### Subhamoy Bhattacharya

### Biography

### Biography

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).

### News

### My publications

### Publications

wind turbines in nonhomogeneous soils-emphasis on

soil/monopile interface characteristics, Earthquakes and Structures

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.

presented to provide a fast and reasonably accurate natural frequency estimation suitable for

preliminary design or verification of Finite Element calculations. Previous research model

led the problem using Euler?Bernoulli beam model where the foundation is represented by two springs(lateral and rotational). In contrast, this study improves on previous efforts by incorporating across-coupling

stiffness thereby modelling the foundation using three springs. Furthermore,this study also derive s the natural frequency using Timoshenko beam model by including rotary inertia and shear deformation.The

results oftheproposedmodelarealsocomparedwithmeasuredvaluesofthenaturalfrequencyoffour

OWTsobtainedfromtheliterature.TheresultsshowthattheTimoshenkobeammodeldoesnotimprove

the resultssignificantly andtheslenderbeamassumptionmaybesufficient. Thecross-couplingspring

term hasasignificant effectonthenaturalfrequencythereforeneedstobeincludedintheanalysis.The

model predicts the natural frequency of existing turbines with reasonable accuracy.

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.

offshore wind turbine support structures, Wind Energy

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

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 De ) 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.

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.

available to carry out long-term prediction analysis of offshore wind turbine

foundations. Different available methods of analysis are discussed.

from stress-strain of soils: FE assessment of scaling coefficients using the Mobilized Strength Design concept, Geomechanics and Engineering 5 (5) pp. 379-399

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 (µ50). 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.

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.

interaction analysis is proposed, utilising elastodynamic solutions. The method is based on solving a

Lagrangian system of coupled equations for the pile and the soil motions for a range of vibration

frequencies and also by considering the vertical, radial and angular stresses on the pile?soil interface.

The solution extensively uses Bessel functions of the second kind and results are compared with

?nite-element models and ?eld pile load tests. A dimensionless frequency related to the well-known

active length of pile is proposed to separate inertial and kinematic interactions. A formula is also

proposed for estimation of the active length of a pile in a two-layered soil. A speci?c depth is

introduced beyond which soil layering does not have any appreciable effects on dynamic stiffness. It

is commonly (rather arbitrarily) assumed that the ?rst natural frequency of soil strata differentiates

radiation (geometric) damping from hysteretic (material) damping for both types of interactions of the

pile?soil system. In contrast, this paper proposes a new formulation based on relative pile?soil

stiffness and frequency of the pile head loading to differentiate these two classes of damping

behaviour. The application of the formulation is shown through an example.

soils continue to collapse during earthquakes despite

being designed with required factors of safety against

bending due to lateral loads and axial capacity (shaft

resistance and end-bearing). Recent research identi?ed a

few weaknesses in the conventional design approach:

(a) when soil lique?es it loses much of its stiffness and

strength, so the piles now act as long slender columns, and

can simply buckle (buckling instability) under the combined

action of axial load and inevitable imperfections (e.g.

out-of-line straightness, lateral perturbation loads due to

inertia and/or soil ?ow). In contrast, most codes recommend

that piles be designed as laterally loaded beams;

(b) Natural frequency of pile supported structures may

decrease considerably owing to the loss of lateral support

offered by the soil to the pile and the damping ratio of

structure may increase to values in excess of 20 %. These

changes in dynamic properties can have important design

consequences. The immediate need is not only to rewrite

the design code to incorporate these effects, particularly

buckling instability but also to requalify and, if necessary,

strengthen the existing important piled foundations in lique?able

soils. This paper aims to provide a methodology for carrying out requali?cation studies. A practical example is taken to show the application of the methodology

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).