Publications
486 results found
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.
Indraratna B, Kan ME, Potts DM, et al., 2016, Analytical solution and numerical simulation of vacuum consolidation by vertical drains beneath circular embankments, Computers and Geotechnics, Pages: 83-96, ISSN: 0266-352X
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.
Cui W, Gawecka KA, Taborda DMG, et al., 2015, Time-step constraints in transient coupled finite element analysis, International Journal for Numerical Methods in Engineering, Vol: 106, Pages: 953-971, ISSN: 1097-0207
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.
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.
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.
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.
Summersgill FC, Kontoe S, Potts DM, 2015, Finite element investigation of vertical stabilisation piles in a stiff clay excavated slope using a nonlocal strain softening model, XVI ECSMGE Geotechnical Engineering for Infrastructure and Development
Colombero R, Kontoe S, Foti S, et al., 2015, Numerical modelling of wave attenuation through soil, XVI ECSMGE Geotechnical Engineering for Infrastructure and Development
Standing JR, Potts DM, Vollum R, et al., 2015, Investigating the effect of tunnelling on existing tunnels, Underground Design and Construction Conference, Publisher: IOM3, Pages: 301-312
A major research project investigating the effect of tunnelling on existing tunnels has beencompleted at Imperial College London. This subject is always of great concern during theplanning and execution of underground tunnelling works in the urban environment. Many citiesalready have extensive existing tunnel networks and so it is necessary to construct new tunnels ata level beneath them. The associated deformations that take place during tunnelling have to becarefully assessed and their impact on the existing tunnels estimated. Of particular concern is theserviceability of tunnels used for underground trains where the kinematic envelope must not beimpinged upon. The new Crossrail transport line under construction in London passes beneathnumerous tunnels including a number of those forming part of the London Underground network
Yu J, Standing J, Vollum R, et al., 2015, Stress and Strain monitoring at Tottenham Court Road Station, London, UK, Proceedings of the ICE - Structures and Buildings, Pages: 107-117, ISSN: 0965-0911
The redevelopment of Tottenham Court Road Underground Station started in 2011 as part of the Tube Upgrade Plan to improve and increase the capacity of the existing facility. The plan is to upgrade the station by 2016 to meet an estimated demand of more than 200,000 journeys per day once Crossrail is built. During April to November 2011, major structural work was carried out on the Northern Line platform tunnels as part of the station upgrade. This included removing grey cast iron tunnellining segments on the platform side to allow for construction of new cross passages to improve access to the platforms. The upgrade works provided an opportunity to trial in-tunnel instrumentation prior to implementation in other London Underground (LUL) tunnels which interface with the Crossrail project. Mechanical and electrical resistance strain gauges were installed on tunnel segments to make discrete measurements of changes in strain due to unloading as the segments were removed from the tunnel rings. Linear variable differential transformer type displacement transducers were installed to make continuous measurements of the opening and closing of circumferential and longitudinal joints on trackside segments which are left insitu and affected by adjacent excavations. This paper describes the installation process and highlights the lessons learnt for future applications. The insitu strain measurements are presented and compared to the expected response based on laboratory tests conducted on grey cast iron tunnel segments in the 1970s. The changes in strain measured by both types of strain gauges agreed well with the estimated changes assuming full overburden unloading.
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
Colombero R, Kontoe S, Foti S, et al., 2015, Numerical modelling of drop load tests, Soil Dynamics and Earthquake Engineering, Vol: 77, Pages: 279-289, ISSN: 1879-341X
Freitas TMB, Potts DM, Zdravkovic L, 2015, Numerical study on the response of two footings at Bothkennar research site, Geotechnique, Vol: 65, Pages: 155-168, ISSN: 1021-8637
This paper presents a numerical study of the performance of two instrumented surface footings at the Bothkennar Clay research site in the UK. Footing A was loaded to failure over 4 days, reaching a net bearing capacity of qr = 138 kPa; footing B was loaded to 89 kPa, at an identical loading rate, and left to consolidate under maintained load for about 11 years. The preloaded footing was then loaded to failure over 3 days, reaching qr = 204 kPa. The increase in bearing capacity was significantly larger than that expected due to consolidation effects alone, and it is anticipated that the occurrence of creep and other ageing processes may have played a major role in the observed response. The complete loading history of the two footings is simulated by means of coupled axi-symmetric finite-element analyses in which the foundation soil is described using an elastic–viscoplastic model that mimics isotach viscosity. The ground profile and the model parameters are derived based on the extensive laboratory and field test data available in the literature. The numerical analyses are able to describe accurately the footings behaviour during first loading, the development of delayed settlement under maintained load and the increase in bearing capacity due to preloading. The paper emphasises various issues regarding the application of elastic–viscoplastic models to model boundary value problems in conditions close to failure.
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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- Citations: 35
Schroeder FC, Merritt AS, Sørensen KW, et al., 2015, Predicting monopile behaviour for the godewind offshore wind farm, Pages: 735-740
Monopiles used as foundations for offshore wind turbines often have very large diameters and low length to diameter (L/D) ratios. Their response to lateral and moment loading is often modelled based on the API p-y method, although this was developed for slender piles with relatively small diameters. Theoretical studies and field monitoring have shown that this method may not accurately describe monopile behaviour, in particular the lateral load-displacement response for serviceability conditions. This paper presents advanced 3D Finite Element (FE) analyses undertaken during design of the GodeWind offshore wind farmat a sand dominated site in the German North Sea. Two turbine locations with different soil profiles and varying monopile L/D ratios were analysed to compare the load-displacement and moment-rotation curves with the API p-y method. The FE analyses predict a stiffer response of the monopile from initial loading until the design ULS condition, which is consistent with field experience.
Kovacevic N, Menkiti CO, Long M, et al., 2015, Finite element analyses of a cantilever wall in Dublin Boulder Clay, Pages: 3983-3988
Over the last decade many deep excavations were constructed in Dublin Boulder Clay (DBC). Supports ranged from propped temporary retaining walls for 25m digs to permanent cantilevers of lower height. This paper presents finite element back-analysis of an ambitious 7.5m cantilever contiguous pile wall for a residential development. Wall movements were monitored during construction and for more than 9 years afterwards enabling a detailed comparison to be made between prediction and measurements. The DBC is characterised by a form of the Modified Cam-Clay model. Model parameters were derived from an extensive set of data now available. The long monitoring records allowed verification of the predictions after significant pore pressure dissipation. Wall movements were well reproduced in both the short and medium-term. The results give an insight into competing mechanisms and associated long-term predictions provide additional confidence in the design and use of high cantilever walls for permanent structures in DBC.
Grammatikopoulou A, Schroeder FC, Kovacevic N, et al., 2015, Stiffness anisotropy and its effect on the behaviour of deep excavations, Pages: 3875-3880
This paper examines the effect of stiffness anisotropy on the behaviour of a deep excavation in a stiff plastic clay, such as. London Clay. A cross anisotropic non-linear elasto-plastic model which has previously been validated against advanced laboratory tests is used to simulate the London Clay. The predictions of this anisotropic model are compared with those of an isotropic model which has been used extensively in the past to simulate the behaviour of such clays. Three propping systems are considered, a single-propped wall, a double-propped wall and a multi-propped wall. The study examines the effect of stiffness anisotropy on the patterns of behaviour both in the short and long term.
Standing JR, Potts DM, Vollum R, et al., 2015, Research into the effect of tunnelling on existing tunnels, Pages: 515-520
Increasing demands for providing transport systems in the urban environment has led to many tunnelling projects being undertaken worldwide. Many of the cities where new tunnels are to be constructed already have a comprehensive underground network of tunnels for both transport and services. New tunnels often have to be aligned beneath these and frequently there are concerns that their construction may cause unacceptable deformations of Ibe existing tunnels, potentially hindering their serviceability and in the extreme threatening their stability. The Crossrail project, currently underway in London, involves tunnelling beneath numerous existing tunnels. It therefore has provided a great opportunity to study this complex boundary value problem. This paper describes the philosophy behind a comprehensive research project, run in conjunction with the Crossrail construction, which has an emphasis on the response of older tunnels lined with grey cast iron segments. There is a focus on how the Central Line tunnels responded to new twin tunnel construction beneath them. The five main strands of the research arc: field monitoring within and around the existing tunnels; numerical analyses of the field conditions; structural testing of a half-scale grey cast iron segmental ring; numerical analyses of the ring and two-segment tests performed; advanced laboratory testing of London Clay samples taken during installation of field instrumentation. These activities link into each other. Some preliminary results are presented and the main finding to date arc summarised.
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.
Schroeder FC, Merritt AS, Sørensen KW, et al., 2015, Predicting monopile behaviour for the Gode Wind offshore wind farm, Frontiers in Offshore Geotechnics III, Pages: 735-740, ISBN: 9781138028487
Monopiles used as foundations for offshore wind turbines often have very large diameters and low length to diameter (L/D) ratios. Their response to lateral and moment loading is often modelled based on the API p-y method, although this was developed for slender piles with relatively small diameters. Theoretical studies and field monitoring have shown that this method may not accurately describe monopile behaviour, in particular the lateral load-displacement response for serviceability conditions. This paper presents advanced 3D Finite Element (FE) analyses undertaken during design of the Gode Wind offshore wind farmat a sand dominated site in the German North Sea. Two turbine locations with different soil profiles and varying monopile L/D ratios were analysed to compare the load-displacement and moment-rotation curves with the API p-y method. The FE analyses predict a stiffer response of the monopile from initial loading until the design ULS condition, which is consistent with field experience.
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, 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, 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.
Yu J, Standing JR, Potts DM, et al., 2015, Tunnelling induced strains and deformations at Central Line, Crossrail Project infrastructure design and construction, Publisher: ICE Publishing, Pages: 499-518
Yu J, Potts DM, Standing JR, et al., 2015, In situ stress strain measurement at Tottenham Court Road platform tunnel, Crossrail Project infrastructure design and construction, Publisher: ICE Publiahing, Pages: 271-288
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)
Kontoe S, Avgerinos V, Potts DM, 2014, Numerical validation of four analytical solutions and their use for equivalent-linear seismic analysis of circular tunnels, Soil Dynamics and Earthquake Engineering, Vol: 66, Pages: 206-219
The first part of this paper presents an extensive validation of four analytical solutions for the seismic design of circular tunnels. The validation is performed with a quasi-static Finite Element (FE) model which conforms to the assumptions of the analytical solutions. Analyses are performed for a wide range of flexibility ratios, slippage conditions at soil-lining interface, assuming both drained and undrained behaviour. Based on the numerical predictions the relative merits of the considered analytical solutions are discussed and recommendations are given for their use in design. The second part of this paper explores the use of equivalent linear soil properties in analytical solutions as an approximate way of simulating nonlinearity. The results of equivalent linear site response analyses are used as an input for the analytical solutions. The comparison of the analytical predictions with nonlinear numerical analysis results is very satisfactory. The results of this study suggest that analytical solutions can be used for preliminary design using equivalent linear properties and the corresponding compatible strain as an approximate way of accounting for nonlinear soil response.
Martinez R, Schroeder F, Potts DM, 2014, Long term settlement following twin tunnel construction, VIII Congreso Chileno de Ingenieria Geotecnica
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.
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