Publications
249 results found
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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- Citations: 1
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
Pelecanos L, Kontoe S, Zdravkovic L, 2013, Numerical modelling of hydrodynamic pressures on dams, Computers and Geotechnics, Vol: 53, Pages: 68-82
This paper discusses several considerations related to appropriate numerical modelling of the reservoirhydrodynamic pressures on dams. The reservoir is modelled with 8-noded isoparametric displacementbased solid finite elements. The study includes both stiff and flexible dams with vertical and slopedupstream faces under ramp, harmonic and random acceleration loads. The numerical results were comparedand found to be in good agreement with available closed-form solutions. The same approach maybe used in analyses of other waterfront structures such as quay walls.
Han B, Zdravkovic L, Kontoe S, 2013, The stability of the Generalised-α time integration method for dynamic coupled consolidation problems, COMPDYN
This paper investigates the stability of the Generalised-α time integration method (the CH method) for a fully coupled solid-pore fluid formulation. Theoretical stability conditions are derived, which are shown to simplify to the existing ones of the CH method for the one–phase formulation when the solid-fluid coupling is ignored. Finite Element (FE) analyses considering a range of loading conditions are conducted to validate the analytically derived stability condition, showing that the numerical results are in agreement with the theoretical investigation. The CH method is a generalisation of a number of other tine integration schemes and hence the derived stability conditions are relevant for most of the commonly utilised time integration methods for fully coupled two-phase formulation.
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, 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 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.
Freitas TMB, Potts DM, Zdravkovic L, 2013, Some strengths and weaknesses of overstress based elastic viscoplastic models, Springer Series in Geomechanics and Geoengineering, Pages: 107-114, ISSN: 1866-8755
Various elastic viscoplastic models based on the overstress theory by Perzyna [9] have been proposed and are described in the literature. This paper presents the theoretical formulation of this type of models and highlights the abilities and shortcomings of such models to reproduce the time dependent behaviour of clays observed in the laboratory and the implications that these have when used in the analysis of boundary value problems. © Springer-Verlag Berlin Heidelberg 2013.
Potts DM, Zdravkovic L, 2012, Accounting for partial material factors in numerical analysis, Geotechnique
Bodas Freitas TM, Zdravkovic L, Potts DM, 2012, Some strengths and weaknesses of overstress based elasto-viscoplastic models, 2nd International Symposium on Constitutive Modelling of Geomaterials - Advances and New application, Publisher: Springer Series in Geotechnics and Geoengineering, Pages: 107-114
Jurecic N, Zdravkovic L, Jovicic V, 2012, Predicting ground movements in London Clay, Proceedings of the Institution of Civil Engineers - Geotechnical Engineering, Vol: 166, Pages: 466-482, ISSN: 1353-2618
The necessity of accounting for small-strain behaviour of soils in numerical analyses of the serviceability limit states of geotechnical structures is well established both nationally and internationally. This has led to further development of appropriate soil constitutive models, as well as to further advances in accurate laboratory measurements of small-strain stiffness. The current paper considers recent laboratory research into the behaviour of London Clay, performed at Imperial College in conjunction with the major ground works for Heathrow Terminal 5 in London, UK. This research has shown the small-strain response of London Clay to be different to that assumed previously and in particular to that determined from good-quality commercial experiments. Both sets of small-strain stiffness data are applied in this paper in finite-element analyses of a deep excavation and a tunnel construction in London Clay, with the objective of investigating their effect on predicted ground movements.
Freitas TMB, Potts DM, Zdravkovic L, 2012, Implications of the definition of the Φ function in elastic-viscoplastic models, GEOTECHNIQUE, Vol: 62, Pages: 643-648, ISSN: 0016-8505
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- Citations: 8
Bodas Freitas TM, Potts DM, Zdravkovic L, 2012, The effect of creep on the short-term bearing capacity of pre-loaded footings, COMPUTERS AND GEOTECHNICS, Vol: 42, Pages: 99-108, ISSN: 0266-352X
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- Citations: 1
Pelecanos L, Kontoe S, Zdravković L, 2012, Numerical analysis of the seismic response of La Villita dam in Mexico, Pages: 120-130
This paper presents two-dimensional plane-strain static and dynamic finite element analyses of La Villita earth dam in Mexico, which has experienced a number of large earthquakes. Static analyses are employed to simulate the layered construction of the embankment, water impounding and consolidation, whereas dynamic analyses simulate the earthquake events. The static behaviour of the dam is well captured, as evidenced by the predicted and recorded crest settlement. The dynamic behaviour is satisfactorily captured by comparing the accelerations recorded at both the crest and downstream berm of the dam.
Kontoe S, Zdravkovic L, Menkiti CO, et al., 2012, Seismic response and interaction of complex soil retaining systems, Computers and Geotechnics, Vol: 39, Pages: 17-26
This paper examines the seismic response of a large and complex system comprising a Lock chamber and three neighbouring water saving basins (WSBs). The developed two-dimensional plane strain finite element model included the entire system in order to explore the interaction of the various structures. The first set of analyses was undertaken under static conditions to simulate the construction sequence and to establish the stress regime prior to the examined seismic event. Subsequently, dynamic time domain analyses were performed to examine the response of the system to seismic loading. The ground motion was applied into the finite element mesh employing a sub-structuring technique which enables economic modelling of large computation domains and accurate representation of the semi-infinite half-space. The paper explores some aspects of the numerical modelling of such complex systems, such as the impact on the predicted seismic response of the adopted constitutive model and the modelling of hydrodynamic effects. The discussion of the dynamic analyses results focuses on the main Lock structure, looking at the prevailing modes of deformation, the potential lift-off of the base of the Lock walls and the interaction of the main wall with the other retaining structures.
Monroy R, Zdravkovic L, Ridley A, 2012, Random Uncertainty in the Measurement of Matric Potential Using the Miniature Tensiometer, GEOTECHNICAL TESTING JOURNAL, Vol: 35, Pages: 41-49, ISSN: 0149-6115
Potts D, Zdravkovic L, 2012, Computer Analysis principles in geotechnical engineering, ICE manual of Geotechnical Engineering - Volume 1 Geotechnical Engineering Principles, Problematic Soils and Site Investigation, Editors: Burland, Skinner, Brown, Publisher: Institution of Civil Engineers, Pages: 35-57
Tsiampousi A, Zdravkovic L, Potts DM, 2012, Stability of highly overconsolidated soil slopes, II European Conference for Unsaturated Soils (E-UNSAT2012), Pages: 377-382
Taborda DMG, Zdravkovic L, 2012, Application of a Hill-Climbing Technique to the Formulation of a New Cyclic Nonlinear Elastic Constitutive Model, Computers and Geotechnics, Vol: 43, Pages: 80-91
In the field of constitutive modelling of soil behaviour, optimisation techniques have been mostly employed as a calibration tool, particularly when several model parameters lack clear physical meaning. In this paper, however, a procedure based on a Hill-Climbing optimisation algorithm is presented as a form of improving the performance of constitutive models. Specifically, a simple cyclic nonlinear elastic model, which is shown to be unable to simulate adequately the damping ratio measured under small and large strain amplitudes, is modified by applying the Hill-Climbing technique to the determination of a new relationship describing the unloading/reloading behaviour of soil under cyclic loading. The performance of the proposed model is assessed by evaluating its parameters based on three distinct sets of empirical damping ratio curves and computing the corresponding error in their simulation. It is shown that the introduction of the new unloading/reloading expression formulated based on the outcome of the optimisation procedure increases substantially the precision of the constitutive model.
Potts DM, Zdravkovic L, 2011, Application of partial factors of safety in numerical analysis of bearing capacity, Pages: 775-781
One of the issues that arise when using numerical analysis in accordance with Eurocode 7 is how to account for partial factors of safety on material strength. Namely, Eurocode 7 implies that strengths to be used in an analysis should be based on a design value, but reduced by partial factors. This can be done in two ways, but Eurocode 7 gives no guidance as to which one is more appropriate. This paper examines the application of partial factors of safety in a bearing capacity problem. Finite element analyses are performed using both a simple Tresca and a more advanced Modified Cam Clay models to represent soil behaviour, and both approaches for the application of partial factors of safety.
Bodas Freitas TM, Potts DM, Zdravkovic L, 2011, A time dependent constitutive model for soils with isotach viscosity, COMPUTERS AND GEOTECHNICS, Vol: 38, Pages: 809-820, ISSN: 0266-352X
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- Citations: 41
Schuetz R, Potts DM, Zdravkovic L, 2011, Advanced constitutive modelling of shotcrete: Model formulation and calibration, COMPUTERS AND GEOTECHNICS, Vol: 38, Pages: 834-845, ISSN: 0266-352X
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- Citations: 16
Zdravkovic L, Potts DM, St John HD, 2011, Modelling of a 3D excavation in finite element analysis, Pages: 319-335
This paper investigates a number of issues related to the modelling of a retaining structure used to support an excavation in 3D finite element analyses. In particular, the effects of wall stiffness in different coordinate directions and the rotational fixity in the corner of the excavation are examined. Both square and rectangular excavations are analysed and compared with the equivalent axisymmetric and plane strain analyses, normally used as approximations for modelling purposes. The chosen geometry, construction sequence and soil conditions are based on a proposed deep excavation at Moorgate in London (next to the Moor House development), which will form part of an underground station for the Crossrail project. The objective of the study is to provide a detailed assessment of wall and ground movements and structural forces in the wall in the light of different modelling assumptions. The study has wider application to a variety of projects that include the development of infrastructure, the construction of deep basement car parks and buried structures, and the effect that these have on the surrounding areas.
Kontoe S, Zdravkovic L, Potts DM, et al., 2011, On the relative merits of simple and advanced constitutive models in dynamic analysis of tunnels, Geotechnique, Vol: 61, Pages: 815-829, ISSN: 0016-8505
This paper compares simple constitutive models that are widely used in engineering practice with more sophisticated methods in the context of a case study. In particular, four constitutive modelling approaches have been considered: a simple elasto-plastic constitutive model (modified Cam-clay), with and without Rayleigh damping; the same model coupled with a cyclic non-linear model that can simulate pre-yield hysteresis; and finally an advanced kinematic hardening model, which is an improved version of the Al-Tabbaa & Wood two-surface model. These four approaches are used to analyse the seismic response of a section of the Bolu tunnels during the 1999 Duzce earthquake. To shed light on the performance of the constitutive models, simple site response finite-element analyses were first undertaken for the studied site, paying particular attention to the calibration of the Rayleigh damping parameters. The results of these analyses, in terms of maximum shear strain, were then used as input to an analytical elastic method (extended Hoeg method) for calculating the thrust and bending moment acting in the tunnel lining. Finally the results of dynamic time domain plane-strain analyses, employing the four adopted constitutive modelling approaches, are compared against field observations and results obtained by the extended Hoeg method, to investigate the ability of the models, of ranging complexity, to mimic soil response under seismic excitation.
Kontoe S, Zdravkovic L, Potts DM, et al., 2011, Seismic response of complex soil-structure systems, 15th European Conference on Soil Mechanics and Geotechnical Engineering, Pages: 1491-1496
Zdravkovic L, Potts DM, Kontoe S, 2011, Effect of wall stiffness on ground deformations around deep excavations in stiff clay, 15th European Conference on Soil Mechanics and Geotechnical Engineering, Pages: 1599-1604
Zdravković L, Potts DM, 2010, Application of numerical analysis in geotechnical engineering practice, Pages: 69-88, ISSN: 0895-0563
Numerical analysis, in the form of the finite element, finite difference or boundary element method, has become a preferred calculation tool in the design of complex geotechnical structures. Even new codes of practice, such as the European EUROCODE 7, encourage the use of numerical analysis in design. However, as there is no general guidance and agreement in the international community on appropriate numerical solvers, constitutive models and boundary conditions, there are often examples of bad practice in the numerical analysis of geotechnical structures. Through examples of the analyses of real problems, this paper investigates the use of appropriate constitutive models and boundary conditions, to highlight both the successful use and the potential pitfalls of modern numerical analysis. © 2010 ASCE.
Grammatikopoulou A, Kovacevic N, Zdravkovic L, et al., 2010, Finite element analysis of the main embankment at Empingham dam, Pages: 557-562
Empingham dam is a 37m high earth fill embankment which was constructed in the UK in the early 1970's. The embankment was built on a brecciated Upper Lias Clay (ULC) foundation, of fill derived from it. The ULC is a typical stiff plastic clay which shows a post-peak strength loss and as such is prone to progressive failure. The paper describes the finite element back analyses of the embankment behaviour during construction, using a kinematic hardening "bubble" model which accounts for both pre-peak plasticity and post-peak strain softening. © 2010 Taylor & Francis Group, London.
Giannopoulos KP, Zdravkovic L, Potts DM, 2010, A numerical study on the effects of time on the axial load capacity of piles in soft clays, Pages: 595-600
This paper investigates the axial load capacity of a pre-loaded pile installed in soft clay and subjected to vertical loading. Previous studies on pre-loaded shallow footings have shown that the undrained shear strength of soft clays is enhanced with time due to the dissipation of the excess pore water pressures generated during initial loading, as well as due to the soil ageing, after all pore pressures have dissipated, associated with creep. It remains a question whether the pile load capacity is enhanced with time as well, after the effects of installation have settled. The problem is investigated by means of a series of coupled finite element analyses, thus taking account of consolidation processes in the ground, in which the soil is modelled using an elastic-plastic constitutive model. Subsequently a similar set of coupled analyses is performed, using an elastic-viscoplastic model, to examine the effect of creep on pile capacity. © 2010 Taylor & Francis Group, London.
Bournta Z, Zdravkovic L, 2010, Bearing capacity of a surface footing founded on a layered clay subsoil, Pages: 553-556
The ultimate bearing capacity of surface footings resting on homogeneous soils has been studied extensively. Real soil strength profiles beneath footings, however, are not homogeneous but may increase or decrease with depth, or consist of distinct layers having significantly different properties. The current study focuses on the application of the finite element method for the evaluation of the ultimate bearing capacity of a rough rigid footing resting on a layered clay profile. Computations are performed using the finite element analysis software ICFEP (Imperial College Finite Element Program). One set of parametric analyses is carried out for the case of strip and circular footings resting on soft clay overlying stiff clay, in order to examine the effect of the vicinity of the strong clay layer on the bearing capacity factor. Another set of parametric analyses is performed for the case of circular footings resting on a multi-layered soil profile that includes a layer of soft clay resting within the strong clay subsoil, in order to investigate the effect of both the vicinity and the thickness of the soft clay layer on the bearing capacity factor. In both cases comparison of the analyses results with the literature has been made. © 2010 Taylor & Francis Group, London.
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