340 results found
Burd HJ, Taborda D, Zdravkovic L, et al., PISA design model for monopiles for offshore wind turbines: application to a marine sand, Geotechnique, ISSN: 0016-8505
This paper describes a one dimensional (1D) computational model for the analysis and design oflaterally-loaded monopile foundations for offshore wind turbine applications. The model represents themonopile as an embedded beam and specially-formulated functions, referred to as soil reactioncurves, are employed to represent the various components of soil reaction that are assumed to act onthe pile. This design model was an outcome of a recently-completed joint industry research project –known as PISA – on the development of new procedures for the design of monopile foundations foroffshore wind applications. The overall framework of the model, and an application to a stiff glacialclay till soil, is described in a companion paper (Byrne et al. 2019b); the current paper describes analternative formulation that has been developed for soil reaction curves that are applicable tomonopiles installed at offshore homogenous sand sites, for drained loading. The 1D model iscalibrated using data from a set of three dimensional finite element analyses, conducted over acalibration space comprising pile geometries, loading configurations and soil relative densities thatspan typical design values. The performance of the model is demonstrated by the analysis of exampledesign cases. The current form of the model is applicable to homogeneous soil and monotonicloading, although extensions to soil layering and cyclic loading are possible.
Byrne BW, Houlsby GT, Burd HJ, et al., PISA design model for monopiles for offshore wind turbines: application to a stiff glacial clay till, Geotechnique, ISSN: 0016-8505
Offshore wind turbines in shallow coastal waters are typically supported on monopile foundations.Although three dimensional (3D) finite element methods are available for the design of monopiles inthis context, much of the routine design work is currently conducted using simplified one dimensional(1D) models based on the p-y method. The p-y method was originally developed for the relativelylarge embedded length-to-diameter ratio (L/D) piles that are typically employed in offshore oil and gasstructures. Concerns exist, however, that this analysis approach may not be appropriate formonopiles with the relatively low values of L/D that are typically adopted for offshore wind turbinestructures. This paper describes a new 1D design model for monopile foundations; the model isspecifically formulated for offshore wind turbine applications although the general approach could beadopted for other applications. The model draws on the conventional p-y approach, but extends it toinclude additional components of soil reaction that act on the pile. The 1D model is calibrated using aset of bespoke 3D finite element analyses of monopile performance, for pile characteristics andloading conditions that span a predefined design space. The calibrated 1D model provides results thatmatch those obtained from the 3D finite element calibration analysis, but at a fraction of thecomputational cost. Moreover, within the calibration space, the 1D model is capable of delivering highfidelity computations of monopile performance that can be used directly for design purposes. This 1Dmodelling approach is demonstrated for monopiles installed in a stiff overconsolidated glacial clay tillwith a typical North Sea strength and stiffness profile. Although the current form of the model hasbeen developed for homogeneous soil and monotonic loading, it forms a basis from which extensionsfor soil layering and cyclic loading can be developed. The general approach can be applied to otherfoundation and soil-structu
Altuhafi FN, Jardine RJ, Georgiannou VN, et al., 2019, Effects of particle breakage and stress reversal on the behaviour of sand around displacement piles, GEOTECHNIQUE, Vol: 69, Pages: 1029-1030, ISSN: 0016-8505
Jardine R, Buckley R, Byrne B, et al., 2019, Research to improve the design of driven pile foundations in chalk: the ALPACA project, Coastal Structures 2019
Quinteros S, Gundersen A, L'Heureux JS, et al., 2019, Øysand research site: Geotechnical characterisation of deltaic sandy-silty soils, AIMS Geosciences, Vol: 4, Pages: 750-783, ISSN: 2471-2132
This paper describes the geology and geotechnical engineering properties of the fluvial and 18 deltaic gravelly-sandy-silty sediments at Øysand, Norway. Geophysical and geotechnical site 19 investigations carried out between 2016 and 2018 at the site are presented. Field testing included state-20 of-the-practice and state-of-the-art soil characterisation techniques such as total sounding, seismic 21 cone penetration testing, seismic flat dilatometer, multichannel analysis of surface waves, electrical 22 resistivity tomography, ground penetrating radar, piezometers, thermistors strings, slug tests, and 23 permeability tests using a newly developed CPT permeability probe from NGI. Several sampling 24 techniques were used at the site to assess sample quality. Laboratory testing consisted of index tests 25 and advanced triaxial tests with bender elements to estimate shear strength and stiffness. Data 26 interpretation, engineering soil properties and state variables derived from this analysis are presented, 27 along with comments on data quality. Engineering problems investigated at Øysand so far, are related 28 to: the impact of using different CPTU types, sample quality assessment by obtaining soils with state-29 of-the-practice and state-of-the-art techniques (such as gel push sampler and ground freezing), and 30 frost heave susceptibility.
Buckley R, Jardine R, Kontoe S, et al., 2019, The design of axially loaded driven piles in chalk, XVII European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: Icelandic Geotechnical Society
The behaviour of driven piles in chalk is poorly understood; their installation resistance, set-up characteristics and response to cyclic and static loading all warrant further investigation. Current axial capacity design methods have poor reliability, particularly in low-medium density chalk. This paper gives an overviewof research which combined systematic investigations at an onshore chalk site in Kent, UK, with careful analysis of large scale offshore tests. The onshore studies involved reduced-scale open-ended driven piles and heavily instrumented closed-ended Imperial College Piles. The offshore analyses addressed static and dynamic pile tests conducted on full scale open-ended steel tubular piles driven in glacial till and low-to-medium density chalk. The understanding drawn from both streams of research form the basis for a new Chalk ICP-18 effective stress-based design approach, which centres on the key physical phenomena identified: (i) the close correlation between pile resistances and local variations in CPT cone resistance (ii) the marked effect of the relativedepth, h/R*, of the pile tip below any given chalk horizon (iii) the effective stress shaft interface shear failure characteristics and (iv) very significant capacity gains over time. The new method offersbetter predictions of field behaviourwith time than the current industry method.
Jardine R, Kontoe S, Liu T, et al., 2019, The ALPACA research project to improve design of piles driven in chalk, XVII European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: Icelandic Geotechnical Society
Chalk is present under large areas of NW Europe as a low-density, porous,weak carbonate rock. Large numbers of offshore wind turbines, bridgesand port facilities rely on piles driven in chalk. Current European practice assumesultimate shaft resistances that appear low in comparison with the Chalk’s unconfined compression strength and CPT cone resistance rangesand can impact very significantly on project economics. Little guidance is available on pile driveability, set-up or lateral resistance in chalk, or on how piles driven in chalk can sustain axial or lateral cyclic loading. This paper describes the ALPACA (Axial-Lateral Pile Analysis for Chalk Applying multi-scale field and laboratory testing) projectfunded by EPSRC and Industry that is developingnew design guidance through comprehensive field testing at awell-characterised low-to-medium density test site, supported by analysis of other tests. Field experiments on 36driven piles, sixteen of which employ high resolution fibre-optic strain gauges, is supported by advanced laboratory and in-situ testing, as well as theoretical analysis. The field work commenced in October 2017 andwas largely complete inMay2019.
Taborda D, Zdravkovic L, Potts DM, et al., Finite element modelling of laterally loaded piles in a dense marine sand at Dunkirk, Géotechnique, ISSN: 0016-8505
The paper presentsthedevelopment of a three-dimensional finite element model for pile tests in dense Dunkirk sand,conducted as part of the PISA project.The projectwas aimed at developing improved design methods for laterally loadedpiles, as used in offshore wind turbine foundations. The importance of the consistent and integrated interpretation of the soil data from laboratory and field investigations is particularly emphasised. The chosen constitutive model for sand is an enhanced version of the state parameter-based bounding surface plasticity model, which, crucially, is able to reproduce the dependency of sand behaviour on void ratio and stress level. The predictions from three-dimensional finite element analyses,performed before the field tests, show good agreement with the measured behaviour, proving the adequacy of the developed numerical model and the calibration process for the constitutive model. This numerical model directly facilitated the development of new soil reaction curves for use in Winkler-type design models for laterally loaded piles in natural marine sands.
Dominguez-Quintans C, Quinteros VS, Carraro JAH, et al., Quality assessment of a new in-mould slurry deposition method for triaxial specimen reconstitution of clean and silty sands, 7th International Symposium on Deformation Characteristics of Geomaterials - IS Glasgow 2019
An innovative specimen reconstitution technique for sandy and silty soils that simulates underwater deposition is presented and evaluated. The technique is an upgraded version for triaxial testing of the well-established slurry deposition method. This novel setup integrates the reconstitution mould and the mixing tube into a single unit to avoid transferring the sample from the mixing tube to the mould. This subtle, but critical, modification enables reconstitution of very loose specimens as sample transfer disturbance, which can be significant, is eliminated. The quality of specimens prepared by the new reconstitution method was assessed by experiments on a clean sand from the UK (Ham River sand) and a silty sand from Norway (Øysand). The method, as any slurry-based procedure, is capable of producing homogeneous specimens with high initial degree of saturation, even in the absence of back pressure. The procedure is shown to be suitable for sands with or without fines. Moreover, the new method is able to achieve a wide range of initial void ratios, from very loose to very dense, without imposing any particle crushing in the latter case.
Vinck K, Liu T, Ushev E, et al., 2019, An appraisal of end conditions in advanced monotonic and cyclic triaxial testing on a range of geomaterials, 7th International Symposium on Deformation Characteristics of Geomaterials, Publisher: EDP Sciences, Pages: 1-6
Compressing samples between rigid platens, as in triaxial testing, induce non-uniform specimen stress, strain and pore water distributions. Although well recognised historically, the effects of such platen restraints are often disregarded or overlooked when performing or interpreting monotonic and cyclic experiments. This paper presents an updated appraisal of end conditions based on laboratory experiments run on sand, glacial till, intact and puttified chalk as part of offshore piling research projects. Monotonic and cyclic triaxial tests are reported that incorporated local strain and pore pressure sensors and a range of platen configurations. New insights are reported regarding the small-to-large behaviour and undrained cyclic pore water pressure measurement.
Chan DLH, Buckley RM, Liu T, et al., 2019, Laboratory investigation of interface shearing in chalk, 7th International Symposium on Deformation Characteristics of Geomaterials, Publisher: EDP Sciences, Pages: 1-6
<jats:p>Chalk, a soft fine-grained Cretaceous limestone, is encountered across northern Europe where recent offshore windfarm, oil, gas and onshore developments have called for better foundation design methods, particularly for driven piles whose shaft capacities are controlled by an effective stress Coulomb interface failure criterion. Interface type and roughness is known to affect both interface friction angles, δ′ and the magnitude of dilation required for shaft failure to develop. Site-specific interface ring-shear tests are recommended for offshore pile design in sands and clays to account for driven pile shaft materials, roughnesses and shear displacements. However, few such tests have been reported for chalks and it is also unclear whether δ′ angle changes contribute to the striking axial capacity increases, or set-up, noted over time with piles driven in chalk. This paper describes an interface shear study on low-to-medium density chalk from the St. Nicholas-at-Wade research test site in Kent, UK, where extensive field driven pile studies have been conducted [1, 2]. Direct shear and Bishop ring shear apparatus were employed to investigate the influences of interface material and surface roughness, as well as ageing under constant normal effective stresses (σn'). It is shown that the high relative roughness of the interface compared to the chalk grain size results in the ultimate interface shearing angles falling close to the chalk-chalk shearing resistance angles. The δ′ angles also increased by up to 5° over 38 days of ageing.</jats:p>
Liu T, Chen H, Buckley RM, et al., 2019, Characterisation of sand-steel interface shearing behaviour for the interpretation of driven pile behaviour in sands, 7th International Symposium on Deformation Characteristics of Geomaterials, Publisher: EDP Sciences, Pages: 1-6
The installation and loading of steel piles driven in sands modifies both the piles' surface topography and the characteristics of the granular materials present adjacent to the pile shaft. Large-displacement ring shear interface tests incorporating pre-conditioning stages are capable of reproducing such physical processes in the laboratory and can generate case-specific interface design parameters. This paper summarises laboratory research that characterised the interface shearing behaviour of three natural sandy soils retrieved from field test sites (Dunkirk, France; Blessington, Ireland; Larvik, SE Norway) where extensive piling studies on micro and industrial scale driven piles have been carried out. The programme examined the influences of soil characteristics (physical properties and chemical compositions), interface type (mild steel or stainless steel) and surface roughness, and highlighted the significant effects of normal effective stress level and ageing time duration. Remarkable trends of increasing interface friction angles with elevated normal effective stress levels and prolonged ageing were observed. The results from supplementary small-displacement direct shear interface tests and triaxial tests are also reported. The experiments are interpreted with reference to earlier studies to develop an overview of interface shearing characteristics between steels and sandy soils and provide important insights into the mechanisms of axial capacity increases applying to steel piles driven in sands.
Buckley R, Jardine R, Kontoe S, et al., Full-scale observations of dynamic and static axial responses of offshore piles driven in chalk and tills, Géotechnique, ISSN: 0016-8505
This paper describes and interprets tests on piles driven through glacial tills and chalk at a Baltic Sea windfarm, covering an advance trial campaign and later production piling. The trials involved six instrumented 1.37m diameter steel open-ended tubes driven in water depths up to 42m. Three piles were tested statically, with dynamic re-strike tests on paired piles, at 12-15 week ages. Instrumented dynamic driving and re-strike monitoring followed on up to 3.7m diameter production piles. During driving, the shaft resistances developed at fixed depths below sea-bed fell markedly during driving, with particularly sharp reductions occurring in the chalk. Shaft resistances increased markedly after driving and good agreement was seen between long-term capacities interpreted from parallel static and dynamic tests. Analyses employing the sites’ geotechnical profiles show long-term shaft resistances in the chalk that far exceed those indicated by current design recommendations, while newly proposed procedures offer good predictions. The shaft capacities mobilised in the low-plasticity tills also grew significantly over time, within the broad ranges reported for sandy soils. The value of offshore field testing in improving project outcomes and design rules is demonstrated; the approach described may be applied to other difficult seabed conditions.
Jardine R, Buckley R, Byrne B, et al., 2019, Rationalising the design of piles driven in chalk through the ALPACA project, 2nd International Conference on Natural Hazards & Infrastructure
Jardine R, Geotechnics, energy and climate change: The 56th Rankine Lecture, Géotechnique, ISSN: 0016-8505
Geotechnical Engineering has matured sufficiently to contribute to resolving some of society’s grand challenges. The 56th Rankine Lecture considered one of the most pressing global problems: maintaining vital energy supplies while also recognising, mitigating and reducing the climate consequences of fossil fuel consumption. This written version reports geotechnical research relating to these wide-ranging issues, considering paired topics within its three main parts and illustrating these with specific practical examples. Part 1 focuses on supporting offshore hydrocarbon production, considering advances in understanding and designing the driven piles that support most continental shelf platforms, before moving to the large underwater landslides that can affect deeper water developments. Part 2 describes investigations into the geotechnical impact of climate change in a permafrost region and a peatland study that contributes to alleviating flood risks exacerbated by climate change. Part 3 outlines research that is improving the economics of renewable offshore wind energy for multi-pile and monopile supported turbines. Integrating geology and rigorous analysis with advanced laboratory and field experiments is shown to be essential to resolving the complex geotechnical problems considered, as is careful full-scale checking and monitoring. Close cooperation with co-workers from industry and academia was central to the studies described and the contributions of many collaborators are emphasised. The concluding section identifies examples of significant questions from each of the six topic areas that remain to be resolved fully.
Carroll R, Carotenuto P, Dano C, et al., 2019, Field experiments at three sites to investigate the effects of age on steel piles driven in sand, Géotechnique, Pages: 1-21, ISSN: 0016-8505
This paper investigates the influences that steel type, in situ soil properties, water table depth, pile diameter, roughness and driving procedures have on the ageing behaviour of piles driven in sand. Tension tests have been performed on 51 open-ended steel micro-piles, with 48 to 60 mm outside diameter, driven at well-established research sites at Larvik in Norway, Dunkirk in France and Blessington in Ireland to better understand the processes that control axial capacity set-up trends in the field. Mild steel, stainless and galvanised steel micro-piles were driven and left to age undisturbed for periods of between 2 h and 696 days before being subjected to first-time axial tension load tests. In addition to reporting and interpreting these experiments, further investigations of the sites’ geotechnical profiles are reported, including new piezocone and seismic cone penetration soundings as well as laboratory tests. Integration with earlier ageing studies at the same sites with larger (340 to 508 mm outside diameter) open-ended steel piles driven to 7 to 20 m embedments and experiments that varied the piles’ initial surface roughness shows that corrosion, pile scale, roughness, the bonding of soil particles and the driving process can all be highly significant. New insights are gained into the mechanisms that control the axial capacity of piles driven in sand.
Byrne BW, Burd HJ, Gavin K, et al., 2019, PISA: Recent developments in offshore wind turbine monopile design, 1st Vietnam Symposium on Advances in Offshore Engineering, Publisher: Springer
This paper provides a brief overview of the Pile Soil Analysis (PISA) project, recently completed in the UK. The research was aimed at developing new design methods for laterally loaded monopile foundations, such as those supporting offshore wind turbine structures. The paper first describes the background to the project and briefly outlines the key research elements completed. The paper concludes with a brief description of the anticipated impact of the work and describes initiatives that have followed since.
Zdravkovic L, Taborda D, Potts D, et al., Finite element modelling of laterally loaded piles in a stiff glacial clay till at Cowden, Géotechnique, ISSN: 0016-8505
The PISA project was a combined field testing/numerical modelling study with the aim ofdevelopingimproved design procedures for large diameter piles subjected to lateral loading. This paper describes the development ofa three-dimensional finite elementmodel for the medium-scale pile tests that were conducted in Cowden tillas part of the PISA work.The paper places particular emphasis on the consistent interpretation of the soil data determined from the available field and laboratory information.An enhancedversion of the modified Cam clay model was employedin the numerical analyses, featuring a non-linear Hvorslevsurface, a generalised shape for the yield and plastic potential surfaces in the deviatoric planeand a non-linear formulation for the elastic shear modulus.Three-dimensional finite element analyses were performed prior to the field tests.Excellent agreement between the measured and simulated behaviourfora range of pile geometrieswas observed, demonstrating the accuracy of the numerical model and the adequacy of the calibration process for theconstitutive model.The developed numerical modelconfirmed the premise of the PISA design method that site-specific ground characterisation and advanced numerical modelling candirectly facilitate the development of additionalsoil reaction curves for use in new design models for laterally loaded piles in a stiff clay till.
Byrne B, McAdam RA, Burd HJ, et al., Monotonic laterally loaded pile testing in a stiff glacial clay till at Cowden, Géotechnique, ISSN: 0016-8505
This paper describes theresults obtained from a field testing campaign on laterally-loaded monopiles conducted at Cowden, UK, where the soil consists principally of aheavilyoverconsolidatedglacial till. These tests formed part of the PISA projecton the development of improved design methods for monopile foundations for offshore wind turbines. Results obtained for monotonic loading tests on piles of three different diameters (0.273m, 0.762m and 2.0m) are presented. The piles had length-to-diameter ratios (L/D) of between 3 and 10. Thetests includedthe application of monotonic loading incorporating periods of constant load to investigate creep effects,and investigations on the influence of loading rate. Data are presented on measured bending moments and inclinations induced in the piles. Inferred data on lateral displacements of the embedded section of the pilesare determined usingan optimisedstructural model. Thesefield data support the developmentof a new 1D modelling approach forthe design of monopile foundations for offshore wind turbines.They also form a unique database of field measurements in an overconsolidated clay, from lateral loading of piles at a vertical distance abovethe ground surface.
Buckley R, Jardine R, Kontoe S, et al., 2018, Effective stress regime around a jacked steel pile during installation, ageing and load testing in chalk, Canadian Geotechnical Journal, Vol: 55, Pages: 1577-1591, ISSN: 0008-3674
This paper reports experiments with 102 mm diameter closed-ended instrumented Imperial College piles (ICPs) jacked into low- to medium-density chalk at a well-characterized UK test site. The “ICP” instruments allowed the effective stress regime surrounding the pile shaft to be tracked during pile installation, equalization periods of up to 2.5 months, and load testing under static tension and one-way axial cyclic loading. Installation resistances are shown to be dominated by the pile tip loads. Low installation shaft stresses and radial effective stresses were measured that correlated with local cone penetration test (CPT) tip resistances. Marked shaft total stress reductions and steep stress gradients are demonstrated in the vicinity of the pile tip. The local interface shaft effective stress paths developed during static and cyclic loading displayed trends that resemble those seen in comparable tests in sands. Shaft failure followed the Coulomb law and constrained interface dilation was apparent as the pile experienced drained loading to failure, although with a lesser degree of radial expansion than with sands. Radial effective stresses were also found to fall with time after installation, leading to reductions in shaft capacity as proven by subsequent static tension testing. The jacked, closed-ended, piles’ ageing trends contrast sharply with those found with open piles driven at the same site, indicating that ageing is affected by pile tip geometry and (or) installation method.
Zdravkovic L, Jardine R, Taborda DMG, et al., Ground characterisation for PISA pile testing and analysis, Géotechnique, ISSN: 0016-8505
This paper is the first of a set of linked publications on the PISA Joint Industry Research Project, which was concerned with the development of improved design methods for monopile foundations in offshore wind applications. PISA involved large-scale pile tests in overconsolidated glacial till at Cowden, north-east England,and in dense normally consolidated marine sand at Dunkirk, northern France. The paper presents the characterisationof the two sites, whichwas crucial to the design of the field experiments and advanced numerical modelling of the pile-soil interactions. The studies described, which had to be completed at an early stage of the PISA project, added new laboratory and field campaignsto historic investigations at both sites.Theyenabledanaccurate description ofsoilbehaviour from small strains to ultimate statesto be derived, allowing analyses to be undertaken that captured both the serviceability and limit statebehaviour of the test monopiles.
Burd HJ, Beuckelaers WJAP, Byrne BW, et al., New data analysis methods for instrumented medium scale monopile field tests, Géotechnique, ISSN: 0016-8505
The PISA Joint Industry Research Project was concerned with the development of improved design methods for monopile foundations in offshore wind applications. PISA involved large-scale pile tests in overconsolidated glacial till at Cowden, north-east England, andin dense normally consolidated marine sand at Dunkirk, northern France. This paper describes the experimental set up for pile testing, with unique features of load-application mechanisms and built-in fibre optic strain gauges.New procedures are describedfor the interpretation of pile loading data, and specifically for providing precise interpretation of pile displacements.
McAdam RA, Byrne BW, Houlsby GT, et al., Monotonic lateral loaded pile testing in a dense marine sand at Dunkirk, Géotechnique, ISSN: 0016-8505
Theresults obtained from a field testing campaignon laterally-loaded monopiles,conducted ata dense sand site inDunkirk, Northern Franceare described.These tests formed part of thePISA projecton the development of improved design methods for monopile foundations for offshore wind turbines. Results obtained frommonotonic loading tests on piles of three different diameters (0.273m, 0.762m and 2.0m) are presented. The piles had aspectratios (L/D) of between 3 and 10. Thetests consisted principally of the application of monotonic loads,incorporating periods of held constant load to investigate creep effects.The influence of loading ratewas also investigated. Data are presented on the overall load displacement behaviour of each of the test piles. Measured data on bending moments and inclinations induced in the pilesare also provided.Inferences are made forthe displacements in the embedded length of the piles. Thesefield data will support the development of a new 1D modelling approach forthe design of monopile foundations for offshore wind turbines.They also form a unique database of field measurements in a dense sand, from lateral loading of piles at a vertical distance abovethe ground surface.
Jardine RJ, Yang ZX, 2018, Joint research into the behaviour of driven piles, China–Europe Conference on Geotechnical Engineering, Publisher: Springer, Pages: 961-972, ISSN: 1866-8755
Large driven piles are used widely in both onshore and offshore construction. Predicting their limiting capacities and load-displacement behaviour under a range of static and cyclic, axial, lateral and moment loading conditions is critical to many engineering applications. This paper reviews relevant recent joint research by groups at Imperial College London (ICL) and Zhejiang University China (ZJU). Two tracks of enquiry are outlined: (i) assembling and analysing a major and open database of high quality load tests conducted on industrial scale piles at well characterised sites; and (ii) modelling the effective stress regime developed around piles driven in sands. Both avenues of research are vital to enabling scientifically well-founded and yet industrially credible improvements to practical pile design methods. The scope of future joint research is also outlined.
Altuhafi F, Jardine RJ, Georgiannou VN, et al., 2018, Effects of particle breakage and stress reversal on the behaviour of sand around displacement piles, Géotechnique, Vol: 68, Pages: 546-555, ISSN: 0016-8505
The stresses acting in the soil mass adjacent to the tips and shafts of displacement piles during installation and loading in medium-dense sand have been simulated in triaxial stress path tests on Fontainebleau NE34 sand. The very high normal and shear stresses recorded in calibration chamber model pile tests involving the same sand were first reproduced in high-pressure triaxial tests, so changing the sand's physical properties markedly. The behaviour of the mutated sand was then examined in second, lower stress, stages of the same experiments, demonstrating important changes in the sand's mechanical behaviour, including a significant increase in the angle of shearing resistance and a relocation of the sand's critical state line in the e−log p′ plane. Image analysis confirmed changes in the sand particles' micro-characteristics. The particles' size distributions altered and grain surface roughness increased markedly, while particle sphericity was only mildly affected. Similar surface roughness changes were noted between the particulate characteristics of specimens examined after the triaxial laboratory tests and those sampled from around the shafts of the calibration chamber model piles.
Buckley RM, Jardine R, Kontoe S, et al., 2018, Ageing and cyclic behaviour of axially loaded piles driven in chalk, Geotechnique, Vol: 68, Pages: 146-161, ISSN: 1751-7656
This paper reports a programme of static and cyclic loading tests on seven open steel tubes driven in low to medium density chalk at a well characterised test site, describing their response to driving, ageing in situand loading under both static and cyclic conditions. Back analysis of dynamic monitoring identifies the distributions of notably low shaft resistances that develop during installation, showing that these depend strongly on the relative pile tip depth (h/R). The shaft capacities available to ‘virgin’ piles are shown to increase markedly after driving, following a hyperbolic trend that led to a fivefold gain after 250days.Pre-failed piles do not follow the same trend when re-tested. Pile exhumation confirmed that driving remoulded the chalk, creating a puttified zone around the shaft. Excess pore water pressure dissipation, whichis likely to have been rapid during and after driving,led to markedly lower water contents close to the shaft. Axial cyclic testing conducted around 250 days after driving led to a range of responses, from manifesting stable behaviour over 1000 cycles to failing after low numbers of cycles after developing sharp losses of static capacity. The dependence of permanent displacement on the cyclic loading parameters is explored and characterised. The experiments provide the first systematic study of which the Authors are aware into the effects ofundisturbed ageingand cyclic loading onpreviously unfailedpiles driven in chalk. Potential predictive tools may now be tested against the reported field measurements.
Carotenuto P, Meyer VM, Stem PJ, et al., 2018, Installation and axial capacity of the Sheringham Shoal offshore wind farm monopiles - A case history, Pages: 117-122
© 2018 Engineering in Chalk - Proceedings of the Chalk 2018 Conference. All rights reserved. The Sheringham Shoal offshore wind farm is located in the North Sea, approximately 20 km north of the Norfolk coast. At this location, Quaternary sediments of the Swarte Bank, Egmond Ground, Bolders Bank and Botney Cut Formations are found directly on top of the Cretaceous chalk. The chalk is thought to be similar to the high porosity upper Campanian (84-74 million years) chalks found in Norfolk and to be comprised of the Paramoudra and Beeston Chalks. Monopile foundations we re installed at the Sheringham Shoal Offshore Wind Farm in Summer 2011. Low driving resistances were encountered during installation of some of the monopiles, particularly in areas with shallow subcropping chalk. In some instances pile 'run-out' occurred (i.e. continued penetration under the weight of the pile and hammer alone). This paper presents a case history of the available information, including an overview of the geological and geotechnical conditions at the site together with the pile driving records. Results from back-analysis of the driving records to assess the Soil Resistance to Driving (SRD) are presented and provide possible explanations for the observed pile driving responses. The long-term axial capacity of the monopiles is discussed. The properties of the chalk are based on a review of the available literature and site-specific laboratory and field tests.
© 2018 Engineering in Chalk - Proceedings of the Chalk 2018 Conference. All rights reserved. Driving resistance is difficult to predict in chalk strata, with both pile free-fall self-weight 'runs' and refusals being reported. Axial capacity is also highly uncertain after driving. This paper reviews recent research that has explored these topics. Programmes of onshore tests and novel, high-value offshore, experiments involving static, dynamic and cyclic loading are described. The key findings form the basis of the Chalk ICP-18 approach for estimating the driving resistance and axial capacity of piles driven in low-to medium-density chalk. The new approach is presented and the highly significant impact of set-up after driving is emphasised. It is shown that Chalk ICP-18 overcomes the main limitations of the industry's current design guidelines by addressing the underlying physical mechanisms. While further tests are required to enlarge the available test database, the new approach is able to provide better predictions for tests available from suitably characterised sites. A new Joint Industry Project is outlined that extends the research to cover further axial, lateral, static and cyclic loading cases.
Doughty LJ, Buckley RM, Jardine RJ, 2018, Investigating the effect of ageing on the behaviour of chalk putty, Pages: 695-701
© 2018 Engineering in Chalk - Proceedings of the Chalk 2018 Conference. All rights reserved. Substantial recent investment in offshore wind energy developments and other foundation projects in chalk dominated locations has created an urgent need for a better understanding of how driven piles behave in this variable and unpredictable material. Pile driving in chalk is known to create a remoulded zone of chalk 'putty' around the pile which mobilises low shaft resistance on installation; however, shaft capacity has been reported to increase over time through a process referred to as pile 'set-up' or ageing. Although field evidence of ageing has been reported, the potential role of the chalk putty's behaviour in the ageing process is not well understood. This is partly due to a lack of published laboratory testing on chalk samples which have been conditioned to represent the conditions adjacent to a pile after driving. This paper presents selected results from a suite of laboratory tests undertaken at Imperial College London. Specimens from intact Maas-trichtian Chalk were reduced to putty by Proctor compaction applied at natural water content and subjected to either drained or undrained ageing for periods of between 0 and 28 days. Consolidated undrained triaxial tests, with pore water pressure measurement and local strain measurements, were undertaken on the aged specimens, along with thixotropy testing on unconsolidated samples employing a fall cone. An interpretation of the test results within a critical state framework combined with a conceptual small strain stiffness model is presented. The ageing of chalk putty is shown to encompass a combination of consolidation, thixotropic hardening and re-cementation, with small increases in undrained shear strength seen due to thixotropic hardening, and larger increases in strength evident following consolidation.
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.
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