Comprehensive studies on the prediction of unsaturated shear strength were performed using three commonly used empirical procedures: Fredlund et al. approach (published in 1996), Vanapalli et al. approach (published in 1996, and Khalili and Khabbaz method (published in 1998). Shear strength data published in the literature for 15 soils were examined using these procedures. Comparisons between measured and predicted values of unsaturated shear strength are presented for different soil types. The effect of stress state on the prediction of shear strength is also discussed.

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The authors have presented very useful experimental data on soil±water characteristics which can throw light on many aspects. We would like to discuss two issues (a) the relationship between suction and water content or degree of saturation (b) the air-entry value.

Critical state models for unsaturated soils have been proposed in recent years; however, the proposed models have been based on limited experimental data. Compacted specimens have generally been used for research, and the complications of soil fabric resulting from the compaction procedures have brought difficulties into the interpretation of fundamental soil behavior. There is a need to undertake further laboratory research studies on unsaturated soils, particularly to obtain a fundamental understanding of the shear strength and critical state of unsaturated soils by testing soil specimens with simple soil structures. Suction-controlled triaxial drained shear tests on an unsaturated silt were carried out as part of this research program. Specimens were prepared by gradually consolidating the initially slurried soil. The resulting specimens had a relatively simple soil fabric and stress history. The results showed that applying suction to an initially saturated specimen has a similar influence on the stress-strain behavior and critical state characteristics as does increasing its density by applying a higher confining pressure. The critical state lines for the unsaturated soil corresponding to different soil suctions are parallel to those for the saturated soil on the (q : p′ ′ ), (v : p′ ′ ), and (v w : p′ ′ ) planes.Résumé : Des modèles d'état critique ont été proposés au cours des récentes années; cependant, les modèles proposés ont été basés sur des données expérimentales limitées. Des spécimens compactés ont généralement été utilisés pour la recherche, et la complexité de la fabrique du sol résultant des procédures de compactage ont présenté des difficultés dans l'interprétation du comportement fondamental du sol. Il est nécessaire d'entreprendre d'autres recherches en laboratoire sur des sols non saturés, particulièrement pour acquérir une compréhension fondamentale de la résistance au cisaillement et de l'état critique des sols non saturés en faisant des essais sur des spécimens de sols ayant des structures simples. Des essais de cisaillement triaxial drainé avec succion contrôlée sur un silt non saturé ont été réalisés dans le cadre de ce programme de recherche. Les spécimens ont été préparés en consolidant graduellement le sol initialement sous forme de boue. Les spécimens de sol qui en résultent ont une fabrique et une histoire des contraintes relativement simples. Les résultats montrent que l'application d'une succion à un spécimen de sol initialement saturé a une influence sur le comportement contrainte-déformation et sur les caractéristiques de l'état critique semblable à l'accroissement de sa densité par l'application d'une pression de confinement plus élevée. Les lignes d'état critique pour le sol non saturé correspondant à différentes succions sont parallèles à celles du sol saturé sur les plans (q : p′ ′ ), (v : p′ ′ ), et (v w : p′ ′ ).Mots clés : état critique, résistance au cisaillement, sols non saturés, changement de volume, essais triaxiaux.[Traduit par la Rédaction] Wang et al. 218

In arctic regions mass movements in thawing permafrost are common. Although different types of landslides have been identified, one type referred to as a "bimodal flow" has caused a significant amount of interest on account of the unusually high rate of mass wasting associated with this type of flow slide. The flow is characterized by a steep headscarp and a low angle mud flow or tongue at the base of the slide.Although more than one activity contributes to the retreat of the headscarp, the most important process is that of ablation. An obvious anomaly exists between the amount of heat required to sustain the high rates of observed ablation and the amount of heat available from net radiation.The components of the energy balance were identified and a field study was undertaken to quantify these terms. Instrumentation included radiometers, wet and dry thermocouples, anemometers, and a lysimeter to measure the latent heat of condensation of evaporation.The results of this study indicate that all the major terms in the energy balance equation combined to form the source of heat responsible for the high rates of observed ablation. Moreover, they provide a rational basis for controlling bimodal flow slides using different insulation materials.