Publications
486 results found
Cui W, Potts DM, Zdravkovic L, et al., 2018, An alternative coupled thermo-hydro-mechanical finite element formulation for fully saturated soils, Computers and Geotechnics, Vol: 94, Pages: 22-30, ISSN: 0266-352X
Accounting for interaction of the soil’s constituents due to temperature change in the design of geo-thermal infrastructure requires numerical algorithms capable of reproducing the coupled thermo-hydro-mechanical (THM) behaviour of soils. This paper proposes a fully coupled and robust THM formulation for fully saturated soils, developed and implemented into a bespoke finite element code. The flexibility of the proposed formulation allows the effect of some coupling components, which are often ignored in existing formulations, to be examined. It is further demonstrated that the proposed formulation recovers accurately thermally induced excess pore water pressures observed in undrained heating tests.
Wan MSP, Standing JR, Potts DM, et al., 2018, Measured short-term subsurface ground displacements from EPBM tunnelling in London Clay, Geotechnique: international journal of soil mechanics, Vol: 67, Pages: 748-779, ISSN: 0016-8505
Subsurface ground displacements from the construction of the twin-bore Crossrail tunnels in London Clay by earth pressure balance machines (EPBMs) are presented and discussed, complementing a companion paper by the authors that focused on the surface response. Both papers report vertical and horizontal displacements, in this case measured using comprehensive arrays of instruments installed within boreholes in Hyde Park, London. The Crossrail tunnels are deeper than those cited in most UK case histories concerning tunnelling in stiff clay. Clear insights were gained into subsurface displacement mechanisms: an ‘inward’ displacement field was observed around the Crossrail tunnel construction, in contrast to the ‘outward’ displacement field that developed around the shallower Channel Tunnel Rail Link tunnels constructed east of London using similar EPBMs in London Clay. This has important implications when estimating subsurface displacements using currently available empirical methods. Appraisal of the EPBM operational variables suggests that the relative magnitude of face and tail grout pressures to overburden stress is the key factor contributing to the opposing senses of the observed displacement fields. Earlier tunnelling-induced strain softening of the London Clay is evident from greater subsurface incremental volume losses and settlement trough width parameters relating to subsequent tunnel construction.
Pedro A, Zdravkovic L, Potts DM, et al., 2017, Geotechnical characterisation of the Miocene formations at the location of Ivens shaft, Lisbon, Quarterly Journal of Engineering Geology and Hydrogeology, Vol: 51, Pages: 96-107, ISSN: 1470-9236
The design of complex underground structures in an urban environment requires in the first instance an appropriate characterization and interpretation of the ground conditions and of the mechanical behaviour of soil formations in the ground profile. With such information it is then possible to select and calibrate appropriate soil constitutive models for application in advanced numerical analysis, with the objective of predicting the induced ground movements and the potential damage to existing structures and services. This paper provides an interpretation of the site investigation data collected for the numerical analysis and design of the Ivens shaft excavation in Lisbon, Portugal. For the first time a comprehensive set of interpreted data is obtained for two of the main formations in the Lisbon area, Argilas e Calcários dos Prazeres (AP) and Areolas da Estefânia (AE), improving the understanding of their mechanical behaviour and making the data available for application in most soil constitutive frameworks. It is evident from the results that even with careful testing procedures the data may appear to be inconsistent, requiring further assumptions when deriving soil parameters. Such assumptions are discussed and emphasis is placed on the need to combine data from laboratory and field investigations.
Burd HJ, Byrne BW, McAdam R, et al., 2017, Foundation Design of Offshore Wind Structures, TC209 Workshop on Foundation Design of Offshore Wind Structures, 19th International Conference on Soil Mechanics and Geotechnical Engineering
This paper describes the outcome of a recently completed research project – known as PISA – on the development of a new process for the design of monopile foundations for offshore wind turbine support structures. The PISA research was concerned with the use of field testing and three-dimensional (3D) finite element analysis to develop and calibrate a new one-dimensional (1D) design model. The resulting 1D design model is based on the same basic assumptions and principles that underlie the current p-y method, but the method is extended to include additional components of soil reaction acting on the pile, and enhanced to provide an improved representation of the soil-pile interaction behaviour. Mathematical functions – termed ‘soil reaction curves’ – are employed to represent the individual soil reaction components in the 1D design model. Values of the parameters needed to specify the soil reaction curves for a particular design scenario are determined using a set of 3D finite element calibration analyses. The PISA research was focused on two particular soil types (overconsolidated clay till and dense sand) that commonly occur in north European coastal waters. The current paper provides an overview of the field testing and 3D modelling aspects of the project, and then focuses on the development, calibration and application of the PISA design approach for monopiles in dense sand.
Abadias Gomez D, Zdravkovic L, Taborda DMG, et al., 2017, On the implications of advanced monopile design methodologies in offshore wind turbines, Proceedings of the Society for Underwater Technology Offshore Site Investigation and Geotechnics 8th International Conference on “Smarter Solutions for Future Offshore Developments"
The design of Offshore Wind Turbines (OWT) is a complex process involving several stages: wind turbine selection, tower and sub-structure design, as well as foundation design and installation. A successful design requires close interaction between these components in order to satisfy the main design requirements, name-ly the capacity and accumulated rotation for the foundation and dynamic response and fatigue for the whole system. Recent research has revealed that the current design methods for laterally loaded piles, when ap-plied to short and stubby OWT monopiles, underestimate their initial stiffness and capacity. Advanced Fi-nite Element (FE) analysis, with realistic modelling of the ground conditions can accurately reproduce soil response around a monopile, and hence improve the design, ultimately leading to cost reduction of monopile foundations. In the present paper, the impact of economies in foundation design on the overall design of a realistic OWT is explored. The NREL 5 MW baseline wind turbine is modelled through FE analysis under several characteristic design load cases. The advantages of using FE analysis when compared to traditional methods, in particular with respect to capacity and dynamic response, are demonstrated and discussed.
Tsiampousi A, Yu JBY, Standing JR, et al., 2017, Behaviour of bolted cast iron joints, Tunnelling and Underground Space Technology, Vol: 68, Pages: 113-129, ISSN: 0886-7798
The structural testing and finite element (FE) analysis described in this paper were part of a major research project undertaken at Imperial College London to investigate the deformation of bolted segmental grey cast iron (GCI) tunnel linings. A key aim was to quantify how joints influence the behaviour of the lining, through a three-path approach comprising physical experiments, finite element modelling, and field instrumentation. The laboratory results have been used to assess the validity of the tunnel assessment methods used by industry.This study examined joint articulation under the serviceability limit state in the absence of hoop force focussing on factors such as applied bolt preload, the loading direction and the freedom of the circumferential flange to deflect. Two half-scale GCI lining segments were bolted together at the longitudinal flanges to form a bolted arch in a similar fashion to the tests performed by Thomas (1977). Modern instrumentation was implemented to gain detailed measurements quantifying changes in global displacements of the two segments, bolt forces and joint opening under applied loading. For the first time, the physical experiments were conducted contemporaneously with the development of a three-dimensional FE model of the joint. The experimental data and the results from the FE analysis indicate a reduction in joint stiffness as the joint articulates under applied load. It is shown that the presence of a joint has far greater influence on the behaviour of the ‘arch’ than the level of preload applied to the bolts in the joint. The FE analysis allowed the deformation behaviour of the joint under positive and negative bending to be investigated: its response under the two modes differs significantly.
Skiada E, Kontoe S, Stafford P, et al., 2017, Ground motion amplification for canyon topographies with different input motions, 3rd International Conference on Performance-based Design in Earthquake Geotechnical Engineering (PBD-III), Publisher: ISSMGE
It is widelyknown that topographic irregularities influence the surfaceground motions, typically with anenhancement of the response close to convex topographic features,such as ridges and slope crests. Several studies have investigatedthe ground motion at the surface of filled valleys and empty canyons, focusingmainly onthe geometry and the soil characteristics rather than the input excitation.Further investigation of the impact of the input excitation to the ground surface response is needed in order to modifyexisting ground motion prediction models to account for topographic effects. The response of canyons has been previously examined; but mainly focusing on simple wavelet input. This paper considers a fully weathered canyon (i.e., without any in-fill material) aiming to investigate the influence of the input excitationon the surface ground motion through a parametric time-domain finite element (FE) study. A two-dimensional plane-strain model of an idealisedcanyon is considered for vertically propagating SV waves, using both wavelets and recorded earthquakes as input excitation. The model consists of two step-like slopes with slope height (H), in a homogeneous linear elastic soil layer overlying rigid bedrock. Topographic aggravation is presented for several points along the canyon ground surface aiming to derive a pattern of its distribution considering input excitation with different characteristics.
Tsaparli V, Kontoe S, Taborda D, et al., 2017, Liquefaction modelling of a strong motion station in Christchurch, New Zealand, 3rd International Conference on Performance-based Design in Earthquake Geotechnical Engineering (PBD-III), Publisher: International Society of Soil Mechanics and Geotechnical Engineering
Advanced constitutive models can replicate several aspects of soil behaviour, but, due to their complexity and number of parameters, they need more sophisticated and realistic validation under general loading conditions. When modelling liquefaction phenomena, the lack of field monitoring data means that modeltesting, such as centrifuge experiments, is often used as benchmark for the numerical analyses. The 2010-2011 Canterbury earthquake sequence in New Zealand was recorded by a number of strong motion stations at various distances from the earthquake epicentre. Additionally, an extensive field and laboratory programmehassincebecome available, adequately describing the geological, geotechnical and hydrogeological conditions in the area. As such,the performance of a two-surface bounding surface plasticity constitutive model for sands, calibrated based on site-specific laboratory data, is assessed usingfield evidenceof a strong motion stationin fully-coupled effective stress-based finite element analyses. As the real stratigraphy is complex, with layers of silts and clays between the sandy strata, a simpler cyclic non-linearelasticmodel,which can adequately incorporate the basic aspects of dynamic soil behaviour, is also used to model the non-liquefiable strata. To specify the input ground motion at the base of the deposit, the recordedgroundsurface motionata sitewith no evidence of liquefaction isdeconvolved and compared with the outcrop predictions ofa New Zealand-specificground motion prediction equation. The numerical results are compared with the recorded horizontal ground surface acceleration time-history of the 22ndFebruary 2011 seismic event, exhibiting very good agreement.
Tsaparli V, Kontoe S, Taborda D, et al., 2017, The importance of accurate time-integration in the numerical modelling of P-wave propagation, Computers and Geotechnics, Vol: 86, Pages: 203-208, ISSN: 1873-7633
The numerical dissipation characteristics of the Newmark and generalised-α time-integration schemes are investigated for P-wave propagation in a fully saturated level-ground sand deposit, where higher frequencies than those for S-waves are of concern. The study focuses on resonance, which has been shown to be of utmost importance for triggering liquefaction due to P-waves alone. The generalised-α scheme performs well, provided that the time-step has been carefully selected. Conversely, the dissipative Newmark method can excessively damp the response, changing radically the computed results. This implies that a computationally prohibiting small time-step would be required for Newmark to provide an accurate solution.
Tsaparli V, Kontoe S, Taborda D, et al., 2017, An energy-based interpretation of sand liquefaction due to vertical ground motion, Computers and Geotechics, Vol: 90, Pages: 1-13, ISSN: 0266-352X
In several recent earthquakes, high vertical ground accelerations accompanied by liquefaction were observed. Downhole records have also shown that large vertical accelerations do not necessarily originate from the source, but rather get amplified towards the ground surface. Given the advantages of energy-based interpretation of liquefaction triggering due to S-waves, this approach is used together with finite element analyses to investigate vertical motion amplification and ensuing liquefaction. The results show the importance of the post-resonance response cycles, while hysteretic damping based on total stresses, accounting for the water in the pores, is shown to be very low, explaining the observed amplification.
Avgerinos V, Potts DM, Standing JR, et al., 2017, Predicting tunnelling-induced ground movements and interpreting field measurements using numerical analysis: Crossrail case study at Hyde Park, Géotechnique, Vol: 68, Pages: 31-49, ISSN: 0016-8505
Ground response to the construction of the Crossrail tunnels in London Clay beneath Hyde Park has been modelled numerically using advanced finite-element analyses. The soil model used for modelling the London Clay was a kinematic hardening soil model (named M2-SKH). This model, when used for the St James's Park greenfield site, provided excellent predictions of tunnelling-induced ground movements. Comparison of the results from the analysis of the Hyde Park greenfield site with associated field monitoring data also suggests excellent predictions, even though in this case the tunnels were: of larger diameter; deeper in the London Clay; and constructed with earth-pressure-balance machines. The influence of lining permeability was found to influence significantly short- and longer-term predictions. Interpretation of the predicted surface and subsurface vertical and horizontal displacements due to the construction of the Crossrail tunnels exemplifies how numerical analysis can assist in explaining and identifying potential ambiguities in field measurements.
Gawecka KA, Taborda DMG, Potts DM, et al., 2017, Numerical modelling of thermo-active piles in London Clay, Proceedings of the Institution of Civil Engineers - Geotechnical Engineering, Vol: 170, Pages: 201-219, ISSN: 1353-2618
Thermo-active foundations utilise heat energy stored in the ground to provide a reliable and effective means ofspace heating and cooling. Previous studies have shown that the effects of temperature changes on their response arehighly dependent on their interaction with the surrounding ground. Consequently, it is necessary to consider thisinteraction and include both the thermal and mechanical behaviour of the ground in design. This paper addresses thisissue by performing state-of-the-art finite-element analyses using the Imperial College Finite Element Program, which iscapable of simulating the fully coupled thermo-hydro-mechanical behaviour of porous materials. First, the LambethCollege pile test is analysed to demonstrate the capability of the adopted modelling approach to capture the observedresponse under thermo-mechanical loading. Subsequently, a detailed study is carried out, demonstrating the impact ofcapturing the fully coupled thermo-hydro-mechanical response of the ground, the use of appropriate boundary conditionsand the uncertainty surrounding thermal ground properties. It is demonstrated that the modelling approach has a largeimpact on the computed results, and therefore potentially on the design of thermo-active piles. Conversely, the effects ofthermal conductivity and permeability of the soil are shown not to influence the pile behaviour significantly.
Wan MSP, Standing JR, Potts DM, et al., 2017, Measured short-term ground surface response to EPBM tunnelling in London Clay, Géotechnique, Vol: 67, Pages: 420-445, ISSN: 0016-8505
Earth-pressure-balance machines (EPBMs) were used for the construction of Crossrail tunnels in London, providing opportunities for field investigation of consequent ground response. Analysed results from an instrumented research site in Hyde Park with extensive surface and subsurface monitoring arrays are presented and discussed. The Crossrail tunnels at the site are 34·5 m below ground, deeper than those in most case histories of tunnelling in stiff clay in the UK. This paper characterises the tunnelling-induced ground response, both ‘greenfield’ and in the proximity of the existing Central Line tunnels, dealing with measurements at the ground surface. A companion paper covers the subsurface ground response. Vertical and horizontal ground surface displacements were obtained from manual precise levelling and micrometer stick measurements. Several key findings will benefit future tunnelling projects involving EPBMs. Volume loss values measured at the instrumented site were low, being less than 0·8% and 1·4% for the first and second tunnel drives respectively, higher values being associated with ground softening from the first tunnel construction. Smaller volume losses were recorded in the vicinity of the existing Central Line tunnels, compared with the greenfield location, suggesting that their presence inhibited the development of ground movements. Asymmetric settlement troughs developed due to either the nearby pre-existing tunnels or the construction of the first tunnel. Marginally smaller values of trough width parameter, Ky, were determined for these deeper tunnels compared with previous greenfield ground case histories. Resultant vectors of ground surface displacement were directed to well-defined point-sinks above the tunnel axis level.
Tsiampousi A, Smith PGC, Potts DM, 2017, Coupled consolidation in unsaturated soils: from a conceptual model to applications in boundary value problems, Computers and Geotechnics, Vol: 84, Pages: 256-277, ISSN: 0266-352X
The paper presents the Finite Element formulation of the equations proposed by Tsiampousi et al. (2016) for coupled consolidation in unsaturated soils. Their coupling is discussed in relation to a conceptual model which divides soil behaviour into zones ranging from fully saturated to dry states. The numerical simulation of a laboratory experiment involving drainage of water from a vertical column of sand is used to validate the equations. Finally, the example of rainfall infiltration into a cut slope highlights how aspects of the conceptual model are reflected in the numerical analysis of boundary value problems involving unsaturated soils.
Tsiampousi A, Smith PGC, Potts DM, 2017, Coupled consolidation in unsaturated soils: an alternative approach to deriving the Governing Equations, Computers and Geotechnics, Vol: 84, Pages: 238-255, ISSN: 0266-352X
The equations governing coupled consolidation in unsaturated soils are known to contain additional parameters when compared to the equations for saturated soils. Nonetheless, the variation of these parameters with suction or degree of saturation is not generally agreed upon. The paper introduces a novel approach to deriving general equations for each of these parameters and their variation, and explains that, for consistency with the constitutive and soil-water retention curve models adopted, these general equations need to be transformed into case-specific expressions. Finally, a conceptual model is presented highlighting how the behaviour of unsaturated soil reflects aspects of its water content.
Afshan S, Yu JBY, Standing JR, et al., 2017, Ultimate capacity of a segmental grey cast iron tunnel lining ring subjected to large deformations, Tunnelling and Underground Space Technology, Vol: 64, Pages: 74-84, ISSN: 0886-7798
Understanding the behaviour of existing tunnels subjected to in-service deformations, as a result of the construction of underground works (e.g. new tunnels) in their proximity, is of importance in order to safeguard infrastructure within the urban environment. The associated deformations that take place during tunnelling have to be carefully assessed and their impact on the existing tunnels needs to be considered. A half-scale segmental grey cast iron (GCI) tunnel lining ring was tested as part of an extensive research project investigating the impact of new tunnel excavations on existing tunnels conducted at Imperial College London. A sophisticated experimental arrangement was developed to deform the ring in a variety of modes under combined displacement and load control. This paper reports on experiments carried out to assess its structural response when subjected to large deformations. The tests reported are the first to be conducted on a realistic scale model under carefully controlled conditions, and provide valuable insight into the behaviour of a GCI segmental ring during distortions commonly observed in reality. Details of the experiments, including the adopted test set-up and the instrumentation employed, are presented. The measured bending moments around the ring, as a result of the applied deformations, are determined and compared with those predicted using the well-known equations given by Morgan (1961) and Muir Wood (1975), often used in industry, as well as those obtained assuming an elastic continuous ring.
Summersgill FC, Kontoe S, Potts D, 2017, On the use of nonlocal regularisation in slope stability problems, Computers and Geotechnics, Vol: 82, Pages: 187-200, ISSN: 0266-352X
This study examines the use of nonlocal regularisation in a coupled consolidation problem of an excavated slope in a strain softening material. The nonlocal model reduces significantly the mesh dependency of cut slope analyses for a range of mesh layouts and element sizes in comparison to the conventional local approach. The nonlocal analyses are not entirely mesh independent, but the predicted response is much more consistent compared to the one predicted by local analyses. Additional Factor of Safety analyses show that for drained conditions the nonlocal regularisation eliminates the mesh dependence shown by the conventional local model.
Summersgill FC, Kontoe S, Potts D, 2017, A critical assessment of nonlocal strain softening methods in biaxial compression, International Journal of Geomechanics (ASCE), Vol: 17, ISSN: 1532-3641
When modelling the development of slip surfaces in strain softening soils, the finite element method can suffer from mesh dependency and a lack of convergence of the analysis. The calculation of strains using a nonlocal method avoids the independent softening of a solitary point that can cause these symptoms. Nonlocal methods calculate the strain at a point with reference to the strains at calculation points surrounding it. In this study, a biaxial compression analysis is used to compare the original and two modified nonlocal methods to local strain softening analysis. Three nonlocal parameters are introduced and their influence on the nonlocal strain calculation is investigated. Meshes with different discretisations permit a comparison of the mesh dependency of these methods and a sensitivity analysis of nonlocal parameters. All the nonlocal methods are found to have significantly less mesh dependency than the local method. Both modified nonlocal methods exhibit less mesh dependency than the original approach, but one of the two modified methods, the Over-nonlocal method, is shown to be unstable as the mesh is refined for certain values of the nonlocal parameters.
Skiada E, Kontoe S, Stafford P, et al., 2017, Canyon topography effects on ground motion, 16th World Conference on Earthquake Engineering, Publisher: WCEE
It is broadly known that topographic irregularities effect ground motions, with a particular enhancement of the ground response close to convex topographic features such as ridges and slope crests. Although there are many studies investigatingthe ground motion in the vicinity of slope crests, the response at the toe has not been studied in great detail, as the toe ground motion is normally considered to be smaller than that of the crest. However, for canyon topographies further investigation of the ground motion at the slope toe, where a more complicated response is expected due to the interaction of the canyon sides, is needed. The response of semi-circular and semi-elliptical canyons has been previously examined; butmainly focusing on valleys filled with soft materials. This paper considers a fully weathered canyon (i.e., without any in-fill material) aiming to investigate the influence of a canyon’s width on the surface ground motion through a parametric time-domain finite element (FE) study. A two-dimensional plane-strain model of an idealised canyon is considered for vertically propagating SV waves, using wavelets as input excitation. The model consists of two step-like slopes with slope height (H), in a homogeneous linear elastic soil layer overlying rigid bedrock. The analyses focus first on the canyon slope areas, where the ground motion is altered depending upon the proximity to the topographic irregularity, identifying the main parameters that effect the response. Results are also presented for several points along the canyon ground surface showing that the distribution of topographic aggravation varies significantly with canyon width.
Yu J, Standing J, Vollum R, et al., 2017, Experimental investigations of bolted segmental grey cast iron lining behaviour, Tunnelling and Underground Space Technology, Vol: 61, Pages: 161-178, ISSN: 0886-7798
The need for the research reported in this paper was driven by the Crossrail project in London for which new tunnels were constructed close to numerous existing operational tunnels of the London Underground (LU) network.This research is based on experimental work conducted on half-scale grey cast iron (GCI) tunnel lining segments with chemical composition similar to the Victorian age GCI segments in the LU network. This paper discusses the deformation behaviour of the bolted segmental lining under the influence of factors such as overburden pressure, bolt preload and presence of grommets at small distortions. The measured behaviour of the segmental lining is compared against the calculated response of a continuous lining based on the assumption of elasticity.The industry practice for tunnel lining assessment is to calculate the induced bending moment in the tunnel lining using an elastic continuum model, while adopting a reduced lining stiffness to take into account the presence of the joints. Case studies have recorded that both loosening and tightening of lining bolts have been used as mitigation measures to reduce the impact of new tunnel excavations on existing GCI tunnels.The experimental work on the half-scale GCI lining has shown that a bolted segmental lining behaves as a continuous ring under small distortions imposed when subjected to hoop forces relevant to the depth of burial of LU tunnels. In the presence of hoop force, joint opening was minimal and the magnitude of preload in the bolts had little impact on the behaviour of the lining. It is therefore concluded that disturbance of the bolts in existing tunnels is not recommended as a mitigation measure as in addition to being ineffective it is both time consuming and introduces the risk of damaging the tunnel lining flanges.
Grammatikopoulou A, Schroeder FC, Brosse AM, et al., 2017, On the use of constitutive models in numerical analysis of offshore structures, 8th Int. Conf. of the Society of Underwater Technology, Offshore Site Investigation and Geotechnics (OSIG2017), Pages: 423-430
Kovacevic N, Menkiti C, Long M, et al., 2017, Finite element back-analysis of an anchored wall in Dublin Boulder-Clay, 19th Int. Conf. On Soil Mech. and Geot. Eng.
Tsiampousi A, Zdravkovic L, Potts DM, 2017, Numerical study of the effect of soil–atmosphere interaction on the stability and serviceability of cut slopes in London clay, Canadian Geotechnical Journal, Vol: 54, Pages: 405-418, ISSN: 1208-6010
The stability of cut slopes is greatly influenced by seasonal pore water pressure variations underthe combined effect of rainfall and vegetation. However, predicting soil-atmosphere interactionis not straightforward, due to the complexity of both the boundary conditions involved and thehydro-mechanical behaviour of soils, which is coupled and highly nonlinear, rendering the use ofnumerical tools, such as finite element analysis, necessary. The paper discusses the numericalmodelling of soil-atmosphere interaction and presents the analysis of a slope cut in London clayin a highly vegetated area. The whole life cycle of the slope is considered with phases of lowand high water demand vegetation and vegetation clearance. The analysis results indicate thatdense vegetation is associated with high factors of safety, but may induce large differentialdisplacements which are likely to affect the serviceability of the slope. Vegetation clearance,however, may initiate instability, highlighting the need for effective vegetation management inorder to achieve a balance between serviceability and ultimate limit states. Although the caseconsidered is representative of South East England, it introduces the necessary tools forrealistic numerical analysis of soil-atmosphere interaction.
Pedro A, Zdravkovic L, Potts DM, et al., 2016, Derivation of model parameters for numerical analysis of the Ivens shaft excavation, Engineering Geology, Vol: 217, Pages: 49-60, ISSN: 1872-6917
The prediction of induced ground movements and the potential damage to existing structures and services is paramount when building deep excavations in an urban environment. In order to obtain a reasonable prediction advanced constitutive models need to be employed, so that the behaviour of the soil can be adequately reproduced under different stress conditions. The calibration of such models is complex and often requires optimisation, as a large number of parameters need to be determined from the available ground investigation data, while also ensuring their consistency with the initial ground conditions. This paper presents the calibration process of advanced constitutive models employed to simulate the excavation of the Ivens shaft in Lisbon, Portugal. The data from both historic and new laboratory and field testing is employed in the calibration procedure. In order to assess and validate the suitability of the derived model parameters, a back-analysis of the nearby Baixa-Chiado metro station excavation is carried out and its results are presented and discussed.
Tsiampousi A, Zdravkovic L, Potts DM, 2016, Soil-atmosphere interaction in unsaturated cut slopes, 3rd European Conference on Unsaturated Soils, Publisher: EDP Sciences, ISSN: 2267-1242
Interaction between atmosphere and soil has only recently attracted significant interest. Soil-atmosphereinteraction takes place under dynamic climatic conditions, which vary throughout the year and are expected to sufferconsiderable alterations due to climate change. However, Geotechnical Analysis has traditionally been limited tosimplistic approaches, where winter and summer pore water pressure profiles are prescribed. Geotechnical Structures,such as cut slopes, are known to be prone to large irreversible displacements under the combined effect of wateruptake by a complex vegetation root system and precipitation. If such processes take place in an unsaturated materialthe complexity of the problem renders the use of numerical analysis essential. In this paper soil-atmosphereinteraction in cut slopes is studied using advanced, fully coupled partially saturated finite element analyses. The effectof rainfall and evapotranspiration is modelled through sophisticated boundary conditions, applying actualmeteorological data on a monthly basis. Stages of low and high water demand vegetation are considered for a periodof several years, before simulating the effect of vegetation removal. The analysis results are presented with regard tothe serviceability and stability of the cut slope.
Tsaparli V, Kontoe S, Taborda D, et al., 2016, Vertical ground motion and its effects on liquefaction resistance of fully saturated sand deposits, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 472, Pages: 1-21, ISSN: 1364-5021
Soil liquefaction has been extensively investigated over the years with the aim to understand its fundamental mechanism and successfully remediate it. Despite the multi-directional nature of earthquakes, the vertical seismic component is largely neglected, as it is traditionally considered to be of much lower amplitude than the components in the horizontal plane. The 2010–2011 Canterbury earthquake sequence in New Zealand is a prime example that vertical accelerations can be of significant magnitude, with peak amplitudes well exceeding their horizontal counterparts. As research on this topic is very limited, there is an emerging need for a more thorough investigation of the vertical motion and its effect on soil liquefaction. As such, throughout this study, uni- and bidirectional finite-element analyses are carried out focusing on the influence of the input vertical motion on sand liquefaction. The effects of the frequency content of the input motion, of the depth of the deposit and of the hydraulic regime, using variable permeability, are investigated and exhaustively discussed. The results indicate that the usual assumption of linear elastic response when compressional waves propagate in a fully saturated sand deposit does not always hold true. Most importantly post-liquefaction settlements appear to be increased when the vertical component is included in the analysis.
Gawecka KA, Taborda DMG, Potts DM, et al., 2016, Effects of transient phenomena on the behaviour of thermo-active piles, 1st International Conference on Energy Geotechnics
Skiada E, Kontoe S, Stafford P, et al., 2016, Canyon Depth Effect on Surface Ground Motion, 1st International Conference on Natural Hazards & Infrastructure
Topographic effects are rarely accounted for in seismic design codes, despite their potential to significantly modify surfaceground motions. This paper investigates the influence of a canyon’s slope height on the surface ground motion through aparametric time-domain Finite Element (FE) study. A two-dimensional plane-strain model of an idealised canyon isconsidered for vertically propagating SV waves, using wavelets as input excitation. The model consists of two step-likeslopes with slope height (H), in a homogeneous linear elastic soil layer overlying rigid bedrock. The analysis results showthat the distribution of topographic aggravation at the ground surface varies significantly with normalized canyon depthover the input wavelength (H/λ) and it does not necessarily reach a maximum at a specific H/λ ratio, as has been suggestedin previous studies. The validity of this conclusion is investigated for different depths to bedrock and soil layer properties.
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.
Avgerinos V, Potts DM, Standing JR, 2016, The use of kinematic hardening models for predicting tunnelling-induced ground movements in London Clay, Geotechnique: international journal of soil mechanics, Vol: 66, Pages: 106-120, ISSN: 0016-8505
The use of a kinematic hardening soil model for predicting short- and long-term ground movements due to tunnelling in London Clay is investigated. The model is calibrated against oedometer and triaxial tests on intact samples from different units of the London Clay. The calibrated model is then used in finite-element analysis to simulate the field response at St James's Park during excavation of the Jubilee Line Extension tunnels. The finite-element predictions compare well with the available field monitoring data. The importance of using consistent initial conditions for this complex boundary value problem in conjunction with the model parameters selected is highlighted. The stiffness response of different regions of the finite-element mesh indicates that the rate at which the stiffness degrades and the stiffness response further away from the tunnel boundary affect the short-term predictions significantly. The long-term predictions confirm that the compression characteristics of the soil control the magnitude of the consolidation settlements and its permeability the shape of the long-term settlement profiles.
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