is based on his two remarkable contributions of original Cam Clay and the promulgation of centrifuge modelling in geotechnical engineering beyond its origins in Russia. It is appropriate therefore that the final chapter in this book is devoted to the application of the principles of critical state soil mechanics by means of centrifuge experiments conducted under conditions of stress similitude. This is a rewarding book, full of insights both technical and personal. It reinforces ideas described in the original Schofield and Wroth book on Critical State Soil Mechanics, and in his 1980 Rankine Lecture. For the unconverted, it is an invitation to re-examine your basic understanding of soil behaviour. For the converted who might be tempted to dismiss the book too lightly, it is a call to ensure that our teaching, and the vocabulary and nomenclature we use in describing strength models for soil, reflect accurately the underlying concepts. Mark F. Randolph The University of Western Australia
About the Author
Professor. A. N. Schofield, FRS, FREng, FICE In 1993, on the joint nomination of the Presidents of ICE and the Royal Society, the ICE Council awarded Andrew Schofield the James Alfred Ewing Gold Medal for special meritorious contributions to the science of engineering in the field of research. He has also been awarded the US Army Distinguished Civilian Service Award.