94 results found
Gawecka K, Taborda D, Potts D, et al., Finite element modelling of heat transfer in ground source energy systems with heat exchanger pipes, International Journal of Geomechanics, ISSN: 1532-3641
Araujo Santos LM, Coelho PALF, Ramos RC, et al., Characterisation of liquefiable sands using the hollow cylinder apparatus, XVII European Conference on Soil Mechanics and Geotechnical Engineering
Sailer E, Taborda D, Zdravkovic L, et al., Long-term thermal performance of a thermo-active retaining wall, XVII European Conference on Soil Mechanics and Geotechnical Engineering
Liu R, Taborda D, Gawecka K, et al., Computational study on the effects of boundary conditions on the modelled thermally induced axial stresses in thermo-active piles, XVII European Conference on Soil Mechanics and Geotechnical Engineering
Sailer E, Taborda DMG, Zdravkovic L, et al., 2019, Assessing the impact of vertical heat exchangers on the response of a retaining wall, 7th International Symposium on Deformation Characteristics of Geomaterials (IS-Glasgow 2019), Publisher: EDP Sciences, ISSN: 2267-1242
Shallow geothermal energy systems, e.g. borehole heat exchangers or thermo-active structures, provide sustainable space heating and cooling by exchanging heat with the ground. When installed within densely built urban environments, the thermo-hydro-mechanical (THM) interactions occurring due to changes in ground temperature, such as soil deformation and development of excess pore water pressures, may affect the mechanical behaviour of adjacent underground structures. This paper investigates the effects of vertical heat exchangers installed near a deep basement by performing fully coupled THM finite element analyses using the Imperial College Finite Element Program. Different heat exchanger configurations are considered and their influence on the response of the basement wall is assessed in two-dimensional plane strain analyses, where different methods of modelling the heat sources in this type of analysis are employed to evaluate their effect on the temperature field and the non-isothermal soil response.
Sailer E, Taborda D, Zdravkovic L, et al., 2019, Fundamentals of the coupled thermo-hydro-mechanical behaviour of thermo-active retaining walls, Computers and Geotechnics, Vol: 109, Pages: 189-203, ISSN: 0266-352X
Geotechnical structures can be employed to provide renewable and cost-effective thermal energy to buildings. To date, limited field studies regarding thermo-active retaining walls exist and therefore their mechanical response under non-isothermal conditions requires further research to comprehend their behaviour. This paper investigates the response of a hypothetical thermo-active diaphragm wall by performing finite element analysis to characterise in detail its short and long term response. The soil-structure interaction mechanisms arising from the coupled thermo-hydro-mechanical nature of soil behaviour are for the first time identified and shown to be complex and highly non-linear. Subsequently, simpler modelling approaches are used to isolate and quantify the impact of the various identified mechanisms on the design of thermo-active retaining walls. It is concluded that simpler approaches tend to overestimate structural forces developing due to temperature changes in the retaining wall, while severely underestimating the associated ground movements, which are highly influenced by the development of thermally-induced excess pore water pressures. Furthermore, the results suggest that the behaviour of thermo-active retaining walls is highly transient in nature, as a result of the high rates of heat transfer and pore water pressure dissipation under plane strain assumptions.
Tsaparli V, Kontoe S, Taborda D, et al., 2019, A case study of liquefaction: demonstrating the application of an advanced model and understanding the pitfalls of the simplified procedure, Géotechnique, ISSN: 0016-8505
The complexity of advanced constitutive models often dictates that their capabilities are only demonstrated in the context of model testing under controlled conditions. In the case of earthquake engineering and liquefaction in particular, this restriction is magnified by the difficulties in measuring field behaviour under seismic loading. In this paper, the well documented case of the Canterbury Earthquake Sequence in New Zealand, for which extensive field and laboratory data are available, is utilised to demonstrate the accuracy of a bounding surface plasticity model in fully-coupled finite element analyses. A strong motion station with manifestation of liquefaction and the second highest peak vertical ground acceleration during the Mw 6.2 February 2011 event is modelled. An empirical assessment predicted no liquefaction for this station, making this an interesting case for rigorous numerical modelling. The calibration of the model aims at capturing both the laboratory tests and the field measurements in a consistent manner. The characterisation of the ground conditions is presented, while, to specify the bedrock motion, the records of two stations without liquefaction are deconvolved and scaled to account for wave attenuation with distance. The numerical predictions are compared to both the horizontal and vertical acceleration records and other field observations, showing a remarkable agreement, also demonstrating that the high vertical accelerations can be attributed to compressional resonance. The results provide further insights into the underperformance of the simplified procedure.
Norambuena R, Tsaparli V, Kontoe S, et al., 2019, The effect of irregular seismic loading on the validity of the simplified liquefaction procedures, Obras y Proyectos, Pages: 42-50, ISSN: 0718-2805
Soil liquefaction has been one of the major hazards for civil engineering projects relating to earthquakes. The simplified liquefaction procedure which is used to assess liquefaction susceptibility in practice is still based on semi-empirical methods. These rely on the assumption that irregular seismic motions can be represented fully by an equivalent number of cycles of uniform stress amplitude, which is based on the peak acceleration measured at ground surface. Most methodologies used to calculate the equivalent number of cycles are based on Miner's damage concept developed for the fatigue analysis of metals. Several researchers have questioned the validity of this concept, as soils have a highly non-linear response. The present work investigates numerically the concept of the equivalent uniform amplitude cycles. Effective stress-based non-linear finite element analyses are performed with a modified bounding surface plasticity model that allows to realistically simulate liquefaction, reproducing the cyclic strength of sands accurately. The seismic response of a 15 m deep uniform level-ground sand deposit is simulated with full hydro-mechanical coupling to establish the benchmark extent of liquefaction zone. In parallel, the analyses are repeated assuming drained conditions to compute the irregular time-histories, which are then converted to an equivalent number of uniform amplitude cycles. The constant amplitude series are then applied in single element simple shear test simulations, with initial conditions those corresponding to the 7 m depth in the deposit. The results in terms of the predicted triggering of liquefaction are contrasted to the predictions of the fully coupled benchmark analyses at the corresponding depth to assess the validity of the Seed et al. (1975) methodology, based on Miner's cumulative damage concept.
Taborda D, Zdravkovic L, Potts DM, et al., Finite element modelling of laterally loaded piles in a dense marine sand at Dunkirk, Géotechnique, ISSN: 0016-8505
The paper presentsthedevelopment of a three-dimensional finite element model for pile tests in dense Dunkirk sand,conducted as part of the PISA project.The projectwas aimed at developing improved design methods for laterally loadedpiles, as used in offshore wind turbine foundations. The importance of the consistent and integrated interpretation of the soil data from laboratory and field investigations is particularly emphasised. The chosen constitutive model for sand is an enhanced version of the state parameter-based bounding surface plasticity model, which, crucially, is able to reproduce the dependency of sand behaviour on void ratio and stress level. The predictions from three-dimensional finite element analyses,performed before the field tests, show good agreement with the measured behaviour, proving the adequacy of the developed numerical model and the calibration process for the constitutive model. This numerical model directly facilitated the development of new soil reaction curves for use in Winkler-type design models for laterally loaded piles in natural marine sands.
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.
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.
© 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
© 2018 The Author(s) 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.
Zdravkovic L, Taborda D, Potts D, et al., Finite element modelling of laterally loaded piles in a stiff glacial clay till at Cowden, Géotechnique, ISSN: 0016-8505
The PISA project was a combined field testing/numerical modelling study with the aim ofdevelopingimproved design procedures for large diameter piles subjected to lateral loading. This paper describes the development ofa three-dimensional finite elementmodel for the medium-scale pile tests that were conducted in Cowden tillas part of the PISA work.The paper places particular emphasis on the consistent interpretation of the soil data determined from the available field and laboratory information.An enhancedversion of the modified Cam clay model was employedin the numerical analyses, featuring a non-linear Hvorslevsurface, a generalised shape for the yield and plastic potential surfaces in the deviatoric planeand a non-linear formulation for the elastic shear modulus.Three-dimensional finite element analyses were performed prior to the field tests.Excellent agreement between the measured and simulated behaviourfora range of pile geometrieswas observed, demonstrating the accuracy of the numerical model and the adequacy of the calibration process for theconstitutive model.The developed numerical modelconfirmed the premise of the PISA design method that site-specific ground characterisation and advanced numerical modelling candirectly facilitate the development of additionalsoil reaction curves for use in new design models for laterally loaded piles in a stiff clay till.
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
© 2018 The Authors 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.
Byrne B, McAdam RA, Burd HJ, et al., Monotonic laterally loaded pile testing in a stiff glacial clay till at Cowden, Géotechnique, ISSN: 0016-8505
This paper describes theresults obtained from a field testing campaign on laterally-loaded monopiles conducted at Cowden, UK, where the soil consists principally of aheavilyoverconsolidatedglacial till. These tests formed part of the PISA projecton the development of improved design methods for monopile foundations for offshore wind turbines. Results obtained for monotonic loading tests on piles of three different diameters (0.273m, 0.762m and 2.0m) are presented. The piles had length-to-diameter ratios (L/D) of between 3 and 10. Thetests includedthe application of monotonic loading incorporating periods of constant load to investigate creep effects,and investigations on the influence of loading rate. Data are presented on measured bending moments and inclinations induced in the piles. Inferred data on lateral displacements of the embedded section of the pilesare determined usingan optimisedstructural model. Thesefield data support the developmentof a new 1D modelling approach forthe design of monopile foundations for offshore wind turbines.They also form a unique database of field measurements in an overconsolidated clay, from lateral loading of piles at a vertical distance abovethe ground surface.
Zdravkovic L, Jardine R, Taborda DMG, et al., Ground characterisation for PISA pile testing and analysis, Géotechnique, ISSN: 0016-8505
This paper is the first of a set of linked publications on the PISA Joint Industry Research Project, which was concerned with the development of improved design methods for monopile foundations in offshore wind applications. PISA involved large-scale pile tests in overconsolidated glacial till at Cowden, north-east England,and in dense normally consolidated marine sand at Dunkirk, northern France. The paper presents the characterisationof the two sites, whichwas crucial to the design of the field experiments and advanced numerical modelling of the pile-soil interactions. The studies described, which had to be completed at an early stage of the PISA project, added new laboratory and field campaignsto historic investigations at both sites.Theyenabledanaccurate description ofsoilbehaviour from small strains to ultimate statesto be derived, allowing analyses to be undertaken that captured both the serviceability and limit statebehaviour of the test monopiles.
McAdam RA, Byrne BW, Houlsby GT, et al., Monotonic lateral loaded pile testing in a dense marine sand at Dunkirk, Géotechnique, ISSN: 0016-8505
Theresults obtained from a field testing campaignon laterally-loaded monopiles,conducted ata dense sand site inDunkirk, Northern Franceare described.These tests formed part of thePISA projecton the development of improved design methods for monopile foundations for offshore wind turbines. Results obtained frommonotonic loading tests on piles of three different diameters (0.273m, 0.762m and 2.0m) are presented. The piles had aspectratios (L/D) of between 3 and 10. Thetests consisted principally of the application of monotonic loads,incorporating periods of held constant load to investigate creep effects.The influence of loading ratewas also investigated. Data are presented on the overall load displacement behaviour of each of the test piles. Measured data on bending moments and inclinations induced in the pilesare also provided.Inferences are made forthe displacements in the embedded length of the piles. Thesefield data will support the development of a new 1D modelling approach forthe design of monopile foundations for offshore wind turbines.They also form a unique database of field measurements in a dense sand, from lateral loading of piles at a vertical distance abovethe ground surface.
Burd HJ, Beuckelaers WJAP, Byrne BW, et al., New data analysis methods for instrumented medium scale monopile field tests, Géotechnique, ISSN: 0016-8505
The PISA Joint Industry Research Project was concerned with the development of improved design methods for monopile foundations in offshore wind applications. PISA involved large-scale pile tests in overconsolidated glacial till at Cowden, north-east England, andin dense normally consolidated marine sand at Dunkirk, northern France. This paper describes the experimental set up for pile testing, with unique features of load-application mechanisms and built-in fibre optic strain gauges.New procedures are describedfor the interpretation of pile loading data, and specifically for providing precise interpretation of pile displacements.
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
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
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, 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
© 2017 The Authors 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.
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.
Byrne BW, McAdam RA, Burd H, et al., 2017, 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"
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