Publications
133 results found
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.
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.
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.
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.
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.
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.
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 J, 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, 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.
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, 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.
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.
Vinck K, Liu T, Jardine RJ, et al., 2022, Advanced in-situ and laboratory characterisation of the ALPACA chalk research site, Géotechnique, ISSN: 0016-8505
Low-to-medium density chalk at St Nicholas at Wade, UK, is characterised by intensive testing to inform the interpretation of axial and lateral tests on driven piles. The chalk de-structures when taken to large strains, especially under dynamic loading, leading to remarkably high pore pressures beneath penetrating CPT and driven pile tips, weak putty annuli around their shafts and degraded responses in full-displacement pressuremeter tests. Laboratory tests on carefully formed specimens explore the chalk's unstable structure and markedly time and rate-dependent mechanical behaviour. A clear hierarchy is found between profiles of peak strength with depth of Brazilian tension (BT), drained and undrained triaxial and direct simple shear (DSS) tests conducted from in-situ stress conditions. Highly instrumented triaxial tests reveal the chalk's unusual effective stress paths, markedly brittle failure behaviour from small strains and the effects of consolidating to higher than in-situ stresses. The chalk's mainly sub-vertical jointing and micro-fissuring leads to properties depending on specimen scale, with in-situ mass stiffnesses falling significantly below high-quality laboratory measurements and vertical Young's moduli exceeding horizontal stiffnesses. While compressive strength and stiffness appear relatively insensitive to effective stress levels, consolidation to higher pressures closes micro-fissures, increases stiffness and reduces anisotropy.
Liu T, Ahmadi-Naghadeh R, Vinck K, et al., 2022, An experimental investigation into the behaviour of de-structured chalk under cyclic loading, Géotechnique, ISSN: 0016-8505
Low-to-medium density chalk can be de-structured to soft putty by high-pressure compression, dynamic impact or large-strain repetitive shearing. These process all occur during pile driving and affect subsequent static and cyclic load-carrying capacities. This paper reports undrained triaxial experiments on de-structured chalk, which shows distinctly time-dependent behaviour as well as highly non-linear stiffness, well-defined phase transformation (PT) and stable ultimate critical states under monotonic loading. Its response to high-level undrained cyclic loading invokes both contractive and dilative phases that lead to pore pressure build-up, leftward effective stress path drift, permanent strain accumulation, cyclic stiffness losses and increasing damping ratios that resemble those of silts. These outcomes are relatively insensitive to consolidation pressures and are distinctly different to those of the parent intact chalk. The maximum number of cycles that can be sustained under given combinations of mean and cyclic stresses are expressed in an interactive stress diagram which also identifies conditions under which cycling has no deleterious effect. Empirical correlations are proposed to predict the number of cycles to failure and mean effective stress drift trends under the most critical cyclic conditions. Specimens that survive long-term cycling present higher post-cyclic stiffnesses and shear strengths than equivalent ‘virgin’ specimens.
Buckley R, Jardine R, Kontoe S, 2021, In situ testing in low-medium density structured chalk, 6th International Conference on Geotechnical and Geophysical Site Characterization
Buckley R, Kontoe S, Jardine R, et al., 2021, Pile driveability in low-to-medium density chalk, Canadian Geotechnical Journal, Vol: 58, Pages: 650-665, ISSN: 0008-3674
Pile driving in low- to medium-density chalk is subject to significant uncertainty. Predictions of “chalk resistance to driving” (CRD) often vary considerably from field driving behaviour, with both pile refusals and free falls under zero load being reported. However, recent field studies have led to better understanding of the processes that control the wide range of behaviour seen in the field. This paper describes the primary outcomes of the analysis of dynamic tests at an onshore and an offshore site and uses the results to propose a new method to predict CRD. The method is based on phenomena identified experimentally: the relationship between cone penetration resistance and CRD, the attenuation of local stresses as driving advances, and the operational effective stress interface shear failure characteristics. The proposed method is evaluated through back-analyses of driving records from independent pile installation cases that were not included in developing the method, but involved known ground conditions, hammer characteristics, and applied energies. The proposed method is shown to lead to more reliable predictions of CRD than the approaches currently applied by industry.
Buckley R, Byrne BW, Doherty JP, et al., 2021, Measurements of distributed strain during impact pile driving, Piling 2020, Publisher: ICE Publishing
This paper reports the use of optical Fibre Bragg Grating (FBG) sensors to monitor the stress waves generated below ground during pile driving, combined with measurements using conventional pile driving analyzer (PDA) sensors mounted at the pile head. Fourteen tubular steel piles with a diameter of 508 mm and embedded length to diameter ratios of 6 to 20 were impact driven at an established chalk test site in Kent, UK. The pile shafts were instrumented with multiple FBG strain gauges and pile head PDA sensors, which monitored the piles’ responses under each hammer blow. A high frequency (5 kHz) fibre optic interrogator allowed a previously unseen resolution of the stress wave propagation along the pile. Estimates of the base soil resistances to driving and distributions of shaft shear resistances were found through signal matching that compared time series of pile head PDA measurements and FBG strains measured below ground surface. Numerical solutions of the one-dimensional wave equation were optimised by taking account of the data from multiple FBG gauges, leading to significant advantages that have potential for widespread application in cases where high resolution strain measurements are made.
Tsaparli V, Kontoe S, Taborda D, et al., 2020, Resonance as the source of high vertical accelerations: field demonstration and impact on offshore wind turbines, 4th International Symposium on Frontiers in Offshore Geotechnics
Recent studies have demonstrated the significance of the vertical seismic acceleration component for offshore wind turbines, as their low natural period in this direction can result in significant excitation, potentially making this load case design-driving. Unexpectedly high vertical ground accelerations, well exceeding their horizontal counterparts, have also been recorded in a number of recent seismic events. This study explores the concept of resonance between the vertical seismic component and the natural frequency for compressional waves of fully saturated soil deposits, which can aggravate further the vertical accelerations at the top of structures of interest, using numerical analysis and monitoring data. The site response at a strong motion station that registered the second highest peak ground vertical acceleration during the 2011 Mw 6.2 Christchurch earthquake in New Zealand is modelled in finite element analyses. Two different depths are also considered: the first one is truncated at the interface of the softer surficial deposits with the stiff gravel horizon. This has been shown to be adequate for S-wave propagation modelling. Conversely, the second one models the full depth to bedrock. Despite the number of uncertainties involved, the results validate the concept of resonance in compression against field measurements and demonstrate the importance of the modelled depth in the case of vertical site response analysis.
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