This substantial book is presented in 10 chapters and eight appendices, and is supplemented by >0.5 GB of downloadable computer files, including some compressed files, grouped into six folders. The authors must have been frustrated as they tried to extract actual values from reference book figures, because they include in the downloadable material Microsoft Excel files of data and graphs that they used to produce the figures in the book. Also contained in the downloadable material are laboratory test data, case-history data, programs, calibrations, source code, executable code and software verification files.
Chapter 1, Introduction (34 pp., 8% of the text), presents the critical state approach as a comparison of soil behaviour as a function of density; basically, ‘dense soils are strong and dilatant, loose soils weak and compressible’. A framework is needed to determine how a particular soil will behave from its current state (expressed as void ratio or density) based on its critical friction angle (a soil property). Most of Chapter 1 is descriptions of nine liquefaction case studies and a summary of lessons gained from them.
Chapter 2, Dilatancy and the state parameter (65 pp., 15% of the text), describes the framework for soil behaviour and the state parameter approach. Dilatancy is the tendency of soils to change volume while being sheared. Two definitions of dilation are in use: an absolute definition (the change in volumetric strain incurred since the initial condition) and a rate definition (the ratio of rate of volume change with rate of shear strain increase). The two definitions are related mathematically, of course, with the absolute being the integral of the rate definition.
The discussion of critical state provides useful insight by pointing out that traditional geotechnical practice uses soil density to differentiate a single geological material as if it were different materials, each with properties that must be determined by independent testing (e.g. 36° dense state friction angle but 32° loose-state friction angle). The idea that soil has a critical void ratio was identified in 1936, but it was not until 1968 that the theoretical development demonstrating soil density to be a state parameter rather than a soil property was published. The state parameter, ψ, is the difference between the void ratio of the present in situ condition, e, and the critical void ratio, ec, or ψ = e − ec. The critical state condition is satisfied by e = ec or ψ = 0. Chapter 2 also contains discussions regarding the influence of soil fabric, over-consolidation ratio and sample size on soil properties. Subsequent major sections include evaluating soil behaviour with the state parameter, determining the critical state, uniqueness, soil properties, plane strain tests for soil behaviour and soil behaviour from triaxial properties.
Chapter 3, Constitutive modelling for liquefaction (42 pp., 9% of the text), introduces the reasoning for modelling soil stress–strain behaviour, including noting the key simplifications and idealization that must be made, followed by historical background, representing the critical state with critical void ratio and mean effective stress, specific ways modelling has been performed (including references to downloadable spreadsheet files to supplement the book and enhance its utility to researchers and practitioners) and comparison of theoretical results with experimental data. Two key simplifications described in Chapter 3 are isotropy and small strain theory. Soils are notoriously anisotropic; however, anisotropy is a detail about the soil rather than a fundamental premise that will invalidate a model. Small strain theory is familiar to engineers and appropriate in the elastic range. Soil behaviour involves large strain resulting in plastic behaviour and is used in calculating void ratio associated with incremental volumetric strains. Large strain also must be used in evaluating in situ tests.
Chapter 4, Determining state parameter in situ (60 pp., 13% of the text), begins with recognition of the difficulty of sampling cohesionless soils without inducing disturbance, which leads to penetration tests being used as a surrogate for sampling, both standard penetration test (SPT) blow counts and cone penetration test (CPT) electronic data. The description of penetration tests includes commentary that the results must be inverted from knowledge of the load or response of the soil (penetration resistance) to the soil properties or state, which usually is input to the problems being solved. The focus of Chapter 4 is the inversion process, which tends to be called ‘interpretation’ of the penetration resistance data.
Chapter 5, Soil variability and characteristic states (21 pp., 5% of the text), describes the effects imposed by real soils with vertical and lateral variability on the question: What state best characterizes the overall response of the soil to loading that may lead to liquefaction? Chapter 5 subdivides this question into two issues: (1) What is the real distribution of state in situ? (2) What value of the given distribution characterizes behaviour of the soil mass? These two issues are addressed with case history results and stochastic approximations with resulting responses. The summary section in this chapter acknowledges that much work remains to be done to understand the effect of soil variability on the performance of both soils and foundations under cyclic loads.
Chapter 6, Static liquefaction and post-liquefaction strength (89 pp., 20% of the text), the longest chapter in the book, is justified by the simple statement that ‘static liquefaction largely controls stability, even when the loading is cyclic’. Furthermore, it is relatively straightforward to understand: ‘excess pore water pressure is caused by plastic strain’ and ‘static liquefaction is an aspect of soil behavior that fits simply within the state parameter framework’. Two methods of triggering static liquefaction are of practical relevance: (1) undrained failure in monotonic shear; (2) post-earthquake liquefaction. The first is caused by an increase in the stress ratio caused by slope steepening (toe erosion) or a decrease in mean effective stress through seepage pressures if the soil is loose enough. The second can be caused by redistribution of pore pressures during dissipation of excess pore pressures caused by cyclic loading.
Substantial amounts of data from laboratory experiments available for downloading in spreadsheet format are discussed in Chapter 6. Tests include triaxial extension, simple shear and plane strain compression. Data trends for peak undrained shear strength, su, and residual shear strength, sr, are presented and discussed. The steady-state approach to liquefaction is described and analysed, and then validated, followed by a discussion of its deficiencies. Full-scale experience leading to the empirical approach is discussed, including analyses of residual strength values. The important question ‘How dense is dense enough?’ is discussed with laboratory test data, CPT charts and project-specific studies. The chapter closes with post-liquefaction residual strength and liquefaction assessment for silts, and finally a summary that notes that ‘while the critical state framework is indeed reliable and something that allows understanding of static liquefaction in practical situations, the simplicity of the steady-state school is an illusion. Rather it is essential to consider the range of in-situ conditions, the actual soil properties, the likely effects of drainage and the corresponding material behavior.’
Chapter 7, Cyclic-stress induced liquefaction (65 pp., 15% of the text), has the subtitle ‘cyclic mobility and softening’. The earthquake-induced liquefaction ground failures in 1964 in Niigata and Alaska commanded the attention of geotechnical engineering with realization that cyclic action can cause liquefaction. The difference between static- and cyclic-induced liquefaction is that static liquefaction affects only soils that are sufficiently loose that the strain hardening of the soil is inadequate to support the increased stress, whereas cyclic loading tends to densify all soils. Shear-related dilation of soils that are dense at the initiation of cyclic loading acts to offset densification-induced pore pressure buildup, such that cyclic softening, rather than brittle collapse, occurs. This chapter identifies two issues that confused the understanding of liquefaction in the early years of study of this topic: (1) a case-history approach focusing on geological history; (2) general disregard of dilation. The first issue led to classification of liquefaction susceptibility without consideration of mechanics, whereas the second issue led to consideration of liquefaction at a condition of zero effective stress.
Experimental data, cyclic behaviour of sands, cyclic behaviour of silts, cyclic rotation of principal stress and a series of discussions on trends in cyclic simple shear behaviour are included in Chapter 7. A discussion of the Berkeley School approach, followed by identification of deficiencies of this approach and the state parameter view of the Berkeley approach, lead to a theoretical framework for cyclic loading. Chapter 7 concludes with a short section on soil fabric, before the summary that notes that the empirical approach requires a number of corrections and adjustments for applying existing knowledge in different situations, whereas the critical state process is simpler and based on in situ conditions, actual soil properties, likely effects of drainage and the corresponding material behaviour.
Chapter 8, Finite element modelling of soil liquefaction (19 pp., 4% of the text), begins with recognition that the previous chapters led to understanding soil behaviour and obtaining soil properties, but were not directly useful for analysing a dam or foundation. The finite element method is introduced as being able to model real-world problems, not just laboratory tests, with elasto-plastic analysis with public domain finite element software included in the group of downloadable files associated with this book. The finite element method can accommodate two types of nonlinear behaviour important in liquefaction analyses: material nonlinearity in which soil stiffness evolves with deformations, and geometric nonlinearity where the solution is affected by higher order terms of displacement gradients. The software is accompanied by verification files.
Chapter 9, Practical implementation of critical state approach (38 pp., 8% of the text), is one of the primary differences between the second edition and the first edition. It is a procedure for engineering application of the principles and practices described in the earlier chapters. Both laboratory and in situ tests are required: in situ tests to measure state, because undisturbed samples cannot be obtained reliably, and laboratory tests on reconstituted samples to measure properties, because in situ tests cannot distinguish between the effects of state and properties. The guidance in Chapter 9 focuses on (1) scoping field investigations and laboratory testing, (2) deriving soil properties from laboratory test data, (3) choosing CPT equipment, (4) interpreting in situ CPT and shear-wave velocity data and (5) applying the results to problems in soils ranging from sands to silts. References are made to spreadsheets available in the downloadable files, which include compressed (zipped) files that are included within other compressed files. For this review only a few of the numerous files were opened; Microsoft Excel spreadsheets that were opened had multiple worksheets, so it is clear that they contain an enormous amount of data and plots. Included with the guidance in Chapter 9 is Section 9.3.8, Reading this section is not enough (page 421); the reader is strongly encouraged to actually open the spreadsheet files and use them as part of gaining a proper understanding of the critical state approach to liquefaction analysis.
Chapter 10, Concluding remarks (15 pp., 3% of the text), is a commentary on aspects where further research and progress are needed. The main headings in this chapter are model uncertainty and soil variability, state as a geological principle, in situ state determination, laboratory strength tests on undisturbed samples, soil plasticity and fabric, relationship to current practice, what next?, and finally do download! The obvious value of having access to data and associated graphs, as mentioned by the authors, is that the effects of changing properties can be seen in graphic outputs to facilitate understanding.
Following the text are Appendices A–H (203 pp.), references (18 pp.) and index (14 pp.). The appendix titles are (A) stress and strain measures, (B) laboratory testing to determine the critical state of sands, (C) NorSand derivatives, (D) numerical implementation of NorSand, (E) calibration chamber test data, (F) some case histories involving liquefaction flow failure, (G) seismic liquefaction case histories and (H) CamClay as a special case of NorSand. This reference book will be a valuable addition to the geotechnical library of any researcher or practitioner working on projects that involve soil liquefaction.