359 results found
Pan K, Liu XM, Yang ZX, et al., 2021, Undrained Cyclic Response of K<inf>0</inf>-Consolidated Stiff Cretaceous Clay under Wheel Loading Conditions, Journal of Geotechnical and Geoenvironmental Engineering, Vol: 147, ISSN: 1090-0241
Optimal whole life design for railways, highways, runways, and metro lines requires an accurate assessment of how their underlying geomaterials respond to large numbers of wheel-loading cycles. This paper presents an experimental study on a natural UK stiff clay with a cyclic triaxial (CT) and hollow cylinder apparatus (CHCA) that imposed K0 and wheel-loading stress conditions. The focus is on Gault clay, a high overconsolidation ratio (OCR) marine clay deposited in the Cretaceous, whose mechanical behavior is significantly anisotropic and in situ K0 values exceed unity. The clay outcrops under sections of most major highways radiating out of London, as well as the HS1 and new HS2 high-speed railways. The experimental investigation explored how the principal stress rotation implicit in wheel loading increases the magnitudes and changes the sign of vertical strain accumulation, as well as accelerating resilient modulus degradation and accentuating stress-strain hysteresis, all of which affect pavement or rail-track serviceability. The clay's deformation and pore pressure responses are categorized into stable, metastable, and unstable patterns. Comparisons with related studies on low OCR, low K0 soft clay from Wenzhou in southeastern China, confirm the Gault clay's generally stiffer prefailure behavior and different cyclic response. The stiff clay's greater brittleness is also emphasized; particle reorientation occurs readily along distinct shear bands, leading to dramatic shear strength reductions that have a major impact on slope and foundation stability and call for appropriate caution in practical design.
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
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.
Buckley R, Jardine R, Byrne B, et al., 2020, Pile behaviour in low-medium density chalk: preliminary results from the ALPACA project, 4th International Symposium on Frontiers in Offshore Geotechnics
Jardine R, Ushev E, Liu T, 2020, The anisotropic stiffness and shear strength characteristics of a stiff glacial till, Journal of Geotechnical and Geoenvironmental Engineering - ASCE, Vol: 146, Pages: 1-18, ISSN: 0733-9410
Glacial tills are widespread across North America, northern and central Asia, and northern Europe, where they are also found under the Baltic, North, and Norwegian Seas. Their geological and geotechnical characterization is important to a wide range of onshore and offshore engineering projects. One aspect of tills on which little has been reported is their mechanical anisotropy. This paper reports coordinated hollow cylinder apparatus (HCA) tests, triaxial shearing, and small-strain stress probing experiments, supported by index testing, on high-quality samples of natural low-to-medium plasticity, high overconsolidation ratios (OCR) stiff clay-till from the Bolders Bank Formation at Cowden, near Hull in the UK. Material variability and sampling bias are inevitably introduced by the till’s erratic gravel particles and fissure systems, and these aspects are addressed carefully. The experiments investigated the till’s stiffness and shear strength anisotropy from its limited linear elastic range up to ultimate failure, showing that stiffnesses are higher in the horizontal direction than in the vertical and that higher undrained shear strengths develop under passive horizontal loading than active vertical loading. Comparisons are made between the till’s patterns of anisotropy and those applying to previously studied sediments, and reference is made to in situ stiffness measurements. The important implications of anisotropic behavior for geotechnical design and the interpretation of field tests are emphasized.
Byrne BW, Houlsby GT, Burd HJ, et al., 2020, PISA design model for monopiles for offshore wind turbines: application to a stiff glacial clay till, Geotechnique, Vol: 70, Pages: 1030-1047, ISSN: 1021-8637
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
Burd HJ, Taborda D, Zdravkovic L, et al., 2020, PISA design model for monopiles for offshore wind turbines: application to a marine sand, Geotechnique, Vol: 70, Pages: 1048-1066, ISSN: 0016-8505
This paper describes a one-dimensional (1D) computational model for the analysis and design of laterally loaded monopile foundations for offshore wind turbine applications. The model represents the monopile as an embedded beam and specially formulated functions, referred to as soil reaction curves, are employed to represent the various components of soil reaction that are assumed to act on the 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 for offshore wind applications. The overall framework of the model, and an application to a stiff glacial clay till soil, is described in a companion paper by Byrne and co-workers; the current paper describes an alternative formulation that has been developed for soil reaction curves that are applicable to monopiles installed at offshore homogeneous sand sites, for drained loading. The 1D model is calibrated using data from a set of three-dimensional finite-element analyses, conducted over a calibration space comprising pile geometries, loading configurations and soil relative densities that span typical design values. The performance of the model is demonstrated by the analysis of example design cases. The current form of the model is applicable to homogeneous soil and monotonic loading, although extensions to soil layering and cyclic loading are possible.
Taborda D, Zdravkovic L, Potts DM, et al., 2020, Finite-element modelling of laterally loaded piles in a dense marine sand at Dunkirk, Geotechnique: international journal of soil mechanics, Vol: 70, Pages: 1014-1029, ISSN: 0016-8505
The paper presents the development of a three-dimensional finite-element model for pile tests in dense Dunkirk sand, conducted as part of the PISA project. The project was aimed at developing improved design methods for laterally loaded piles, 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.
Zdravkovic L, Taborda D, Potts D, et al., 2020, Finite-element modelling of laterally loaded piles in a stiff glacial clay till at Cowden, Geotechnique: international journal of soil mechanics, Vol: 70, Pages: 999-1013, ISSN: 0016-8505
The PISA project was a combined field testing/numerical modelling study with the aim of developing improved design procedures for large-diameter piles subjected to lateral loading. This paper describes the development of a three-dimensional finite-element model for the medium-scale pile tests that were conducted in Cowden till as 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 enhanced version of the modified Cam clay model was employed in the numerical analyses, featuring a non-linear Hvorslev surface, a generalised shape for the yield and plastic potential surfaces in the deviatoric plane and 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 behaviour for a range of pile geometries was observed, demonstrating the accuracy of the numerical model and the adequacy of the calibration process for the constitutive model. The developed numerical model confirmed the premise of the PISA design method that site-specific ground characterisation and advanced numerical modelling can directly facilitate the development of additional soil 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., 2020, Monotonic laterally loaded pile testing in a stiff glacial clay till at Cowden, Géotechnique, Vol: 70, Pages: 970-985, ISSN: 0016-8505
This paper describes the results obtained from a field testing campaign on laterally loaded monopiles conducted at Cowden, UK, where the soil consists principally of a heavily overconsolidated glacial till. These tests formed part of the PISA project on 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 (0·273 m, 0·762 m and 2·0 m) are presented. The piles had length-to-diameter ratios (L/D) of between 3 and 10. The tests included the 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 piles are determined using an optimised structural model. These field data support the development of a new one-dimensional modelling approach for the 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 above the ground surface.
McAdam RA, Byrne BW, Houlsby GT, et al., 2020, Monotonic lateral loaded pile testing in a dense marine sand at Dunkirk, Géotechnique, Vol: 70, Pages: 986-998, ISSN: 0016-8505
The results obtained from a field testing campaign on laterally loaded monopiles, conducted at a dense sand site in Dunkirk, northern France are described. These tests formed part of the PISA project on the development of improved design methods for monopile foundations for offshore wind turbines. Results obtained from monotonic loading tests on piles of three different diameters (0·273 m, 0·762 m and 2·0 m) are presented. The piles had length-to-diameter ratios (L/D) of between 3 and 10. The tests consisted principally of the application of monotonic loads, incorporating periods of held constant load to investigate creep effects. The influence of loading rate was 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 piles are also provided. Inferences are made for the displacements in the embedded length of the piles. These field data will support the development of a new one-dimensional modelling approach for the 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 above the ground surface.
Burd HJ, Beuckelaers WJAP, Byrne BW, et al., 2020, New data analysis methods for instrumented medium scale monopile field tests, Géotechnique, Vol: 70, Pages: 961-969, 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, and in 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 described for the interpretation of pile loading data, and specifically for providing precise interpretation of pile displacements.
Zdravkovic L, Jardine R, Taborda DMG, et al., 2020, Ground characterisation for PISA pile testing and analysis, Géotechnique, Vol: 70, Pages: 945-960, 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 characterisation of the two sites, which was 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 campaigns to historic investigations at both sites. They enabled an accurate description of soil behaviour from small strains to ultimate states to be derived, allowing analyses to be undertaken that captured both the serviceability and limit state behaviour of the test monopiles.
Burd H, Abadie C, Byrne B, et al., 2020, Application of the PISA design model to monopiles embedded in layered soils, Geotechnique, Vol: 70, Pages: 1067-1082, ISSN: 1021-8637
The PISA design model is a procedure for the analysis of monopile foundations for offshore windturbine applications. This design model has been previously calibrated for homogeneous soils; thispaper extends the modelling approach to the analysis of monopiles installed at sites where the soilprofile is layered. We describe a computational study on monopiles embedded in layered soilconfigurations comprising selected combinations of soft and stiff clay and sand at a range of relativedensities. The study comprises (i) analyses of monopile behaviour using detailed three dimensional(3D) finite element analysis, and (ii) calculations employing the PISA design model. Results from the3D analyses are used to explore the various influences that soil layering has on the performance ofthe monopile. The fidelity of the PISA design model is assessed by comparisons with data obtainedfrom equivalent 3D finite element analyses, demonstrating a good agreement in most cases. Thiscomparative study demonstrates that the PISA design model can be applied successfully to layeredsoil configurations, except in certain cases involving combinations of very soft clay and very densesand.
Buckley R, Jardine R, Kontoe S, et al., 2020, Full-scale observations of dynamic and static axial responses of offshore piles driven in chalk and tills, Géotechnique, Vol: 70, Pages: 657-681, 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, Hight D, Potts D, 2020, Integrated research into the foundation behaviour of offshore energy production platforms, Geotechnical Engineering Journal of the SEAGS and AGSSEA, ISSN: 0046-5828
Buckley R, McAdam R, Byrne B, et al., 2020, Optimisation of impact pile driving using optical fibre Bragg grating measurements, Journal of Geotechnical and Geoenvironmental Engineering, Vol: 146, Pages: 1-15, ISSN: 0733-9410
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 (5kHz) 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 onedimensional 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.
Carroll R, Carotenuto P, Dano C, et al., 2020, Field experiments at three sites to investigate the effects of age on steel piles driven in sand, Géotechnique, Vol: 70, Pages: 469-489, 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.
Yang Z, Gao Y, Jardine R, et al., 2020, Large deformation finite element simulation of displacement pile installation experiments in sand, ASCE Soil Mechanics and Foundation Division Journal, Vol: 146, ISSN: 0044-7994
Displacement piles are driven to support a wide rangeof structures. Predicting their axial limiting capacities and load-displacement behavior is critical to many such engineering applications. While field load tests may be conducted to check design assumptions, such tests can prove expensive and difficult to generalize. Numerical analyses undertaken to support design face uncertaintyover the potentially important effects of pile installationasno well-developed method exists to predict the stresses applying during and after driving. Recent experiments have provided evidence regarding the stresses and strains developed around displacement piles during installation in sandthat can help guiderepresentative numerical modeling. This paper contributes to this development by reporting large displacement numerical analysesand linking these to high-quality experiments. The Arbitrary Lagrangian-Eulerian (ALE) options available in ABAQUS/Explicit have been employed to simulate highly instrumented calibration chamber tests made with closed-ended pilespenetrated into sand. Predictions for the stress components developed during and afterpile installation are presented, along with measurements made by other authors of the corresponding strain fields. The simulations’ broad agreement with the available experimental evidenceindicatesthat the adopted ALE techniqueand soil modeling approach are appropriate for pile installation analysis in sands.
Jardine R, 2020, Geotechnics, energy and climate change: The 56th Rankine Lecture, Géotechnique, Vol: 70, Pages: 3-59, 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.
Altuhafi FN, Jardine RJ, Georgiannou VN, et al., 2019, Effects of particle breakage and stress reversal on the behaviour of sand around displacement piles, Géotechnique, Vol: 69, 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, Publisher: Karlsruhe: Bundesanstalt für Wasserbau, Pages: 923-930
Large numbers of offshore wind turbines, near-shore bridges and port facilities are supported by driven piles. The design and installation of such piles is often problematic in Chalk, a low-density, porous, weak carbonate rock, which is present under large areas of NW Europe. There is little guidance available to designers on driveability, axial capacity, the lateral pile resistance which dominates offshore wind turbine monopile behaviour, or on how piles can sustain axial or lateral cyclic loading. This paper describes the ALPACA project which involves comprehensive field testing at a low-to-medium density chalk research test site. The project is developing new design guidance through comprehensive field testing and analysis combined with in-situ testing campaigns and advanced static-and-cyclic laboratory testing on high quality block and rotary core samples.
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.
Byrne BW, Burd HJ, Zdravkovic L, et al., 2019, PISA: new design methods for offshore wind turbine monopiles, Revue Francaise de Geotechnique, ISSN: 0181-0529
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.
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.
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.
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.
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