Book review

Back Analysis in Rock Engineering,Shunsuke Sakurai,ISRM BookSeries,Volume4(2017),CRCPress/Balkema.ISBN:9781138028623(Hbk),9781315375168(ebook)

This volume is the fourth in the International Society for Rock Mechanics and Rock Engineering(ISRM)Book Series,edited by Professor Xia-Ting Feng,ISRM Past-President and Vice Editor-in-Chief of this journal.The author,Professor Shunsuke Sakurai,of Kobe University,Japan,has also had a long and distinguished association with the ISRM,having served as the Society’s ninth President in the period 1995-99.

The publication of a dedicated book on back analysis in rock engineering is to be especially welcomed.As the author notes in Chapter 2,the input data used in back analyses are measured values of quantities such as displacements,strains,stresses and pressures in rock masses,while the calculated outputs are the mechanical parameters of rock masses such as deformation moduli,the strength parameters,permeabilities and sometimes the initial states of stress.This is in contrast to the more usual case of forward analysis in which the stresses,strains and displacements induced in the rock mass are calculated using the mechanical properties of the rock mass and the initial stresses as inputs.Thus,back analysis calculations are the reverse of forward analysis calculations.As the author says,“field measurement data are only numbers unless they are properly interpreted.Therefore,the most important aspect of field measurements is the quantitative interpretation of measurement results.For this purpose,back analyses must be a powerful tool”.

The class of analyses discussed in this book is what this reviewer prefers to call formal back analyses.In geotechnical engineering practice,the term “back analysis”is sometimes used more loosely to refer to a curve fitting approach based on the observational method in which soil or rock mass parameters are selected,more or less by “trial and error”,to provide a fit to measured data,most frequently displacements.However,the formal back analyses discussed in this book are based on the equations of engineering mechanics,notably stress-strain relationships.Sakurai identifies two distinct approaches to carrying out these back analyses.The inverse approach requires a mathematical formulation in a reverse manner to that used in forward analysis,so that it is only available for linear elastic materials whose stress-strain relationships may be expressed in linear form.The direct approach,on the other hand,avoids the inversion of the mathematical equations of stress analysis so that it can be readily applied to any nonlinear problem.The direct approach is based on the discrepancy between the field measurement data and the corresponding numerically evaluated values.The problem then becomes one of minimizing a generally nonlinear error function that is used to define the discrepancy.

In his acknowledgements,the author notes that the book has been prepared on the basis of the outcomes of the theoretical and experimental research carried out in the Rock Mechanics Laboratory of Kobe University over the last 40 years.(This reviewer must acknowledge the fact that he has known Professor Sakurai over that period of time,considers him to be a personal friend,and has been his guest in Kobe.)Accordingly,a high proportion of the references given are to publications by the author and his former students.While this approach can be understood and accepted on one level,it may equally well be argued that the book may have benefitted from the inclusion of a wider range of back analysis results and examples based on the approaches and publications of the several other groups who have made significant contributions to the general topic of back analysis in geotechnical engineering.

This 223-page book includes 23 chapters of varying lengths,the shortest of them(other than for the introductory chapters),covering only three or four pages.In the reviewer’s opinion,some of the shorter chapters could well have been incorporated into other chapters to produce more substantive chapters on particular topics such as slope stability.In fact,before he had undertaken to prepare this review,the reviewer had taken the liberty of raising this issue with Professor Sakurai,particularly in respect of the quite short Chapter 21 dealing with back analysis for determining strength parameters.Professor Sakurai informed the reviewer that he had,indeed,carried out more substantive work on this important topic than that appearing in Chapter 21 of his book.However,a paper based on that work has been accepted for inclusion in a forthcoming conference but,for copyright reasons,that material could not be included in the present book.While this is understood from the perspective of the publishers,it is not especially helpful in terms of the ISRM’s and this journal’s broader aim of disseminating significant new developments among the international rock engineering and geotechnical engineering communities.

One of the most important contributions made by Professor Sakurai to the general topic of back analysis is his development with his students in the early 1990s,of the universal back analysis method discussed in Chapter 9.This method assesses the stability of tunnels and other underground excavations by incorporating measured non-elastic strains into the back analysis,and using Sakurai’s critical strain approach introduced and discussed in Chapters 5-7.In the reviewer’s opinion,some other especially important chapters in the book are Chapter 4 on the observational method;Chapter 8 on back analysis in tunnel engineering practice;Chapter 18 on the back analysis of slopes allowing for anisotropy;Chapter 22 on the use of back analysis in assessing the stability of slopes;and Chapter 23 on the monitoring of slope stability using the global positioning system(GPS).This last chapter was prepared by Professor Sakurai’s former student,Professor Norikazu Shimizu,who is currently an ISRM Vice-President at Large and served as the coordinator of the group which developed the ISRM Suggested Method for monitoring rock displacements using GPS.

The author,Professor Shunsuke Sakurai,the ISRM Book Series Editor,Professor Xia-Ting Feng,the ISRM,and the publisher,CRC Press/Balkema,are all to be congratulated on the publication of a much-needed book on the increasingly important topic of Back Analysis in Rock Engineering.In the opening section setting out the aims and scope of his book,the author says,“This book is dedicated to practicing engineers working in rock engineering practice,as well as for graduate students studying and doing research on rock mechanics and rock engineering.The aim of the book is to make the engineers and students understand how to apply the theory of rock mechanics to engineering practice,in order to achieve the rationaldesign and construction of rock structures such as tunnels,underground caverns,and slopes,and to assess not only the stability of them during/after construction,but also to ensure the safety of the workers.”In this reviewer’s opinion,the author has succeeded in his aim by producing an invaluable book oriented towards rock engineering practice but with a fundamental basis in engineering mechanics.