Now showing 1 - 10 of 19
  • Publication
    Statistical review of CPT data and implications for pile design
    (CPT 10, 2010-05-09) ;
    Natural soil variability can result in a range of pile capacities at a specific test site. Soil variability is ideally determined using in-situ testing techniques such as the CPT. This paper presents a series of CPT profiles from a dense sand site in Wicklow, Ireland. The qc value is detrended using a quadratic profile with depth and the scatter in the remaining data is quantified using the coefficient of variation (COV). In addition, the autocorrelation of each CPT trace is quantified using the scale of fluctuation. The measured values from the site, including the trend, COV and scale of fluctuation are subsequently incorporated into a Monte Carlo simulation that generates multiple realisations of the CPT data from an underlying lognormal distribution. These realisations are applied to a simple pile design model to calculate the variability in capacity that arises from the site specific variability. Spatial averaging is shown to be highly influential in reducing the uncertainty in pile resistance, with long piles and small scale of fluctuations demonstrating the lowest COV in the calculated capacity.
  • Publication
    Soil properties at the UCD geotechnical research site at Blessington
    Over the past ten years, the Geotechnical Research Group (GRG) at University College Dublin have developed a research site at Blessington, County Wicklow, for the purpose of testing foundation systems. This paper presents the results of field and laboratory tests conducted to obtain the geotechnical parameters of Blessington sand. The in-situ tests included cone penetration and dilatometer tests. Sonic coring was performed in three boreholes at the site and complete recovery was obtained in boreholes up to 14 m deep. Additional disturbed samples were taken from trial pits which were up to 6 m deep. The classification tests performed on samples compared favourably with those inferred from correlations with in-situ test data. The strength, stiffness and mineralogy were also determined by a suite of laboratory tests including SEM imagery, triaxial tests and ring shear testing. The accuracy of conventional correlations in predicting the laboratory measured parameters is discussed.
  • Publication
    An investigation into the effect of scour on the natural frequency of an Offshore Wind Turbine
    Rapid expansion of the offshore wind industry has stimulated a renewed interest in the behaviour of offshore piles. There is widespread acceptance in practice that pile design methods developed for the offshore oil and gas industry may not be appropriate for designing wind turbine foundations. To date, the majority of offshore wind turbines are supported by large diameter monopiles. These foundations are sensitive to scour which can reduce their ultimate capacity and alter their dynamic response. In this paper, the use of a vibration-based method to monitor scour is investigated. The effect of scour on the natural frequency of a model monopile was measured in a scale model test. A spring-beam finite element numerical model was developed to examine the foundation response. The model, which used springs tuned to the small-strain stiffness of the sand, was shown to be capable of capturing the change in frequency observed in the scale test. This numerical procedure was extended to investigate the response of a full-scale wind turbine over a range of soil densities, which might be experienced at offshore development sites. Results suggest that wind turbines founded in loose sand would exhibit the largest relative reductions in natural frequency resulting from scour.
      817Scopus© Citations 142
  • Publication
    The Geotechnical Challenges Facing the Offshore Wind Sector
    (American Society of Civil Engineers, 2011-03) ; ;
    The offshore wind sector is undergoing rapid expansion across Europe, driven by the demand for renewable energy and the uncertainty regarding fossil fuel supplies. The proposed wind farm developments are creating significant geotechnical challenges, particularly in terms of efficient foundation design. The majority of wind farms constructed to date have been founded in water depths of less than thirty meters. However 70% of the proposed turbines over the next 10 years will be located in water between 30 and 70m, which increases the lateral loads and moments applied to the foundations. This paper outlines the geotechnical design considerations for turbines located in relatively deep water. The empirical design models commonly used for monopiles and jackets are briefly discussed, in order to highlight the limitations in existing design procedures, originally formulated for the oil and gas sector.
      591Scopus© Citations 6
  • Publication
    The Shaft Capacity of Displacement Piles in Clay: A State of the Art Review
    (Springer Verlag, 2011-07) ;
    The rapid expansion of the offshore wind sector, coupled with increasing demand for high rise structures, has placed renewed demand on the driven piling market. In light of this industry growth, this paper reviews the evolution of design approaches for calculating the shaft capacity of displacement piles installed in cohesive soils. The transition from traditional total stress design towards effective stress methods is described. Complex stress–strain changes occur during pile installation, equalisation and load testing and as a consequence, the selection of parameters for use in conventional earth-pressure type effective stress approaches is not straight-forward. These problems have led to the development of empirical correlations between shaft resistance and in situ tests, such as the cone penetration tests. However, many of these approaches are limited because they were developed for specific geological conditions. Significant insight into pile behaviour has been obtained from recent model pile tests, which included reliable measurements of radial effective stresses. These tests have allowed factors such as friction fatigue and interface friction to be included explicitly in design methods. Whilst analytical methods have been developed to investigate pile response, these techniques cannot yet fully describe the complete stress–strain history experienced by driven piles. The use of analytical methods in examining features of pile behaviour, such as the development of pore pressure during installation and the effects of pile end geometry on pile capacity, is discussed.
      4970Scopus© Citations 33
  • Publication
    Shaft Capacity of Open-Ended Piles in Clay
    This paper describes an experimental investigation designed to assess the impact of pile end condition on the capacity of piles installed in soft clay. A series of field tests are described in which instrumented open-ended and closed-ended model piles were jacked into soft clay. The radial stresses, pore pressures, and load distribution were recorded throughout installation, equalization, and load-testing. Although the total stress and pore pressure developed during installation were related to the degree of soil plugging, the radial effective stress that controls the shaft resistance was shown to be independent of the mode of penetration. The long-term shaft capacity of the open-ended pile was closely comparable to that developed by closed-ended piles, suggesting a limited influence of end condition on the fully equalized shaft resistance. In contrast to the shaft resistance, the base capacity was highly dependent on the degree of plugging.
      1604Scopus© Citations 66
  • Publication
    Cyclic and Rapid Axial Load Tests on Displacement Piles in Soft Clay
    Offshore piles are subjected to complex loading regimes, which include both rapidly applied static and cyclic loads. This note describes an experimental investigation conducted in order to assess the factors influencing the response of offshore piles to these loading conditions. The tests were performed using instrumented model piles installed in soft clay. During cyclic loading, the piles demonstrated a transition from stable to unstable behaviour when the applied loads reached a specific load threshold. Stable behaviour was defined when increments of plastic displacement decreased as the number of load cycles increased. During stable behaviour, radial effective stresses at the pile-soil interface remained constant. During unstable behaviour, pore pressures at the pile-soil interface rose as the number of cycles increased. This resulted in reduced radial effective stresses and progressively increasing displacement rates. Due to the presence of these excess pore pressures, the shaft resistance recorded during static load tests, performed after a period of unstable cyclic loading, were lower than those measured on piles where the pore pressure was fully equalised. However, the axial resistance was seen to be rate dependent. Fast loading of the pile resulted in reductions of pore water pressure at the soil-pile interface and enhanced shaft resistance which could overcome the negative effect caused by cyclic loading.
      1052Scopus© Citations 6
  • Publication
    Laterally loaded monopile design for offshore wind farms
    (Institution of Civil Engineers, 2011-12) ;
    Expansion of the offshore UK wind energy sector has stimulated renewed interest in the response of piles to lateral and moment loads. This paper compares the state of the art in foundation design with current industry trends in offshore wind turbine construction. The historical evolution of pile design for lateral loading is described in detail, focusing on the American Petroleum Institute guidelines used by the offshore sector. The limitations of these design codes are discussed in light of the specific requirements for the wind sector. Recent research efforts attempting to bridge the gap between practice and industry are highlighted and further research needs are identified.
      6059Scopus© Citations 147
  • Publication
    The Development and Testing of an Instrumented Open-Ended Model Pile
    (ASTM International, 2010-01) ; ;
    This paper describes the development of a model instrumented open-ended (pipe) pile. The importance of model geometry and separating the shaft, annular and plug load, and horizontal effective stresses is discussed. A detailed description of the construction of the twin-walled open-ended pile is presented. Particular attention was given to protecting the fragile instrumentation from the rigours of installation and the effects of water ingress. Calibration procedures, which were used to verify the instrument reliability, are also discussed. The final section describes field tests conducted in both loose sand and medium-dense sand deposits, which are used to validate the instrument performance.
      663Scopus© Citations 20
  • Publication
    Piles for offshore wind turbines: A state of the art review
    (Institution of Civil Engineers, 2011-08-01) ; ;
    The paper considers the current state of the art for estimating the pull-out capacity of driven open-ended piles used to support wind turbine foundations founded on sand. The latest edition of the American Petroleum Institute guidelines for pile design includes a conventional earth pressure approach and four alternative cone penetration test (CPT) methods for estimating pile shaft resistance in sand. A database of open-ended pile tests was used to assess the predictive reliability of the design approaches. While the earth pressure approach was unreliable, exhibiting bias with pile slenderness and sand relative density, the CPT methods were shown to provide improved and relatively consistent estimates of pile capacity. However, the tension loads experienced by wind turbine foundations are significantly higher than those applied to piles in the database. When the CPT methods were used to estimate the pile length required to support a 5 MW turbine installed in typical offshore soil conditions, the CPT methods provided a wide range of predicted pile lengths. The reasons for this divergence are discussed and an alternative framework for considering driven pile shaft resistance is put forward.
      1481Scopus© Citations 58