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PREFACE:

Civil engineers build on or in the earth’s surface. Most of the earth’s land surface comprises notoriously hazardous geomaterials called ‘unsaturated soils’. These soils are a hazard to earth structures and earth-supported structures because on wetting, by rain or other means, they can expand or collapse with serious consequences for cost and safety. This book puts the mechanics and engineering of unsaturated soils into a logical framework for civil engineering analysis and design.

 It also explains the laboratory and field testing and research that are the logical basis of this modern approach to safe construction in these hazardous geomaterials. A search of the Institution of Civil Engineers’ library web site returns a total of 462 titles of books, conference proceedings and articles on ‘soil mechanics’. 

A search for ‘unsaturated soil mechanics’, however, returns only 66 titles, or just over 14 per cent of the total. Of these only two of the titles are text books. This is in spite of the fact that most of the soils on our planet’s land surface are unsaturated. The small proportion of unsaturated soil titles is hardly surprising, however. This is because post-Terzaghi soil mechanics had its origins in northern Europe and North America where soils involved in civil engineering construction are mainly saturated or nearsaturated. So naturally the science developed on the (simplified) assumption of saturation.

With the rapid development, however, of China, India, Central and South America, and Africa (covering regions where foundation soils are rarely saturated near the surface), geotechnical engineers can no longer ignore the complication of unsaturated soils and the challenges they present.

 Accordingly, the international geotechnical community has responded. Since the 7th International Conference on Expansive Soils held in Dallas, USA, in 1992, there have been over 10 major conferences or workshops up to and including the 3rd International Conference on Unsaturated Soils (UNSAT, 2002) held in Recife, Brazil, in 2002, plus the 4th International Conference on Unsaturated Soils held in Carefree, Arizona in 2006 (the international conferences are convened at 4 yearly intervals). 

These conferences have brought into the public domain the results of research conducted mainly at universities. But the teaching at universities of unsaturated soil mechanics and engineering is still relatively rare and when covered is confined mainly to post graduate courses including short courses for continuing professional development. Part of the problem of teaching the subject is the lack of up-to-date textbooks. There are two books as mentioned above.

There is the classic textSoil Mechanics for Unsaturated Soilsby Fredlund and Rahardjo (1993) which starts from the concepts of agronomy and soil science and moves on to unsaturated soil mechanics, based mainly on the Mohr–Coulomb failure criterion, and the problems of unsaturated soil slopes.

 Also, there is the more recent textUnsaturated Soil Mechanicsby Lu and Likos (2004) which concentrates more on flow in unsaturated soils. Both books are excellent and are major aids to the teaching of unsaturated soil mechanics.

 Based on our teaching, research and test equipment development, particularly at Hong Kong University of Science and Technology and at GDS Instruments, we felt, however, that there was a need for a third text book – one where modern soil testing of unsaturated soil behaviour, including measurement of the state-dependent soil–water characteristic curve (SDSWCC) and anisotropic soil stiffness, is highlighted; where constitutive modelling, which is at the heart of modern soil mechanics, is central and its’ power in numerical analysis demonstrated; and where real applications in unsaturated soil slope engineering field trials are described. Accordingly, we wrote this state-of-the-art book which is in four parts as follows:

• Part 1: Physical and flow characteristics of unsaturated soils;

 • Part 2: Collapse, swelling, strength and stiffness of unsaturated soils;

 • Part 3: State-dependent elasto-plastic modelling of unsaturated soils; 

• Part 4: Field trials and numerical studies in slope engineering of unsaturated soils

Each part is prefaced with a frontispiece of images that represent the content of that part. The images are then followed by synopses of each chapter in that part. 

These synopses give overviews of each chapter and explain how they enhance geotechnical knowledge. The reader can immediately judge from this the benefit to be gained by looking into a particular part or chapter.

 This state-of-the-art book is unique because it illustrates the importance of state-dependent soil–water characteristic curves and how to measure them, describes how to measure small-strain anisotropic soil stiffness, develops state-dependent elasto-plastic constitutive modelling as well as conjunctive modelling of rain infiltration, and studies the behaviour of loose fills from  unsaturated to saturated states.

 A major part of the book addresses the hugely challenging geotechnical problem of unsaturated soil slope stability. 

This state-of-the-art book was written at Churchill College, Cambridge where Charles Ng was Overseas Fellow in 2005. The book is dedicated to Prof. Delwyn G. Fredlund, unsaturated soil mechanics pioneer