Residual soils are an important group of materials covering large areas of the planet; those derived from the in situ tropical weathering of bedrock profiles are of particular importance in the developing countries of Africa and Asia. Residual soils are recognised as having characteristics that may vary considerably from the norms of sedimentary or transported soils, on which textbook approaches to geotechnical problems are traditionally based. It is therefore very welcome to see a book such as Dr Wesley's appear on the market. This reviewer followed Laurence Wesley at the Indonesian Institute of Road Engineering, some years after he left, where he was a well-remembered and respected figure for his ground-breaking work on the residual volcanic soils of Java. This work has proved to be the starting point for many researchers, including the current reviewer, in the field of tropical and residual soils.
Dr Wesley has limited himself to dealing with residual soils comprising ‘saturated clay soils' with examples drawn largely from his experience in volcanic materials. The majority of residual soil-like materials are therefore excluded from his consideration, as well as all the materials in the potentially very difficult residually weathered soil–rock zone. Dr Wesley also excludes partially saturated materials from consideration. In surprising contrast however, he includes commentary on the lake clays of Mexico and Bandung, which are not true residual materials. They are transported residually weathered clays, which is a very different category of material, particularly as regards structure. To be fair, Dr Wesley does say in his introduction that a more suitable title might have been ‘Geotechnical Engineering in Residual Soils – A Personal View'.
There are an initial two chapters on the key aspects of residual soil behaviour and their characterisation and classification. These are followed by three chapters on residual soil behaviour in terms of pore pressure and water table; consolidation and settlement; and shear strength. The sixth chapter covers site investigation techniques, and is followed by two chapters on the practical application of the special characteristics of tropical soils in terms of bearing capacity, earth pressures and slope engineering. Chapter 9 is then devoted to volcanic soils in particular, followed by chapter 10 on other residual soils. The final short chapter 11 concentrates on the compaction of residual soils.
The slightly unusual sequence of chapters may be a consequence of the origin of this book as a series of lectures. Nevertheless, they contain excellent commentary and examples of the likely problems to be encountered in understanding and working with residual soils. As is to be expected in a personal view of a complex group of materials, there are some areas of debate. A particular surprise is the omission from the book of any reference at all to the comprehensive Working Party Report on Tropical Residual Soils by the Engineering Group of the Geological Society.
One general difficulty is that by focusing principally on ‘soil' behaviour, the book does not address the need for a total earth science approach to the geotechnical engineering of residually weathered materials that encompasses aspects of geology, geomorphology, hydrology, pedology and rock mechanics, as well as soil mechanics.
Because of the influence of differing scales of relict structure and fabric in residual soils, it is essential to differentiate between mass, material and microscopic scale soil behaviours and to be clear about what scale at which testing and modelling should be conducted – much more so than in conventional sedimentary soils. In chapter 2 in his grouping system for residual soils Dr Wesley has very sensibly subdivided his group A soils into those influenced by structure and those not influenced. However, he strangely has not extended this subdivision into his other two groups; group B, soils influenced by conventional clay minerals and group C, soils influenced by non-conventional clay minerals.
Dr Wesley acknowledges and discusses the apparently high moisture contents in some volcanic soils; however, he could have taken this further in terms of soil-mineral structure and the concepts of a ‘working moisture content' at earth-working temperatures as opposed to standard drying temperatures. Similarly, there is a need to be much more aware of the various states of disturbance, from slightly disturbed to disturbed to totally remoulded, in the application of test results to practical civil engineering projects. The effects of disturbance on structured residual soils make the recovery of truly undisturbed samples an extremely problematical exercise. The careful selection of appropriate sampling and testing techniques is particularly important in residual soils and there is perhaps a slight imbalance in the limited discussion in this book on test method selection and the use of in situ testing techniques when compared, for example, to the extensive coverage of oedometer consolidation. Surprisingly little or no mention is made of using the shear box as a testing tool for establishing the characteristics of undisturbed residual soils.
Dr Wesley very wisely indicates, however, that geotechnical design in residual soil environments should be accomplished by observational or by semi-empirical methods, rather than by traditional classical methods. A key element in this approach is the accurate collection of relatively easily obtainable and inexpensive site information which may be correlated between similar sites, or otherwise adapted for design purposes. There might, therefore, have been a greater emphasis in the book on the establishment of logical and accurate data collection and description techniques.
In summary the book that Dr Wesley has written is an excellent personal view of the residual soils that he has encountered; it should not, however, be considered, as the title and associated publicity might imply, as a definitive book on the geotechnics of residual soils. This book is a very welcome and thought-provoking commentary on the very distinctive geotechnical character of residual soils and an essential book for an experienced geotechnical practitioner to have at hand when dealing with igneous residual soils, particularly those of volcanic origin.
