Publications
486 results found
Sailer E, Taborda D, Zdravkovic L, et al., 2019, 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., 2019, 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
Ghiadistri G, Zdravković L, Potts DM, et al., 2019, Calibration of a double structure constitutive model for unsaturated compacted soils, 7th International Symposium on Deformation Characteristics of Geomaterials (IS-Glasgow 2019), Publisher: EDP Sciences, Pages: 1-6
This paper describes a calibration procedure for the double structure constitutive model ICDSM (Imperial College Double Structure Model), developed for highly expansive clays, when the model is applied to MX-80 bentonite. Firstly, the model calibration process is discussed and organised in a number of hierarchical steps. These steps involve the estimation of the macrostructural parameters that can be derived from oedometer, isotropic and triaxial laboratory data. Estimation of the microstructural parameters is more challenging due to the limited knowledge of an expansive clay’s fabric and of the physico-chemical phenomena that control its evolution upon wetting. Nevertheless, this paper discusses the available sources of data and identifies the appropriate information that is needed to characterise the micro-structural behaviour of the bentonite. Finally, through the simulation of a swelling pressure test on a bentonite plug, the hydration of the material is studied as a hydro-mechanical coupled process. Particular attention is devoted to the evolution of the stress state of the sample, which is compared to the experimental measurements in order to demonstrate that the constitutive model accurately reproduces the expansive behaviour of MX-80 bentonite.
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.
Pedro A, Zdravkovic L, Potts D, et al., 2019, Numerical modelling of the Ivens shaft construction in Lisbon, Portugal, Proceedings of the ICE - Geotechnical Engineering, Vol: 172, Pages: 263-282, ISSN: 1353-2618
The traditional use of shafts is that of simple geotechnical structures, built to provide direct access to the subsoil to either access existing facilities, or to provide a launch chamber for new excavations. More recently, shafts have been constructed with complex geometries and directly incorporated as part of a permanent basement or metro station. One such complex structure is the Ivens shaft in Lisbon, Portugal, which is yet to be built. The construction of this shaft represents a geotechnical challenge due to its complex shape and location in central Lisbon, surrounded by several historic buildings and having in its close vicinity the Baixa-Chiado metro station. This paper presents a sophisticated 3D numerical simulation of the Ivens shaft construction, with a focus on assessing its effect on the existing buildings and the metro station and on the structural forces induced in its lining. The importance of integrating in the analysis the appropriate constitutive modelling of the different formations, the realistic initialisation of the ground conditions and the stiffness of the buildings, is discussed in detail. Although this is a class A prediction of a specific case study, it provides an insight into the general soil-structure interaction of a complex underground excavation.
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.
Wan MSP, Standing JR, Potts DM, et al., 2019, Pore water pressure and total horizontal stress response to EPBM tunnelling in London Clay, Géotechnique, Vol: 69, Pages: 434-457, ISSN: 0016-8505
The ground response, in terms of surface and subsurface displacements, to twin-bore Crossrail tunnel construction beneath a research monitoring site in Hyde Park, London, using earth-pressure-balance machines (EPBMs) in London Clay, has recently been reported in two companion papers by the authors. This third paper presents and discusses corresponding changes in pore water pressure and total horizontal stress measured using multi-level piezometers and pushed-in spade cells. The three papers together provide a comprehensive and completely unique field monitoring case history of the short-term ground response to EPBM tunnelling in London Clay, making them invaluable for validating future numerical analyses. The fully grouted vibrating-wire piezometers were able to measure the rapid pore water pressure changes around the tunnels as they were constructed. Five distinct immediate pore water pressure responses are identified, induced by different stages of the tunnel drives as the EPBMs approached and passed the instruments. The responses are correlated with tunnel-boring machine operation variables and a postulated arching mechanism, identified for the first time through field measurements. The sense and magnitude of changes in horizontal total stress were reasonable and are correlated with overall pore water pressure changes. Both responses are linked where possible with measured subsurface displacements and generally correlate well, at least qualitatively. Limitations to the measurements and influencing factors are also discussed.
Byrne BW, Burd HJ, Zdravkovic L, et al., 2019, PISA Design Methods for Offshore Wind Turbine Monopiles, Offshore Technology Conference
Byrne BW, McAdam RA, Burd H, et al., 2019, PISA: new design methods for offshore wind turbine monopiles, Proceedings of the Society for Underwater Technology Offshore Site Investigation and Geotechnics 8th International Conference on “Smarter Solutions for Future Offshore Developments"
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.
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.
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.
Zdravković L, Potts DM, Bodas Freitas TM, 2019, Extending the life of existing infrastructure, Pages: 136-143
Contemporary geotechnical design is increasingly faced with a demand for extending the life of ageing infrastructure. Typical examples are foundation systems for re-development projects in congested urban environments, road and rail infrastructure and flood defences. The latter structures in particular are also projected to have to sustain rising sea-levels due to the effects of climate change. To enable such a design, the changes in the mechanical behaviour of the foundation soil over the period from first construction to the current state must be quantified. This transient period is governed by the time-related process of consolidation and creep in the ground. Considering examples of earthfill embankments, this paper discusses the facets of soil behaviour governing both the short- and long-term design of existing infrastructure embankments, using advanced numerical analysis.
Ghiadistri GM, Zdravkovic L, Potts DM, et al., 2019, Modelling the behaviour of swelling clays in a Geological Disposal Facility (GDF)
This paper discusses the numerical modelling of the buffer material in a Geological Disposal Facility (GDF), by simulating the FEBEX in-situ experiment. The test was conducted over 18 years at the Grimsel site in Switzerland, under the conditions of a real GDF, with compacted bentonite blocks used as the buffer material and a heater replacing the nuclear waste canister. Particular emphasis in the paper is given to the constitutive modelling of the FEBEX bentonite, highlighting the importance of accounting in the model for the double porosity structure of the compacted bentonite. Furthermore, the coupled thermo-hydro-mechanical (THM) finite element analysis also emphasises the importance of realistic modelling of the evolution of the hydraulic permeability of the bentonite with the changing suctions. The analysis results demonstrate substantial agreement between numerical predictions and FEBEX field measurements in terms of the buffer’s THM evolution. The area of the host formation affected by the test is also defined and investigated in order to provide useful information for the design of a GDF.
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.
Cui W, Potts DM, Zdravkovic 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.
Avgerinos V, Potts DM, Standing JR, 2018, Numerical investigation of the effects of tunnelling on existing tunnels., Géotechnique, Vol: 67, Pages: 808-822, ISSN: 0016-8505
Construction of the Crossrail tunnels just beneath the existing Central line tunnels at the northern side of Hyde Park provided the impetus for this paper. A basic three-dimensional (3D) finite-element (FE) model was developed to study a general case of a new tunnel (NT) crossing perpendicularly below an existing tunnel (ET). A series of 3D FE analyses was carried out and the results presented in this paper reveal some of the interaction effects. Changes in hoop forces, bending moments and lining deformations of the ET due to excavation of the NT are discussed. Conclusions are drawn about how the relative position of the excavation face of the NT in relation to the ET's axis affects the latter's behaviour. Cross-sectional and longitudinal deformations of the ET are discussed, leading to recommendations for field monitoring of similar interaction cases. Two parametric studies were also carried out to quantify the effects of the magnitude of the earth pressure balance machine face pressure and the longitudinal stiffness of the ET on the predicted behaviour of the ET due to construction of the NT.
Zdravkovic L, Tsiampousi A, Potts DM, 2018, On the modelling of soil-atmosphere interaction in cut and natural slopes, 7th International Conference on Unsaturated Soils
The need to predict the consequences of atmospheric conditions on the stability of slopes is widely evident from numerous examples of slope failures around the world, which often result in material and human loss.Equally, the serviceability conditions of cut slopes very much depend onthe heave mobilised byexcavation, the magnitude of which is partly governed by the hydraulic boundary conditions.Soil-atmosphere interaction is complex, involving precipitation and evapotranspiration across the slope surface, and acts in ad-dition to theground water regime within the slope body. As a consequence, calculation tools cannot be overly simplified if realistic predictions are expected. This paper provides an overview of recent research at Imperial College in modellingunsaturatednatural and cut slopes, using finite element analysis and advanced constitutive models and boundary conditions.
Kirkham AD, Tsiampousi A, Potts DM, 2018, Temperature-controlled oedometer testing on compacted bentonite, 7th International Conference on Unsaturated Soils
A new temperature-controlled oedometer has been designed at Imperial College London and commissioned to investigate the thermo-hydro-mechanicalbehaviour of soils. Temperature control is achieved by submerging the specimenin a water bath. The water temperature is regulated byheaters positionedradially around the specimen, or by an external unit. The temperature can be varied between 5°C and 85°C. The temperature gradient across the specimenis minimised by circulating water beneath the specimenthrough a hollow plate.A thermo-mechanical, elastic, finite element model of the equipment has been produced using the Imperial College Finite Element Program (ICFEP). The experimental results are used to develop and validate the numerical model. The model is then used to inform and improve the experimental testing programme.The accuracy of temperature control has already been established. The testing programme includes heating tests at constant applied stress, and loading tests at discrete temperature values. Of particular interest is thermally-inducedoverconsolidation behaviour. The experimental results are used to verify the existing numerical framework and to establish the effect of temperature on the behaviourofsaturated soil.
Ghiadistri GM, Potts DM, Zdravkovic L, et al., 2018, A new double structure model for expansive clays, 7th International Conference on Unsaturated Soils
The behaviourof compacted bentonite upon hydration is numerically investigated here by simu-lating a swelling pressure teston aMX-80 bentonitesample. Two constitutive modelsareemployed in the analysis: the “Imperial College Single-Structure Model” (ICSSM)andthe “Imperial College Double-Structure Model” (ICDSM), the latterspecifically developed for expansive clays. It is shown that the latter exhibits a considerably improved performance as it is able to accurately capture the swelling pressure developed in the materialupon wetting. Nevertheless, a limited knowledge of the evolution of the material’s fabric, notably at the micro-scale,is an obstacle for deriving with certainty some of the model parameters. This issue is high-lightedhere by performing analyses of theswelling pressure test with two sets ofmaterial characterisations, with model parameters differinginthe derivation of the microstructural component.Both analyses show a very good match with the testdata, but it is difficult to justify one set of microstructural parametersoverthe other. The paper emphasises what aspects of experimental research could be helpful in studying the fabric of compacted bentonite upon wetting, and hence improve the calibration procedure of thedouble-structure mod-el.
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.
Summersgill F, Kontoe S, Potts DM, 2018, Stabilisation of excavated slopes in strain softening materials with piles, Géotechnique, Vol: 68, Pages: 626-639, ISSN: 0016-8505
The use of a row of discrete vertical piles is an established method, successfully used to remediate failure of existing slopes and to stabilise potentially unstable slopes created by widening transport corridors. This paper challenges the assumptions made in current design procedures for these piles, which treat the pile only as an additional force or moment and simplify soil–pile interaction. Two-dimensional plane-strain finite-element analyses were performed to simulate the excavation of a slope in a stiff clay and the interaction of vertical piles within the slope. A non-local strain-softening model was employed for the stiff clay to reduce the mesh dependency of the solution. An extensive parametric study was performed to systematically examine the impact of pile position, dimensions (length and diameter) and time of pile construction on the stability of a cutting in London Clay, which was chosen as a representative strain-softening material. A variety of different failure mechanisms were identified, depending on pile location, dimensions and time of construction. The variability of the pile and slope interaction that was modelled suggests that an oversimplification during design could miss the critical failure mechanism or provide a conservative stabilisation solution. Given the prevalence of stiff clay slopes in the UK, increased capacity requirements of transport infrastructure and the age of slopes in this material, an informed and more realistic design of stabilisation piles will become increasingly necessary.
Kontoe S, Summersgill F, Potts D, et al., 2018, Stabilisation of excavated slopes with piles in soils with distinctly different strain softening behaviour, 9th European Conference on Numerical Methods in Geotechnical Engineering (NUMGE)
The majority of existing design procedures for slope stabilization with piles treat the pile only as an additional force or moment acting on the critical slip surface of the unstabilised slope, effectively ignoring any interaction of the pile with the evolution of the failure mechanism. This paper presents a numerical investigation that challenges this assumption, demonstrating the importance of the soil-pile interaction. Two dimensional plane-strain hydro-mechanically coupled finite element analyses were performed to simulate the excavation of a slope, considering materials with both a strain softening and non-softening response. The impact of pile position and time of pile construction on the stability of a cutting were parametrically examined, comparing and contrasting the findings for the different material types. The results suggest that an oversimplification during design regarding the soil/pile interaction could either entirely miss the critical failure mechanism (unconservative) or provide a conservative stabilisation solution.
Skiada E, Kontoe S, Stafford P, et al., 2018, Ground surface amplification for canyon topographies excited with bi-directional earthquake records, 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
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
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
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
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