Professor Subhamoy Bhattacharya (Suby) holds the chair in Geomechanics at the University of Surrey since October 2012 and is also a visiting fellow at the University of Bristol. He previously held the position of Senior Lecturer in Dynamics at University of Bristol, Departmental Lecturer in Engineering Science at the University of Oxford, Junior Research Fellow of Somerville College (University of Oxford), College Lecturer at Brasenose College and Lady Margaret Hall (University of Oxford). Professor Bhattacharya earned his doctorate from the University of Cambridge investigating failure mechanisms of piles in seismically liquefiable soils.
Professor Bhattacharya had many happy years working in the Civil/Offshore Engineering consultancy: Staff engineer and project manager at Fugro Geo Consulting Limited (2003 to 2005), Consulting Engineering Services (I) Limited (now Jacobs).
Offshore Wind Turbines are a complex, dynamically sensitive structure owing to their irregular mass and stiffness distribution and complexity of the loading conditions they need to withstand. There are other challenges in particular locations such as typhoon, hurricane, earthquake, sea-bed current, tsunami etc. As offshore wind turbines have stringent Serviceability Limit State (SLS) requirements and need to be installed in variable, and often complex ground conditions, their foundation design is challenging. Foundation design must be robust due to the enormous cost of retrofitting in a challenging environment should any problem occurs during the design lifetime. Traditionally, engineers use conventional types of foundation system such shallow Gravity-Based Foundations (GBF), suction caissons or slender pile or monopile owing to prior experience with designing such foundations for the oil and gas industry. For offshore wind turbine, however, new types of foundations are being considered for which neither prior experience nor guidelines exist. One of the major challenges is to develop a method to de-risk the life cycle of offshore wind turbines in diverse met-ocean and geological conditions. The paper, therefore, has the following aims: (a) Provide an overview of the complexities and the common SLS performance requirements for offshore wind turbine; (b) Discuss the use of physical modelling for verification and validation of innovative design concepts, taking into account all possible angles to de-risk the project. (c) Provide examples on applications of scaled model tests.
Suction Bucket Jackets (SBJs) need to be fundamentally designed to avoid rocking modes of vibration about the principal axes of the set of foundations and engineered towards sway-bending modes of tower vibration. Whether or not such type of jackets exhibit rocking modes depends on the vertical stiffness of the caissons supporting them. This paper therefore derives closed form solutions for vertical stiffness in three types of ground profiles: linear, homogenous, and parabolic. The expressions are applicable to suction caissons having an aspect ratio (depth: diameter) between 0.2 and 2 (i.e. 0.2
Resilience of bridges in seismic zones can be realised by taking the advantage of rocking isolation which aims at reducing the permanent drifts after a seismic event. The seismic forces at the base of the bridge can be reduced by allowing uplift in the foundation when subjected to ground shaking. Conventional monolithic connection of bridge pier to the foundation often leads to severe damages (or even collapse) during high magnitude earthquakes. In this context, this article proposes a novel seismically resilient pier footing which rocks on elastomeric pads and external restrainers (provided by shape memory alloy bars). Seismic performance of a typical existing overpass motorway bridge is compared with the proposed rocking isolation concept. The proposed technique shows good re-centering capability during earthquakes with negligible residual drifts. Furthermore, it is also observed that forces in the pier and size of pier footing are reduced as compared with the reference bridge considered in this study.
The paper examines the behaviour of buried continuous pipelines crossing strike-slip faults using experimental and numerical modelling. A newly developed experiment setup is presented along with the derivation of relevant scaling laws and non-dimensional terms governing global response of continuous pipelines to strike-slip faulting. Four model tests are carried out to understand the performance of the pipelines and the results are presented through the derived non-dimensional framework. Three-dimensional (3D) Finite Element (FE) model is also undertaken to simulate buried continuous pipelines crossing strike-slip faults and is calibrated against the model test results and a field case record for validation and verification. A parametric study is also carried out to better understand the parameters influencing the response of buried continuous pipelines to strike-slip faults and to also investigate the effects of pipe end conditions on their behaviour. API 5L X70 steel pipe with 490 MPa of yield strength was used in the numerical parametric study. Two different scenarios based on fault crossing angle of the pipe (β) were considered in the parametric study: (a) pipelines in tension and bending; (b) pipelines in compression and bending. The experimental and numerical results show that the longitudinal pipe strains under strike-slip faulting are strongly dependent on six parameters: (a) normalized fault displacements (represented by δ/D where δ is the fault displacement and D is the pipe diameter which is also an indication of soil strain in the mobilised zone); (b) ratio of pipe diameter to wall thickness (D/t); (c) fault crossing angle of the pipe (β); (d) relative soil-pipe stiffness (kD4/EI); (e) ratio of burial depth to pipe diameter (H/D) and (f) pipe end conditions. Finally, practical implications of the study are discussed.
One of the challenges faced by the engineering profession is to meet the energy requirement of an increasingly prosperous world. Nuclear power was considered as a reliable option until the Fukushima Daiichi Nuclear Power Plant (NPP) disaster which eroded the public confidence. This short paper shows that offshore wind turbines (due to its shape and form, i.e. heavy rotating mass resting at the top of a tall tower) have long natural vibration periods (>3.0 s) and are less susceptible to earthquake dynamics. The performance of near-shore wind turbines structures during the 2011 Tohoku earthquake is reviewed. It has been observed that they performed well. As NPPs are often sited close to the sea, it is proposed that a small wind farm capable of supplying emergency backup power along with a NPP can be a better safety system (robust and resilient system) in avoiding cascading failures and catastrophic consequences.
Offshore wind farms are a collection of offshore wind turbines (OWTs) and are currently being installed in seismically active regions. An OWT consists of a long slender tower with a top-heavy fixed mass (Nacelle) together with a heavy rotating mass (Hub and blades) and is always exposed to variable environmental wind and wave loads. For dynamic analysis, an OWT can also be seen as an inverted pendulum (with over 25%–50% of the total mass concentrated in the upper 3rd of the tower), yet it is not granted that their seismic response is dominated by the first mode. Guidelines for the design of such special structures are not explicitly mentioned in current codes of practice. The aim of this technical note is to identify the design issues and provide a rational background for the seismic analysis. Where feasible, further research work that is needed is also identified and discussed. •Considerations for seismic design.•Design return period.•Types of seismic analysis.•Selection of input motion.
Life line structures such as elevated flyovers and rail over bridges should remain functional after an earthquake event to avoid possible traffic delays and risk to general public. Generally, restraining the structure by reducing the degrees of freedom often cause serious damages that occurs during a seismic event through yielding of the structural components. By allowing the structure to rock through uplift using suitable arrangements can be a plausible seismic resilient technique. In this context, this article proposes a novel seismic resilient pile supported bridge pier foundation, which uses elastomeric pads installed at top of pile cap. The effect of pile soil interaction along with ground response analysis is also incorporated in the full bridge model adopted for the study. One dimensional equivalent linear site response analyses were performed to arrive at the amplified/attenuated ground motions along the depth of soil.The seismic performance of the proposed bridge with new rocking isolation concept is compared with existing bridge located in medium seismic zone of India. With the help of non-linear dynamic time history analysis and nonlinear static pushover analysis, the bridge modelled using the proposed novel rocking isolation technique shows good re-centering capability during earthquakes with negligible residual drifts and uniform distribution of ductility demand along the piers of the bridge considered in this study. •Proposed a novel rocking resilient bridge pier foundation system using elastomeric pads with pile foundation.•Developed a full 3D finite element model incorporating site specific soil-pile-interaction and ground response analysis.•Proposed bridge has improved seismic resilience in terms of recentering capability and reduced residual drifts, over the existing bridge pier system.
An efficient finite element nonlinear model has been applied to examine the lateral behavior of real-world monopiles supporting Offshore Wind Turbines (OWTs) chosen from five different offshore wind farms currently under operating service in Europe, in the aim to accurately estimate the natural frequency of these slender structures which is function of the interaction of their foundations with the subsoil. After a brief introduction giving the advantages of wind power energy as a reliable alternative to fossil fuel based one, the paper focuses on the importance of the concept of natural frequency as a primary indicator in designing the foundations of OWTs and gives the target range of frequencies where the natural frequency should lie for a safe design. Then, an analytical expression of an OWT natural frequency is presented in function of soil monopile interaction through monopile head springs characterized by lateral stiffness K_L, rotational stiffness K_R and cross-coupling stiffness, where their different constituting terms are discussed. The nonlinear pseudo 3D Finite Element vertical slices model has been used to analyze the lateral behavior of monopiles supporting OWTs of the different wind farm sites considered. Through the monopiles head movements (displacements and rotations) K_L, K_R and K_LR were obtained and soon substituted in the analytical expression of natural frequency for comparison. The results of comparison between computed and measured natural frequencies showed an excellent agreement for ones and slight deviations for the others. This confirms the convenience of the finite element model used for the accurate estimation of the monopile head stiffness.
The number of offshore wind turbine farms in seismic regions has been increasing globally. The seismic performance of steel monopile-supported wind turbines, which are the most popular among viable structural systems, has not been investigated thoroughly and more studies are needed to understand the potential vulnerability of these structures during extreme seismic events and to develop more reliable design and assessment procedures. This study investigates the structural performance assessment of a typical offshore wind turbine subjected to strong ground motions. Finite element models of an offshore wind turbine are developed and subjected to unscaled natural seismic records. For the first time, the sensitivity to earthquake types (i.e. crustal, inslab, and interface) and the influence of soil deformability and modeling details are investigated through cloud-based seismic fragility analysis. It is observed that monopile-supported offshore wind turbines are particularly vulnerable to extreme crustal and interface earthquakes, and the vulnerability increases when the structure is supported by soft soils. Moreover, a refined structural modeling is generally necessary to avoid overestimation of the seismic capacity of offshore wind turbines.
A simplified design procedure for foundations of offshore wind turbines is often useful as it can provide the types and sizes of foundation required to carry out financial viability analysis of a project and can also be used for tender design. This paper presents a simplified way of carrying out the design of monopiles based on necessary data (i.e. the least amount of data), namely site characteristics (wind speed at reference height, wind turbulence intensity, water depth, wave height and wave period), turbine characteristics (rated power, rated wind speed, rotor diameter, cut-in and cut-out speed, mass of the rotor-nacelle-assembly) and ground profile (soil stiffness variation with depth and soil stiffness at one diameter depth). Other data that may be required for final detailed design are also discussed. A flowchart of the design process is also presented for visualisation of the rather complex multi-disciplinary analysis. Where possible, validation of the proposed method is carried out based on field data and references/guidance are also drawn from codes of practice and certification bodies. The calculation procedures that are required can be easily carried out either through a series of spreadsheets or simple hand calculations. An example problem emulating the design of foundations for London Array wind farm is taken to demonstrate the proposed calculation procedure. The data used for the calculations are obtained from publicly available sources and the example shows that the simplified method arrives at a similar foundation to the one actually used in the project.
Bridges are a part of vital infrastructure, which should operate even after the disaster to keep the emergency services running. There have been numerous bridge failures due to the liquefaction during major past earthquakes. Among other categories of failures, mid span collapse (without the failure of abutments) of pile supported bridges, founded in liquefiable deposits are still observed even in most recent earthquakes. This mechanism of collapse is attributed to the effects related to the differential elongation of natural period of the individual piers, during liquefaction. A shake table investigation has been carried out in this study to verify mechanisms behind midspan collapse of pile supported bridges in liquefiable deposits. A typical pile supported bridge is scaled down and its foundations pass through the liquefiable loose sandy soil and rest in dense gravel layer. White noise motions of increasing acceleration magnitude have been applied to initiate progressive liquefaction and to characterize the dynamic features of the bridge. It has been found that as the liquefaction sets in the soil, the natural frequency of individual bridge support reduces with the highest reduction occurring near the central spans. As a result, there is differential lateral displacement and bending moment demand on the piles. It has also been observed that for the central pile, the maximum bending moment in the pile will occur at a higher elevation, as compared to that of the interface of soils of varied stiffness, unlike the abutment piles. The practical implications of this research are also highlighted.
Very large diameter steel tubular piles (up to 10 m in diameter, termed as XL or XXL monopiles) and caissons are currently used as foundations to support offshore Wind Turbine Generators (WTG) despite limited guidance in codes of practice. The current codes of practice such as API/DnV suggest methods to analysis long flexible piles which are being used (often without any modification) to analyse large diameter monopiles giving unsatisfactory results. As a result, there is an interest in the analysis of deep foundation for a wide range of length to diameter (L/D) ratio embedded in different types of soil. This paper carries out a theoretical study utilising Hamiltonian principle to analyse deep foundations ( L/ 2 D≥ ) embedded in three types of ground profiles (homogeneous, inhomogeneous and layered continua) that are of interest to offshore wind turbine industry. Impedance functions (static and dynamic) have been proposed for piles exhibiting rigid and flexible behaviour in all the 3 ground profiles. Through the analysis, it is concluded that the conventional Winkler-based approach (such as p–y curves or Beanon-Dynamic Winkler Foundations) may not be applicable for piles or caissons having aspect ratio less than about 10 to 15. The results also show that, for the same dimensionless frequency, damping ratio of large diameter rigid piles is higher than long flexible piles and is approximately 1.2–1.5 times the material damping. It is also shown that Winkler-based approach developed for flexible piles will under predict stiffness of rigid piles, thereby also under predicting natural frequency of the WTG system. Four wind turbine foundations from four different European wind farms have been considered to gain further useful insights.
Choosing appropriate foundations for supporting offshore wind turbines is one of the uncertainties in the future rounds of offshore wind power development. Offshore wind turbines are dynamically sensitive structures as the global natural frequency of the whole system is very close to the forcing frequencies (due to the environmental loads and the associated frequencies due to the rotor). This particular aspect is important for designing foundations for Round 2 and Round 3 offshore wind farms in the UK. It must be mentioned here that monopile foundations have been commonly used to support offshore wind turbine generators (WTGs), but this type of foundation encounters economic and technical limitations for larger WTGs in water depths exceeding 30m. Therefore offshore wind farm projects are increasingly turning to alternative multipod foundations (for example tetrapod, jacket, tripods) or on shallow foundations to reduce the environmental effects of piling noise. However the characteristics of these foundations under dynamic loading or long term cyclic wind turbine loading are not fully understood. This keynote lecture summarizes the results from a series of scaled model tests of the overall wind turbine system (including the foundations).
The offshore wind turbines (OWTs) are dynamically sensitive, whose fundamental frequency can be very close to the forcing frequencies activated by the environmental and turbine loads. Minor changes of support conditions may lead to the shift of natural frequencies, and this could be disastrous if resonance happens. To monitor the support conditions and thus to enhance the safety of OWTs, a model updating method is developed in this study. A hybrid sensing system was fabricated and set up in the laboratory to investigate the long-term dynamic behaviour of the OWT system with monopile foundation in sandy deposits. A finite element (FE) model was constructed to simulate structural behaviours of the OWT system. Distributed nonlinear springs and a roller boundary condition are used to model the soil-structure-interaction (SSI) properties. The FE model and the test results were used to analyze the variation of the support condition of the monopile, through an FE model updating process using Estimation of Distribution Algorithms (EDAs). The results show that the fundamental frequency of the test model increases after a period under cyclic loading, which is attributed to the compaction of the surrounding sand instead of local damage of the structure. The hybrid sensing system is reliable to detect both the acceleration and strain responses of the OWT model and can be potentially applied to the remote monitoring of real OWTs. The EDAs based model updating technique is demonstrated to be successful for the support condition monitoring of the OWT system, which is potentially useful for other model updating and condition monitoring applications.
To support large wind turbines in deeper waters (30-60 m) jacket structures are currently being considered. As offshore wind turbines (OWT’s) are effectively a slender tower carrying a heavy rotating mass subjected to cyclic/dynamic loads, dynamic performance plays an important role in the overall design of the system. Dynamic performance dictates at least two limit states: Fatigue Limit State (FLS) and overall deformation in the Serviceability Limit State (SLS). It has been observed through scaled model tests that the first eigen frequency of vibration for OWTs supported on multiple shallow foundations (such as jackets on 3 or 4 suction caissons) corresponds to low frequency rocking modes of vibration. In the absence of adequate damping, if the forcing frequency of the rotor (so called 1P) is in close proximity to the natural frequency of the system, resonance may occur affecting the fatigue design life. A similar phenomenon commonly known as “ground resonance” is widely observed in helicopters (without dampers) where the rotor frequency can be very close to the overall frequency causing the helicopter to a possible collapse. This paper suggests that designers need to optimise the configuration of the jacket and choose the vertical stiffness of the foundation such that rocking modes of vibration are prevented. It is advisable to steer the jacket solution towards sway-bending mode as the first mode of vibration. Analytical solutions are developed to predict the eigen frequencies of jacket supported offshore wind turbines and validated using the finite element method. Effectively, two parameters govern the rocking frequency of a jacket: (a) ratio of super structure stiffness (essentially lateral stiffness of the tower and the jacket) to vertical stiffness of the foundation; (b) aspect ratio (ratio of base dimension to the tower dimension) of the jacket. A practical example considering a jacket supporting a 5MW turbine is considered to demonstrate the calculation procedure which can allow a designer to choose a foundation. It is anticipated that the results will have an impact in choosing foundations for jackets.
Floating offshore wind turbines are complex dynamic structures, and detailed analysis of their loads require coupled aero-servo-hydro-elasto-dynamic simulations. However, time domain approach used for such analysis is slow, computationally expensive and requires detailed data about the wind turbine. Therefore, simplified approaches are necessary for feasibility studies, front-end engineering design (FEED) and the early phases of detailed design. This paper aims to provide a methodology with which the designer of the anchors can easily and quickly assess the expected ultimate loads on the foundations. For this purpose, a combination of a quasi-static wind load analysis and Morison’s equation for wave load estimation using Airy waves is employed. Dynamic amplification is also considered and design load cases are established for ultimate limit state (ULS) design. A simple procedure is also presented for sizing suction caisson anchors. All steps are demonstrated through an example problem and the Hywind case study is considered for such purpose.
Offshore wind turbines (OWTs) have emerged as a reliable source of renewable energy, witnessing massive deployment across the world. While there is a wide range of support foundations for these structures, the monopile and jacket are most utilised so far; their deployment is largely informed by water depths and turbine ratings. However, the recommended water depth ranges are often violated, leading to cross-deployment of the two foundation types. This study firstly investigates the dynamic implication of this practice to incorporate the findings into future analysis and design of these structures. Detailed finite element (FE) models of Monopile and Jacket supported offshore wind turbines are developed in the commercial software, ANSYS. Nonlinear Soil springs are used to simulate the soil-structure interactions (SSI) and the group effects of the jacket piles are considered by using the relevant modification factors. Modal analyses of the fixed and flexible-base cases are carried out, and natural frequencies are chosen as the comparison parameters throughout the study. Secondly, this study constructs a few-parameters SSI model for the two FE models developed above, which aims to use fewer variables in the FE model updating process without compromising its simulation quality. Maximum lateral soil resistance and soil depths are related using polynomial equations, this replaces the standard nonlinear soil spring model. The numerical results show that for the same turbine rating and total height, Jacket supported OWTs generally have higher first-order natural frequencies than Monopile supported OWTs, while the reverse is true for the second-order vibration modes, for both fixed and flexible foundations. This contributes to future design considerations of OWTs. On the other hand, with only two parameters, the proposed SSI model has achieved the same accuracy as that using the standard model with seven parameters. It has the potential to become a new SSI model, especially for the identification of soil properties through the model updating process.
Offshore wind turbine (OWT) foundations are subjected to a combination of cyclic and dynamic loading arising from wind, wave, 1P (rotor frequency) and 2P/3P (blade passing frequency) loads. Under cyclic/dynamic loading, most soils change their characteristics. Cyclic behaviour (in terms of change of shear modulus change and accumulation of strain) of a typical silica sand (RedHill 110) was investigated by a series of cyclic simple shear tests. The effects of application of 50,000 cycles of shear loading having different shear strain amplitude, cyclic stress ratio (ratio of shear to vertical stress), and vertical stress were investigated. Test results were reported in terms of change in shear modulus against the number of loading cycles. The results correlated quite well with the observations from scaled model tests of different types of offshore wind turbine foundations and limited field observations. Specifically, the test results showed that; (a) Vertical and permanent strain (accumulated strain) is proportional to shear strain amplitude but inversely proportional to the vertical stress and relative density; (b) Shear modulus increases rapidly in the initial cycles of loading and then the rate of increase diminishes and the shear modulus remains below an asymptote. Discussion is carried out on the use of these results for long term performance prediction of OWT foundations.
The complexity of the loads acting on the offshore wind turbines (OWTs) structures and the significance of investigation on structure dynamics are explained. Test results obtained from a scaled wind turbine model are also summarized. The model is supported on monopile, subjected to different types of dynamic loading using an innovative out of balance mass system to apply cyclic/dynamic loads. The test results show the natural frequency of the wind turbine structure increases with the number of cycles, but with a reduced rate of increase with the accumulation of soil strain level. The change is found to be dependent on the shear strain level in the soil next to the pile which matches with the expectations from the element tests of the soil. The test results were plotted in a non-dimensional manner in order to be scaled to predict the prototype consequences using element tests of a soil using resonant column apparatus.
Suction caissons are currently considered as an alternative to monopile foundations for met masts and offshore wind turbines. This paper presents the results of a series of centrifuge tests conducted on cyclically loaded suction caissons in very dense dry sand. Two representative caisson foundations were modelled at a 1200 scale in a geotechnical centrifuge and were subjected to a number of different cyclic loading regimes, for up to 12 000 cycles, both of which add to previous data sets available in the literature. During each test, changes in stiffness, the accumulation of rotation and settlement of the system were measured. It was found that the rotational caisson stiffness increased logarithmically with the number of loading cycles, but to a much lower extent than previously reported for monopiles. Similarly the accumulation of rotation was also observed to increase with number of cycles and was well described using a power relationship. An aggregation of rotation was also observed during two-way tests and is believed to be caused by the initial loading cycles that create a differential stiffness within the local soil. Predictions were then made as to the behaviour of a prototype structure based upon the observed test results and established influence parameters.
Offshore wind turbines are subjected to multiple dynamic loads arising from the wind, waves, rotational frequency (1P) and blade passing frequency (3P) loads. In the literature, these loads are often represented using a frequency plot where the power spectral densities (PSDs) of wave height and wind turbulence are plotted against the corresponding frequency range. The PSD magnitudes are usually normalized to unity, probably because they have different units, and thus, the magnitudes are not directly comparable. In this paper, a generalized attempt has been made to evaluate the relative magnitudes of these four loadings by transforming them into bending moment spectra using site-specific and turbine-specific data. A formulation is proposed to construct bending moment spectra at the mudline, i.e. at the location where the highest fatigue damage is expected. Equally, this formulation can also be tailored to find the bending moment at any other critical cross section, e.g. the transition piece level. Finally, an example case study is considered to demonstrate the application of the proposed methodology. The constructed spectra serve as a basis for frequency-domain fatigue estimation methods available in the literature
In this paper, we introduce a novel algorithm for morphing any accelerogram into a spectrum matching one. First, the seed time series is re-expressed as a discrete Volterra series. The first-order Volterra kernel is estimated by a multilevel wavelet decomposition using the stationary wavelet transform. Second, the higher-order Volterra kernels are estimated using a complete multinomial mixing of the first-order kernel functions. Finally, the weighting of every term in this Volterra series is optimally adapted using a Levenberg–Marquardt algorithm such that the modified time series matches any target response spectrum. Comparisons are made using the SeismoMatch algorithm, and this reweighted Volterra series algorithm is demonstrated to be considerably more robust,matching the target spectrum more faithfully. This is achieved while qualitatively maintaining the original signal’s non-stationary statistics, such as general envelope, time location of large pulses, and variation of frequency content with time.
The collapse of the Showa Bridge during the 1964 Niigata earthquake features in many publications as an iconic example of the detrimental effects of liquefaction. It was generally believed that lateral spreading was the cause of failure of the bridge. This hypothesis is based on the reliable eye witness that the bridge failed 1 to 2 minutes after the earthquake started which clearly ruled out the possibility that inertia (during the initial strong shaking) was the contributor to the collapse. Bhattacharya (2003), Bhattacharya and Bolton (2004), Bhattacharya et al (2005) reanalyzed the bridge and showed that the lateral spreading hypothesis cannot explain the failure of the bridge. The aim of this short paper is to collate the research carried out on this subject and reach conclusions based on analytical studies and quantitative analysis. It is being recognised that precise quantitative analysis can be difficult due to lack of instrumented data. However, as engineers, we need to carry out order-of-magnitude calculations to discard various failure hypotheses.
Many major river bridges were constructed in highgly active seismic areas of India much before the seismic code development. Bridges are lifelines infrastructure and as a result, it is necessary to requalify/reasses these structures in the light of the new and improved understanding of seismic resistant design philosophies. The aim of the paper is to develop a simplified methodology to carry out scenario based seismic requalification of major river bridges supported on caisson foundations (aslo known as Well Foundation). An example problem of Saraighat Bridge located in high Himalayan seismic zone is considered to demonstrate the application of the methodology. Field investigation and advanced laboratory tests on soil samples from the bridge site were carried out. The test results reveal that the soil is susceptible to liquefaction and as a result, soil structure interaction analyses are carried out. It is shown that good performance of these type of bridges depend on the displacement response of the pier head so as not to cause unseating of the decks. It is concluded, owing to the large stiffness of the foundations, bridges supported on caisson foundations may not adversely affected by liquefaction induced effects.
Stiffness and damping properties of soil are essential parameters for any dynamic soil structure interaction analysis. Often the required stiffness and damping properties are obtained from the empirical curves. This paper presents the stiffness and damping properties of two naturally occurring sandy soils collected from a river bed in a highly active seismic zone in the Himalayan belt. A series of resonant column tests are performed on the soil specimens with relative densities representative of the field and with varying confining pressures. The results are compared with the available empirical curves. Furthermore, a ground response analysis study is also carried out for a bridge site in the region using both empirical curves and experimentally obtained curves. It has been observed that the application of empirical modulus and damping curves in ground response prediction often leads to underestimation of the seismic demands on the structures.
A time-frequency approach based on wavelet transform is employed to examine the transient vibration characteristics of two 2×2 pile-group models tested in a shake table and subjected to three different records consisting of: white noise input and two differently scaled records from the 2011 Christchurch earthquake. In contrast to the conventional Fourier transform, the proposed method has the advantage of being capable of monitoring the temporal variation in structural frequencies and frequency content of ground motion due to liquefaction. Results are presented in time-frequency planes that enable displaying these time-varying processes in an effective way. It is found that the onset of liquefaction can have a substantial effect on the vibration characteristics, resulting in a reduction of natural frequencies in the range of 34- 51%, and shift of frequency content of the ground motion towards lower frequencies, along with narrowing of its overall frequency bandwidth. A final discussion on the practical implications of the main findings highlights that such non-stationary phenomena have important effects on the seismic response of pile-supported structures founded in liquefiable soils.
This article proposes solutions for stiffness estimation of Double-D shaped caisson foundations embedded in three different types of ground profiles (stiffness variation along the depth: homogeneous, linear and parabolic). The approach is based on three dimensional finite element analyses and is in line with the methodology adopted in Eurocode 8-Part 5 (2004)- lumped spring approach. The method of extraction of various stiffness values from the finite element model is described and followed by obtaining the closed form solutions. Parametric study revealed the nominal effect of embedment length of Double-D caisson and hence only the width and diameter effects are included in the suggested formulations. The obtained closed form solutions are presented in terms of multiplication factors for Double-D caissons. Final stiffness terms for a given width and diameter of a Double-D caisson can be conveniently estimated by multiplying the proposed formulations to the circular shaft solutions available in literature. Applicability of the proposed formulations is demonstrated by considering a typical bridge pier supported by Double-D caissons. The proposed formulations requires minimum amount of input parameters and can be used during the tender design to arrive at the required geometry of such foundations.
An active sequence of earthquakes (foreshock, main-shock, and aftershocks) hit the Kumamoto area (Japan) in April 2016, resulting in 69 deaths and considerable economic loss. The earthquakes induced numerous ground failures and cascading geo-hazards, causing major damage to important infrastructures. The main damage patterns include: (a) surface rupture with widespread subsidence of the surface ground, resulting in damage and disruption to transport infrastructure; (b) landslide and slope failure of mountains causing severe damage, collapse and near-collapse of bridges; and (c) liquefaction in some areas of Kumamoto City. Following the earthquakes, field surveys were conducted to study the damages and to understand the main cause of the observed failures. This technical note provides a summary of the geotechnical and infrastructural damage in Kumamoto and the lessons learnt and future research needs are also highlighted.
Many offshore wind turbines are supported by large diameter piles (known as monopiles) and are subjected to large number of cyclic and dynamic loads. There are evidences suggesting that foundation stiffness are changing with cycles of loading and this may lead to changes in the natural frequency of the system with the potential for unplanned system resonances. There are other consequences such as excessive tilt leading to expensive repair or even complete shutdown. Therefore, it is vital to understand the long-term response of wind turbine foundation so that a method to predict the change in frequency and tong term tilt could be established. This paper aims to present the experimental work of small scale physical modelling and Discrete Element Modelling (DEM) of the interaction between a monopile and the surrounding soil. Changes in soil stiffness under cyclic loading of various strain amplitudes were examined for both physical modelling and DEM. Micro-mechanics of soils underlying the soil stiffness change was investigated using DEM. Variation of force distribution along the mono-pile under cyclic loading was analysed to show the influence of monopile stability.
Liquefaction is one of the leading seismic actions to cause extensive damage to buildings and infrastructure during earthquakes. In many historic cases, plastic hinge formations in piles were observed at inexplicable locations. This project investigates the behaviour of piled foundations within soils susceptible to liquefaction using numerical analysis carried out in Abaqus in terms of plastic hinge development. Three different soil profiles were considered in this project by varying the thickness of both the liquefiable and non-liquefiable layers, pile length, free and fixed head pile conditions. Modelling a single pile as a beam-column element carrying both axial and El-Centro record earthquake loading produced results of the seismic behaviour of piles that could be assessed by Force-Based Seismic Design (FBSD) approaches. The displacements and deformations induced by dynamic loads were analysed for piles affected by liquefaction and the results used to demonstrate the pile capacity and discuss the damage patterns and location of plastic hinges. Parametric studies generally demonstrate that plastic hinge formation occurs at the boundaries of the liquefiable and non-liquefiable layers; however, the location can be affected by a variety of factors such as material properties, pile length and thickness of liquefying soil layer.
Offshore Wind Turbines (OWTs) are dynamically sensitive structures and as a result estimating the natural frequency of the whole system taking into effect the flexibility of the foundation is one of major design considerations. The natural frequency is necessary to predict the long-term performance as well as the fatigue life. Currently, jackets supported on multiple foundations (such as piles or suction caissons) are being considered to support WTG (Wind Turbine Generators) for deeper water developments. This paper presents a practical method to compute the natural frequency of a jacket supporting WTG by incorporating Soil-Structure-Interaction (SSI) based on closed form solutions. The formulation presented can be easily programmed in a spreadsheet type program and can serve as a convenient way to obtain natural frequency with least amount of input. The basis of this method is the Euler-Bernoulli beam theory where the foundations are idealized with a set of linear springs. In this method, a 3-Dimensional jacket is first converted into a two 2-Dimensional problem along the orthogonal planes of vibration which are essentially the principle axes of the foundation geometry. Subsequently, the jacket is converted into an equivalent beam representing its stiffness and a formulation is presented to find an equivalent beam for entire tower-jacket system. Using energy methods, an equivalent mass of the RNA (Rotor Nacelle Assembly)-tower-jacket system is also calculated and fixed base frequency of the jacket is estimated. To consider the flexibility effects of the foundation, a formulation for an equivalent rotational spring of the foundation is developed. A method to incorporate the mass of the transition piece is also presented. Finally, a step-by-step application of the methodology is presented by taking example problems from the literature which also serves the purpose of validation and verification.
Fault rupture is one of the main hazards for continuous buried pipelines and the problem is often investigated experimentally and numerically. While experimental data exists for pipeline crossing strike-slip and normal fault, limited experimental work is available for pipeline crossing reverse faults. This paper presents results from a series of tests investigating the behaviour of continuous buried pipeline subjected to reverse fault motion. A new experimental setup for physical modelling of pipeline crossing reverse fault is developed and described. Scaling laws and non-dimensional groups are derived and subsequently used to analyse the test results. Three-dimensional Finite Element (3D FE) analysis is also carried out using ABAQUS to investigate the pipeline response to reverse faults and to simulate the experiments. Finally, practical implications of the study are discussed.
Suction caissons have recently been considered as a cost effective alternative to conventional foundations for offshore met-masts and wind turbines. Such foundation arrangements are suitable for applications within water depths of 20-30m. Most offshore structures have stringent serviceability limit states imposed on their design dictating the allowable structural deflections and accumulated rotations throughout its operational life. This paper summarises the findings from a series of scale model tests and identifies key factors which influence the serviceability performance of a caisson founded offshore structure. These tests were conducted using representative caisson models in loose sand under single-g conditions, replicating a fully drained prototype condition. These experiments recorded the rotational foundation stiffness (soil structure interaction), the evolution of foundation stiffness under cyclic loading and the accumulation of structural rotation with loading cycles. It was discovered that the foundation stiffness was dependent on the local soil strain, and under cyclic loading would increase in a logarithmic manner. In addition it was found that under cyclic loading, a caisson system will retain and accumulate structural rotation following with a power relationship. From these observations it was possible to produce an analytical model and describe the changing serviceability state of a prototype structure with loading cycles.
The collapse of piled foundations in liquéfiable soil has been observed in the majority of recent strong earthquakes. This paper critically reviews the current understanding of pile failure in liquéfiable deposits, making reference to modern design codes such as JRA (1996), and taking the well-documented failure of the Showa Bridge in the 1964 Niigata earthquake as an example of what must be avoided. It is shown that the current understanding cannot explain some observations of pile failure.
Methane hydrate (MH) becomes a promising new energy in some countries including China and Japan due to its huge reservation. The key mission is to find the safe and efficient exploitation method. The exploitation processes will cause stress changes, which may induce submarine landslides and failures of engineering projects. This chapter described some state-of-art exploitation methods reproduced in laboratory and in numerical modelling to understand the responses of soils during exploitation process. These studies could provide valuable guidance for real life projects.
The aim of this chapter is to provide an overview of an overall layout of a wind farm to appreciate the multidisciplinary nature of the subject. The fundamental concepts and understanding of other disciplines and fields not directly related to civil engineering design but are necessary to carry out the design are also described with references for further study. The challenges in design of foundation are highlighted.
In practice, analysis of laterally loaded piles is often carried out using a “Beam on Non-linear Winkler Foundation method” whereby the lateral pile-soil interaction is modelled as a set of non-linear springs (also known as p y curves). During seismic liquefaction, the saturated sandy soil changes its state from a solid to a thick fluid like material (solid suspension), which in turn alters the shape of the p-y curve. Typically, p-y curves for non-liquefied soil looks like a convex curve with initial stiff slope which reduces with pile-soil relative displacement (y). However, recent research conclusively showed that p-y curve for liquefied soil has a different shape, i.e., upward concave with near-zero initial stiffness (due to the loss of particle to particle contact) up to a certain displacement (y), beyond which the stiffness increases due to reengaging of the sand particles. This paper presents a practical method for construction of the newly proposed p-y curves along with an example.
Offshore wind turbines (OWTs) are new types of structures with no track record of long-term performance. This chapter discusses the challenges in the design of OWTs with a focus on the Chinese waters. The main loads acting on a wind turbine are discussed together with the specific case of Chinese waters where in some location typhoon/cyclone and earthquake governs. The relatively soft and complex multilayering ground conditions in the Chinese water are also discussed. The challenges associated with dynamic soil structure interaction are also highlighted.
In practice, analysis of laterally loaded piles is carried out using beams on non-linear Winkler springs model (often known as p-y method) due to its simplicity, low computational cost and the ability to model layered soils. In this approach, soil-pile interaction along the depth is characterized by a set of discrete non-linear springs represented by p-y curves where p is the pressure on the soil that causes a relative deformation of y. p-y curves are usually constructed based on semi-empirical correlations. In order to construct API/DNV proposed p-y curve for clay, one needs two values from the monotonic stress-strain test results i.e., undrained strength (su) and the strain at 50% yield stress (ε50). This approach may ignore various features for a particular soil which may lead to un-conservative or over-conservative design as not all the data points in the stress-strain relation are used. However, with the increasing ability to simulate soil-structure interaction problems using highly developed computers, the trend has shifted towards a more theoretically sound basis. In this paper, principles of Mobilized Strength Design (MSD) concept is used to construct a continuous p-y curves from experimentally obtained stress-strain relationship of the soil. In the method, the stress-strain graph is scaled by two coefficient NC(for stress) and MC(for strain) to obtain the p-y curves. MC and NC are derived based on Semi-Analytical Finite Element approach exploiting the axial symmetry where a pile is modelled as a series of embedded discs. An example is considered to show the application of the methodology.
Offshore wind turbines (OWTs) are dynamically loaded structures and therefore the estimation of the natural frequency is an important design calculation to avoid resonance and resonance related effects (such as fatigue). Monopiles are currently the most used foundation type and are also being considered in deeper waters (>30 m) where a stiff transition piece will join the monopile and the tapered tall tower. While rather computationally expensive, high fidelity finite element analysis can be carried to find the Eigen solutions of the whole system considering soil–structure interaction; a quick hand calculation method is often convenient during the design optimisation stage or conceptual design stage. This paper proposes a simplified methodology to obtain the first natural frequency of the whole system using only limited data on the WTG (Wind Turbine Generator), tower dimensions, monopile dimensions and the ground. The most uncertain component is the ground and is characterised by two parameters: type of ground profile (i.e. soil stiffness variation with depth) and the soil stiffness at one monopile depth below mudline. In this framework, the fixed base natural frequency of the wind turbine is first calculated and is then multiplied by two non-dimensional factors to account for the foundation flexibility (i.e. the effect of soil–structure interaction). The theoretical background behind the model is the Euler–Bernoulli and Timoshenko beam theories where the foundation is idealised by three coupled springs (lateral, rocking and cross-coupling). 10 wind turbines founded in different ground conditions from 10 different wind farms in Europe (e.g. Walney, Gunfleet sand, Burbo Bank, Belwind, Barrow, Kentish flat, Blyth, Lely, Thanet Sand, Irene Vorrink) have been analysed and the results compared with the measured natural frequencies. The results show good accuracy (errors below 3.5%). A step by step sample calculation is also shown for practical use of the proposed methodology.
Offshore wind turbine structures (OWTs) are dynamically sensitive due to their shape and form (slender column supporting a heavy rotation mass) and also due to the different forcing functions (wind, wave, and turbine loading) acting on the structures. Designers need to ensure that the first Eigen natural frequency is not close to forcing frequencies to avoid dynamic associated effects such as resonance and fatigue damage. Such damages may result in higher maintenance costs and a lower service life. Therefore, it is crucial to get the best prediction of the first natural frequency during the early stages of a project. Other design requirements include the serviceability limit state (SLS) criteria which imposes strict pile head deflection and rotation limits. These calculations require foundation stiffness and the aim of this chapter is to provide practical methods to predict the stiffness of the foundations for any ground profile (nonuniform or layered soils) through the use of standard methods. The foundation stiffness values can then be used as an input to predict the first natural frequency of OWT system as well as checking SLS requirements. An example problem is taken to show the application of the method.
Understanding the behaviour of soils under cyclic/dynamic loading has been one of the challenging topics in geotechnical engineering. The response of liquefiable soils has been well studied however, the post liquefaction behaviour of sands needs better understanding. In this paper, the post liquefaction behaviour of sands is investigated through several series of multi-stage soil element tests using a cyclic Triaxial apparatus. Four types of sand were used where the sands were first liquefied and then monotonically sheared to obtain stress-strain curves. Results of the tests indicate that the stress-strain behaviour of sand in post liquefaction phase can be modelled as a bi-linear curve using three parameters: the initial shear modulus ( ), critical state shear modulus ( ), and post-dilation shear strain ( ) which is the shear strain at the onset of dilation. It was found that the three parameters are dependent on the initial relative density of sands. Furthermore, it was observed that with the increase in the relative density both and increase and decreases. The practical application of the results is to generate p-y curves for liquefied soil.
One of the major uncertainties in the design of offshore wind turbines is the prediction of long term performance of the foundation i.e. the effect of millions of cycles of cyclic and dynamic loads on the foundation. This technical note presents a simple and easily scalable loading device that is able to apply millions of cycles of cyclic as well as dynamic loading to a scaled model to evaluate the long term performance. Furthermore, the device is economic and is able to replicate complex waveforms (in terms of frequency and amplitude) and also study the wind and wave misalignment aspects. The proposed test methodology may also suffice the requirements of Technology Readiness Level (TRL) Level 3–4 i.e. Experimental Proof of Concept validation as described by European Commission. Typical long term test results from two types of foundations (monopile and twisted jacket on piles) are presented to show the effectiveness of the loading device.
Offshore wind turbines (OWTs) are considered as an important element of the future energy infrastructure. The majority of operational OWTs are founded on monopiles in water depths up to 30 m. Alternative foundation arrangements, however, are needed for future development rounds in deeper waters. To date, there have been no long-term observations of the performance of these relatively novel structures, although the monitoring of a limited number of OWTs has indicated a departure of the system dynamics from the design requirements. Lack of data concerning long-term performance indicates a need for detailed investigation to predict the future performance of such structures. Arguably this can be best carried out through small-scale laboratory experimental investigation, whose results are interpreted based on appropriate scaling laws. In this chapter, scaling laws are derived for the design of such model tests, which can be used for studying the long-term performance of small-scale wind turbines and prediction of the prototype response.
The purpose of this paper is to investigate the effects of liquefaction on modal parameters (frequency and damping) of pile-supported structures. Four physical models, consisting of two single piles and two 2 × 2 pile groups, were tested in a shaking table where the soil surrounding the pile liquefied because of seismic shaking. The experimental results showed that the natural frequency of pile-supported structures may decrease considerably owing to the loss of lateral support offered by the soil to the pile. On the other hand, the damping ratio of structure may increase to values in excess of 20%. These findings have important design consequences: (a) for low-period structures, substantial reduction of spectral acceleration is expected; (b) during and after liquefaction, the response of the system may be dictated by the interactions of multiple loadings, that is, horizontal, axial and overturning moment, which were negligible prior to liquefaction; and (c) with the onset of liquefaction due to increased flexibility of pile-supported structure, larger spectral displacement may be expected, which in turn may enhance P-delta effects and consequently amplification of overturning moment. Practical implications for pile design are discussed.
FPSO [Floating Production Storage and Offloading] structures have been accepted as a sustainable economic solution for deepwater development projects. Short to medium length (typically 15 to 25m) large diameter driven piles are often used to anchor FPSOs. The loading in such piles during a storm can be resolved into two components: (a) Lateral load, which is one-way cyclic; (b) Tensile (upward) load, which is typically only a few percentage of the lateral load. The greatest uncertainty in the analysis is the load carrying capacity of the pile, since the cyclic storm loading results in progressive degradation of the soil (sand or clay) supporting the pile. Thus understanding the degradation of the supporting oil is critical, for a safe, economic design. This paper thus hastwo aims: (a) to propose criteria and considerations for design of such piles; (b) to set out simple modifications in the p-y formulation that will provide a safe working envelope for the full range of ground conditions likely to be encountered at different sites. A parallel is also drawn to the approach routinely used by the geotechnical earthquake engineering profession, and reported centrifuge tests have been used to validate the proposed modification.
Under cyclic loading, most soils change their characteristics. Cyclic behaviour (change of shear modulus and accumulated strain) of the RedHill 110 sand was investigated by a series of cyclic simple shear tests. The effects of application of 50,000 cycles of shear loading with different shear strain amplitudes and vertical stresses were investigated. The results correlated quite well with the observations from scaled model tests of different types of offshore wind turbine foundations and limited field observations. Specifically, the test results showed that shear modulus increases rapidly in the initial loading cycles and then the rate of increase diminishes; the rate of increase depends on strain amplitude, initial relative density and vertical pressure. Complementary DEM simulations were performed using PFC2D to analyse the micromechanics underlying the cyclic behaviour of soils. It shows that the change of soil behaviour strongly related to the rotation of principle axes of fabric and degree of fabric anisotropy.
The aim of this chapter is to provide a summary of the numerical methods available to carry out long-term prediction analysis of offshore wind turbine foundations. Different available methods of analysis are discussed.
Unreinforced masonry (URM) buildings exhibited extreme vulnerability during past earthquakes though these are shelters of majority population in many earthquake prone developing countries. Most of the current retrofitting techniques used for such structures are either expensive or requires highly skilled labor or sophisticated equipment for implementation. On the other hand, the retrofitting technique proposed in this paper is economical and easy-to-apply. This paper aims at examining the performance of the retrofitting technique using polypropylene (PP) band. The displacement controlled lateral deformation has been investigated experimentally. The monotonic load-displacement behaviors of URM wall and the wall retrofitted with PP band are compared. It was found that URM wall retrofitted by PP band improves the ductility and energy absorption capacity by three times, and two times, respectively. Performance of a full-scale masonry building retrofitted with PP band in Nepal during last Gorkha earthquake of April 25, 2015, has also been presented in this paper. It was observed that the PP band retrofitted masonry building survived while the nearby many buildings experienced severe damage and some of them collapsed. This study demonstrates the efficacy and practicability of use of PP band for improving seismic resistance of URM structure.
Offshore wind turbine (OWT) foundations are subjected to a combination of cyclic and dynamic loading arising from wind, wave, rotor and blade shadowing. Under cyclic loading, most soils change their characteristics including stiffness, which may cause the system natural frequency to approach the loading frequency and lead to unplanned resonance and system damage or even collapse. To investigate such changes and the underlying micromechanics, a series of cyclic simple shear tests were performed on the RedHill 110 sand with different shear strain amplitudes, vertical stresses and initial relative densities of soil. The test results showed that: (a) Vertical accumulated strain is proportional to the shear strain amplitude but inversely proportional to relative density of soil; (b) Shear modulus increases rapidly in the initial loading cycles and then the rate of increase diminishes and the shear modulus remains below an asymptote; (c) Shear modulus increases with increasing vertical stress and relative density, but decreasing with increasing strain amplitude. Coupled DEM simulations were performed using PFC2D to analyse the micromechanics underlying the cyclic behaviour of soils. Micromechanical parameters (e.g. fabric tensor, coordination number) were examined to explore the reasons for the various cyclic responses to different shear strain amplitudes or vertical stresses. Both coordination number and magnitude of fabric anisotropy contribute to the increasing shear modulus.
Soil-Structure-Interaction (SSI) for offshore wind turbine supporting structures is essentially the interaction of the foundation/foundations with the supporting soil due to the complex set of loading. This paper reviews the different aspects of SSI for different types of foundations used or proposed to support offshore wind turbines. Due to cyclic and dynamic nature of the loading that acts on the wind turbine structure, the dominant SSI will depend to a large extent on the global modes of vibration of the overall structure. This paper summarises the modes of vibration of offshore wind turbines structures supported on different types of foundations based on observations from scaled model tests and numerical analysis. As these are new structures with limited monitoring data, field records are scarce. Field records available in the public domain are also used to compare with the experimental findings.
Simplified design procedures for offshore wind turbine (OWT) support structures are necessary in the concept and tender design stages for assessing the financial viability. Building upon the previous research on simplified monopile design, this paper presents a similar method to design jacket supported OWTs founded on piles or suction caissons. The method is based on the minimum and necessary data, namely the site characteristics (wind speed at reference height, wind turbulence intensity, water depth, wave height and wave period), turbine characteristics (rated power, rated wind speed, rotor diameter, cut-in and cut-out speed, mass of the rotor-nacelle-assembly) and ground profile (soil stiffness variation with depth and soil strength). A flowchart of the design process is also presented for visualisation of the rather complex multi-disciplinary analysis. Insights are given for 3 and 4 legged jackets. The calculation procedures that are required can be easily carried out either through a series of spreadsheets or simple hand calculations. An example problem emulating the design of jacket on piles is taken from literature to demonstrate the proposed calculation procedure. The data used for the calculations are obtained from publicly available sources and the example shows that the simplified method arrives at a similar foundation to the one reported.
Liquefaction is an important seismic hazard that can cause extensive damage and high economic impact during earthquakes. Despite the extensive research, methodologies, and approaches for managing liquefaction for pile supported structures, failures of structures due to liquefaction have continued to occur to this day. The main aim of this paper is to develop a simplified methodology to reduce potential structural damage of structures founded in soils susceptible to liquefaction. In order to implement a successful remediation technique, the current methods for pile failure in liquefiable soils and remediation schemes of earthquake-induced liquefaction are critically reviewed and discussed. The cementation and lattice structure techniques to reduce the liquefaction hazard are proposed, while numerical analysis for unimproved and stabilised soil profiles using Finite Element Method (FEM) is carried out to simulate the analysis of both stabilisation techniques. The results showed that the both techniques are effective and economically viable for reduction or avoidance of potential structural damage caused by liquefied soil and can be used in isolation or in combination, depending on the ground profile and pile type.
This article presents a comprehensive study of dynamic soil properties [namely, initial shear modulus-Gmax; normalized shear modulus reduction (G/Gmax); and damping ratio (D) variation curves] and pore water pressure parameters of a river bed sand (Brahmaputra sand), sampled from a highly active seismic region (northeast India). Two independent high quality apparatus (resonant column-RC and cyclic triaxial-CTX) are adopted in the study. Resonant column apparatus was used to obtain the small strain properties (up to 0.1%) while CTX equipment was adopted to obtain the high strain properties along with the pore water pressure parameters. The results obtained from both the equipment are combined to provide a comprehensive data of dynamic soil properties over wide range of strains. A modified hyperbolic formulation was suggested for efficient simulation of G/Gmax and D variations with shear strain. Based on the CTX results, a pore water pressure generation model is presented. Furthermore, a nonlinear effective stress ground response study incorporating the pore water pressure generation, is performed using the recorded earthquake motions of varying peak bed rock acceleration (PBRA) in the region, to demonstrate the applicability of proposed dynamic soil properties and pore pressure parameters. High amplification for low PBRA ground motions (˂ 0.10 g) was observed and attenuation of seismic waves was witnessed beyond a PBRA of 0.10 g near the surficial stratum due to the induced high strains and the resulting high hysteretic damping of the soil. Also, increased excess pore pressure generation with increased PBRA of the input motion was observed and the considered soil stratum is expected to liquefy beyond a PBRA of 0.1 g. The established properties can be handy to the design engineers during seismic design of structures in the northeast Indian region.
The seismic performance of four pile-supported models is studied for two conditions: (i) transient to full liquefaction condition i.e. the phase when excess pore pressure gradually increases during the shaking; (ii) full liquefaction condition i.e. defined as the state where the seismically-induced excess pore pressure equalises to the overburden stress. The paper describes two complementary analyses consisting of an experimental investigation carried out at normal gravity on a shaking table and a simplified numerical analysis, whereby the soil-structure interaction (SSI) is modelled through non-linear Winkler springs (commonly known as p-y curves). The effects of liquefaction on the SSI are taken into account by reducing strength and stiffness of the non-liquefied p-y curves by a factor widely known as p-multiplier and by using a new set of p-y curves. The seismic performance of each of the four models is evaluated by considering two different criteria: (i) strength criterion expressed in terms of bending moment envelopes along the piles; (ii) damage criterion expressed in terms of maximum global displacement. Comparison between experimental results and numerical predictions shows that the proposed p-y curves have the advantage of better predicting the redistribution of bending moments at deeper elevations as the soil liquefies. Furthermore, the proposed method predicts with reasonable accuracy the displacement demand exhibited by the models at the full liquefaction condition. However, disparities between computed and experimental maximum bending moments (in both transient and full liquefaction conditions) and displacement demands (during transient to liquefaction condition) highlight the need for further studies.
© 2015 by ASME.The dynamic response of the supporting structure is critical for the in-service stability and safety of offshore wind turbines (OWTs). The aim of this paper is to first illustrate the complexity of environmental loads acting on an OWT and reveal the significance of its structural dynamic response for the OWT safety. Second, it is aimed to investigate the long-term performance of the OWT founded on a monopile in dense sand. Therefore, a series of well-scaled model tests have been carried out, in which an innovative balance gear system was proposed and used to apply different types of dynamic loadings on a model OWT. Test results indicated that the natural frequency of the OWT in sand would increase as the number of applied cyclic loading went up, but the increasing rate of the frequency gradually decreases with the strain accumulation of soil around the monopile. This kind of the frequency change of OWT is thought to be dependent on the way how the OWT is cyclically loaded and the shear strain level of soil in the area adjacent to the pile foundation. In this paper, all test results were plotted in a nondimensional manner in order to be scaled up to predict the consequences for prototype OWT in sandy seabed.
Monopile foundations have been commonly used to support offshore wind turbine generators (WTGs), but this type of foundation encounters economic and technical limitations for larger WTGs in water depths exceeding 30m. Offshore wind farm projects are increasingly turning to alternative multipod foundations (for example tetrapod, jacket and tripods) supported on shallow foundations to reduce the environmental effects of piling noise. However the characteristics of these foundations under dynamic loading or long term cyclic wind turbine loading are not fully understood. This paper summarises the results from a series of small scaled tests (1:100, 1:150 and 1:200) of a NREL (National Renewable Energy Laboratory) on three types of foundations: monopiles, symmetric tetrapod and asymmetric tripod. The test bed used consists of either kaolin clay or sand and up to 1.4 million loading cycles were applied. The results showed that the multipod foundations (symmetric or asymmetric) exhibit two closely spaced natural frequencies corresponding to the rocking modes of vibration in two principle axes. Furthermore, the corresponding two spectral peaks change with repeated cycles of loading and they converge for symmetric tetrapods but not for asymmetric tripods. From the fatigue design point of view, the two peaks for multipod foundations broaden the range of frequencies that can be excited by the broadband nature of the environmental loading (wind and wave) thereby impacting the fatigue. The system life (number of cycles to failure) may effectively increase for symmetric foundations as the two peaks will tend to converge. However, for asymmetric foundations the system life may continue to be affected adversely as the two peaks will not converge. In this sense, designers should prefer symmetric foundations to asymmetric foundations.
Offshore Wind Turbine (OWT) support structures need to satisfy different Limit States (LS) such as Ultimate LS (ULS), Serviceability LS, Fatigue LS and Accidental LS. Furthermore, depending on the turbine rated power and the chosen design (all current designs are soft-stiff), target natural frequency requirements must also be met. Most of these calculations require the knowledge of the stiffnesses of the foundation which, especially in the case of large turbines in intermediate waters (30 to 60 meters), might need to be configured using multiple foundation elements. For this reason, this paper studies, for a homogeneous elastic halfspace, the static stiffnesses of groups of polygonally arranged non-slender suction bucket foundations in soft soils modeled as rigid solid embedded foundations. A set of formulas for correcting the stiffnesses obtained from isolated foundation formulation are proposed. It is shown through the study of several multi-megawatt OWTs that, as expected, group effects becomes more relevant as spacing decreases. Also, group effects are sensitive mainly to shear modulus of soil, foundation shape ratio and diameter, and the number of foundations. The results obtained from the soil-structure system show that ignoring group effects may add significant errors to the estimation of OWT fundamental frequencies and leads to either overestimating or underestimating it by 5%. This highlights the importance of adequately modeling the interaction between elements of closely-separated multi-bucket foundations in soft soils, when current guidelines specify the target fundamental frequency to be at least 10% away from operational 1P and blade passing frequencies (2P/3P frequencies).
The paper presents a simplified approach to determine the post-cyclic deformation of liquefied sloping grounds. The approach uses instability curves derived from undrained multi-stage (cyclic+monotonic) triaxial tests. It is shown that the salient aspects of the post-liquefaction deformation can be expressed as a function of the state parameter ψ, defined as the void ratio difference between the current state of the soil and its critical state at the same mean stress level, and amplitude of accumulated cyclic strain. As the proposed approach predicts deformations, rather than residual strength or factor of safety, the method can be used for the definition of the performance criteria following a performance-based design approach. The application of the proposed method is illustrated through a real case study.
Pile supported river bridges still continue to collapse after most major earthquakes in the event of liquefaction. The identified failure mechanisms of piles in liquefied soil are: bending failure due to the inertial loads of the superstructure and kinematic loads due to the lateral spreading of soil; shear failure due to shear loads; buckling instability failure due to vertical loads and associated imperfections; settlement failure due to loss of effective stress in the liquefied zone and finally failure due to the effects related to the elongation of natural period of the piers (also referred to as dynamic failure). This paper revisits the collapse of Shengli Bridge (due to 1976 Tangshan Earthquake) and Panshan Bridge (due to 1975 Haicheng Earthquake) based on the aforementioned failure mechanisms. It has been concluded that pile supported bridges in liquefiable soil can collapse due to each of these five failure mechanisms or due to a suitable combination thereof. It is therefore quite imperative to design pile foundations in liquefiable soil by taking all the failure mechanisms into consideration. The simplified calculation procedure presented in this paper can also be used to carry out the design of bridge piles in the liquefiable soil.
In practice, laterally loaded piles are most often modelled using a ‘Beam-on-Nonlinear-Winkler-Foundation’ (BNWF) approach. While well calibrated p-y curves exist for non-liquefied soils (e.g. soft clay and sands), the profession still lacks reliable p-y curves for liquefied soils. In fact, the latter should be consistent with the observed strain-hardening behaviour exhibited by liquefied samples in both element and physical model tests. It is recognised that this unusual strain-hardening behaviour is induced by the tendency of the liquefied soil to dilate upon undrained shearing, which ultimately results in a gradual decrease of excess pore pressure and consequent increase in stiffness and strength. The aim of this paper is twofold. First it proposes an easy-to-use empirical model for constructing stress-strain relationships for liquefied soils. This only requires three soil parameters which can be conveniently determined by means of laboratory tests, such as a cyclic triaxial and cyclic simple shear tests. Secondly, a method is illustrated for the construction of p-y curves for liquefiable soils from the proposed stress-strain model. This involves scaling of stress and strain into compatible soil reaction p and pile deflection y, respectively. The scaling factors for stress and strain axis are computed following an energy-based approach, analogous to the upper-bound method used in classical plasticity theory. Finally, a series of results from centrifuge tests are presented, whereby p-y curves are back-calculated from available experimental data and qualitatively compared with that proposed by the authors.
Large diameter caissons are being considered as plausible foundations for supporting offshore wind turbines (OWTs) where reductions in overall cost and environmentally friendly installation methods are expected. The design calculations required for optimization of dimensions/sizing of such caissons are critically dependent on the foundation stiffness as it is necessary for SLS (Serviceability Limit State), FLS (Fatigue Limit State), and natural frequency predictions. This paper derives closed form expressions for the 3 stiffness terms (Lateral stiffness KL, Rotational Stiffness KR and Cross-Coupling term KLR) for suction caissons having aspect ratio between 0.5 and 2 (i.e. 0.5
Monopiles are currently the preferred option for supporting offshore wind turbines (OWTs) in water depths up to about 40 m. Whilst there have been significant advancements in the understanding of the behaviour of monopiles, the guidelines on the prediction of long term tilt (Serviceability Limit State, SLS) under millions of cycles of loads are still limited. Observations and analysis of scaled model tests identify two main parameters that governs the progressive tilt of monopiles: (a) Loading type (one-way or two-way) which can be quantified by the ratio of the minimum to maximum mudline bending moments (Mmin/Mmax); (b) factor of safety against overturning i.e. the ratio of the maximum applied moment (Mmax) to the moment carrying capacity of the pile or Moment of Resistance (MR) and therefore the ratio Mmax/MR. Due to the nature of the environmental loads (wind and wave) and the operating conditions of the turbine, the ratio Mmin/Mmax changes. The aim of this paper is to develop a practical method that can predict the nature of loading for the following governing load cases: Normal Operating Conditions, Extreme Wave Load scenario, and Extreme Wind Load scenario. The proposed method is applied to 15 existing wind farms in Europe where (Mmin/Mmax) and (Mmax/MR) are evaluated. The results show that the loading ratio is sensitive to the water depth and turbine size. Furthermore, under normal operating conditions, most of the wind turbine foundations in shallow waters are subjected to one-way loading and in deeper waters and under extreme conditions the loading is marginally two-way. Predictions for the nature of loading for large wind turbines (8MW and 10MW) in deeper waters are also presented. The results from this paper can be used for planning scaled model tests and element tests of the soil.
Pile supported river bridge failures are still observed in liquefiable soils after most major earthquakes. One of the recurring observations is the mid span collapse of bridges (due to pier failure) with decks falling into the river while the piers close to the abutment and the abutment itself remain stable. This paper proposes a mechanism of the observed collapse. It has been shown previously through experiments and analytically that the natural period of bridge piers increases as soil liquefies. Due to the natural riverbed profile (i.e. increasingly higher water depth towards the center of the river), the increase in natural period for the central piers is more as compared to the adjacent ones. Correspondingly, the displacement demand on the central pier also increases as soil progressively liquefies further promoting differential pier-cap displacements. If the pier-cap seating lengths for decks are inadequate, it may cause unseating of the decks leading to collapse. The collapse of Showa Bridge (1964 Niigata earthquake) is considered to demonstrate the mechanism. The study suggests that the bridge foundations need to be stiffened at the middle spans to reduce additional displacement demand.
The dynamic free field response of two stratified deposits with different stiffness ratios between the top and the bottom layer was analysed by shaking table testing. The granular deposits were contained in a laminar shear box and subjected to a wide set of dynamic inputs with different frequency content. Two exploratory modal testing techniques were employed to measure the natural frequency of the individual layers and the results were employed in the calculation of the fundamental period of the overall stratified profile by an extended variant of the Madera procedure . The dynamic response was investigated in relation to the frequency content of the dynamic excitation, the granular material properties and the stiffness characteristics of the enclosing container. The measured dynamic stiffness for the mono-layered and the bi-layered sand deposits compare well with previous empirical curves for sands increasing the confidence in the shaking table and shear stack testing as tools of dynamic investigation of granular media.
Offshore wind turbine foundations are subject to 107 to 108 cycles of loadings in their designed service life. Recent research shows that under cyclic loading, most soils change their properties. Discrete Element Modelling of cyclic simple shear tests was performed using PFC2D to analyse the micromechanics underlying the cyclic behaviours of soils. The DEM simulation were first compared with previous experimental results. Then asymmetric one-way and two-way cyclic loading pattern attained from real offshore wind farms were considered in the detailed parametric study. The simulation results show that the shear modulus increases rapidly in the initial loading cycles and then the rate of increase diminishes; the rate of increase depends on the strain amplitude, initial relative density and vertical stress. It shows that the change of soil behaviour is strongly related to the variation of coordination number, rotation of principal stress direction and evolution of degree of fabric anisotropy. Loading asymmetry only affects soil behaviours in the first few hundred of cycles. In the long term, the magnitude of (γmax - γmin) rather than loading asymmetry dominates the soil responses. Cyclic loading history may change the stress-strain behaviour of a soil to an extent dependent on its initial relative density.
Offshore wind turbines are considered as an important element of the future energy infrastructure. There is currently a surge in the construction of such facilities in Europe, yet there is no track record of long-term performance of these structures. Offshore wind turbines are dynamically sensitive structures because of the very nature of the structural form (tall and slender) and the different types of dynamic and cyclic loading imposed on them. Lack of data concerning long-term performance indicates a need for detailed investigation to predict the future performance of such structures. Arguably this can be best carried out through small-scale well-controlled laboratory experimental investigation. In this paper, scaling laws are derived for the design of such model tests for studying the long-term performance. Non-dimensional groups that need to be preserved are identified while carrying out these tests. The effectiveness of these chosen non-dimensional groups is investigated by carrying out controlled tests on a 1:100 scale offshore wind turbine. Typical experimental data are presented.
Modes of vibration play a dominant role in the design of WTG (Wind Turbine Generator) support structures. It is necessary to choose the overall system frequency such that the modes of vibration do not coincide with the rotor frequencies as well as the wave frequencies. WTG supported on multiple foundations (such as jackets or seabed frames) may exhibit rocking modes of vibration if the vertical stiffness of the foundation is not large enough which in turn may have serious implications on the fatigue performance of the overall structure. From the O&M (Operation and Maintenance) point of view, it is necessary to design the overall system to have sway-bending as the dominant mode of vibration. This paper develops a formulation for obtaining foundation (for both piles and shallow suction caissons) sizes and spacing such that rocking vibrations are prevented and sway-bending vibrations are achieved. Expressions for the minimum vertical stiffness of foundations are proposed for different configurations: square base, symmetrical (equilateral) triangle, or asymmetrical (isosceles) triangle. Verification of the method is carried out through finite element analysis and a step-by-step solved example is taken to show the application of the formulation. It is hoped that the formulation will assist designers to optimize the foundation arrangement and provide preliminary sizing for tender design.
Physical modelling of scaled models is an established method for understanding failure mechanisms and verifying design hypothesis in earthquake geotechnical engineering practice. One of the requirements of physical modelling for these classes of problems is the replication of semi-infinite extent of the ground in a finite dimension model soil container. This chapter is aimed at summarizing the requirements for a model container for carrying out seismic soil-structure interactions (SSI) at 1-g (shaking table) and Ng (geotechnical centrifuge at N times earth's gravity).
This thesis addresses the major problem of high turbidity water in drinking water treatment to small rural and urban populations. This problem is attributed to polluted surface water sources and untreated piped water supply systems. In this research, the problem is addressed in the context of the Gilgit-Baltistan located in the extreme north of Pakistan, however, the solutions proposed may be applicable in other parts of the world with similar conditions of glacial melt water as a source for gravity fed drinking water delivery systems. Glaciers, lakes and seasonal snow deposits are the principal sources of all flowing water in the Gilgit-Baltistan (GB) region. High river water flows in summer, due to snow and glacial melt, which result in flooding and high turbidity water in almost all surface water sources in GB. Main surface drinking water sources in the region include rivers, lakes, springs and traditional drinking water channels and shallow water pits. Slow sand filter, due to the simple operational, maintenance requirements and high biological treatment efficiency is an attractive technology. However, due to its vulnerability to high suspended solids loadings this could not be applied in Gilgit-Baltistan, where more than 70% of the surface water sources has turbid water ranging from 500-3000 NTUs for 6 months (IUCN 2003). In order to cope with such water quality issues, dosing of a coagulant at upstream of the pre-filtration stage is commonly used worldwide. However, due to high costs and skilled personnel requirements, this is not suitable for small rural drinking water supply systems, such as in Gilgit-Baltistan. The successful use of gravel filters as pre-treatment have been investigated worldwide and in Gilgit-Baltistan, however, in few glacial water sources having finer particles conventional up-flow gravel filters failed to reduce turbidity levels up-to-the range suitable for slow sand filters. The objectives of this study were to: assess dirking water quality in rural and urban settlements, critically review of the physical characteristics of glacial water and to investigate efficiency as well as the effectiveness of up-flow gravel filters in series (UGFS) as pre-treatment in the context of Gilgit-Baltistan. In order to achieve the research aims an experimental plant was built at Mominabad, Hunza. The experimental plant was designed on the bases of the literature review, studies, past experience of up-flow roughing filters in GB and typical design guideline values for up-flow roughing filters. The experimental plant consisted of three series connected with the existing sedimentation tank. Each series was further divided into 3 sub-stages of up-flow filters, with different types, gradings, and depth of filter media. Different types of filter media used in the experimental plant consisted upon commercial charcoal, natural granite gravel, burnt bricks, natural limestone gravel and river bed gravel. The depth of filter media in each stage of the series was kept 0.9 m, with a different type of filter medium of 0.3 m depth. The filter compartment having burnt bricks, commercial charcoal, and limestone media were placed in different positions in each series. Water quality analysis was carried out for three months for turbidity levels revealed that 80 to 99 percent reduction in turbidity levels were achieved at the end of all URF-series in the experimental plant (p=0.000) Performance of burnt bricks, charcoal, and lime stone filter medium at first stage of the URF series coupled with granite filter medium in other sub-stages was found best in reducing turbidity levels of 340 NTUs and 939 NTUs in winter and summer seasons where 94 and 88 percent reduction was observed at outlet of the URF Stage-3 respectively A baseline water quality survey was carried out in the initial phase of the research to assess the bacteriological and physio-chemical quality of improved and traditional drinking water delivery systems, spring sources in rural and urban areas and proposed water source for the experimental plant in Mominabad Hunza. Grab samples were collected from selected representative points of the improved and traditional drinking water systems. Membrane filtration technique was used to assess bacteriological contamination in water samples. Conductivity, TDS, temperature, and pH was measured by a conductivity meter (Eutech Instruments, Cyberscan Con 11). Turbidity was measured by nephelometric turbidity tubes provided with Del-aqua water testing kit. Chemical analysis of urban drinking water supply systems and experimental plant site was analyzed on a spectrophotometer. The rural baseline results showed that with the exception of spring water, almost all the rural water supply systems and sources were biologically contaminated. Out of 284 samples, only 68 (24 %) samples had zero counts for E.coli in 100 mL sample and were fit for human consumption as per WHO Guideline Values (GVs) and National Standards Drinking Water Quality set by Government of Pakistan. (WHO, 2011; PEPA, 2008). Urban baseline water quality results showed that out of the 89 water samples, collected from different points of the urban water supply networks, 20 (22 %) samples were found to be fit for human consumption. However, chemical parameter in most of the cases were below the recommended guideline values of WHO (WHO, 2011) for drinking water in developing countries and National Drinking Water Quality Standards (PEPA, 2008), except in water samples taken from Gilgit town water supply networks, where the concentration of chromium, iron and nitrate was found on higher side as per the recommended values. The research revealed that almost all surface drinking water sources and piped water supply systems were not fit for human consumption with different degree of feacal contamination. High concentration of chromium, nitrite, and iron were observed in some urban drinking water sources. Experimental plant monitoring results indicated that up-flow multi-layer roughing filters in series was a suitable option for pretreatment under low flow rates for high turbidity glacier waters for community managed drinking water supply systems. Experimental plant results showed the effective removal of bacteriological (E.coli) and chemical (Nitrate, Nitrite, Phosphate, and Ammonia. Overall research results indicate that up-flow multi-layer roughing filter in series is a suitable option for pretreatment of highly turbidity glacial waters and is effective to remove bacterial and chemical contamination present in source water. URF-series-3, where burnt bricks, limestone and charcoal were placed in first stage of the series, reduced turbidity level higher than the other URF series. Research conclude that URF could be an appropriate and environment friendly option for rural water supply systems for glacial water sources, such as in Gilgit-Baltistan.