Publications
140 results found
Möller JK, Taborda DMG, Kontoe S, et al., 2024, A shear history model for capturing the liquefaction resistance of sands at various cyclic stress ratios, Computers and Geotechnics, Vol: 166, Pages: 105940-105940, ISSN: 0266-352X
Various constitutive formulations have been developed over the years to reproduce the cyclic resistance of sands.A common challenge for existing models is the accurate simulation of the cyclic strength of sands for a widerange of initial conditions and different cyclic stress levels when adopting a single calibration. Many liquefactionmodels tend to overpredict the resistance of the soil under large-amplitude loading, while underestimating thestrength at low-amplitude cyclic shearing. This manifests itself in slopes of simulated cyclic resistance ratiocurves (CRR-curves) which are steeper than experimental studies indicate. This paper provides a discussion onthe effects of large-amplitude and low-amplitude cyclic shearing on a granular material based on micromechanical and experimental investigations presented in the literature. A constitutive model with a shear-historythreshold is proposed, which accounts for a shift of the apparent angle of phase transformation under cyclicloading. In addition, a novel expression for a deviatoric fabric tensor is introduced to describe the evolution ofshear-induced fabric anisotropy while a soil is dilating and contracting. Combining these two features in oneformulation within the bounding surface plasticity framework enables an accurate prediction of cyclic strength ofsands under a wide range of cyclic stress ratios.
Cathie D, Jardine R, Silvano R, et al., 2023, Axial capacity ageing trends of large diameter tubular piles driven in sand, Soils and Foundations, Vol: 63, ISSN: 0038-0806
The paper examines dynamic pile test data from 25 high-quality offshore cases, where end-of-initial driving (EoID) and beginning-of-restrike (BoR) instrumented dynamic monitoring was undertaken on tubular piles driven in sands at well-characterised sites after known setup periods. The static resistances derived from signal matching by two independent specialist teams using different software are compared with CPT-based pile capacity calculations, providing the first axial capacity and setup dataset for large offshore piles driven in sand. Complementary re-analyses are made from three onshore/nearshore sites where dynamic and static testing was conducted on comparable piles. Open-ended tubular steel piles with 0.3–3.5 m diameters driven in (mainly dense) sands are all shown to develop marked setup, which is most active over the first 2–10 days. All piles show similar outcomes 20–30 days after installation. However, the larger diameter offshore piles’ dynamic tests indicate no further setup after 30 days, while smaller diameter piles at onshore/nearshore sites continue to display further marked capacity growth. Comparisons of the axial shaft capacities inferred from signal matching with CPT-based design methods provides insights into the performance of the design methods. A trend for long-term pile shaft set-up to decrease with increasing diameter is identified and ascribed principally to the diameter-dependent constrained dilatancy that develops under axial loading at the pile-sand interface.
Solans D, Kontoe S, Zdravković L, 2023, Comparison of a tailings material and a natural sand in seismic site response analyses, Soil Dynamics and Earthquake Engineering, Vol: 175, ISSN: 0267-7261
This article investigates the seismic response of a tailings sand material in the context of site response analysis. The computed response is contrasted with that obtained for an equivalent deposit consisting of a well-characterised natural sand. Initially, the fundamental properties of both materials are used to interpret and compare their behaviour in the framework of Critical State Soil Mechanics (CSSM). Subsequently, both materials' monotonic and cyclic responses are calibrated for an advanced bounding surface plasticity model. Finally, the impact of different calibration approaches is examined through site response analyses under a strong motion. This process identifies fundamental differences between the two geo-materials, impacting the single elements simulations, especially the accuracy of undrained cyclic triaxial tests. The site response analyses examined herein show the impact of the calibrations adopted for both materials in terms of acceleration response spectra, displacements, and excess of pore water pressures (PWP). Although a relatively deep phreatic surface is adopted, the cyclic resistance and permeability of both materials plays a dominant role in the site response characteristics, controlling the excess PWP generation and hence the non-linear behaviour in the soil deposit. Finally, parametric studies are also conducted to explore the impact of permeability of the sands in the site response.
Koronides M, Kontoe S, Zdravkovic L, et al., 2023, Numerical simulation of SSI free and forced vibration experiments on real scale structures of different stiffness, Soil Dynamics and Earthquake Engineering, Vol: 175, Pages: 1-21, ISSN: 0267-7261
Time domain finite element (FE) analysis is a powerful tool for the study of Soil-Structure-Interaction (SSI) phenomena, but it requires a rigorous calibration of all aspects of the numerical model. This study presents three-dimensional (3D) FE analyses that are calibrated and validated against real scale free and forced vibration experiments on the prototype structure of EUROPROTEAS which is founded on soft alluvial sediments. The proposed calibration procedure exploits data recorded during experiments on structures with different structural stiffness, that mobilised SSI effects at different intensities. Particular focus is placed on the modelling of the soil-foundation interface, where zero thickness elastoplastic interface elements are used to allow foundation separation from the soil. A novel approach to simulate contact imperfections (gaps) between the foundation and the adjacent soil is proposed. The results demonstrate the significant impact of the interface gaps and soil nonlinearity on the response of the examined SSI systems, highlighting the importance of a rigorous model calibration.
Wen K, Kontoe S, Jardine RJ, et al., 2023, An axial load transfer model for piles driven in chalk, Journal of Geotechnical and Geoenvironmental Engineering, Vol: 149, Pages: 1-17, ISSN: 1090-0241
Chalk is encountered under large areas of Northern Europe and other locations worldwide, and a wide range of onshore and offshore structures are founded on piles driven in chalk. The safe and economical design of these structures is challenging because of current uncertainties regarding their axial capacity and load-displacement behavior. This work builds on recent research into the axial capacity of open-ended driven piles by proposing new shaft and base load transfer models for chalk that employ geotechnical properties measured directly in laboratory and in-situ tests. First, this study set out a new closed-form elastic analysis of the initial tension loading response. Then, a new nonlinear shaft model was proposed that captures the radial variation in properties induced by pile installation in chalk and uncouples the piles’ stiffness responses from their ultimate local shaft resistances, which are predicted independently. A new base model was also outlined that predicts the loading response based on the chalk’s small-strain stiffness and cone penetration test (CPT) cone resistances. The models are shown to offer good load-displacement predictions for 0.139-m to 1.8-m diameter piles tested between 20 and 600 days after driving at several sites. Improved reliability and accuracy are demonstrated through comparison with existing load-transfer methods.
Moller JK, Kontoe S, Taborda D, 2023, Combination of kinematic and inertial loads acting on monopile foundations for offshore wind turbines, Symposium on Energy Geotechnics 2023
Vinck K, Liu T, Mawet J, et al., 2023, Field tests on large scale instrumented piles driven in Chalk: results and interpretation, Canadian Geotechnical Journal, Vol: 60, Pages: 1475-1490, ISSN: 0008-3674
The design of large open steel piles driven at chalk sites suffers from considerable uncertainty, leading to major difficulties in many significant onshore and offshore projects. This paper describes recent instrumented driving, monotonic testing to failure and re-strike tests conducted on large open steel piles driven in primarily low- to medium-density chalk at a site in North-western France. The experiments are described and interpreted with reference to a high-quality site characterisation, dynamic and static methods of test analysis and alternative predictive design approaches. Important new conclusions flow regarding driving behaviour, the set-up that took place over up to 65 days after installation and the resistances available under compression and tension loading. Surprisingly large differences are shown between tension and compression shaft capacity which are postulated to be due to Poisson straining in the steel pile shaft and its interaction with the surrounding chalk mass. The field tests contribute to building a high-quality dataset that allows proposed axial capacity design methods to be tested and potentially refined to provide reliable and representative design tools.
Zhu J, Li X, Liang J, et al., 2023, Effects of pulse-like ground motions on tunnels in saturated poroelastic soil for obliquely incident seismic waves, Soil Dynamics and Earthquake Engineering, Vol: 173, Pages: 1-21, ISSN: 0267-7261
It has been widely recognised that near-fault ground motions can be distinctly different from far-fault ground motions, in terms of their amplitudes and spectral characteristics, and there are a number of studies investigating the effects of near-fault ground motions on the structural response. However, only a few studies focus on underground tunnels, and most of them consider vertically incident seismic waves, neglecting the wave passage effect which is critical for long tunnels. This paper investigates the consequences of near-fault pulse-like ground motions on the seismic tunnel response, with an example of a tunnel embedded in saturated poroelastic soil. The input motions are represented by obliquely incident P1 waves, and the wave passage effect along the longitudinal direction is considered by a 2.5D modelling technique. The seismic tunnel response for near-fault pulse-like and far-fault ground motions is compared. Additionally, artificial ground motions, which have consistent acceleration response spectra with the near-fault pulse-like ground motions, but without velocity pulses, are generated to gain further insight into the contribution of velocity pulses. It is shown that the near-fault pulse-like ground motions can noticeably increase the tunnel internal forces, due to large seismic energy associated with the velocity pulses. For pulse-like ground motions with similar acceleration response spectra, the fling-step velocity pulse can be more detrimental to the tunnel than the forward-directivity velocity pulse. Moreover, the amplification effect of the near-fault pulse-like ground motions on the tunnel internal forces tends to be more prominent for large vertical angles of incidence, highlighting the significance of considering the oblique incidence case for seismic design.
Wen K, Kontoe S, Jardine RJ, et al., 2023, Assessment of time effects on capacities of large-scale piles driven in dense sands, Canadian Geotechnical Journal, Vol: 60, Pages: 1015-1035, ISSN: 0008-3674
This paper considers the axial resistances of open-ended, highly instrumented, 763 mm diameter steel pipe piles driven in sands for the EURIPIDES (EURopean Initiative on PIles in DEnse Sands) project at a well characterised research site at Eemshaven, in the northern Netherlands. It offers new analyses of previously unreported dynamic tests and considers their relationship to four heavily instrumented static compression tests. Rigorous signal matching employing two distinct pile-soil interaction models is reported, supported by careful sensitivity analyses, to interpret the recorded driving signals. The back-calculated shaft resistance profiles show good agreement between the models as well as calculations performed with a global wave equation analysis approach. The study highlights the need to account for the internal soil column resistance. The combined interpretation of the dynamic and static test data indicates a 50% gain in shaft resistance over the ten days after driving and threefold shaft capacity growth over a total period of 533 days after driving. The outcomes have important implications for driven pile design and field quality monitoring; the case history contributes an important benchmark in the study of long-term set-up trends.
Wen K, Kontoe S, Jardine R, et al., 2023, A new load transfer model for axially loaded piles driven in chalk, 9th International SUT Offshore Site Investigation and Geotechnics Conference Innovative Geotechnologies for Energy Transition, Publisher: Society for Underwater Technology, ISSN: 0141-0814
Solans D, Kontoe S, Zdravkovic L, 2023, Impact of foundation layer characteristics on the seismic response of a tailings dam, 10th European Conference on Numerical Methods in Geotechnical Engineering, Publisher: International Society for Soil Mechanics and Geotechnical Engineering
The foundation layer thickness and stiffness impact the site response by influencing the fundamental frequenciesand vibration modes in soil structure interaction (SSI) problems. From a practical perspective, the geotechnical characterisation of earthfill dams is typically focused on the borrow materials comprising the dam, while the foundation materials are often under-characterised, with the depth to the bedrock commonly only approximately estimated. In the seismic response of dams, these unknowns may also impact the deformation patterns affecting the overall stability of the dam. A back-analysis of seismic recorded data for an existing tailings sand dam is performed, to determine the thickness and stiffness of the soil foundation layer byfinite element analysis. A cyclic non-linear model (CNL) is employed in the Finite Element analyses which consider different depths to bedrock and soil stiffness profiles. The results suggest satisfactory agreement with the recorded data in terms of acceleration response spectra and amplification ratios and highlight the impact of the foundation layer characteristics on the overall dam response.
Koronides M, Kontoe S, Zdravkovic L, et al., 2023, Numerical simulation of soil-structure interaction experiments on shallow founded structures for different mass configurations, 10th European Conference on Numerical Methods in Geotechnical Engineering, Publisher: International Society for Soil Mechanics and Geotechnical Engineering
Soil-Structure Interaction (SSI) phenomena and foundation rocking can modify the structural response signifi-cantly with respect to the response predicted adopting the fixed-base assumption. The importance of SSI and rocking depends,among other factors, on the structural mass and the distribution of static stresses at the soil-foundation interface. Within thiscontext, an experimental campaign was carried out aiming to investigate the SSI effects on the response of a 3m x 3m x 5m steel-framed structure. The prototype structure, called EUROPROTEAS, was founded on a shallow footing at the well-characterisedEuroseistest site, while its mass was either 18Mgr or 9Mgr. The present study simulates free vibration experiments, placingparticular emphasis on soil nonlinearity and soil-foundation interface. A novel approach to simulate gaps at the soil-foundationinterface, foundation rocking and to manipulate interface stresses under static conditions is presented. The three aspects areshown to significantly affect the response, while they are found to be more important for the lighter structure.
Wen K, Kontoe S, Jardine RICHARD, et al., 2023, Non-linear finite-element analysis of axially loaded piles driven in chalk, 10th European Conference on Numerical Methods in Geotechnical, Publisher: International Society for Soil Mechanics and Geotechnical Engineering
Driven piles are often employed to support onshore and offshore structures at low-density, porous weak carbonatechalk sites, which are encountered across Northern Europe and under the North and Baltic seas. Their efficient design is limitedby uncertainties regarding their ultimate axial capacity and load-displacement behaviour. Intensive axial testing has been under-taken recently for the ALPACA Joint Industry Project on piles driven at a UK chalk site, in conjunction with comprehensivechalk characterisation studies. This paper presents PLAXIS-2D numerical simulations of such piles' axial loading behaviour.The simulation accounts for three distinct zones of chalk identified around the pile shafts after installation. These comprise athin annular zone of de-structured, puttified, chalk and a second, thicker, annular zone of highly fractured chalk; both havedifferent mechanical properties compared to the surrounding parent intact chalk mass. The FE analyses investigate how shaftresistance, axial capacity and load-displacement behaviour develop differently in compression and tension tests.
Ma S, Kontoe S, Taborda D, 2023, On the impact of soil permeability in the numerical simulation of seismically induced liquefaction, 10th European Conference on Numerical Methods in Geotechnical Engineering, Publisher: International Society for Soil Mechanics and Geotechnical Engineering
Moller JK, Kontoe S, Taborda D, et al., 2023, Resonance in offshore wind turbine systems due to seismic loading and extensive soil liquefaction, 10th European Conference on Numerical Methods in Geotechnical Engineering, Publisher: International Society for Soil Mechanics and Geotechnical Engineering
Pedone G, Kontoe S, Zdravkovic L, et al., 2023, A sensitivity study on the mechanical properties of interface elements adopted in finite element analyses to simulate the interaction between soil and laterally loaded piles, 10th European Conference on Numerical Methods in Geotechnical Engineering, Publisher: International Society for Soil Mechanics and Geotechnical Engineering
An increasing number of offshore energy structures have been built recently on driven piles, ranging from jack-et piles with typical length-to-diameter (L/D) ratios of 10-40 to monopiles with far lower L/D ratios. The load-displacementbehaviour of these foundations can be investigated by means of Finite Element (FE) analyses, for instance following the designmethodology developed by the PISA Joint Industry Project (JIP). A challenging aspect of the modelling, for piles loaded eitheraxially or laterally, is the simulation of the behaviour at the soil-pile interface with the adoption of suitable formulations for theinterface elements and with representative mechanical properties. This paper presents a sensitivity study conducted on both theelastic and plastic properties of interface elements adopted in FE analyses of laterally loaded piles driven in chalk. The studybenefited from the extensive field and laboratory test results collected during the ALPACA JIP and the corresponding piletests. The aim of the paper is to provide guidance for numerical modelling on the selection of the most appropriate mechanicalproperties of interface elements to be used in the analyses of soil-pile interaction under lateral loading.
Moller JK, Kontoe S, Taborda D, 2023, Numerical investigation of energy dissipation in liquefiable soil deposits, SECED 2023 Conference
Pedone G, Kontoe S, Zdravkovic L, et al., 2023, Numerical modelling of laterally loaded piles driven in low-to-medium density fractured chalk, Computers and Geotechnics, Vol: 156, Pages: 1-18, ISSN: 0266-352X
Chalk’s sensitive, variable nature poses difficulties for foundation designers. It can present as weak rock and yet be de-structured to very weak putty by dynamic or high-pressure loading. The development of multiple offshore wind farms at north European chalk sites led to the recent ALPACA Joint Industry Programme, which undertook intensive material characterisation and large-scale field testing at St Nicholas at Wade (SNW), Kent, UK to capture and better understand the behaviour of piles driven in fractured low-to-medium density chalk. Noting that lateral loading response is a vital design concern for monopile and jacket supported structures, this paper focuses on 3D Finite Element (FE) modelling of ALPACA’s monotonic lateral loading field tests on open-ended driven tubular steel piles. The brittle chalk is modelled with a strain-softening Mohr-Coulomb model combined with nonlocal regularisation, calibrated meticulously against the ALPACA characterisation dataset. A second, simpler modelling approach, adopting a perfectly-plastic Mohr-Coulomb model, is also explored as a simplified practical alternative. Both approaches can match field lateral capacity and bending moment distributions, after taking due account of pile installation and chalk fracturing effects. The analyses indicate how robust, accurate and cost-effective lateral loading design may be approached for low-to-medium density fractured chalks.
Buckley R, Jardine R, Kontoe S, et al., 2023, Axial cyclic loading of piles in low to medium density chalk, Geotechnique, ISSN: 1021-8637
Comprehensive field investigations into the axial cyclic loading behaviour of open-steel pipe piles driven and aged in low-to-medium density chalk identify the conditions under which behaviour is stable, unstable or metastable. Post-cycling monotonic tests confirmed that stable cycling enhanced pile capacity marginally, while unstable cases suffered potentially large losses of shaft capacity. Metastable conditions led to intermediate outcomes. The patterns by which axial deflections grew under cyclic loading varied systematically with the normalised loading parameters and could be captured by simple fitting expressions. Cyclic stiffnesses also varied with loading conditions, with the highest operational shear stiffnesses falling far below the in-situ seismic test values. The monotonic and cyclic axial responses of the test piles were controlled by the behaviour of, and conditions within, the reconsolidated, de-structured, chalk putty annuli formed around pile shafts during driving. Fibre-optic strain gauges identified progressive failure from the pile tip upwards. Large factors of safety were required for piles to survive repetitive loading under high-level, two-way, conditions involving low mean loads, while low amplitude one-way cycling had little impact. A simple ‘global’ prediction procedure employing interface shear and cyclic triaxial tests is shown to provide broadly representative predictions for field behaviour.
Jardine R, Buckley R, Liu T, et al., 2023, The axial behaviour of piles driven in chalk, Geotechnique, Pages: 1-12, ISSN: 0016-8505
This paper describes research into the poorly understood axial behaviour of piles driven in chalk. Comprehensive dynamic and monotonic axial testing on 27, mostly instrumented, piles undertaken for the ALPACA Joint Industry Projects is reported and interpreted covering: diameters between 139mm and 1.8m; lengths from 3 to 18m; different pile material types; tip and groundwater conditions, and ages after driving. The experiments show the factors that influence resistance most strongly are: (i) pile end-conditions, (ii) slenderness ratio and flexibility, (iii) shaft material, (iv) age after driving, (vi) relative water table depth, and (vii) whether loading is compressive or tensile. Varying the factors systematically identified a remarkable average long-term shaft resistance range from below 11 kPa to more than 200 kPa for piles driven at the same low-to-medium density chalk test site in Kent (UK). Dynamic and static analyses demonstrate that soil resistances to driving (SRD) were generally well-predicted by the Chalk ICP-18 short-term formulation. Considering the piles’ long-term behaviour, the Chalk ICP-18 approach over-predicted capacity, while the widely used CIRIA approach proved over-conservative for most cases. The research enabled the development of a revised ‘ALPACA-SNW’ long-term capacity assessment method that matches the test outcomes far more faithfully.
Liu T, Ferreira PMV, Vinck K, et al., 2023, The behaviour of a low- to medium-density chalk under a wide range of pressure conditions, Soils and Foundations, Vol: 63, ISSN: 0038-0806
Experiments are described which provided the basis for advanced numerical modelling of large-scale axial and lateral pile tests undertaken chalk to assist the design of offshore wind and other projects in northern Europe. The research explored the mechanical behaviour of chalk from a UK research site under effective cell pressures up to 12.8 MPa. When sheared from low confining pressures the chalk’s interparticle bonds contribute a large proportion of the peak deviator stresses available to specimens that crack, bifurcate and dilate markedly after failing at relatively small strains. Progressively more ductile behaviour is seen as pressures are raised, with failures being delayed until increasingly large strains and stable critical states are attained. Loading invokes very stiff responses within the chalk’s (Y1) linear elastic limits and behaviour remains stiff, although non-linear, up to large-scale (Y3) yield points. Near-elliptical Y1 and Y3 yield loci can be defined in q-p′ stress space and a critical state v-p′ curve is identified. The chalk’s initially bonded, high porosity, structure is explored by normalising the shearing and compression state paths with reference to both critical state and intrinsic compression lines. The results have important implications for pile test analysis and practical design in this challenging geomaterial.
Kourelis I, Kontoe S, Buckley R, et al., 2022, An assessment of pile driveability analyses for monopile foundations, 11th International Conference on Stress Wave Theory and Design and Testing Methods for Deep Foundations (SW2022)
Several methodologies to predict the static soil resistance to driving (SRD) available in the literature have found wideuse in the offshore industry over the last decades. These range from simple methods that require few soil strengthparameters to more advanced semi-empirical methods that correlate the driving resistance to cone penetration testmeasurements. These methods were primarily developed based on driving records for piles less than 2.5m in diameteri.e. much smaller than the monopiles currently used in the offshore wind industry today. The aim of this study is toevaluate the accuracy of some of the most widely used SRD prediction methods when employed for driveabilityanalysis of large diameter monopile foundations, by comparing the predicted SRD profiles with the driving records of6.5m diameter monopiles installed in the Danish region of the North Sea.
Solans D, Kontoe S, Zdravkovic L, 2022, Comparison of the monotonic and cyclic response of tailings sands with a reference natural sand, 4rth International Conference on Performance-based Design in Earthquake Geotechnical Engineering, Publisher: Springer
This article considers the monotonic and cyclic behaviour of a tailings sand material, tested over a wide range of confining pressures and relative densities. The collated experimental data are used to interpret the behaviour of this material in the framework of Critical State Soil Mechanics (CSSM) and to calibrate an advanced bounding surface plasticity model. The response of this man-made material is then contrasted, within the same framework, to that of Toyoura sand, which is a well-characterised reference natural sand.This process aims to identify the principal behavioural differences between the two types of geo-materials and the implications that these may have on the calibration process of an advanced constitutive model. Ultimately, the process also supports the numerical modelling of Tailings Storage Facilities under both monotonic and cyclic/earthquake loading.
Koronides M, Kontoe S, Zdravkovic L, et al., 2022, Numerical simulation of real-scale vibration experiments of a steel frame structure on a shallow foundation, 4rth International Conference on Performance-based Design in Earthquake Geotechnical Engineering
Cathie D, Jardine RICHARD, Silvano R, et al., 2022, Pile setup in sand – the "PAGE" joint industry project, 11th International Conference on Stress Wave Theory and Design and Testing Methods for Deep Foundations (SW2022)
The reliability of long-term axial capacity predictions for large, offshore-scale, piles is uncertain. Current databases of static load tests include very few entries with diameters ≥ 1m, and none >2m. Also, most of the available tests were conducted at relatively early ages after driving. The PAGE Joint Industry Project addressed this knowledge gap by collating and analysing dynamic driving data from 25 offshore piles with 1.6 to 3.4m outside diameters and contrasting these with dynamic re-strike tests conducted between 1h and 1 year after driving. Systematic signal matching was performed with two independent codes that applied different soil models and the outcomes were compared with predictions from modern CPT-based static capacity design methods. Additional supporting analyses were performed on other piles, where static and dynamic tests had been conducted, to help assess the relationships between statically and dynamically measured resistances. Piles with 0.3 to 3.5m outside diameters followed broadly common trends overthe first 30 days after driving, with shaft capacities approximately doubling. While smaller (<1m) diameter piles driven at onshore/nearshore sites display marked further capacity growth, larger offshore piles showed little additionalcapacity gain after 30 days. The CPT-based Unified offshore pile design method offered conservative predictions for long-term shaft resistance, while no bias was apparent with the ICP-05 approach. An inverse relationship was identified between long-term shaft setup and diameter, which is ascribed to enhanced dilatancy applying at the pile-sand interface. The base capacities interpreted from dynamic analyses consistently fell far below the monotonic loading capacities predicted by current design methods and showed no significant trend to increase over time.
d’Oriano V, Kontoe S, 2022, Dynamic Properties of Organic Soils, 4rth International Conference on Performance-based Design in Earthquake Geotechnical Engineering
Kontoe S, 2022, The seventeenth Mallet-Milne lecture, BULLETIN OF EARTHQUAKE ENGINEERING, Vol: 20, Pages: 2821-2823, ISSN: 1570-761X
Kontoe S, Summersgill F, Potts D, et al., 2022, On the effectiveness of slope stabilising piles for soils with distinct strain-softening behaviour, Geotechnique, Vol: 72, Pages: 309-321, ISSN: 1021-8637
The stabilisation of slopes with rows of discrete vertical piles is a commonly adopted method for both cuttings as well as embankment slopes. The majority of existing design procedures consider the pile only as an additional force or moment acting on the critical slip surface of the un-stabilised slope. Based on simplified models, existing design methodologies effectively ignore any interaction of the pile with the evolution of the failure mechanism, while they do not consider important aspects of soil behaviour for slope stability relating to strain softening response. This paper presents a numerical investigation that challenges the above-mentioned simplifications, 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 entirely miss the critical failure mechanism.
Koronides M, Kontoe S, Zdravkovic L, et al., 2022, 'Numerical simulations of field soil-structure interaction experiments on a shallow founded steel frame structure, 3rd international Conference on Natural Hazards & Infrastucture
Ahmadi-Naghadeh R, Liu T, Vinck K, et al., 2022, A laboratory characterisation of the response of intact chalk to cyclic loading, Géotechnique, ISSN: 0016-8505
This paper reports the cyclic behaviour of chalk, which has yet to be studied comprehensively. Multiple undrained high-resolution cyclic triaxial experiments on low-to-medium density intact chalk, along with index and monotonic reference tests, define the conditions under which either thousands of cycles could be applied without any deleterious effect, or failure can be provoked under specified numbers of cycles. Intact chalk's response is shown to differ from that of most saturated soils tested under comparable conditions. While chalk can be reduced to putty by severe two-way displacement-controlled cycling, its behaviour proved stable and nearly linear visco-elastic over much of the one-way, stress controlled, loading space examined, with stiffness improving over thousands of cycles, without loss of undrained shear strength. However, in cases where cyclic failure occurred, the specimens showed little sign of cyclic damage before cracking and movements on discontinuities lead to sharp pore pressure reductions, non-uniform displacements and the onset of brittle collapse. Chalk's behaviour resembles the fatigue response of metals, concretes and rocks, where micro-shearing or cracking initiates on imperfections that generate stress concentrations; the experiments identify the key features that must be captured in any representative cyclic loading model.
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