Publications
140 results found
Pedro A, Santos P, Araujo Santos L, et al., 2016, Potential sources of error in bender element testing of reconstituted sand samples, XV Congresso Nacional de Geotecnia, Publisher: Congresso Nacional de Geotecnia
Santos P, Pedro A, Coelho P, et al., 2016, Characterisation of the initial stiffness of Coimbra Sand - Batch I, XV Congresso Nacional de Geotecnia, Publisher: Congresso Nacional de Geotecnia
Araujo Santos L, Coelho PALF, Taborda DMG, 2016, Influence of the variability of Coimbra Sand index properties on its permeability, XV Congresso Nacional de Geotecnia, Publisher: Congresso Nacional de Geotecnia
Cui W, Gawecka KA, Potts DM, et al., 2016, Numerical analysis of coupled thermo-hydraulic problems in geotechnical engineering, Geomechanics for Energy and the Environment, Vol: 6, Pages: 22-34, ISSN: 2352-3808
Ground sources energy systems, such as open-loop systems, have been widely employed in recent years due to their economic and environmental benefits compared to conventional heating and cooling systems. Numerical modelling of such geothermal system requires solving a coupled thermo-hydraulic problem which is characterised by a convection-dominated heat transfer which can be challenging for the Galerkin finite element method (GFEM). This paper first presents the coupled thermo-hydraulic governing formulation as well as the coupled thermo-hydraulic boundary condition, which can be implemented into a finite element software. Subsequently, the stability condition of the adopted time marching scheme for coupled thermo-hydraulic analysis is established analytically. The behaviour of highly convective problems is then investigated via a series of analyses where convective heat transfer along a soil bar is simulated, with recommendations on the choice of an adequate discretisation with different boundary conditions being provided to avoid oscillatory solutions. Finally, the conclusions from the analytical and numerical studies are applied to the simulation of a boundary value problem involving an open-loop system, with the results showing good agreement with an approximate solution. The main objective of this paper is to demonstrate that the GFEM is capable of dealing with highly convective geotechnical problems.
Taborda DMG, Potts DM, Zdravkovic L, 2016, On the assessment of energy dissipated through hysteresis in finite element analysis, Computers and Geotechnics, Vol: 71, Pages: 180-194, ISSN: 0266-352X
The accurate reproduction of the hysteretic behaviour exhibited by soils under cyclic loading is a crucial aspect of dynamic finite element analyses and is typically described using the concept of damping ratio. In this paper, a general algorithm is presented for assessing the damping ratio simulated by any constitutive model based on the registered behaviour in three-dimensional stress-strain space. A cyclic nonlinear elastic model capable of accurately reproducing a wide range of features of soil behaviour, including the variation of damping ratio with deformation level, is chosen to illustrate the capabilities of the proposed algorithm. The constitutive model is described and subsequently employed in two sets of finite element analyses, one involving the dynamic response of a sand deposit subjected to different types of motion and another focussing on the simulation of a footing subjected to cyclic vertical loading. The application of the presented algorithm provides insight into the processes through which energy is dissipated through hysteresis.
Azeiteiro RJN, Coelho PALF, Taborda DMG, et al., 2015, Dissipated energy in undrained cyclic triaxial tests, 6th International Conference on Earthquake Geotechnical Engineering
Energy-based methods are an emerging tool for the evaluation of liquefaction potential. Thesemethods relate excess pore water pressure build-up to seismic energy dissipated per unit volume.Further development of these methods require their validation through laboratory testing. In thispaper, a comprehensive study of energy dissipated during cyclic triaxial tests is undertaken. Resultsof undrained cyclic triaxial tests performed on air-pluviated samples of Hostun sand prepared withdifferent initial densities and subjected to several confining pressures and loading amplitudes arepresented. The energy dissipated per unit volume is estimated from the experimental results andcorrelated to the generated excess pore water pressure. The correlation between those quantitiesappear to be independent of the initial relative density of the sample, isotropic consolidation pressureand cyclic stress ratio used in the tests. Moreover, the relationship between observed doubleamplitudeaxial strain and the energy dissipated per unit volume is examined. It is found that thisrelationship is greatly dependent on the relative density of the sample.
Tsaparli V, Kontoe S, Taborda D, et al., 2015, Numerical Modelling of Multi-directional Earthquake Loading and Its Effect on Sand Liquefaction, 6th International Conference on Earthquake Geotechnical Engineering
Earthquakes generate multi-directional ground motions, two components in the horizontal direction and one in the vertical. Nevertheless, the effect of vertical motion on site response analysis has not been the object of extensive research. The 2010/2011 Canterbury sequence of seismic events in New Zealand is a prime example among other earlier field observations strongly corroborating that the vertical acceleration may have a detrimental effect on soil liquefaction. Consequently, this study aims to provide insight into the influence of the input vertical motion on sand liquefaction. For this reason, two ground motions, with very different frequency contents, are used as the input excitations. Non-linear elasto-plastic plane strain fully coupled effective stress-based finite element analyses are conducted to investigate the occurrence of liquefaction in a hypothetical fully saturated Fraser River Sand deposit. The results indicate that the frequency content of the input motion is of utmost importance for the response of sands to liquefaction when the vertical loading is considered.
Byrne BW, McAdam RA, Burd HJ, et al., 2015, Field testing of large diameter piles under lateralloading for offshore wind applications, 16th European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: ICE Publishing
Offshore wind power in the UK, and around Europe, has the potential to deliver significant quantities of renewable energy. The foundation is a critical element in the design. The most common foundation design is a single large diameter pile, termed a monopile. Pile diameters of between 5m and 6m are routinely used, with diameters up to 10m or more, being considered for future designs. Questions have been raised as to whether current design methods for lateral loading are relevant to these very large diameter piles. To explore this problem a joint industry project, PISA, co-ordinated by DONG Energy and the Carbon Trust, has been established. The aim of the project is to develop a new design framework for laterally loaded piles based on new theoretical developments, numerical modelling and bench-marked against a suite of large scale field pile tests. The project began in August 2013 and is scheduled to complete during 2015. This paper briefly outlines the project, focusing on the design of the field testing. The testing involves three sizes of pile, from 0.27m in diameter through to 2.0m in diameter. Two sites will be used; a stiff clay site and a dense sand site. Tests will include monotonic loading and cyclic loading. A suite of site investigation will be carried out to aid interpretation of the field tests, and will involve in-situ testing, standard laboratory testing and more advanced laboratory testing.
Dubasaru V, Zdravkovic L, Taborda DMG, et al., 2015, Influence of pile raft stiffness on building behaviour in a tunnel-pile clash scenario, 16th European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: ICE Publishing
In a modern urban environment, the underground space becomes increasingly congested due to the high value of the land that forces the new infrastructure projects to be constructed deeper into the ground. For each new project, the potential of both expected and unexpected clashes between new tunnel alignments and the foundations of the existing structures becomes more probable. However, to date, the research on tunnel-pile clashes has been scarce. In the current study, the effects of such a situation are studied by carrying out finite element analyses for a scenario that is typical in the London ground profile. A parametric study was conducted to investigate the influence of the pile raft bending stiffness on the building settlement and the change in piles’ axial forces. It is shown that an increased raft bending stiffness helps to transfer the load from the trimmed pile to the adjacent piles, thus reducing the settlement of the trimmed pile. In the process of tunnel excavation, the pile settles due to the soil-induced downdrag and the loss of both its base and part of its shaft capacity. It is concluded that the tunnel-pile clash has a large impact on the surface structure, piles and tunnel itself.
Martinez Calonge D, Gawecka KA, Zdravkovic L, et al., 2015, Development of a new temperature-controlled triaxial apparatus for saturated soils, 16th European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: ICE Publishing
In recent years, the study of the Thermo-Hydro -Mechanical (THM) behaviour of geomaterials has become a growing area in geotechnical engineering due to the increasing interest in energy geostructures and underground nuclear waste disposal. Advanced laboratory testing is essential in gaining an understanding of the THM behaviour of soils and solving these complex geomechanical problems. This paper describes the development of a new triaxial apparatus at the Imperial College Geotechnics Laboratory, capable of testing saturated soils at temperatures up to 85°C and pressures up to 800kPa. In order to aid its design, numerical analysis of the thermal response of the cell was conducted using the Imperial College Finite Element Program (ICFEP) with its newly developed THM capabilities.
Cui W, Gawecka KA, Potts DM, et al., 2015, Numerical modelling of open-loop ground source energy systems, 16th European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: ICE Publishing
The environmental and economic benefits of utilising the ground for extracting and storing heat have been known for a long time. However, only recently have government sustainability policies and rising energy prices encouraged the use of this renewable energy resource. In open-loop systems water is abstracted from one well and re-injected into another after exchanging energy with a building’s heating/cooling system using a heat pump. In order to guarantee a good performance of the system, it is fundamental that the possibility of thermal breakthrough occurring is minimised, i.e. that the temperature of the water being abstracted remains unaffected by the injection of warmer/cooler water at the other well. In this paper, the Imperial College FiniteElement Program (ICFEP), which is capable of simulating fully coupled thermo-hydro-mechanical behaviour of porous materials, was used to perform two-dimensional analyses of open-loop ground source heat systems. The parametric studies carried out highlight the relative impact on the occurrence of thermal break-through of the hydraulic ground conditions andthe geometric characteristics of the system, providing an invaluable insight into possible improvements to the current design procedure.
Mantikos V, Tsiampousi A, Taborda DMG, et al., 2015, Numerical interpretation of the coupled hydromechanical behaviour of expansive clays in constant volume column tests, 16th European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: ICE Publishing
Experimental and numerical studies of the behaviour of expansive clays have been attracting increasing interest, due to their good sealing properties, which render them ideal to be used as engineered barriers (buffers) in both active (e.g. nuclear) and non-active waste disposal facilities. Both large scale and aboratory scaled experiments indicate that the sealing capabilities of the buffer are fundamentally governed by its volumetric behaviour when wetted. In this paper, a constant volume column infiltration test, perform ed under is othermal conditions on compacted MX80 bentonite, is modelled numerically using the Imperial College Finite Element Program (ICFEP). A modified version of the Barcelona Basic Model is used to simulate the behaviour of the buffer, which is inherently partly saturated. The numerical results agree well with the observed experimental data, especially with regard to the advancement of the wetting front. A detailed interpretation of the computed evolutions with time of stress state, suction and void ratio at different elevations along the sample’s axis is carried out, providing insight into the complex hydro-mechanical response of the buffer during the experiment. Indeed, even though the overall volume of the sample was kept constant, a region of localised dilation, which induced the contraction of other zones of the material, was observed to advance simultaneously with the wetting front along the height of the soil column.
Tsaparli V, Kontoe S, Taborda D, et al., 2015, Numerical investigation of the effect of the irregular nature of seismic loading on the liquefaction resistance of saturated sand deposits, SECED 2015 Conference: Earthquake Risk and Engineering towards a Resilient World
Doherty P, Igoe D, Murphy G, et al., 2015, Field validation of fibre Bragg grating sensors for measuring strain on driven steel piles, GEOTECHNIQUE LETTERS, Vol: 5, Pages: 74-79, ISSN: 2049-825X
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Sailer E, Taborda DMG, Keirstead J, 2015, Assessment of Design Procedures for Vertical Borehole Heat Exchangers, 40th Workshop on Geothermal Reservoir Engineering
The use of ground source energy systems is a well-established method to provide low cost heating to buildings, diversify the energy mix and help meeting increasingly stricter sustainability targets. However, considerable uncertainties remain over their efficient design, with several standards, guidelines and manuals being proposed over the last few years. This paper aims at providing insight into the implications to the design of a vertical borehole heat exchanger of the adoption of different design procedures. The hypothetical case of a typical dwelling located in London, UK, is analysed in order to highlight the impact on the final design of the chosen methodology. Moreover, a parametric study using an analytical design procedure was performed to point out the influence of various factors, such as borehole characteristics and thermal properties of the ground. It is shown that there are considerable discrepancies between design methods and that uncertainties in some input parameters, such as the thermal properties of the ground, which for relatively small systems are often selected from tables rather than measured in situ, may have a substantial influence on the length of borehole required.
Byrne BW, McAdam R, Burd HJ, et al., 2015, New design methods for large diameter piles under lateral loading for offshore wind applications, Frontiers in Offshore Geotechnics III, Pages: 705-710, ISBN: 9781138028487
Offshore wind turbines are typically founded on single large diameter piles, termed monopiles. Pile diameters of between 5mand 6mare routinely used, with diameters of up to 10 m, or more, being considered for future designs. There are concerns that current design approaches, such as the p - y method, which were developed for piles with a relatively large length to diameter ratio, may not be appropriate for large diameter monopiles. A joint industry project, PISA (PIle Soil Analysis), has been established to develop new design methods for large diameter monopiles under lateral loading. The project involves three strands of work; (i) numerical modelling; (ii) development of a new design method; (iii) field testing. This paper describes the framework on which the new design method is based. Analyses conducted using the new design method are compared with methods used in current practice.
Zdravković L, Taborda DMG, Potts DM, et al., 2015, Numerical modelling of large diameter piles under lateral loading for offshore wind applications, Pages: 759-764
There is currently a significant focus on developing offshore wind power in the UK and Europe. The most common foundation type for wind turbines is a single large diameter pile, termed a monopile, on which the turbine is located. As the diameter of such piles is envisaged to increase in future installations, there are concerns that current design methods are not applicable. To explore this problem, the joint industry project PISA has been established, with the aim to develop a new design framework for laterally loaded piles utilised in the offshore wind industry, based on new theoretical developments, numerical modelling and large scale field pile testing. This paper presents an overview of numerical modelling undertaken as part of the project.
Byrne BW, McAdam R, Burd HJ, et al., 2015, New design methods for large diameter piles under lateral loading for offshore wind applications, Pages: 705-710
Offshore wind turbines are typically founded on single large diameter piles, termed monopiles. Pile diameters of between 5mand 6mare routinely used, with diameters of up to 10 m, or more, being considered for future designs. There are concerns that current design approaches, such as the p − y method, which were developed for piles with a relatively large length to diameter ratio, may not be appropriate for large diameter monopiles. A joint industry project, PISA (PIle Soil Analysis), has been established to develop new design methods for large diameter monopiles under lateral loading. The project involves three strands of work; (i) numerical modelling; (ii) development of a new design method; (iii) field testing. This paper describes the framework on which the new design method is based. Analyses conducted using the new design method are compared with methods used in current practice.
Zdravković L, Taborda DMG, Potts DM, et al., 2015, Numerical modelling of large diameter piles under lateral loading for offshore wind applications, Frontiers in Offshore Geotechnics III, Pages: 759-764, ISBN: 9781138028487
There is currently a significant focus on developing offshore wind power in the UK and Europe. The most common foundation type for wind turbines is a single large diameter pile, termed a monopile, on which the turbine is located. As the diameter of such piles is envisaged to increase in future installations, there are concerns that current design methods are not applicable. To explore this problem, the joint industry project PISA has been established, with the aim to develop a new design framework for laterally loaded piles utilised in the offshore wind industry, based on new theoretical developments, numerical modelling and large scale field pile testing. This paper presents an overview of numerical modelling undertaken as part of the project.
Cui W, Gawecka KA, Potts DM, et al., 2015, Investigations on numerical analysis of coupled thermo-hydraulic problems in geotechnical engineering, International Symposium on Energy Geotechnics (1st.: 2015: Barcelona)
Taborda DM, Zdravkovic L, Kontoe S, et al., 2014, Computational study on the modification of a bounding surface plasticity model for sands, Computers and Geotechnics, Vol: 59, Pages: 145-160, ISSN: 0266-352X
The accurate simulation of complex dynamic phenomena requires the availability of advanced constitutive models capable of simulating a wide range of features of soil behaviour under cyclic loading. One possible strategy is to improve the capabilities of existing bounding surface plasticity models, as this framework is characterised by its modularity and flexibility. As a result, specific components of the formulation of this type of model may be adjusted to improve the reproduction of any aspect of soil behaviour deemed essential to the problem being analysed. In this paper, a series of computational studies are performed in order to establish the impact of expanding a bounding surface plasticity model for sands on its modelling capabilities and to suggest ways of mitigating the associated increase in complexity. Changes to three distinct aspects of the selected constitutive model are examined: the shape of the Critical State Line in p′ − e space, the expression used for calculating the hardening modulus and the form of the yield surface. It is shown that the introduced changes have the potential to increase significantly the ability to control how certain aspects of soil response, such as degradation of stiffness and flow liquefaction with limited deformation, are reproduced by the model. Moreover, this paper presents a systematic approach to the expansion of this type of constitutive model, establishing how alterations to the formulation of a model may be assessed in terms of improved accuracy and potential benefits.
Araujo Santos LM, Coelho PALF, Taborda DM, 2014, Validation of the use of the Hollow Cylinder Apparatus in the study of the behaviour of sandy soils, XIV Congresso Nacional de Geotecnia (in Portuguese)
Hughes T, Taborda DMG, Zdravkovíc L, et al., 2014, Predicting the settlement of footings on sand using a bounding surface plasticity model, Delft, 8th European Conference on Numerical Methods in Geotechnical Engineering, NUMGE 2014, Publisher: Taylor and Francis - Balkema, Pages: 675-680
Despite having been the subject of extensive research, the prediction of load-settlement response of footings on sand deposits currently yields relatively poor and unreliable results. This paper aims at providing insight into this problem by examining the ability of a bounding surface plasticity model to predict the response of four distinct footings on Perth sand, which were tested as part of an international prediction event. Given the complexity of the chosen constitutive model and the relatively limited ground information available, emphasis is placed on the calibration procedure. In particular, the contributions of specific components of the constitutive model, such as the ability to simulate stiffness degradation at small deformation levels and the adopted formulation for the plastic hardening modulus, are investigated in a parametric study. Axisymmetric finite element analyses demonstrate that the response of the footings measured in the field is adequately simulated using the selected constitutive model. © 2014 Taylor & Francis Group.
Taborda DMG, Tsiampousi A, 2014, Development of a hysteretic soil-water retention model using an optimisation technique, Sydney, NSW, 6th International Conference on Unsaturated Soils, UNSAT 2014, Publisher: Taylor and Francis - Balkema, Pages: 965-971
Experimental testing has shown that unsaturated soils exhibit hysteretic retention behaviour upon cycles of drying and wetting. This paper describes the general formulation of a hysteretic soilwater retention curve model, according to which the shape of the scanning paths is defined based on the relative positions of the current point, its projection on the corresponding primary curve, designated as image point, and the position at which the scanning path was initiated. However, rather than prescribing an analytical expression relating the hydraulic response at the current point with that at the image point, a Hill-Climbing algorithm is employed to characterise this component of the model. The use of this optimisation technique enables a pattern to be established based on experimental data, which is then converted into the analytical formulation of the model. The resulting expression is shown to accurately reproduce the experimentally observed behaviour. © 2014 Taylor & Francis Group.
Measham PG, Taborda DMG, Zdravković L, et al., 2014, Numerical simulation of a deep excavation in London Clay, Delft, 8th European Conference on Numerical Methods in Geotechnical Engineering, NUMGE 2014, Publisher: Taylor and Francis - Balkema, Pages: 771-776
The requirement for accurate modelling of the small-strain stiffness behaviour of soils in numerical analysis has been driven by the need to establish serviceability limit states for geotechnical structures. A common approach when tackling this problem is to employ a non-linear elastic constitutive model coupled with an appropriate failure criterion. The latter establishes the shear strength of the material and allows the evaluation of plastic deformations at large strains, while the former typically reproduces the effect of stress and strain levels on the shear and bulk stiffness of the soil. This paper evaluates distinct strategies for reproducing the stiffness of a material within the context of a new small-strain stiffness model. After introducing the constitutive model and describing its key features, a procedure to determine its parameters is proposed and demonstrated for London Clay. Subsequently, the relative impact of the different methods of interpreting the effect of changes in strain path direction on the stiffness of the material is investigated by performing finite element analyses of a deep excavation in London Clay. © 2014 Taylor & Francis Group, London.
Araujo Santos LM, Coelho PALF, Taborda DM, 2013, Local and Global Instrumentation in Hollow Cylinder Testing: Redundancy or Accuracy, International Conference on Engineering
Coelho PALF, Azeiteiro RJN, Marques VD, et al., 2013, Challenges to the laboratory evaluation of field liquefaction resistance, 18th International Conference on Soil Mechanics and Geotechnical Engineering, Pages: 1459-1462
Soil liquefaction is one of the most feared phenomena in Geotechnical Earthquake Engineering, due to the serious damage it can cause to modern societies in seismically active regions. Even if significant progress has been achieved in the laboratory evaluation of liquefaction resistance of sandy soils, considerable challenges still remain. These mostly result from the complex and variable nature of earthquakes, which apply non-uniform and multidirectional cyclic loading to soils and is therefore difficult to reproduce in laboratory testing. This paper aims at discussing the impact of the loading conditions imposed during soil testing. In particular, the effects of using uniform, axial and unidirectional cyclic loading are considered. The results show that the existence of a singular peak load of larger amplitude considerably affects the number of cycles required to liquefy the soil, with the location of that peak being of upmost importance for the evaluation of liquefaction resistance. Also, it was found that the loading mode affects the cyclic response of the soil, with fewer cycles being necessary to liquefy the soil under radial cyclic loading. Moreover, the liquefaction resistance of sand is substantially reduced when multidirectional loading is used.
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.
Araujo Santos LM, Coelho PALF, Taborda DMG, 2012, The use of the Hollow Cylinder Apparatus in the characterisation of liquefaction-related phenomena in Coimbra Sand, XIII Congresso Nacional de Geotecnia (in Portuguese)
The study of liquefaction phenomena is frequently based on laboratory tests, in which samples of sand are subjected to a state of stress that is distinct from the state induced by earthquakes. When compared to conventional cylindrical samples, the study of hollow cylinder specimens subjected to cyclic torsional shear allows a better approximation of the real situation. This paper highlights the main features andcapabilities of the Hollow Cylinder Apparatus (HCA) in the study of samples of saturated sand subjected to generalised undrained loading, detailing the techniques employed in the preparation of samples and characteristics of the equipment available at the Geotechnical Laboratory of the University of Coimbra.Additionally, this paper describes preliminary tests conducted on samples of Coimbra sand, which has been the object of an extensive characterisation program carried out at the aforementioned laboratory.
Guerreiro P, Kontoe S, Taborda D, 2012, Comparative study of stiffness reduction and damping curves, 15th World Conference on Earthquake Engineering
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