29 results found
Cui W, Potts DM, Zdravković L, et al., 2019, Formulation and application of 3D THM-coupled zero-thickness interface elements, Computers and Geotechnics, Vol: 116, Pages: 1-11, ISSN: 0266-352X
Interface elements are frequently employed in finite element (FE) analyses to represent soil-structure interfaces or rock joints. The modelling of coupled thermo-hydro-mechanical (THM) problems in geotechnical engineering requires equally a coupled and robust THM formulation for interface elements. This paper presents such a formulation which is capable of reproducing the coupled THM behaviour of discontinuities and soil-structure boundaries, and is compatible with other types of finite elements used to discretise the soil and structural domains (e.g. solid and shell elements). The coupled THM three-dimensional (3D) zero-thickness interface element is implemented into the bespoke FE code employed in this research and its features are verified using a number of numerical exercises. To demonstrate their performance, the proposed interface elements are employed in the simulation of the coupled THM behaviour of a fissured triaxial sample subjected to a thermal load and the influence of the presence of fissures on soil behaviour is presented.
Cui W, Tsiampousi A, Potts D, et al., Numerical modelling of time-dependent thermally induced excess pore fluid pressures in a saturated soil, Journal of Geotechnical and Geoenvironmental Engineering - ASCE, ISSN: 0733-9410
A temperature rise in soils is usually accompanied by an increase in excess pore fluid pressure due to the differential thermal expansion coefficients of the pore fluid and the soil particles. To model the transient behaviour of this thermally induced excess pore fluid pressure in geotechnical problems, a coupled THM formulation was employed in this study, which accounts for the non-linear temperature-dependent behaviour of both the soil permeability and the thermal expansion coefficient of the pore fluid. Numerical analyses of validation exercises (where there is an analytical solution), as well as of existing triaxial and centrifuge heating tests on Kaolin clay, were carried out in the current paper. The obtained numerical results exhibited good agreement with the analytical solution and experimental measurements respectively, demonstrating good capabilities of the applied numerical facilities and providing insight into the mechanism behind the observed evolution of the thermally induced pore fluid pressure. The numerical results further highlighted the importance of accounting for the temperature-dependent nature of the soil permeability and the thermal expansion coefficient of the pore fluid, commonly ignored in geotechnical numerical analysis.
Ghiadistri G, Zdravković L, Potts DM, et al., 2019, Calibration of a double structure constitutive model for unsaturated compacted soils, 7th International Symposium on Deformation Characteristics of Geomaterials (IS-Glasgow 2019), Publisher: EDP Sciences, Pages: 1-6
This paper describes a calibration procedure for the double structure constitutive model ICDSM (Imperial College Double Structure Model), developed for highly expansive clays, when the model is applied to MX-80 bentonite. Firstly, the model calibration process is discussed and organised in a number of hierarchical steps. These steps involve the estimation of the macrostructural parameters that can be derived from oedometer, isotropic and triaxial laboratory data. Estimation of the microstructural parameters is more challenging due to the limited knowledge of an expansive clay’s fabric and of the physico-chemical phenomena that control its evolution upon wetting. Nevertheless, this paper discusses the available sources of data and identifies the appropriate information that is needed to characterise the micro-structural behaviour of the bentonite. Finally, through the simulation of a swelling pressure test on a bentonite plug, the hydration of the material is studied as a hydro-mechanical coupled process. Particular attention is devoted to the evolution of the stress state of the sample, which is compared to the experimental measurements in order to demonstrate that the constitutive model accurately reproduces the expansive behaviour of MX-80 bentonite.
Cui W, Potts DM, Zdravkovic L, et al., 2018, A coupled thermo-hydro-mechanical finite element formulation for curved beams in two-dimensions, Computers and Geotechnics, Vol: 103, Pages: 103-114, ISSN: 0266-352X
To enable the use of beam elements in the modelling of coupled thermo-hydro-mechanical (THM) geotechnical problems, a fully coupled and robust THM formulation is required. This paper presents such a formulation which allows both fluid flow and heat transfer along a 2D curved beam, while ensuring compatibility with coupled THM solid elements commonly used to discretise soils. Verification exercises and application with the proposed coupled beam element are carried out to demonstrate its satisfactory behaviour. The results of these analyses are compared against closed form solutions, solutions obtained using solid elements, and field measurements, showing an excellent agreement.
Mantikos V, Tsiampousi A, Standing JR, 2018, Swelling behaviour of an expansive clay at high suction, 7th International Conference on Unsaturated Soils, Publisher: UNSAT
Deep geological disposal designs for nuclear waste often include an engineered barrier to protect the waste canistersand prevent leakage. The long-term safety of the repository relies on studies of the buffermaterial.Oedometer tests provide values ofdesign parameters fornumerical simulations. Anewly-developed oedometer with automated suction control is presented to assist in the investigation of the coupled hydro-me-chanical-volumetric behaviour of an expansive clay, namely a natural sodium bentonite. The displacement-controlled device was developed to apply suctionover a range of10 MPa to 300 MPausing a divided-flow humidity-generator. The device allows the application of combined stress and suction states, and continuous stress paths of constant volume, stress or suction. The development of the new oedometer is described. Results obtained during the preliminary tests are evaluated through comparison with experimental data from similar tests found in the literature. The current method benefits from continuous control of suction with servo-control of relative humidity using calibrated capacitance hygrometers. The system self-compensates for minor temper-ature changes and therefore the requirement for thermal insulation is not as crucial as in vapour equilibrium methods.
Ghiadistri GM, Potts DM, Zdravkovic L, et al., 2018, A new double structure model for expansive clays, 7th International Conference on Unsaturated Soils
The behaviourof compacted bentonite upon hydration is numerically investigated here by simu-lating a swelling pressure teston aMX-80 bentonitesample. Two constitutive modelsareemployed in the analysis: the “Imperial College Single-Structure Model” (ICSSM)andthe “Imperial College Double-Structure Model” (ICDSM), the latterspecifically developed for expansive clays. It is shown that the latter exhibits a considerably improved performance as it is able to accurately capture the swelling pressure developed in the materialupon wetting. Nevertheless, a limited knowledge of the evolution of the material’s fabric, notably at the micro-scale,is an obstacle for deriving with certainty some of the model parameters. This issue is high-lightedhere by performing analyses of theswelling pressure test with two sets ofmaterial characterisations, with model parameters differinginthe derivation of the microstructural component.Both analyses show a very good match with the testdata, but it is difficult to justify one set of microstructural parametersoverthe other. The paper emphasises what aspects of experimental research could be helpful in studying the fabric of compacted bentonite upon wetting, and hence improve the calibration procedure of thedouble-structure mod-el.
Zdravkovic L, Tsiampousi A, Potts DM, 2018, On the modelling of soil-atmosphere interaction in cut and natural slopes, 7th International Conference on Unsaturated Soils
The need to predict the consequences of atmospheric conditions on the stability of slopes is widely evident from numerous examples of slope failures around the world, which often result in material and human loss.Equally, the serviceability conditions of cut slopes very much depend onthe heave mobilised byexcavation, the magnitude of which is partly governed by the hydraulic boundary conditions.Soil-atmosphere interaction is complex, involving precipitation and evapotranspiration across the slope surface, and acts in ad-dition to theground water regime within the slope body. As a consequence, calculation tools cannot be overly simplified if realistic predictions are expected. This paper provides an overview of recent research at Imperial College in modellingunsaturatednatural and cut slopes, using finite element analysis and advanced constitutive models and boundary conditions.
Kirkham AD, Tsiampousi A, Potts DM, 2018, Temperature-controlled oedometer testing on compacted bentonite, 7th International Conference on Unsaturated Soils
A new temperature-controlled oedometer has been designed at Imperial College London and commissioned to investigate the thermo-hydro-mechanicalbehaviour of soils. Temperature control is achieved by submerging the specimenin a water bath. The water temperature is regulated byheaters positionedradially around the specimen, or by an external unit. The temperature can be varied between 5°C and 85°C. The temperature gradient across the specimenis minimised by circulating water beneath the specimenthrough a hollow plate.A thermo-mechanical, elastic, finite element model of the equipment has been produced using the Imperial College Finite Element Program (ICFEP). The experimental results are used to develop and validate the numerical model. The model is then used to inform and improve the experimental testing programme.The accuracy of temperature control has already been established. The testing programme includes heating tests at constant applied stress, and loading tests at discrete temperature values. Of particular interest is thermally-inducedoverconsolidation behaviour. The experimental results are used to verify the existing numerical framework and to establish the effect of temperature on the behaviourofsaturated soil.
Cui W, Tsiampousi A, Potts DM, et al., 2018, Finite element modelling of excess pore fluid pressure around a heat source buried in saturated soils, London, 9th European Conference on Numerical Methods in Geotechnical Engineering, Publisher: Taylor Francis Group, Pages: 741-749
The structural testing and finite element (FE) analysis described in this paper were part of a major research project undertaken at Imperial College London to investigate the deformation of bolted segmental grey cast iron (GCI) tunnel linings. A key aim was to quantify how joints influence the behaviour of the lining, through a three-path approach comprising physical experiments, finite element modelling, and field instrumentation. The laboratory results have been used to assess the validity of the tunnel assessment methods used by industry.This study examined joint articulation under the serviceability limit state in the absence of hoop force focussing on factors such as applied bolt preload, the loading direction and the freedom of the circumferential flange to deflect. Two half-scale GCI lining segments were bolted together at the longitudinal flanges to form a bolted arch in a similar fashion to the tests performed by Thomas (1977). Modern instrumentation was implemented to gain detailed measurements quantifying changes in global displacements of the two segments, bolt forces and joint opening under applied loading. For the first time, the physical experiments were conducted contemporaneously with the development of a three-dimensional FE model of the joint. The experimental data and the results from the FE analysis indicate a reduction in joint stiffness as the joint articulates under applied load. It is shown that the presence of a joint has far greater influence on the behaviour of the ‘arch’ than the level of preload applied to the bolts in the joint. The FE analysis allowed the deformation behaviour of the joint under positive and negative bending to be investigated: its response under the two modes differs significantly.
Tsiampousi A, Smith PGC, Potts DM, 2016, 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 containadditional parameters when compared to the equations for saturated soils. Nonetheless, thevariation of these parameters with suction or degree of saturation is not generally agreedupon. The paper introduces a novel approach to deriving general equations for each of theseparameters and their variation, and explains that, for consistency with the constitutive andsoil-water retention curve models adopted, these general equations need to be transformedinto case-specific expressions. Finally, a conceptual model is presented highlighting how thebehaviour of unsaturated soil reflects aspects of its water content.
Tsiampousi A, Smith PGC, Potts DM, 2016, Coupled consolidation in unsaturated soils: From a conceptual model to applications in boundary value problems, Computers and Geotechnics, Vol: 84, Pages: 256-277, ISSN: 1873-7633
The paper presents the Finite Element formulation of the equations proposed by Tsiampousi etal. (2016) for coupled consolidation in unsaturated soils. Their coupling is discussed in relationto a conceptual model which divides soil behaviour into zones ranging from fully saturated todry states. The numerical simulation of a laboratory experiment involving drainage of waterfrom a vertical column of sand is used to validate the equations. Finally, the example of rainfallinfiltration into a cut slope highlights how aspects of the conceptual model are reflected in thenumerical analysis of boundary value problems involving unsaturated soils.
Tsiampousi A, Zdravkovic L, Potts DM, 2016, A 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.
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, 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.
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, 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
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-0, in London clay, 8th European Conference on Numerical Methods in Geotechnical Engineering (NUMGE), Publisher: CRC PRESS-TAYLOR & FRANCIS GROUP, Pages: 209-214
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
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.
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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