Publications
140 results found
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.
- Abstract
- Open Access Link
- Cite
- Citations: 10
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.
- Abstract
- Open Access Link
- Cite
- Citations: 3
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
Tsaparli V, Kontoe S, Taborda D, et al., 2018, Liquefaction triggering due to compressional waves: validation through field records, 16th European Conference on Earthquake Engineering
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
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, 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
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
Ferreira D, Pedro AMG, Almeida e Sousa J, et al., 2018, Development and validation of a numerical tool for modelling the variability of rock and soil massifs, XVI National Conference on Geotechnics
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.
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.
Tsaparli V, Kontoe S, Taborda DMG, et al., 2017, An energy-based interpretation of sand liquefaction due to vertical ground motion, Computers and Geotechnics, Vol: 90, Pages: 1-13, ISSN: 0266-352X
In several recent earthquakes, high vertical ground accelerations accompanied by liquefaction were observed. Downhole records have also shown that large vertical accelerations do not necessarily originate from the source, but rather get amplified towards the ground surface. Given the advantages of energy-based interpretation of liquefaction triggering due to S-waves, this approach is used together with finite element analyses to investigate vertical motion amplification and ensuing liquefaction. The results show the importance of the post-resonance response cycles, while hysteretic damping based on total stresses, accounting for the water in the pores, is shown to be very low, explaining the observed amplification.
- Abstract
- Open Access Link
- Cite
- Citations: 13
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.
Tsaparli V, Kontoe S, Taborda D, et al., 2017, Liquefaction modelling of a strong motion station in Christchurch, New Zealand, 3rd International Conference on Performance-based Design in Earthquake Geotechnical Engineering (PBD-III), Publisher: International Society of Soil Mechanics and Geotechnical Engineering
Advanced constitutive models can replicate several aspects of soil behaviour, but, due to their complexity and number of parameters, they need more sophisticated and realistic validation under general loading conditions. When modelling liquefaction phenomena, the lack of field monitoring data means that modeltesting, such as centrifuge experiments, is often used as benchmark for the numerical analyses. The 2010-2011 Canterbury earthquake sequence in New Zealand was recorded by a number of strong motion stations at various distances from the earthquake epicentre. Additionally, an extensive field and laboratory programmehassincebecome available, adequately describing the geological, geotechnical and hydrogeological conditions in the area. As such,the performance of a two-surface bounding surface plasticity constitutive model for sands, calibrated based on site-specific laboratory data, is assessed usingfield evidenceof a strong motion stationin fully-coupled effective stress-based finite element analyses. As the real stratigraphy is complex, with layers of silts and clays between the sandy strata, a simpler cyclic non-linearelasticmodel,which can adequately incorporate the basic aspects of dynamic soil behaviour, is also used to model the non-liquefiable strata. To specify the input ground motion at the base of the deposit, the recordedgroundsurface motionata sitewith no evidence of liquefaction isdeconvolved and compared with the outcrop predictions ofa New Zealand-specificground motion prediction equation. The numerical results are compared with the recorded horizontal ground surface acceleration time-history of the 22ndFebruary 2011 seismic event, exhibiting very good agreement.
Tsaparli V, Kontoe S, Taborda D, et al., 2017, The importance of accurate time-integration in the numerical modelling of P-wave propagation, Computers and Geotechnics, Vol: 86, Pages: 203-208, ISSN: 1873-7633
The numerical dissipation characteristics of the Newmark and generalised-α time-integration schemes are investigated for P-wave propagation in a fully saturated level-ground sand deposit, where higher frequencies than those for S-waves are of concern. The study focuses on resonance, which has been shown to be of utmost importance for triggering liquefaction due to P-waves alone. The generalised-α scheme performs well, provided that the time-step has been carefully selected. Conversely, the dissipative Newmark method can excessively damp the response, changing radically the computed results. This implies that a computationally prohibiting small time-step would be required for Newmark to provide an accurate solution.
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.
Goncalves Pedro AM, Ferreira D, Coelho PALF, et al., 2017, Modelling the influence of rock variability on geotechnical structures, 19th International Conference on Soil Mechanics and Geotechnical Engineering, Publisher: ICSMGE
The importance of variability in soils and rocks has been widely acknowledged and the object of numerous studies. Inthis paper, the impact of modelling rock variability is assessed by performing numerical analysis of the excavation of a deep circulartunnel in a rock mass characterised by a uniform and isotropic stress state. The variability is introduced in the analyses by consideringrandom fields of variable parameters generated using the Local Average Subdivision (LAS) method. The results of the performedanalyses are compared against the results of a traditional deterministic approach where uniform properties are considered, both interms of displacements around the excavation and forces in the tunnel lining, so that the influence of introducing variability can beassessed. Finally, a parametric study is carried out in order to evaluate the influence of the parameters adopted in the generation ofrandom fields, such as the standard deviation, the spatial correlation distance and the ratio of anisotropy.
Liu T, Aghakouchak A, Taborda DMG, et al., 2017, Advanced laboratory characterization of a fine marine sand from Dunkirk, France, 19th International Conference on Soil Mechanics and Geotechnical Engineering, Publisher: ICSMGE, Pages: 439-442
Dense fine marine sand is encountered at the Dunkirk ZIP Les Huttes test site located in northern France that has beenemployed extensively for research into pile behaviour. Laboratory testing of the sand is required to fully characterise site conditionsand determine parameter inputs for analysing the field pile experiments. This paper summarises some of the comprehensivelaboratory testing programmes undertaken to investigate the sand’s mechanical behaviour, including stress-strain relationships,stiffness and strength anisotropy, cyclic behaviour, and interface shear properties. The paper first reviews the site’s geotechnicalconditions and their potential variations over time. The stringent laboratory requirements that are necessary for the accuratemeasurement of shear stiffness, strength, and creep strains are then discussed, before presenting illustrative results regarding thesand’s small strain stiffness and time-dependent behaviour. The importance of reproducing site conditions and stress states are alsoaddressed in relation to integrating the laboratory research with field observations and analyses of both recent and historical pilingexperiments at the Dunkirk test site.
Coelho PALF, Azeiteiro RJN, Costa ALD, et al., 2017, Effects of earthquake-induced liquefaction: integrated research tools towards optimum reduction of society vulnerability, 19th International Conference on Soil Mechanics and Geotechnical Engineering
Azeiteiro RJN, Coelho PALF, Taborda DMG, et al., 2017, Critical state-based interpretation of the monotonic behaviour of Hostun sand, Journal of Geotechnical and Geoenvironmental Engineering, Vol: 143, Pages: 1-1, ISSN: 0733-9410
A series of bender element tests and drained and undrained monotonic triaxial compression and extension tests were performed on air-pluviated samples of Hostun sand. Samples were prepared to different initial void ratios, consolidated under various isotropic and anisotropic stress states, and sheared using different stress paths and a wide range of deformations to characterize the sand’s stress-strain response. The results suggest that the sand’s small-strain behavior essentially depends on the current void ratio and mean effective stress. Within the medium to large strain range, a state-parameter approach in conjunction with the critical-state framework can successfully predict the distinctive states of the sand’s monotonic response, namely the phase-transformation, peak-stress-ratio, and critical states. Furthermore, the data are used to examine a stress-dilatancy relationship often incorporated in constitutive models. The characterization presented herein aims at assisting the efficient calibration of numerical models and provides insight into this sand’s behavior, thus supporting the interpretation of results of physical modeling involving this sand. This paper highlights the importance of characterizing sand’s behavior over the full strain range and shows that accurate predictions of the critical state and small-strain stiffness are crucial to assess other aspects of the sand’s behavior.
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.
Azeiteiro RN, Coelho PALF, Taborda DMG, et al., 2016, Energy-based evaluation of liquefaction potential under non-uniform cyclic loading, Soil Dynamics and Earthquake Engineering, Vol: 92, Pages: 650-665, ISSN: 1879-341X
Uniform cyclic loading is commonly used in laboratory tests to evaluate soil resistance to earthquake-induced liquefaction, even if the cyclic stresses induced by earthquakes in the field are highly irregular. This paper discusses the use of stress and energy-based approaches to evaluate the liquefaction resistance of sand under irregular loading. Results of undrained cyclic triaxial tests including a large-amplitude singular peak loading cycle are presented and compared to those obtained using uniform loading. Although samples are subjected to loading patterns which would have been deemed equivalent by conventional stress-based methods, the number of cycles required to trigger liquefaction strongly depends on the amplitude and location of the peak within the loading history. Conversely, a unique relationship exists between the accumulation of dissipated energy per unit volume, computed using stress and strain measurements, and the observed residual pore water pressure build-up for all tests, throughout the entire cyclic loading application. This demonstrates that conventional laboratory tests using uniform loading conditions can be employed to determine liquefaction resistance if their interpretation is carried out based on energy principles.
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.
Tsaparli V, Kontoe S, Taborda D, et al., 2016, Vertical ground motion and its effects on liquefaction resistance of fully saturated sand deposits, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 472, Pages: 1-21, ISSN: 1364-5021
Soil liquefaction has been extensively investigated over the years with the aim to understand its fundamental mechanism and successfully remediate it. Despite the multi-directional nature of earthquakes, the vertical seismic component is largely neglected, as it is traditionally considered to be of much lower amplitude than the components in the horizontal plane. The 2010–2011 Canterbury earthquake sequence in New Zealand is a prime example that vertical accelerations can be of significant magnitude, with peak amplitudes well exceeding their horizontal counterparts. As research on this topic is very limited, there is an emerging need for a more thorough investigation of the vertical motion and its effect on soil liquefaction. As such, throughout this study, uni- and bidirectional finite-element analyses are carried out focusing on the influence of the input vertical motion on sand liquefaction. The effects of the frequency content of the input motion, of the depth of the deposit and of the hydraulic regime, using variable permeability, are investigated and exhaustively discussed. The results indicate that the usual assumption of linear elastic response when compressional waves propagate in a fully saturated sand deposit does not always hold true. Most importantly post-liquefaction settlements appear to be increased when the vertical component is included in the analysis.
Gawecka KA, Taborda DMG, Potts DM, et al., 2016, Effects of transient phenomena on the behaviour of thermo-active piles, 1st International Conference on Energy Geotechnics
Cui W, Gawecka KA, Taborda DMG, et al., 2016, Time-step constraints in transient coupled finite element analysis, International Journal for Numerical Methods in Engineering, Vol: 106, Pages: 953-971, ISSN: 0029-5981
In transient finite element analysis, reducing the time-step size improves the accuracy of the solution. However, a lower bound to the time-step size exists, below which the solution may exhibit spatial oscillations at the initial stages of the analysis. This numerical 'shock' problem may lead to accumulated errors in coupled analyses. To satisfy the non-oscillatory criterion, a novel analytical approach is presented in this paper to obtain the time-step constraints using the θ-method for the transient coupled analysis, including both heat conduction-convection and coupled consolidation analyses. The expressions of the minimum time-step size for heat conduction-convection problems with both linear and quadratic elements reduce to those applicable to heat conduction problems if the effect of heat convection is not taken into account. For coupled consolidation analysis, time-step constraints are obtained for three different types of elements, and the one for composite elements matches that in the literature. Finally, recommendations on how to handle the numerical 'shock' issues are suggested.
- Abstract
- Open Access Link
- Cite
- Citations: 30
Han B, Zdravkovic L, Kontoe S, et al., 2016, Numerical investigation of the response of the Yele rockfill dam during the 2008 Wenchuan Earthquake, Soil Dynamics and Earthquake Engineering, Vol: 88, Pages: 124-142, ISSN: 1879-341X
In this paper the seismic response of a well-documented Chinese rockfill dam, Yele dam, is simulated and investigated employing the dynamic hydro-mechanically (HM) coupled finite element (FE) method. The objective of the study is to firstly validate the numerical model for static and dynamic analyses of rockfill dams against the unique monitoring data on the Yele dam recorded before and during the Wenchuan earthquake. The initial stress state of the dynamic analysis is reproduced by simulating the geological history of the dam foundation, the dam construction and the reservoir impounding. Subsequently, the predicted seismic response of the Yele dam is analysed, in terms of the deformed shape, crest settlements and acceleration distribution pattern, in order to understand its seismic behaviour, assess its seismic safety and provide indication for the application of any potential reinforcement measures. The results show that the predicted seismic deformation of the Yele dam is in agreement with field observations that suggested that the dam operated safely during the Wenchuan earthquake. Finally, parametric studies are conducted to explore the impact of two factors on the seismic response of rockfill dams, i.e. the permeability of materials comprising the dam body and the vertical ground motion.
This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.