PrefaceSpecial Issue on Unsaturated Soil Concepts and TheirApplication in Geotechnical Practice

This collection contains contributions from leading researchers on the engineeringbehaviour of unsaturated soils. Many of the major international research groupsinvestigating these topics are represented here. It includes contributions dealing withthe measurement of suction, discussions about how pore-water affects the engineer-ing behaviour of soils and the important role played by soil^water characteristic(water retention) curves. This leads on to applications of unsaturated soil behaviourwithin geotechnical engineering practice, dealing with collapsible soils, foundationsand slopes.The ¢rst contribution deals with one of the most dif¢cult areas in the investigation

of unsaturated soils; the measurement of suction. Tarantino and Mongiovi (Italy)describe the experimental procedures for direct measurement of metric suction.The ability to make direct measurement of pore-water pressure below �100 kPahas been a major breakthrough in the last decade, with devices being developedat Imperial College, London, UK and at University of Saskatchewan, Canada.Tarantino and Mongiovi describe their procedure for preparing one of these devices(the Imperial College tensiometer) to restrict cavitation. Following their procedures,they were able to measure pore-water tensions greater than 2 MPa, a very majorachievement indeed.The contribution from Karube and Kawai (Japan) brings together many of the

concepts developed by Professor Karube and his group at Kobe University, Japanover a long career research unsaturated soils. They look at the way pore-wateris held within unsaturated soils and consider how stresses are carried by the soilskeleton and the water. They consider how these concepts can be used within anelasto-plastic model for unsaturated soil behaviour. They point out that currentmodels cannot handle hysterisis between wetting and drying, an important featureof unsaturated behaviour. They go on to present a model for water retention(soil^water characteristic) curves that does consider hysterisis and also the effectof changes in void ratio.Sillers, Fredlund and Zakerzadeh (Canada) continue the discussion about

soil^water characteristic curves by looking at the various models that can be usedto ¢t experimental data. They review models from Gardner’s ¢rst equation in 1956through to later, more complex models. The models are set within a consistentframework. Derivations and derivatives are presented. I am sure it will becomethe de¢nitive reference for those who need to check the equations for a particular

Geotechnical and Geological Engineering 19: 185^187, 2001. 185# 2001 Kluwer Academic Publishers. Printed in the Netherlands.

model. They conclude that the models using three ¢tting parameters provide the best¢t over a wide range of suctions.Models for soil^water characteristic curves are only useful if we have experimental

data on which to base them. Agus, Leong and Rahardjo (Singapore) present a largenumber of experimental soil^water characteristic curves determined for two typesof residual soil from Sigapore. They present data for eight different sites. This dataset allows them to relate the parameters of the soil^water characteristic curvesto index properties. They conclude that the relationships derived are suitable to pro-vide a quick preliminary estimate of a soil^water characteristic curve.The importance of soil^water characteristic curves is emphasized by another con-

tribution dealing with this topic. Aung, Rahardjo, Leong and Toll (Singapore) inves-tigate the relationship between mercury intrusion porosimetry measurements andsoil^water characteristic curves. The porosimetry measurements are presented assoil^air characteristic curves. The slopes of the soil^air characteristic curves arefound to be similar to the slopes of the soil^water characteristic curves. The equiv-alent pore diameters calculated from the mercury entry value and the air entry valueappear to be related. Therefore, it is suggested that porosimetry data can be used toconstruct an estimate of the soil^water characteristic curve.Romero, Gens and Lloret (Spain) look at temperature effects on both the water

retension (soil^water characteristic) curves and the permeability of water (hydraulicconductivity). This is an important issue in the design of engineering barriers forradioactive waste disposal. However, experimental dif¢culties become even greaterwhen the additional variable of temperature is included. From their very high qualityexperimental data they ¢nd that there is a shift in the soil^water characteristic curveif the temperature is increased from 22�C to 80�C. An equation is presented for¢tting soil^water characteristic curves, which includes the temperature effect. Theyalso ¢nd that water permeability is not dependent on temperature but is highlydependent on void ratio and degree of saturation.The ¢rst application of unsaturated soil mechanics in the collection is presented by

Houston, Houston, Zapata and Lawrence (USA). They review the problems associ-ated with unsaturated collapsible soil with particular reference to the impact ofurbanisation in arid regions. They use their considerable experience of identifyingand characterising collapsible soils to give very practical guidelines on how to tacklethe problem. They note that estimating the extent and degree of wetting is the mostdif¢cult task and suggest ways to do this. They also discuss possible mitigationmeasures.Cameron (Australia) investigates an equally dif¢cult problem; design of shallow

footings on expansive/shrinkable soils. He looks particularly at the effects of treesand the in£uence they have on suction pro¢les in the ground. He reports on measuredsuction pro¢les adjacent to trees in a semi-arid region of Australia. It is shown thatthe drying effect due to trees extends below what would normally be consideredas the zone within which movements occur. He presents preliminary design guide-lines for estimating the extra settlement caused by the proximity of trees.

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186 PREFACE

To complete the collection, Rahardjo, Li, Toll and Leong (Singapore) examine theeffect of antecedent rainfall on slope stability. They present a case study on theNanyang Technical University campus in Singapore where a spate of twentylandslides occurred following a single large storm event. However, they contrastthis with similar size storm events that had not led to landslides. The differencein the case that did produce landslides was the amount of rain that had fallenon preceding days. Numerical modelling showed that the additional antecedentrainfall was suf¢cient to lower the factor of safety of a slope to close to unity, con-sistent with the failures that occurred.I hope this colection is seen to be valuable both in contributing to the research

effort on unsaturated soil behaviour and also in showing how unsaturated soil con-cepts can be applied within geotechnical engineering practice. We still have muchto learn about unsaturated soils. Nevertheless, we are making signi¢cant headwayin establishing rational approaches to dealing with these complex circumstances.

David TollEditor

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