Book review: Unsaturated soils: A fundamental interpretation of soil behaviour.

Abstract

Civil engineering structures interact with the ground and some structures are composed entirely of ground-derived materials. In this context, the ground is as much an engineering material as concrete and steel. We will be concerned principally with those fine-grained clays and silts, or soils containing coarser sand and gravel particles but with a significant percentage of fines, which constitute the ground. Soilstructure interaction must be taken into account for structures such as foundations, roads and tunnels; and the behaviour of earth structures such as earth dams and slopes requires the development of sound prin- ciples on which to base analyses. There is a fundamental need to interpret the ground conditions and geology in an engineering context, to provide warning of natural disasters such as landslides, to deal with environmental issues such as landfills or contaminant migration in the ground and groundwater, amongst other equally important issues. Soil mechanics is a specialist branch of civil engineering that addresses the investigation, anal- ysis and prediction of soil behaviour. Yet, even among practising engineers there is often lack of awareness of the intricacies of the subject, with solutions to problems appearing to emanate from a black box. Unsaturated soil, which is the focus of this book, is an important, complex and not well-understood component of soil mechanics that presents geotechnical engineers with many challenges. The study of unsaturated soils is an ab- sorbing, practical subject linking fundamental science to nature. While soils, in general, are inherently variable and not readily amenable to analysis, unsaturated soils raise the complexity to a higher level. This book provides a perspective of unsaturated soils based on recent research, and shows how this dovetails with the general discipline of soil mechanics. Thermodynamic principles underpin the analyses and allow the development of a rigorous theoretical model. In developing the model, reference is made to branches of physics and chemistry and analogous behavioural trends. In this respect, imagination is not considered a vice but must be tempered with realism and experimental evidence. Attempt is made to clearly distinguish between verifiable conclusions and imagination while pushing ideas as far as we dare. Where assumptions are made and where potential errors arise in testing, these are highlighted, but we do not apologise for raising peoples blood pressure if it generates meaningful discussion. Geotechnical engineers have a different perspective of the ground than other profession- als involved in associated fields. However, there is a great deal of overlap in the disciplines of geology, environmental science and soil science; and professionals in these fields have a lot of knowledge and expertise to share. Nevertheless, each has a different view of the ground and the most important problems to be solved. It is only in recent times that the significance of suction in unsaturated soils has transcended from soil science applications, involving plant water deficiency, into geotechnical engineering applications. There are also different approaches to problems within geotechnical engineering. A practising geotechnical engineer dealing with day-to-day problems is likely to be more interested in a prescriptive, quick solution than the niceties of a detailed analysis. The use of California Bearing Ratio (CBR) tests for the design of road pavements exemplifies this approach; though highly empirical, such tests nevertheless provide a basis for achieving a satisfactory design. Researchers in the field of soil mechanics sometimes adopt a similar approach to a problem, arguing that it is not possible to fully analyse soil behaviour as the materials are not manufactured under controlled conditions. Further, soils do not obey simple stressstrain rules and, importantly, exhibit time-dependent behaviour. Thus, unless proven otherwise, a pragmatist would argue that tried and tested methods be adopted in solving simple problems. However, other researchers strive for a better understanding of the underlying principles behind soil behaviour and try to answer the question: what is actually happening? This more realistic approach attempts to discover what the world is actually like with protagonists arguing that a better understanding should lead to better predictive models and ultimately to cost savings. The development of predictive models based on a sound theoretical footing is a necessity in allaying the perception that geotechnical engineering comprises empiricism and guesswork. Arguably, geotechnical engineers, possibly more than most other associated profes- sionals, must keep up with current research. However, research, by definition, is at the forefront of knowledge and opinions will vary on the methods and products of endeav- ours to reach solutions to problems. It is often only with years of verification testing and experience that a consensus is achieved and research is accepted as valid, though subject to review and improvement with time. The accumulation of research promotes and progresses concepts and established theories may turn out to be special cases of more far-reaching ones. The prime scientific example is Newtons laws, which Einstein showed to be a limited case of relativity. On a less grand scale, Terzaghis well-established effec- tive stress equation for saturated soils is shown in Chapter 8 to be a special case of a more general equation for unsaturated soils relating the three-phase relationship of stress and pressure to the volumes of the phases. In this context research is a self-developing system of investigation in which errors and idealisations are, sooner or later, detected by experiment and more realistic analysis. Findings are customarily put forward fraught with errors and unperceived assumptions that are gradually worked out before the underlying theory is accepted. We hope this is not the case here, but if we could predict the future we might not have written the book. It is imperative, however, that theory fits well with sound, repeatable experimental data. This is a major thrust of this book. We hope you find the book interesting and readable.