Publications
48 results found
Tsiampousi A, Smith PGC, Potts DM, 2017, Coupled consolidation in unsaturated soils: an alternative approach to deriving the Governing Equations, Computers and Geotechnics, Vol: 84, Pages: 238-255, ISSN: 0266-352X
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.
Tsiampousi A, Zdravkovic L, Potts DM, 2017, Numerical study of the effect of soil–atmosphere interaction on the stability and serviceability of cut slopes in London clay, Canadian Geotechnical Journal, Vol: 54, Pages: 405-418, ISSN: 1208-6010
The stability of cut slopes is greatly influenced by seasonal pore water pressure variations underthe combined effect of rainfall and vegetation. However, predicting soil-atmosphere interactionis not straightforward, due to the complexity of both the boundary conditions involved and thehydro-mechanical behaviour of soils, which is coupled and highly nonlinear, rendering the use ofnumerical tools, such as finite element analysis, necessary. The paper discusses the numericalmodelling of soil-atmosphere interaction and presents the analysis of a slope cut in London clayin a highly vegetated area. The whole life cycle of the slope is considered with phases of lowand high water demand vegetation and vegetation clearance. The analysis results indicate thatdense vegetation is associated with high factors of safety, but may induce large differentialdisplacements which are likely to affect the serviceability of the slope. Vegetation clearance,however, may initiate instability, highlighting the need for effective vegetation management inorder to achieve a balance between serviceability and ultimate limit states. Although the caseconsidered is representative of South East England, it introduces the necessary tools forrealistic numerical analysis of soil-atmosphere interaction.
Pedone G, Tsiampousi A, Cotecchia F, et al., 2016, Effects of soil-vegetation-atmosphere interaction on the stability of a clay slope: a case study, 3rd European Conference on Unsaturated Soils, Publisher: EDP Sciences, ISSN: 2267-1242
Deep and slow landslide processes are frequently observed in clay slopes located along the Southern Apennines (Italy). A case study representative of these processes, named Pisciolo case study, is discussed in the paper. The geo-hydro-mechanical characteristics of the materials involved in the instability phenomena are initially discussed. Pluviometric, piezometric, inclinometric and GPS monitoring data are subsequently presented, suggesting that rainfall infiltration constitutes the main factor inducing slope movements. The connection between formation of landslide bodies and slope-atmosphere interaction has been demonstrated through a hydro-mechanical finite element analysis, whose results are finally reported in the work. This analysis has been conducted employing a constitutive model that is capable of simulating both saturated and unsaturated soil behaviour, as well as a boundary condition able to simulate the effects of the soil-vegetation-atmosphere interaction.
Mantikos V, Ackerley S, Kirkham AD, et al., 2016, Investigating soil-water retention characteristics at high suctions using Relative Humidity control, 3rd European Conference on Unsaturated Soils, Publisher: EDP Sciences, Pages: 10007-10007
A technique for controlling relative humidity (RH) is presented, which involves supplying a sealed chamber with a continuous flow of air at a computer-regulated RH. The desired value of RH is achieved by mixing dry and wet air at appropriate volumes and is measured for servo-control at three locations in the chamber with capacitive RH sensors and checked with a sensitive VAISALA sensor. The setup is capable of controlling RH steadily and continuously with a deviation of less than 0.2% RH. The technique was adopted to determine wetting soil-water retention curves (SWRC) of statically compacted London Clay, under both free-swelling and constant volume conditions. The RH within the chamber was increased in a step-wise fashion, with each step maintained until vapour equilibrium between the chamber atmosphere and the soil samples was established. Independent filter paper measurements further validate the method, while the obtained retention curves complement those available in the literature for lower ranges of suction.
Tsiampousi A, Zdravkovic L, Potts DM, 2016, Soil-atmosphere interaction in unsaturated cut slopes, 3rd European Conference on Unsaturated Soils, Publisher: EDP Sciences, ISSN: 2267-1242
Interaction between atmosphere and soil has only recently attracted significant interest. Soil-atmosphereinteraction takes place under dynamic climatic conditions, which vary throughout the year and are expected to sufferconsiderable alterations due to climate change. However, Geotechnical Analysis has traditionally been limited tosimplistic approaches, where winter and summer pore water pressure profiles are prescribed. Geotechnical Structures,such as cut slopes, are known to be prone to large irreversible displacements under the combined effect of wateruptake by a complex vegetation root system and precipitation. If such processes take place in an unsaturated materialthe complexity of the problem renders the use of numerical analysis essential. In this paper soil-atmosphereinteraction in cut slopes is studied using advanced, fully coupled partially saturated finite element analyses. The effectof rainfall and evapotranspiration is modelled through sophisticated boundary conditions, applying actualmeteorological data on a monthly basis. Stages of low and high water demand vegetation are considered for a periodof several years, before simulating the effect of vegetation removal. The analysis results are presented with regard tothe serviceability and stability of the cut slope.
Mantikos V, Tsiampousi A, Taborda DMG, et al., 2015, Numerical interpretation of the coupled hydromechanical behaviour of expansive clays in constant volume column tests, 16th European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: ICE Publishing
Experimental and numerical studies of the behaviour of expansive clays have been attracting increasing interest, due to their good sealing properties, which render them ideal to be used as engineered barriers (buffers) in both active (e.g. nuclear) and non-active waste disposal facilities. Both large scale and aboratory scaled experiments indicate that the sealing capabilities of the buffer are fundamentally governed by its volumetric behaviour when wetted. In this paper, a constant volume column infiltration test, perform ed under is othermal conditions on compacted MX80 bentonite, is modelled numerically using the Imperial College Finite Element Program (ICFEP). A modified version of the Barcelona Basic Model is used to simulate the behaviour of the buffer, which is inherently partly saturated. The numerical results agree well with the observed experimental data, especially with regard to the advancement of the wetting front. A detailed interpretation of the computed evolutions with time of stress state, suction and void ratio at different elevations along the sample’s axis is carried out, providing insight into the complex hydro-mechanical response of the buffer during the experiment. Indeed, even though the overall volume of the sample was kept constant, a region of localised dilation, which induced the contraction of other zones of the material, was observed to advance simultaneously with the wetting front along the height of the soil column.
Standing JR, Potts DM, Vollum R, et al., 2015, Investigating the effect of tunnelling on existing tunnels, Underground Design and Construction Conference, Publisher: IOM3, Pages: 301-312
A major research project investigating the effect of tunnelling on existing tunnels has beencompleted at Imperial College London. This subject is always of great concern during theplanning and execution of underground tunnelling works in the urban environment. Many citiesalready have extensive existing tunnel networks and so it is necessary to construct new tunnels ata level beneath them. The associated deformations that take place during tunnelling have to becarefully assessed and their impact on the existing tunnels estimated. Of particular concern is theserviceability of tunnels used for underground trains where the kinematic envelope must not beimpinged upon. The new Crossrail transport line under construction in London passes beneathnumerous tunnels including a number of those forming part of the London Underground network
Taborda DMG, Tsiampousi A, 2014, Development of a hysteretic soil-water retention model using an optimisation technique, Sydney, NSW, 6th International Conference on Unsaturated Soils, UNSAT 2014, Publisher: Taylor and Francis - Balkema, Pages: 965-971
Experimental testing has shown that unsaturated soils exhibit hysteretic retention behaviour upon cycles of drying and wetting. This paper describes the general formulation of a hysteretic soilwater retention curve model, according to which the shape of the scanning paths is defined based on the relative positions of the current point, its projection on the corresponding primary curve, designated as image point, and the position at which the scanning path was initiated. However, rather than prescribing an analytical expression relating the hydraulic response at the current point with that at the image point, a Hill-Climbing algorithm is employed to characterise this component of the model. The use of this optimisation technique enables a pattern to be established based on experimental data, which is then converted into the analytical formulation of the model. The resulting expression is shown to accurately reproduce the experimentally observed behaviour. © 2014 Taylor & Francis Group.
Zdravkovic L, Potts DM, Tsiampousi A, 2014, Obtaining factors of safety from a finite element analysis of unsaturated soils, 6th International Conference on Unsaturated Soils (UNSAT), Publisher: CRC PRESS-TAYLOR & FRANCIS GROUP, Pages: 553-559
Tsiampousi A, Vitsios I, Zdravkovic L, et al., 2014, Effect of previous stress history and vegetation on the coefficient of earth pressure at-rest, K<sub>0</sub>, in London clay, 8th European Conference on Numerical Methods in Geotechnical Engineering (NUMGE), Publisher: CRC PRESS-TAYLOR & FRANCIS GROUP, Pages: 209-214
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Tsiampousi A, Zdravkovic L, Potts DM, 2013, Variation with time of the factor of safety of slopes excavated in unsaturated soils, Computers and Geotechnics, Vol: 48, Pages: 167-178
Tsiampousi A, Zdravkovic L, Potts DM, 2013, A new Hvorslev surface for critical state type unsaturated and saturated constitutive models, Computers and Geotechnics, Vol: 48, Pages: 156-166
Tsiampousi A, Zdravkovic L, Potts DM, 2013, A three-dimensional hysteretic soil-water retention curve, Geotechnique, Vol: 63, Pages: 155-164, ISSN: 0016-8505
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.
Tsiampousi A, Zdravkovic L, Potts DM, 2012, Stability of highly overconsolidated unsaturated slopes, 2nd European Conference on Unsaturated Soils, Pages: 377-382
Tsiampousi A, Zdravkovic L, Potts DM, 2012, Stability of highly overconsolidated soil slopes, II European Conference for Unsaturated Soils (E-UNSAT2012), Pages: 377-382
Tsiampousi A, Zdravkovic L, Potts DM, 2010, Modelling of the hysteretic soil-water retention curve of unsaturated soils, 7th European Conference on Numerical Methods in Geotechnical Engineering, Pages: 331-336
Tsiampousi A, Zdravkovic L, Potts DM, 2009, Modelling of overconsolidated unsaturated soils, 4th Asia Pacific Conference on Unsaturated Soils, Publisher: CRC Press, Pages: 673-678
Constitutive models for unsaturated soils are most commonly based on conventional critical state type models for saturated soils. The effect of partial saturation is taken into account through the intro-duction of an additional stress variable which depends on soil suction. The modified Cam-Clay surface is usu-ally assumed for the formulation of the yield and the plastic potential surfaces in the mean stress – deviatoric stress space. In this paper an existing model for unsaturated soils is used to simulate drained triaxial tests per-formed on highly overconsolidated unsaturated silt. Even though the model adopts an improved expression for the yield surface that allows a variety of shapes to be introduced, none of them can accurately simulate the observed soil behaviour in the laboratory tests. A new version of the model is, therefore, proposed and pre-sented here. Following the formulation and implementation of these alterations, analyses of laboratory ex-periments on artificial silt are presented demonstrating the improved simulation of soil behaviour.
Zdravkovic L, Tsiampousi A, Potts DM, 2007, Effect of wall and soil permeability on the long-term ground movements adjacent to a deep excavation, 10th International Conference on Numerical Methods in Geomechanics, Pages: 589-594
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