Publications
249 results found
Byrne BW, Burd HJ, Zdravkovic L, et al., 2019, PISA Design Methods for Offshore Wind Turbine Monopiles, Offshore Technology Conference
Byrne BW, McAdam RA, Burd H, et al., 2019, PISA: new design methods for offshore wind turbine monopiles, Proceedings of the Society for Underwater Technology Offshore Site Investigation and Geotechnics 8th International Conference on “Smarter Solutions for Future Offshore Developments"
Cui W, Gawecka KA, Taborda D, et al., 2019, Time-step constraints for finite element analysis of two-dimensional transient heat diffusion, Computers and Geotechnics, Vol: 108, Pages: 1-6, ISSN: 0266-352X
In a FE analysis of transient heat transfer, a lower limit of the time-step size exists below which numerical oscillations of temperatures may occur. Although time-step constraints for simulating 1D heat diffusion have been well established in the literature, the conclusions cannot be directly applied to 2D cases. In this paper, both analytical and computational studies are carried out to obtain the time-step constraints for 2D linear and quadratic elements. It is noted that in the simulation of 2D heat diffusion employing quadratic elements is not always beneficial. Recommendations are provided on selecting the numerical scheme to minimise numerical oscillations.
Ghiadistri GM, Zdravkovic L, Potts DM, et al., 2019, Modelling the behaviour of swelling clays in a Geological Disposal Facility (GDF)
This paper discusses the numerical modelling of the buffer material in a Geological Disposal Facility (GDF), by simulating the FEBEX in-situ experiment. The test was conducted over 18 years at the Grimsel site in Switzerland, under the conditions of a real GDF, with compacted bentonite blocks used as the buffer material and a heater replacing the nuclear waste canister. Particular emphasis in the paper is given to the constitutive modelling of the FEBEX bentonite, highlighting the importance of accounting in the model for the double porosity structure of the compacted bentonite. Furthermore, the coupled thermo-hydro-mechanical (THM) finite element analysis also emphasises the importance of realistic modelling of the evolution of the hydraulic permeability of the bentonite with the changing suctions. The analysis results demonstrate substantial agreement between numerical predictions and FEBEX field measurements in terms of the buffer’s THM evolution. The area of the host formation affected by the test is also defined and investigated in order to provide useful information for the design of a GDF.
Zdravković L, Potts DM, Bodas Freitas TM, 2019, Extending the life of existing infrastructure, Pages: 136-143
Contemporary geotechnical design is increasingly faced with a demand for extending the life of ageing infrastructure. Typical examples are foundation systems for re-development projects in congested urban environments, road and rail infrastructure and flood defences. The latter structures in particular are also projected to have to sustain rising sea-levels due to the effects of climate change. To enable such a design, the changes in the mechanical behaviour of the foundation soil over the period from first construction to the current state must be quantified. This transient period is governed by the time-related process of consolidation and creep in the ground. Considering examples of earthfill embankments, this paper discusses the facets of soil behaviour governing both the short- and long-term design of existing infrastructure embankments, using advanced numerical analysis.
Byrne BW, Burd HJ, Gavin K, et al., 2019, PISA: Recent developments in offshore wind turbine monopile design, 1st Vietnam Symposium on Advances in Offshore Engineering, Publisher: Springer
This paper provides a brief overview of the Pile Soil Analysis (PISA) project, recently completed in the UK. The research was aimed at developing new design methods for laterally loaded monopile foundations, such as those supporting offshore wind turbine structures. The paper first describes the background to the project and briefly outlines the key research elements completed. The paper concludes with a brief description of the anticipated impact of the work and describes initiatives that have followed since.
Sailer E, Taborda DMG, Zdravkovic L, et al., 2019, Numerical modelling of thermo-active shafts, 2nd Symposium in Energy Geotechnics, Pages: 97-104, ISSN: 1866-8755
© Springer Nature Switzerland AG 2019. Geotechnical structures, such as foundation piles, retaining walls and tunnel linings, are increasingly employed to produce geothermal energy for space heating and cooling. However, the exchange of heat between the structure and the ground induces additional structural forces and contributes to further structural and ground movements, which may affect the serviceability and stability of such structures. While numerous field and numerical studies exist regarding the response of geothermal piles, no investigations have been carried out to characterise the response of thermo-active shafts. This paper presents a numerical study of the short and long term behaviour of hypothetical thermo-active shafts through fully coupled thermo-hydro-mechanical (THM) finite element (FE) analyses using the Imperial College Finite Element Program (ICFEP), where the effect of changing the structure’s geometric characteristics is investigated.
Gawecka KA, Potts DM, Cui W, et al., 2018, A coupled thermo-hydro-mechanical finite element formulation of one-dimensional beam elements for three-dimensional analysis, Computers and Geotechnics, Vol: 104, Pages: 29-41, ISSN: 0266-352X
Finite element (FE) analysis in geotechnical engineering often involves entities which can be represented as one-dimensional elements in three-dimensions (e.g. structural components, drains, heat exchanger pipes). Although structural components require an FE formulation accounting only for their mechanical behaviour, for the latter two examples, a coupled approach is necessary. This paper presents the first complete coupled thermo-hydro-mechanical FE formulation for one-dimensional beam elements for three-dimensional analysis. The possibility of deactivating each of the systems enables simulation of both coupled and uncoupled behaviour, and hence a range of engineering problems. The performance of these elements is demonstrated through various numerical simulations.
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Cui W, Potts DM, Zdravković 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.
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- Citations: 3
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.
Stone KJL, Arshi HS, Zdravkovic L, 2018, The use of bearing plate to enchance the lateral capacity of monopiles in sand, Journal of Geotechnical and Geoenvironmental Engineering, Vol: 144, ISSN: 1090-0241
Offshore foundation systems are constantly evolving to meet the needs of developments in the energy sector. These developments may be induced by the requirements of moving into ever deeper water for hydrocarbon recovery or creating foundation systems for renewable energy sources such as offshore wind farms. One such approach is that foundation systems are developed that combine several foundation elements to create a hybrid system. In this way it may be possible to develop a foundation system that is more efficient for the combination of vertical and lateral loads associated with the offshore environment, and in particular wind-powered generators. This paper presents the results from a physical and numerical modeling program undertaken to investigate the performance of hybrid, monopiled footing foundations under combined monotonic loading conditions in sand.
Cui W, Gawecka KA, Potts DM, et al., 2018, A Petrov-Galerkin finite element method for 2D transient and steady state highly advective flows in porous media, Computers and Geotechnics, Vol: 100, Pages: 158-173, ISSN: 0266-352X
A new Petrov-Galerkin finite element method for two-dimensional (2D) highly advective flows in porous media, which removes numerical oscillations and retains its precision compared to the conventional Galerkin finite element method, is presented. A new continuous weighting function for quadratic elements is proposed. Moreover, a numerical scheme is developed to ensure the weighting factors are accurately determined for 2D non-uniform flows and 2D distorted elements. Finally, a series of numerical examples are performed to demonstrate the capability of the approach. Comparison against existing methods in the simulation of a benchmark problem further verifies the robustness of the proposed method.
Kontoe S, Han B, Zdravkovic L, et al., 2018, The importance of compressional deformation in three dimensional site response analysis, 16th European Conference on Earthquake Engineering
Pelecanos L, Kontoe S, Zdravkovic L, 2018, The effects of non-linear dam-foundation interaction on the seismic response of earth dams, 16th European Conference on Earthquake Engineering
Han BO, Zdravkovic L, Kontoe S, 2018, Analytical and numerical investigation of site response due to vertical ground motion, Geotechnique: international journal of soil mechanics, Vol: 68, Pages: 467-480, ISSN: 0016-8505
Owing to the repeatedly observed strong vertical ground motions and compressional damage of engineering structures in recent earthquakes, the multi-directional site response analysis is increasingly critical for the seismic design of important structures, such as nuclear power plants and high earth dams. However, the site response to the vertical component of the ground motion has not been the subject of detailed investigation in the literature. Therefore, in this paper, the vertical site response due to vertical ground motion is investigated by employing both analytical and numerical methods. First, a one-dimensional frequency domain analytical solution, which can be employed for vertical site response analysis in practice, is studied and compared against time domain finite-element (FE) analyses for the two extreme soil state conditions (i.e. undrained and drained conditions). The vertical site response is further investigated with hydro-mechanically (HM) coupled FE analysis, considering solid–fluid interaction. The parametric studies undertaken show that the predicted vertical site response is strongly affected by the parameters characterising the hydraulic phase, namely, soil permeability and soil state conditions, both in terms of frequency content and amplification. The subsequent corresponding quantitative investigation, of the frequency content and amplification function of the vertical site response, shows that depending on the soil permeability the response is dominated by the two types of compressional waves (fast and slow waves). Notably, the parametric studies identify a range of permeability that significantly affects dynamic soil properties in terms of P-wave velocities, damping ratios and vertical site response, and this range is relevant for geotechnical earthquake engineering applications. It is therefore recommended that coupled consolidation analysis is necessary to simulate this effect accurately at such permeability-dependent intermediate
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
Potts D, Cui W, Gawecka KA, et al., 2018, Numerical modelling of coupled thermo-hydro-mechanical problems: Challenges and pitfalls, 9th European Conference on Numerical Methods in Geotechnical Engineering, Publisher: Taylor Francis Group
Taborda DMG, Potts DM, Zdravkovic L, et al., 2018, Incorporating the state parameter into a simple constitutive model for sand, London, 9th European Conference on Numerical Methods in Geotechnical Engineering, Publisher: Taylor Francis Group, Pages: 327-334
Sailer E, Taborda DMG, Zdravkovic L, et al., 2018, Factors affecting the thermo-mechanical response of a retaining wall under non-isothermal conditions, London, 9th European Conference on Numerical Methods in Geotechnical Engineering, Publisher: Taylor Francis Group, Pages: 741-749
Cui W, Potts DM, Zdravkovic L, et al., 2018, An alternative coupled thermo-hydro-mechanical finite element formulation for fully saturated soils, Computers and Geotechnics, Vol: 94, Pages: 22-30, ISSN: 0266-352X
Accounting for interaction of the soil’s constituents due to temperature change in the design of geo-thermal infrastructure requires numerical algorithms capable of reproducing the coupled thermo-hydro-mechanical (THM) behaviour of soils. This paper proposes a fully coupled and robust THM formulation for fully saturated soils, developed and implemented into a bespoke finite element code. The flexibility of the proposed formulation allows the effect of some coupling components, which are often ignored in existing formulations, to be examined. It is further demonstrated that the proposed formulation recovers accurately thermally induced excess pore water pressures observed in undrained heating tests.
Pedro A, Zdravkovic L, Potts DM, et al., 2017, Geotechnical characterisation of the Miocene formations at the location of Ivens shaft, Lisbon, Quarterly Journal of Engineering Geology and Hydrogeology, Vol: 51, Pages: 96-107, ISSN: 1470-9236
The design of complex underground structures in an urban environment requires in the first instance an appropriate characterization and interpretation of the ground conditions and of the mechanical behaviour of soil formations in the ground profile. With such information it is then possible to select and calibrate appropriate soil constitutive models for application in advanced numerical analysis, with the objective of predicting the induced ground movements and the potential damage to existing structures and services. This paper provides an interpretation of the site investigation data collected for the numerical analysis and design of the Ivens shaft excavation in Lisbon, Portugal. For the first time a comprehensive set of interpreted data is obtained for two of the main formations in the Lisbon area, Argilas e Calcários dos Prazeres (AP) and Areolas da Estefânia (AE), improving the understanding of their mechanical behaviour and making the data available for application in most soil constitutive frameworks. It is evident from the results that even with careful testing procedures the data may appear to be inconsistent, requiring further assumptions when deriving soil parameters. Such assumptions are discussed and emphasis is placed on the need to combine data from laboratory and field investigations.
Sailer E, Taborda DMG, Zdravkovic, 2017, A new approach to estimating temperature fields around a group of vertical ground heat exchangers in two-dimensional analyses, Renewable Energy, Vol: 118, Pages: 579-590, ISSN: 1879-0682
Vertical ground heat exchangers (VHEs), in the form of either Borehole Heat Exchanger (BHE) or thermo-active piles, are increasingly being deployed to provide low cost and sustainable heating and cooling to buildings. These are often installed within densely built urban environments, where adjacent foundation systems and underground structures can be affected by soil temperature changes induced by the heat exchangers. Therefore, they need to be considered in the geotechnical design of such structures, which typically involves carrying out two dimensional finite element plane strain analyses in order to assess their stability and performance. In such a scenario, it is common to model a line of heat exchangers as a planar source with one infinite dimension and a heat flux rate calculated by dividing the design extraction rate of a single heat exchanger by their spacing in the out-of-plane direction. This study shows that this approach largely overestimates the generated temperature field and proposes a simplified but accurate procedure to estimate the required heat flux to be applied to the planar heat sources in a 2D analysis. For this purpose, a correction factor, Full-size image (<1 K), is introduced which is shown to depend on geometric parameters and thermal ground properties.
Burd HJ, Byrne BW, McAdam R, et al., 2017, Foundation Design of Offshore Wind Structures, TC209 Workshop on Foundation Design of Offshore Wind Structures, 19th International Conference on Soil Mechanics and Geotechnical Engineering
This paper describes the outcome of a recently completed research project – known as PISA – on the development of a new process for the design of monopile foundations for offshore wind turbine support structures. The PISA research was concerned with the use of field testing and three-dimensional (3D) finite element analysis to develop and calibrate a new one-dimensional (1D) design model. The resulting 1D design model is based on the same basic assumptions and principles that underlie the current p-y method, but the method is extended to include additional components of soil reaction acting on the pile, and enhanced to provide an improved representation of the soil-pile interaction behaviour. Mathematical functions – termed ‘soil reaction curves’ – are employed to represent the individual soil reaction components in the 1D design model. Values of the parameters needed to specify the soil reaction curves for a particular design scenario are determined using a set of 3D finite element calibration analyses. The PISA research was focused on two particular soil types (overconsolidated clay till and dense sand) that commonly occur in north European coastal waters. The current paper provides an overview of the field testing and 3D modelling aspects of the project, and then focuses on the development, calibration and application of the PISA design approach for monopiles in dense sand.
Abadias Gomez D, Zdravkovic L, Taborda DMG, et al., 2017, On the implications of advanced monopile design methodologies in offshore wind turbines, Proceedings of the Society for Underwater Technology Offshore Site Investigation and Geotechnics 8th International Conference on “Smarter Solutions for Future Offshore Developments"
The design of Offshore Wind Turbines (OWT) is a complex process involving several stages: wind turbine selection, tower and sub-structure design, as well as foundation design and installation. A successful design requires close interaction between these components in order to satisfy the main design requirements, name-ly the capacity and accumulated rotation for the foundation and dynamic response and fatigue for the whole system. Recent research has revealed that the current design methods for laterally loaded piles, when ap-plied to short and stubby OWT monopiles, underestimate their initial stiffness and capacity. Advanced Fi-nite Element (FE) analysis, with realistic modelling of the ground conditions can accurately reproduce soil response around a monopile, and hence improve the design, ultimately leading to cost reduction of monopile foundations. In the present paper, the impact of economies in foundation design on the overall design of a realistic OWT is explored. The NREL 5 MW baseline wind turbine is modelled through FE analysis under several characteristic design load cases. The advantages of using FE analysis when compared to traditional methods, in particular with respect to capacity and dynamic response, are demonstrated and discussed.
Zdravkovic L, 2017, Editorial: tunnelling in the urban environment, GEOTECHNIQUE, Vol: 67, Pages: 747-747, ISSN: 0016-8505
Pelecanos L, Kontoe S, Zdravkovic L, 2017, Steady-state and transient dynamic visco-elastic response of concrete and earth dams due to dam-reservoir interaction, 6th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering, Publisher: ECCOMAS Proceedia, Pages: 3630-3646
The aim of this study is to investigate the effects of dam–reservoir interaction on the dynamic response of dams. Both thin rectangular concrete cantilever and large trapezoidal earth dams are considered with empty and full reservoir.It has recently been shown by Pelecanos et al. [32] that the amplification of accelerations at the crest of the dam depends on the combinations of the frequency of the harmonic acceleration load and the fundamental frequencies of the dam and the reservoir. This study considers transient dynamic loading and selected scenarios of different combinations of the above-mentioned frequencies are examined under random seismic acceleration load.It is shown that for certain cases the amplification of accelerations of the dam can be affected by the presence of the upstream reservoir. In general, thin rectangular concrete cantilever dams are found to be considerably more sensitive to dam–reservoir interaction than large trapezoidal earth dams. Therefore, this investigation examines the significance of dam–reservoir interaction and when this interaction should be taken into consideration or it could be neglected.
Gawecka KA, Taborda DMG, Potts DM, et al., 2017, Numerical modelling of thermo-active piles in London Clay, Proceedings of the Institution of Civil Engineers - Geotechnical Engineering, Vol: 170, Pages: 201-219, ISSN: 1353-2618
Thermo-active foundations utilise heat energy stored in the ground to provide a reliable and effective means ofspace heating and cooling. Previous studies have shown that the effects of temperature changes on their response arehighly dependent on their interaction with the surrounding ground. Consequently, it is necessary to consider thisinteraction and include both the thermal and mechanical behaviour of the ground in design. This paper addresses thisissue by performing state-of-the-art finite-element analyses using the Imperial College Finite Element Program, which iscapable of simulating the fully coupled thermo-hydro-mechanical behaviour of porous materials. First, the LambethCollege pile test is analysed to demonstrate the capability of the adopted modelling approach to capture the observedresponse under thermo-mechanical loading. Subsequently, a detailed study is carried out, demonstrating the impact ofcapturing the fully coupled thermo-hydro-mechanical response of the ground, the use of appropriate boundary conditionsand the uncertainty surrounding thermal ground properties. It is demonstrated that the modelling approach has a largeimpact on the computed results, and therefore potentially on the design of thermo-active piles. Conversely, the effects ofthermal conductivity and permeability of the soil are shown not to influence the pile behaviour significantly.
Han B, Zdravkovic L, Kontoe S, et al., 2017, Numerical investigation of multi-directional site response based on KiK-net downhole array monitoring data, Computers and Geotechnics, Vol: 89, Pages: 55-70, ISSN: 1873-7633
The multi-directional site response of a well-documented downhole array in Japan is numerically investigated with three directional (3-D) dynamic hydro-mechanically (HM) coupled Finite Element (FE) analysis. The paper discusses the challenges that 3-D modelling poses in the calibration of a cyclic nonlinear model, giving particular emphasis on the independent simulation of the shear and volumetric deformation mechanisms. The employed FE model is validated by comparing the predicted site response against the recorded motions obtained from the KiK-net downhole array monitoring system in Japan. The results show that, by employing the appropriate numerical model, a good agreement can be achieved between the numerical results and the monitored acceleration response in all three directions simultaneously. Furthermore, the comparison with the recorded response highlights the significance of the independent modelling of the shear and volumetric deformation mechanisms to the improvement of the numerical predictions of multi-directional site response.
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.
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