I N T F R W A T I O N 4 L J O U R N A L FOR NIJMFRI< 'AL A N D ANALYTICAL M E T H O D S I N GFOMECHANICS V O L 16, 891 -894 ( 1 992)

BOOK REVIEW

C O M P U T E R M E T H O D S A N D ADVANCES I N G E O M E -

CHANICS,G. Beer, J. R . Booker and J. P. Car te r (Editors), A. A. Balkema; Rot terdam, Brook- field; Three volumes, 1802 pages.

This two volume set contains the proceedings of the Seventh International Conference on Com- puter Methods and Advances in Geomechanics which was held at Cairns, Australia, in May 1991. The 262 papers contained in the two volume set, cover topics such as testing and instrumenta- tion, joints, localisation, slopes, stability and dis- tinct elements, constitutive modelling, dynamics and tunnels and underground openings as well as topics of more recent interest such as environ- mental geomechanics and expert systems.

The keynote papers to the conference begin the first volume, and reflect the wide range of topics now involved in the field of geomechanics. Profes- sor Ted Brown's paper, which was the opening address a t the conference, deals with Geomech- anics in Australia. He outlines the history of geo- mechanics and presents a personal view of what he considers to be the greatest contributions that Australian researchers have made in the field. Also included are the keynote addresses of Professor Chandra Desai (constitutive modelling), Professor John Booker (analytic methods) and Professor Kerry Rowe (environmental geotechnology). These papers deal with the latest developments and directions in fields for which the authors are renown.

New directions in the development of software and Computer Aided Design (CAD) are repres- ented by papers which deal with expert systems that can aid in the design of foundations in com- pression or uplift or in selecting the best computer program for carrying out rock excavation design. Automatic generation of finite element meshes, CAD systems for the modelling of subsurface geo- logical structures and database systems for storing triaxial test data are among some of the areas of software design that are considered,

In the section dealing with engineering appli- cations, there are several papers which deal with piled foundations, and cover such topics as down- drag forces, dynamic behaviour, group behaviour,

piled strips, piles in Gibson soils, and the behavi- our of piles bearing onto thin soil layers. Other topics of interest were excavation and the behavi- our of rafts and embankments.

In many of these papers, modifications have been made to simple theories in order to tit better the observed field behaviour. For example, in the paper by Chow and Chin, which deals with down- drag forces in piles, a compressible base layer is introduced beneath the pile base, rather than as- suming it to be totally rigid. This simple change greatly improves the prediction of pile behaviour. Hirayama takes account of non-homogeneity in- duced into the soil by pile movements, while Poulos analyses piled strip foundations by making some empirical assumptions concerning the dis- placements caused at a pile when an element in the strip is loaded.

However, in some cases simplified methods are not capable of reproducing field behaviour and so full finite element analyses are used. Kimura et a/. carry out 3D analysis of laterally loaded piles, Matsui and Oda analyse vertically loaded piles bearing onto thin layers, Kulathilaka and Donald analyse excavation problems as do Tamano et al. who take special account of the lowering of the water table due to the excavation process. Sakai et al. use Cam clay models to simulate embank- ment construction. Most of these finite element analyses have to employ non-linear constitutive laws in order to correctly simulate actual field behaviour, and in some cases special elements as well have to be introduced.

The section which deals with experimental stud- ies, testing and field instrumentation, contains sev- eral papers which deal with backfiguring soil prop- erties from either laboratory or field tests. In doing this, some analytical model needs to be proposed and, therefore, the calculation of the soil properties will be dependent on the model used. The papers by Cogill (for calculation of elastic moduli in pavements) or by Ohta et al. (correction factors for undrained shear strength as found by pressureme- ters) are such examples.

Use of the appropriate model is highlighted in this section by two papers by the same author. Toh and Fahey show that they can very closely predict large strain consolidation test results obtained

0 1992 by John Wiley & Sons, Ltd.

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from a centrifuge by the use of large strain consol- idation theory. However, in another paper, Goh and Fahey show that they cannot predict pore pressures around piezocones and pressuremeters well and surmise that this may be due to a model deficiency since the model does not include creep.

In dealing with problems of slope stability, bear- ing capacity or collapse, there are three broad categories represented. Firstly, there are the elasto- plastic finite element or finite difference analyses, secondly, there are the upper bound type solutions in which a failure surface is proposed and, finally, there are solutions which provide both upper and lower bounds to the correct solution.

For assessing slope stability, the upper-bound- type solutions for which failure surfaces are pro- posed and a search carried out to find the most critical surface, still appear to be the most widely used and successful. This is because they are quick and easy to use, and experience with their usage has lead to confidence in applying the results. Giam and Donald present an interesting multiple wedge method of analysis which can be used for slope-stability or bearing-capacity analysis, in which they use a pattern-search technique to find the critical failure surface. The finite element or finite difference approaches are, generally, used only for more complex problems such as a re- inforced slope (Matsui and San) or toppling fail- ures in open-cut mines (Orr et al.) where a ubiqui- tous joint model is used in the finite difference code FLAC.

It is interesting to note that there are no three dimensional slope stability analyses presented, as has been the case at previous conferences. Pre- sumably, this is once again due to practical diffi- culties in applying such analyses; especially, if methods other than upper-bound approaches are used.

Constitutive modelling forms a large section of the first volume, with new models being postulated to describe more closely the behaviour of unusual materials or to describe better the behaviour of soils generally.

Some of the features which are of common interest to authors in this section are creep or viscoelasticity (Silva et al., Bonnier and Troost, and Benedetto and Hameury) and anisotropy (Atukorala and Byrne; Pastor; Whittle; and Xu etal.) . Other topics covered are swelling soils (Alonso et al.), stress history effects (Atkinson and Stallebrass) unsaturated soils (Kohgo et al.) and degradation instabilities in brittle material struc- tures (Frantziskonis).

Many of the models are based on cap models (Zaman and Faruque, Lupo et al.) or extensions of the Cam Clay Model (Silva et al., Atkinson and

Stallebrass, Asaoka and Nakano), although Lade and Kim have presented a model which uses a single yield surface and can take into account hardening or softening of the material.

An interesting discussion is presented by Fourie who proposes that in fitting constitutive models to test results, not enough attention is paid to the end conditions of the sample. Quite often a single (smooth ended) element is used in comparing the numerical model with the experimental results and this may lead to error if the ends, indeed, are rough.

Applying the model to only a single element test may not often provide a full test of the model as pointed out by Fourie and, in fact, very few of the models presented in this section are applied to actual boundary value problems. Models which perform adequately under simple stress conditions may not perform well under complex stress paths or they may involve large amounts of computing time and may not be efficient enough for use in the solution of large boundary value. problems. Such demonstrations of constitutive model applications to engineering problems are given by Asaoka and Nakano, and Alonso et al.

One of the most interesting sections of the pro- ceedings is that dealing with back-analysis and field performance and prediction. Back-analysis involves finding a set of soil or rock properties by adjusting the properties in some way so that the predicted behaviour of the prototype and the nu- merical model are similar. For example, the dis- placements at a chosen set of points on the proto- type may be compared with those of the numerical model. There are, obviously, many pitfalls in car- rying out such an operation, such as a deficiency in the number of pieces of measured data which are used in the back-analysis, or the number of mater- ial properties which are to be determined. This may mean that a ‘unique’ set of parameters may not exist, i.e. there may be several sets which give good fits to the numerical data.

Akutagawa et al. present an interesting paper concerning the use of a Direct Modulus Factoring method which may be used for determining elastic moduli. They examined materials having horizon- tally layered, diagonal, or mosaic regions where each region had a different elastic modulus. These composite materials were subjected to different loading patterns and for displacement measure- ment different points (and directions) were used in the back-analysis. They found that the correct result could not always be found, although in the majority of cases the moduli could be predicted fairly closely. Arai et al. show that a Bayesian method applied to back-analysis of material prop- erties for a consolidation problem (modulus,

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Poisson’s ratio and permeability) were found more easily than when a direct (linear programming) method was used. The two different methods gave different estimates of back-figured material prop- erties, showing that the method of analysis also may affect the results.

Day and Potts apply an elastoplastic finite ele- ment analysis to an excavation problem and, by comparing the predicted with the measured res- ults, point out observed features which were not taken into account in the analysis such as loss of load due to creep in the wall’s tie back anchor. This is one of the real benefits of comparison of field behaviour with predicted behaviour; models can be improved to capture features which were not considered in the initial models.

A common theme running through the section which deals with analytic, semi-analytic and boundary element methods is simplification of the modelling of the problem so that it can be more readily solved by analytic or semi-analytic means.

Examples of this are the papers by Savvidou and Booker, who use Fourier transforms in conjunc- tion with Laplace transforms to simplify the equa- tions governing the heating of a poroelastic mater- ial. Takemiya and Arioka also use Fourier trans- forms to simplify the problem of a horizontally layered material (which may be viscous) that is subjected to harmonic loadings. Zhang and Small also use Fourier transforms to simplify the three- dimensional problem of a loaded raft on a layered material, to that of a problem involving only two spatial dimensions. Kay makes use of Fourier series to simplify the non-symmetric problem which occurs when lateral loading is applied to a gravity platform. With this type of analysis, plastic failure of the soil may also be considered. Griffiths and Lane use a similar approach to analyse the behaviour of a shear vane.

To simplify the problem of determining stresses in the walls of a tunnel which is lined with shot- Crete and reinforced by anchors, Fotieva and Sam- ma1 treat the tunnel as being surrounded by several different elastic regions. The problem can then be simplified using conformal mapping and solved for the case where the inside wall of the tunnel is either smooth or uneven.

Such simplifications can lead to great savings in computational effort and data preparation time, and can often be used in place of much more costly three-dimensional finite element analyses. In many cases these methods provide a quick and simple way of obtaining realistic solutions to engineering problems.

Researchers from those countries whose econ- omies are largely dependent on mining, such as Australia, Canada, China, South Africa, the USSR

and the USA, have contributed a large number of papers to the proceedings. It is evident from the papers, that analysis of underground mining is extremely difficult because of the three dimen- sional nature of the problem, and because of the non-linearity of the materials involved. This has lead to the widespread use of boundary element methods (Barron and Kullmann, Choi et al., Cowling, Vervoort and Yu et al.). However, the limitation to the boundary element method is that it cannot easily take into account non-linearity or fracturing. In such cases, full three-dimen- sional finite element analyses are still needed (Koutsabeloulis et al.).

In some cases two-dimensional analyses are ac- ceptable, and in the case of subsidence prediction, special techniques such as the use of a combination of finite element and displacement discontinuity methods can be used to improve predictive capa- bilities (Siriwardane and Amanat). Friday and Giam use a two-dimensional analysis to model the three-dimensional effects of drains used for stabili- sing open cut mine slopes.

It is also interesting to note that there are a large number of papers dealing with rock bursts or coal outbursts in the section on mining. Barron and Kullmann use a boundary element approach where gas pressures are superimposed onto the total stresses which have been calculated. Pan et al. use viscoelastic and viscoplastic strain softening models, while Yu and Xian propose a theory based on the strain energy in the coal and the swell energy in the gas. Hou and Song use an energy index, which is the ratio of the area under the stress-strain curve for the rock before the peak stress, to the area under the curve after the peak. They have found from experience that rockbursts occur if the energy index is greater than 1.5. Stresses in the rock surrounding the tunnel also determine whether a rockburst will occur and the condition proposed by Hou and Song is that the elastic energy in the wall at a point must exceed the post-peak area (energy) under the stress-strain curve.

Tunnels and underground openings are dis- cussed in Section 13 which contains 16 papers specifically related to this topic although there is some overlap between tunnelling and under- ground mining, which is covered in Section 12.

The main difference between the two areas is that tunnels and underground openings have to remain in service unlike underground mine work- ings which can be abandoned. Rock bolting and linings are often used for tunnels and these cause special problems in the analysis. Combined with this is the fact that the rock surrounding a tunnel may creep or work soften and with time will place

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more load on the linings. Material effects have been examined by Esaki et al., Cividini and Gioda, Pan et al., Rock and Pottler and Yamatomi et al., while ways of dealing with rock bolts are the interest of Ayden and Kawamoto, Swoboda and MarenEe, and Tsubouchi et al.

Soft ground tunnelling presents problems which are quite different from those encountered in hard rock tunnelling. The use of tunnelling shields is necessary and compressed air may have to be used. Adachi et al. use a coupled finite element method to predict deformations associated with soft- ground tunnelling, DeMoor and Taylor use the computer program CRISP (which is based on the Cam Clay model) to model consolidation at the unsupported face of a lined tunnel, and show that the dissipation of the negative pore pressures caused by soil removal can cause flow of the soil into the tunnel. Schweiger et al. investigate the stresses caused in shotcrete linings due to ground- water flow and have found that the water pressures can be almost equal to the full hydrostatic pres- sures.

Environmental engineering is becoming more and more important in recent times and for the first time in the proceedings of the series of Inter- national Conferences on Computer Methods, this is recognised with a special section on Environ- mental, Resource and Groundwater Geomech- anics.

Geomechanics plays an important part in envir- onmental management, and is involved in the design of landfills for domestic waste, disposal sites for toxic waste and atomic waste as well as in the rehabilitation of soil and rock which has been contaminated in the past. Many of these topics are not represented by papers in these proceedings, and in future conference proceedings I should expect that many more papers on environmental topics will be submitted.

Environmental topics which are represented are a random-walk method for predicting pollution migration in jointed rock (Genske et al.) and a three dimensional solution for groundwater flow, heat transport and thermohydromechanical de- formation in rock masses with discrete fractures (Guvanasen and Chan); a problem associated with disposal of radioactive waste. Storage of radio-

active waste in salt domes is one possible solution to the disposal problem, as salt which has not been leached from the ground must be in an area which has a stable groundwater regime. Radioactive waste stored in such an environment would heat the salt and may cause cracking or excessive stress in the salt dome. Heusermann et al. compare tem- perature, deformation and stress measurements obtained in a salt mine with finite element calcu- lations and show that reasonable predictions can be made of all of these quantities. The numerical results indicate that overstressing or cracking would be unlikely with only a small amount of microfracturing taking place. Staudtmeister and Schmidt also look at the deformation of a cavity in rock salt which has been constructed for the pur- poses of storing waste materials.

Many new ideas have been proposed and new directions taken by the researchers whose work is represented in the proceedings. The ease with which these methods may be applied to practical problems and the success which they have in pre- dicting field behaviour will determine which methods become routine design tools in the future and which do not. Many of the newer areas of research such as expert systems, environmental geomechanics and computer aided design may begin to play more significant roles in the future, while faster and more powerful computers may mean that discrete element methods and three- dimensional finite element analyses become more popular.

Having looked at every one of the 262 papers in the two volumes and finding myself unintention- ally becoming engrossed in many of them, I can recommend these volumes to anyone with an inter- est in geomechanics. Once again, the IACMAG Conference Series has resulted in an impressive set of proceedings with an excellent state-of-the-art collection of papers which should be a compulsory acquisition for all university and company librar- ies and no doubt will find its way into many private collections.

JOHN SMALL The University of Sydney

Australia