Abstract:The stability of cut slopes is greatly influenced by seasonal pore-water pressure variations under the combined effect of rainfall and vegetation. However, predicting soil-atmosphere interaction is not straightforward, due to the complexity of both the boundary conditions involved and the hydromechanical behaviour of soils, which is coupled and highly nonlinear, rendering the use of numerical tools, such as finite element analysis, necessary. This paper discusses the numerical modelling of soil-atmosphere interaction and presents the analysis of a slope cut in London clay in a highly vegetated area. The whole life cycle of the slope is considered with phases of low and high water demand vegetation and vegetation clearance. The analysis results indicate that dense vegetation is associated with high factors of safety, but may induce large differential displacements, which are likely to affect the serviceability of the slope. Vegetation clearance, however, may initiate instability, highlighting the need for effective vegetation management to achieve a balance between serviceability and ultimate limit states. Although the case considered is representative of southeast England, it introduces the necessary tools for realistic numerical analysis of soil-atmosphere interaction.Key words: slope stability, serviceability, vegetation, precipitation, soil-atmosphere interaction.Résumé : La stabilité des pentes entaillées est fortement influencée par les variations de pression de l'eau interstitielle saisonnières sous l'effet combiné des précipitations et de la végétation. Cependant, prédire l'interaction sol-atmosphère n'est pas si simple, en raison de la complexité des deux conditions aux limites en cause et le comportement hydromécanique des sols, qui est couplé et fortement non linéaire, ce qui rend l'utilisation d'outils numériques, tels que l'analyse des éléments finis néces-saire. Cet article traite la modélisation numérique de l'interaction sol-atmosphère et présente l'analyse d'une pente entaillée dans l'argile de Londres dans une zone très végétalisée. L'ensemble du cycle de vie de la pente est considéré avec des phases de demande en eau basse et haute de la végétation et la clairance de la végétation. Les résultats de l'analyse indiquent que la végétation dense est associée à des facteurs élevés de sécurité, mais peut induire des déplacements différentiels qui sont susceptibles d'affecter l'état de fonctionnement de la pente. L'enlèvement de la végétation, cependant, peut déclencher l'instabilité, soulignant la nécessité d'une gestion efficace de la végétation afin de parvenir à un équilibre entre la fonctionnalité et les états limites ultimes. Bien que le cas considéré est représentatif du Sud est de l'Angleterre, il présente les outils nécessaires à l'analyse numérique réaliste de l'interaction sol-atmosphère. [Traduit par la Rédaction] Mots-clés : stabilité des pentes, facilité d'entretien, végétation, précipitations, interaction sol-atmosphère.

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A three-dimensional hysteretic soil-water retention curve A. TSIAMPOUSI Ã , L. ZDRAVKOVIĆ a nd D. M. P OTTS One of the most important features in unsaturated soil mechanics is the soil-water retention curve and its coupling to the mechanical component of soil behaviour. It has long been recognised that the retention curve exhibits significant hysteresis, and that it is affected by the specific volume. Several attempts have been made in the past to model this behaviour. A novel approach is proposed herein, which accounts for both the hydraulic hysteresis and the specific volume dependence of the retention relationship in a three-dimensional formulation. The primary and the scanning paths are simple geometric curves, which have a common tangent at the point of intersection, ensuring a smooth transition from scanning to primary paths. A small number of parameters are required to define the primary paths, and no fitting parameters are necessary for generation of the scanning paths. As knowledge of the specific volume and its variation is required, the retention model needs to be employed in conjunction with a constitutive model capable of reproducing the complex behaviour of unsaturated soils. To guarantee consistency with the retention model, the degree of saturation needs to be incorporated in the specific volume-suction relationship adopted within the constitutive model. To accommodate such a feature when absent, a new expression for the soil compressibility with suction as a function of the degree of saturation is proposed. Simulations of laboratory experiments on unsaturated soils, involving cyclic changes of applied suction, demonstrate the effectiveness of the proposed modelling approach.

The equations governing coupled consolidation in unsaturated soils are known to contain additional parameters when compared to the equations for saturated soils. Nonetheless, the variation of these parameters with suction or degree of saturation is not generally agreed upon. The paper introduces a novel approach to deriving general equations for each of these parameters and their variation, and explains that, for consistency with the constitutive and soil-water retention curve models adopted, these general equations need to be transformed into case-specific expressions. Finally, a conceptual model is presented highlighting how the behaviour of unsaturated soil reflects aspects of its water content

Soil-vegetation-atmosphere interaction is long known to induce significant pore pressure variations at shallow depths and associated superficial slope movements. Recent findings suggest that the effect of this interaction may also extend to large depths in natural clay slopes. Multiple examples of weather-induced deep landslide mechanisms can be found in the Southern Apennines (Italy), where slopes are often formed of fissured clays. The relationship between the activity of these landslides and the hydro-mechanical processes due to soil-vegetation-atmosphere interaction was investigated herein by means of a two-dimensional coupled hydro-mechanical finite element analysis. A constitutive model capable of simulating the behaviour of highly overconsolidated clays, in both saturated and unsaturated states, was adopted in the analysis, in conjunction with a boundary condition capable of reproducing the combined effects of rainfall infiltration, evapo-transpiration and run-off. The results of the analysis corroborate the connection between weather conditions, pore pressure variations and slope movements in natural clay slopes. The importance of reproducing adequately the geological history of a natural slope in order to define its current state is also demonstrated.

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The paper presents the Finite Element formulation of the equations proposed by Tsiampousi et al. (2016) for coupled consolidation in unsaturated soils. Their coupling is discussed in relation to a conceptual model which divides soil behaviour into zones ranging from fully saturated to dry states. The numerical simulation of a laboratory experiment involving drainage of water from a vertical column of sand is used to validate the equations. Finally, the example of rainfall infiltration into a cut slope highlights how aspects of the conceptual model are reflected in the numerical analysis of boundary value problems involving unsaturated soils

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