The design of axially loaded piles has been an area of focus for the offshore industry in recent years. A number of studies report substantial increases in the shaft capacity of piles driven in sand, known as pile ageing. The offshore industry has been slow to implement ageing into practice because of uncertainty over the mechanisms controlling ageing and variability on the impact of ageing. This paper presents the results of tests using an instrumented pile with two separate installations considered, one where the pile was load tested 4 days after installation and the second where the load test was performed after an ageing period of 116 days. Data from the installation, ageing period, load testing and pile extraction provide further insights into the mechanisms governing the effect of time on the axial capacity of piles in sand.

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In recent years, there has been a significant push to develop optimized pile designs in the offshore industry, driven by the growth of offshore wind. One of the largest remaining uncertainties governing axial pile design in sands is the influence of ageing effects and how an aged pile responds under axial cyclic loading. This paper presents results from a long-term campaign of field tests on pile ageing and axial cyclic loading in sands, undertaken at the Blessington geotechnical test site in Ireland. Five open-ended driven steel piles were subjected to a total of 33 static and cyclic tension pile load tests undertaken over a 3-year period. The tests were planned and coordinated in order to accurately quantify the effects of ageing and cyclic loading on the pile shaft capacity over time. The tests showed that significant gains in pile shaft capacity occurred over time as a result of pile ageing, in line with results presented by other researchers. Piles subjected to cyclic tension loading remained stable under cyclic loading at load levels much larger than their 1-day capacity. The 1-day capacity appears to offer a lower bound for the degraded pile capacity following tension cyclic loading to failure. The reduction in capacity caused by cyclic loading to failure was related to the pretest increase in capacity caused by ageing. The findings from these field tests in addition to reinterpretation of previous testing indicated that previously published cyclic interaction boundaries may be overly conservative.

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Because of the deltaic nature of the Netherlands, deep soft soil deposits are widespread. Due to the population density exploitation of underground space is vital for commercial developments and transport networks. Piles are used as primary support elements in deep excavations, cut and cover tunnels, quay walls, flood defences and to provide uplift resistance to the base of tunnels and basements. This paper examines empirical correlations linking the Cone Penetration Test (CPT) end resistance qc and the resistance of deep foundations in sand. It is found that correlations between qc and pile end resistance are independent of pile diameter. However, the impact of installation method, residual load, plugging and sand creep should be considered. In the case of shaft resistance, constant correlation factors between qc and average shaft resistance are possible for non-displacement piles. For the case of displacement piles, correlations that include the effects of friction fatigue and pile plugging during installation are recommended.@en

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 in this 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 relatively large embedded length-to-diameter ratio (L/D) piles that are typically employed in offshore oil and gas structures. Concerns exist, however, that this analysis approach may not be appropriate for monopiles with the relatively low values of L/D that are typically adopted for offshore wind turbine structures. This paper describes a new 1D design model for monopile foundations; the model is specifically formulated for offshore wind turbine applications, although the general approach could be adopted for other applications. The model draws on the conventional p-y approach, but extends it to include additional components of soil reaction that act on the pile. The 1D model is calibrated using a set of bespoke 3D finite-element analyses of monopile performance, for pile characteristics and loading conditions that span a predefined design space. The calibrated 1D model provides results that match those obtained from the 3D finite-element calibration analysis, but at a fraction of the computational cost. Moreover, within the calibration space, the 1D model is capable of delivering high-fidelity computations of monopile performance that can be used directly for design purposes. This 1D modelling approach is demonstrated for monopiles installed in a stiff, overconsolidated glacial clay till with a typical North Sea strength and stiffness profile. Although the current form of the model has been developed for homogeneous soil and monotonic loading, it forms a basis from which extensions for soil layering and cyclic loading can be developed. The general approach can be applied to other foundation and soil-structure interaction problems, in which bespoke calibration of a simplified model can lead to more efficient design.@en

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.

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The PISA project explored the development of improved design methods for offshore wind turbine monopile foundations. Medium scale field pile testing for the project took place at Cowden in the UK, where the soil consists of a heavily over-consolidated glacial till, and Dunkirk in France, where the soil consists of a dense marine sand. The main focus of the testing was on monotonic capacity. However, results were also obtained for cyclic loading tests on medium scale piles of two different diameters (0.762 m and 2.0 m), and a length to diameter (L/D) ratio of 5.25. The testing consisted of uniform one-way cycling on 7 separate piles across the two sites at different amplitudes, including tests of over 20000 cycles, as well as two-way cycling on one pile at each site at different amplitudes and frequencies. The one-way tests explored ratcheting phenomena and associated effects, whilst the two-way tests explored stiffness and damping at increasing load amplitudes. This paper presents a summary of the cyclic pile tests that were completed, illustrated by results from the Dunkirk site. Comparisons are made to the PISA monotonic test results, as well as to observations from model testing reported in the literature. These field measurements support the development of new cyclic modelling approaches for offshore monopile foundations.@en

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.

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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.

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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.

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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.

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Over the past 5 years, a substantial research effort aimed at optimising the design of offshore wind turbines has led to significant reductions in the projected cost of developing offshore wind. Optimising the geotechnical design of these structures, through modern analysis techniques such as 3D Finite Element Modelling (FEM), has played a key role in helping to reduce costs. This paper presents a methodology for accurately modelling monopile behaviour using Cone Penetration Test (CPT) data to calibrate the non-linear stress dependent Hardening Soil (HS) model. The methodology is validated by comparing the modelled behaviour to field tests on a range of pile geometries. The paper also demonstrates how the soil-pile reaction response curves can be extracted from the FE model by isolating the stresses on each element of the pile. The contribution of each component to the overall lateral resistance is shown to vary with the pile geometry and is examined using the extracted soil reaction curves.

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Lateral loading is often the governing design criteria for piles supporting offshorewind turbines and with the recent growth of this sector, the reliability of traditional design approaches is receiving renewed interest. To accurately abeb the behavior of a laterally loaded pile requires a detailed understanding of the soil reaction that ismobilized as the lateral deflection of the pile occurs. Currently, the p-y curvemethod iswidely adopted tomodel the response of laterally loaded piles. The limitations of existing p-y formulations arewidely known, and there is acceptance that load tests on large-diameter stiff monopiles are urgently required to formulate appropriate designmethods. However, interpretation of the data frominstrumentation placed on stiffmonopiles is not straightforward. This paper proposes an optimization technique to derive the soil reaction profile along the shaft of instrumented piles, fromwhich the correlated p-y curves can then be obtained. Themethod considers force equilibrium, pile deflection, and additional boundary conditions. A set of fourth-order polynomial equations are abumed to model the soil reaction profile under each load step during a monotonic load test. By minimizing the difference between themeasured and calculated bendingmoment and considering equilibriumof the shear forces acting on the pile, the soil reaction profile and the concentrated tip resistance can be obtained simultaneously. A stiff instrumented test pile installed in over-consolidated sand was load tested and the resultswere used to test the performance of the proposed method. The results are compared with othermethods used in literature and practice. Themethod provides a consistent framework to derive p-y curves from measured strain data. The results of the field test and derived p-y curves confirmed that existing designmethods do not accurately capture the lateral loading response of piles in dense sand.

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The paper describes some recent field test programmes performed across Europe. Given the increased need to develop alternative, clean energy sources the paper presents details of R&D programmes with a focus on reducing costs offshore developments and increases safety. Work on the development of design codes for large diameter piles is presented. New areas of research are highlighted in particular ongoing research is needed to look at effects such as cyclic loading and dynamic interaction. In the offshore field the use of piles that reduce environmental impact is an area of ongoing research that is addressed. Interesting full-scale field tests looking at the problem of negative skin friction are also considered.@en

In recent years, fibre Bragg grating (FBG) sensors have emerged as a relatively new strain sensing technology for civil engineering applications. This paper presents a field trial to assess the feasibility of using FBG sensor arrays to measure strain in driven steel piles. Two FBG arrays were installed in grooves within the wall of an open-ended steel pile such that the finished profile was completely flush with the pile shaft. The pile was then driven into a dense sand deposit using an impact hammer to provide the required installation energy. The FBG gauges were monitored throughout driving in conjunction with accelerometers to quantify the scale of the hammer impacts. The FBG sensors were subjected to hammer blows that yielded pile accelerations between 500 g and 1400 g during installation. The fibre optic sensors were measured throughout driving, where they were observed to respond to the hammer impacts, showing a rapid increase in strain and a return to their initial values between hammer strikes. After installation, a lateral load test was performed with independent load measuring devices. Excellent agreement was observed between the measured moments and those inferred from the FBG strain output. The output of this trial demonstrates that FBG strain sensors are a viable means of measuring load transfer in foundation systems and are suitably robust to withstand high pile driving accelerations.

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Offshore wind power in the UK, and around Europe, has the potential to deliver significant quantities of renewable energy. The foundation is a critical element in the design. The most common foundation design is a single large diameter pile, termed a monopile. Pile diameters of between 5m and 6m are routinely used, with diameters up to 10m or more, being considered for future designs. Questions have been raised as to whether current design methods for lateral loading are relevant to these very large diameter piles. To explore this problem a joint industry project, PISA, co-ordinated by DONG Energy and the Carbon Trust, has been established. The aim of the project is to develop a new design framework for laterally loaded piles based on new theoretical developments, numerical modelling and bench- marked against a suite of large scale field pile tests. The project began in August 2013 and is scheduled to complete during 2015. This paper briefly outlines the project, focusing on the design of the field testing. The testing involves three sizes of pile, from 0.27m in diameter through to 2.0m in diameter. Two sites will be used; a stiff clay site and a dense sand site. Tests will include monotonic loading and cyclic loading. A suite of site investigation will be carried out to aid interpretation of the field tests, and will involve in-situ testing, standard laboratory testing and more advanced laboratory testing.

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There is currently a significant focus on developing offshore wind power in the UK and Europe. The most common foundation type for wind turbines is a single large diameter pile, termed a monopile, on which the turbine is located. As the diameter of such piles is envisaged to increase in future installations, there are concerns that current design methods are not applicable. To explore this problem, the joint industry project PISA has been established, with the aim to develop a new design framework for laterally loaded piles utilised in the offshore wind industry, based on new theoretical developments, numerical modelling and large scale field pile testing. This paper presents an overview of numerical modelling undertaken as part of the project.

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Design models for offshore wind turbine monopiles tend to be based on those developed for flexible piles in the offshore oil and gas sectors. However, wind monopiles tend to be shorter and wider, resulting in a stiffer structure that rotates rather than bends when laterally loaded. New methods have been proposed to calculate the load-displacement response of piles to monotonic lateral loading. Those based on correlations with in situ tests such as the cone penetration test seem particularly promising. There is a dearth of experimental test data where monopiles have been subjected to extensive cyclic loading with which to extend such models to consider these effects. To address this, field tests were performed on two 340 mm diameter driven piles with an embedded length of 2.2 m, which were cyclically loaded with up to 5000 loading cycles. The piles had a slenderness ratio (embedded length over diameter) of 6.5, which is a typical value used in the offshore wind sector. The test results show that the accumulated pile head response primarily depends on the soil strength and stiffness, the previous loading history and loading characteristics. A cyclic loading design procedure consisting of cone penetration test based static design and Miner’s rule based superposition is presented.

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Offshore wind turbines are typically founded on single large diameter piles, termed monopiles. Pile diameters of between 5mand 6mare routinely used, with diameters of up to 10 m, or more, being considered for future designs. There are concerns that current design approaches, such as the p − y method, which were developed for piles with a relatively large length to diameter ratio, may not be appropriate for large diameter monopiles. A joint industry project, PISA (PIle Soil Analysis), has been established to develop new design methods for large diameter monopiles under lateral loading. The project involves three strands of work; (i) numerical modelling; (ii) development of a new design method; (iii) field testing. This paper describes the framework on which the new design method is based. Analyses conducted using the new design method are compared with methods used in current practice.

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Lateral force-displacement (P-y)-based Winkler spring models are commonly applied for the design of piles, P being the soil lateral reaction and y the lateral displacement. Despite their relative simplicity, P-y models can capture important aspects of pile behaviour including non-linear soil stiffness. Several P-y models based on cone penetration tests (CPTs) have been proposed over the last two decades, developed largely using empirical curve fitting to results of field tests, centrifuge modelling and finite-element analyses on relatively flexible piles installed in calcareous sand. However, major uncertainties exist when attempting to extrapolate empirical models for use with soil types and pile geometries outside the database on which they were formulated. There is an urgent need for a reliable P-y method for application to the design of rigid monopiles used extensively for offshore wind projects. A series of field lateral load tests performed on open-ended steel pipe piles driven in dense siliceous sand is reported here. The pile embedment length and load eccentricity were varied to investigate the behaviour of rigid and flexible monopiles. The measured pile response was used to evaluate the performance of a number of recent CPT-based P-y models and an update to an existing power-law model is suggested for rigid monopiles in siliceous sand.

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To date, monopiles are the most popular foundations used in the offshore wind sector, accounting for over 75% of existing turbine foundations. The cyclic lateral loading response of these foundations is one of the most important issues to be considered during the life-cycle design of monopiles. Current design guidelines rely on methods developed from field tests performed on long slender piles typically used in the oil and gas industry. Monopiles used for wind turbines tend to be relatively short and stiff. In order to investigate the response of a rigid monopile to long-term cyclic lateral loading, a series of field model experiments were conducted on instrumented steel pipe piles. The piles, which had a diameter of 340 mm and slenderness ratio of 6.5, were driven into dense sand at the University College Dublin geotechnical test site. Lateral loading tests were performed on two separate piles. In the cyclic lateral load tests, one pile was loaded with circa 5000 cycles at two load amplitudes (approximately 20% and 50% of the static resistance). In the second cyclic load test, more than 3000 load cycles were applied at three load amplitudes (30%, 40% and 70% of the ultimate resistance). The ability of current design models to predict the monopile response is considered.@en