Professor Subhamoy Bhattacharya

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Phone: 01483 68 9534

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Publications

Journal articles

  • Dammala P, Bhattacharya S , Adapa M, Kumar S, Dasgupta K. (2017) 'Scenario based Seismic Re-qualification of caisson supported major bridges – a case study of Saraighat bridge'. Elsevier Soil Dynamics and Earthquake Engineering,
    [ Status: Accepted ]

    Abstract

    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.

  • Dammala P, Adapa M, Bhattacharya S , Nikitas G , Rouholamin M. (2017) 'Dynamic Soil Properties for Seismic Ground Response Studies in Northeastern India'. Elsevier Soil Dynamics and Earthquake Engineering, 100, pp. pp. 357-370.
    [ Status: Accepted ]

    Abstract

    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.

  • Dash S, Rouholamin M, Bhattacharya S . (2017) 'A practical method for construction of p-y curves for liquefiable soils'. Elsevier Soil Dynamics and Earthquake Engineering, 97, pp. pp. 478-481.

    Abstract

    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.

  • Lombardi D, Dash S, Bhattacharya S , Ibraim E, Wood D, Taylor C. (2017) 'Construction of simplified design p-y curves for liquefied soils'. Thomas Telford (ICE Publishing) Geotechnique, 67 (3), pp. pp. 216-227.

    Abstract

    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.

  • Rouholamin M, Bhattacharya S , Orense R. (2017) 'Effect of initial relative density on the post-liquefaction behaviour of sand'. Elsevier Soil Dynamics and Earthquake Engineering, 97, pp. pp. 25-36.

    Abstract

    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.

  • Bhattacharya S , Nikitas G , Arany L, Nikitas N. (2017) 'Soil-Structure Interactions (SSI) for Offshore Wind Turbines'. The Institution of Engineering and Technology IET Engineering and Technology Reference,
    [ Status: Accepted ]

    Abstract

    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.

  • Mohanty P, Dutta S, Bhattacharya S . (2017) 'Proposed mechanism for mid-span failure of pile supported river bridges during seismic liquefaction'. Elsevier Soil Dynamics and Earthquake Engineering,
    [ Status: Accepted ]

    Abstract

    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.

  • Arany L, Bhattacharya S , Macdonald J, Hogan S. (2017) 'Design of monopiles for offshore wind turbines in 10 steps'. Soil Dynamics and Earthquake Engineering, 92, pp. pp. 126-152.

    Abstract

    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.

  • Cox J, Bhattacharya S . (2016) 'Serviceability of Suction Caisson Founded Offshore Structures'. Proceedings of the Institution of Civil Engineers: Geotechnical Engineering, 170 (3), pp. pp. 273-284.

    Abstract

    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.

  • Cui L , Bhattacharya S . (2016) 'Soils-structure interaction for monopile supporting OWT'. Institution of Civil Engineers Engineering and Computational Mechanics,
    [ Status: Accepted ]

    Abstract

    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.

  • Nikitas G , Arany L, Aingaran S, Vimalan J, Bhattacharya S . (2016) 'Predicting long term performance of Offshore Wind Turbines using Cyclic Simple Shear apparatus'. Elsevier Soil Dynamics and Earthquake Engineering, 92, pp. pp. 678-683.
    [ Status: Accepted ]

    Abstract

    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.

  • Bhattacharya S. (2016) 'Static impedance functions for monopiles supporting offshore wind turbines in nonhomogeneous soils-emphasis on soil/monopile interface characteristics'. Earthquakes and Structures,
  • Shadlou M, Bhattacharya S. (2016) 'Dynamic stiffness of monopile supporting offshore wind turbine generators'. Elsevier Soil Dynamics and Earthquake Engineering, 88, pp. 15-32.

    Abstract

    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.

  • Dihoru L, Bhattacharya S, Moccia F, Simonelli AL, Taylor CA, Mylonakis G. (2016) 'Dynamic testing of free field response in stratified granular deposits'. ELSEVIER SCI LTD SOIL DYNAMICS AND EARTHQUAKE ENGINEERING, 84, pp. 157-168.
  • Lombardi D, Bhattacharya S . (2016) 'Evaluation of seismic performance of pile-supported models in liquefiable soils'. Wiley Earthquake Engineering and Structural Dynamics, 45 (6), pp. pp. 1019-1038.

    Abstract

    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.

  • Bhattacharya S. (2016) 'Closed form solution of Eigen frequency of monopile supported offshore wind turbines in deeper waters incorporating stiffness of substructure and SSI'. International Journal of Soil Dynamics and Earthquake Engineering, Volume 83, April 2016, Pages 18–32, pp. 18-32.

    Abstract

    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.

  • Bhattacharya S, Nikitas G, Vimalan N. (2016) 'An innovative cyclic loading device to study long term performance of offshore wind turbines'. Soil Dynamics and Earthquake Engineering, 82, pp. 154-160.

    Abstract

    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.

  • Bhattacharya S, Goda K. (2015) 'Use of offshore wind farms to increase seismic resilience of Nuclear Power Plants'. Soil Dynamics and Earthquake Engineering, 80, pp. 65-68.
  • Cui L, Bhattacharya S. (2015) 'Choice of aggregates for permeable pavements based on laboratory tests and DEM simulations'. International Journal of Pavement Engineering, 18 (2), pp. 162-170.
  • Bhattacharya S, Nikitas G, Yu L, Wang L-Z, Guo Z. (2015) 'Long-term dynamic behavior of monopile supported offshore wind turbines in sand'. Elsevier Theoretical and Applied Mechanics Letters, 5 (2), pp. 80-84.

    Abstract

    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.

  • Bhattacharya S. (2015) 'An analytical model to predict the natural frequency of offshore wind turbines on three-spring flexible foundations using two different beam models'. Soil Dynamics and Earthquake Engineering,
  • Guo Z, Yu L, Wang L, Bhattacharya S, Nikitas G, Xing Y. (2015) 'Model tests on the long-term dynamic performance of offshore wind turbines founded on monopiles in sand'. Journal of Offshore Mechanics and Arctic Engineering, 137 (4)
  • Bhattacharya S. (2014) 'Centrifuge study on the cyclic performance of caissons in sand'. 14 Edition. International Journal of Physical Modelling in Geotechnics, 4 (4), pp. 99-115.
  • Bhattacharya S, Arany L, Macdonald J, Hogan SJ. (2014) 'Simplified critical mudline bending moment spectra of offshore wind turbine support structures'. Wind Energy,
  • Bhattacharya S, Alexander N. (2014) 'Obtaining Spectrum Matching Time Series Using a Reweighted Volterra Series Algorithm (RVSA)'. Bulletin of the Seismological Society of America, 104 (4), pp. 1663-1673.
  • Bhattacharya S, Shadlou S. (2014) 'Dynamic stiffness of pile in a layered elastic continuum'. Geotechnique,
  • Bhattacharya S, Sarkar R, Maheswari BK. (2014) 'Seismic Requalification of Pile Foundations in Liquefiable Soils'. Springer Indian Geotechnical Journal,
  • Bhattacharya S. (2014) 'Experimental and Analytical Study of Seismic Soil-Pile-Structure Interaction in Layered Soil Half-Space'. Journal of Earthquake Engineering,
  • Bhattacharya S. (2014) 'A Critical Review of Retrofitting Methods for Unreinforced Masonry Structures'. International Journal of Disaster Risk Reduction, 7, pp. 51-67.
  • Bhattacharya S, Lombardi D. (2014) 'Modal analysis of pile-supported structures during seismic liquefaction'. Earthquake Engineering and Structural Dynamics, 43 (1), pp. 119-138.
  • Bhattacharya S. (2013) 'Winkler Springs (p-y curves) for pile design from stress-strain of soils: FE assessment of scaling coefficients using the Mobilized Strength Design concept'. Vol. 5, No. 5 (2013) 379-399 Edition. Geomechanics and Engineering, 5 (5), pp. 379-399.
  • Bhattacharya S . (2013) 'Observed Dynamic Soil-Structure Interaction in scale testing of Offshore Wind Turbine Foundations'. Elsevier Soil Dynamics and Earthquake Engineering, 54, pp. pp. 47-60.

    Abstract

    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.

  • Bhattacharya S. (2013) 'Collapse of Showa Bridge revisited'. http://casehistories.geoengineer.org/volume/volume3/issue1/IJGCH_3_1_3.html, 3 (1), pp. 24-35.

    Abstract

    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.

  • Bhattacharya S. (2013) 'Observed Dynamic Soil-Structure Interaction in scale testing of Offshore Wind Turbine Foundations'. Soil Dynamics and Earthquake Engineering, 54, pp. 47-60.
  • Bhattacharya S, Huang Y, Miao Y. (2013) 'Review of Liquefaction induced damage to Soils and Foundation due to the 2011 Tohoku Earthquake'. Chinese Journal of Geotechnical Engineering, 35 (5), pp. 834-840.
  • Bhattacharya S, Cox J, Lombardi D. (2013) 'Dynamics of offshore wind turbines on two types of foundations'. ICE Proceedings of the Institution of Civil Engineers: Geotechnical Engineering,
  • Lombardi D, Bhattacharya S, Wood DM. (2013) 'Dynamic soil–structure interaction of monopile supported wind turbines in cohesive soil'. Soil Dynamics and Earthquake Engineering, 49 Article number 0 , pp. 165 - 180-165 - 180.
  • Bhattacharya S, Goda K. (2013) 'Probabilistic buckling analysis of axially loaded piles in liquefiable soils'. Soil Dynamics and Earthquake Engineering, 45 Article number 0 , pp. 13 - 24-13 - 24.
  • Bhattacharya S, Macabuag J, Gurgain R. (2012) 'Seismic retrofitting of non-engineered masonry in rural Nepal'. 165 Edition. ICE Proceedings of the Institution of Civil Engineers: Structures and Buildings, 6, pp. 273-286.
  • Bhattacharya S, Lombardi D, Dihoru L, Dietz MS, Crewe AJ, Taylor CA. (2012) 'Model Container Design for Soil-Structure Interaction Studies'. Springer Role of Seismic Testing Facilities in Performance-Based Earthquake Engineering, , pp. 135-158.
  • Bhattacharya S. (2012) 'Dynamic analysis of wind turbine towers on flexible foundations'. Shock and Vibration: shock and vibration control - crashworthiness - structural dynamics - impact engineering - sound, 19 (1), pp. 37-56.
  • Bhattacharya S, Krishna AM, Lombardi D, Crewe A, Alexander N. (2012) 'Economic MEMS based 3-axis water proof accelerometer for dynamic geo-engineering applications'. Soil Dynamics and Earthquake Engineering, 36 Article number 0 , pp. 111 - 118-111 - 118.
  • Bhattacharya S, Lombardi D, Wood DM. (2011) 'Similitude relationships for physical modelling of monopile-supported offshore wind turbines'. Ice Virtual Library International Journal of Physical Modelling in Geotechnics, 11 Article number 2 , pp. 58-68.

    Abstract

    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.

  • Bhattacharya S, Adhikari S. (2011) 'Vibrations of wind-turbines considering soil-structure interaction'. Wind and Structures: an international journal, 14 (2)
  • Bhattacharya S, Adhikari S. (2011) 'Experimental validation of soil–structure interaction of offshore wind turbines'. Soil Dynamics and Earthquake Engineering, 31 Article number 5–6 , pp. 805 - 816-805 - 816.
  • Bhattacharya S, Hyodo M, Goda K, Tazoh T, Taylor CA. (2011) 'Liquefaction of soil in the Tokyo Bay area from the 2011 Tohoku (Japan) earthquake'. Soil Dynamics and Earthquake Engineering, 31 Article number 11 , pp. 1618 - 1628-1618 - 1628.
  • Dash SR, Bhattacharya S, Blakeborough A. (2010) 'Bending–buckling interaction as a failure mechanism of piles in liquefiable soils'. Soil Dynamics and Earthquake Engineering, 30 Article number 1–2 , pp. 32 - 39-32 - 39.
  • Govindaraju L, Bhattacharya S. (2010) 'Site-specific earthquake response study for hazard assessment in Kolkata city, India'. Springer Natural Hazards, , pp. 1-23.
  • Bhattacharya S, Adhikari S, Alexander NA. (2009) 'A simplified method for unified buckling and free vibration analysis of pile-supported structures in seismically liquefiable soils'. Soil Dynamics and Earthquake Engineering, 29 Article number 8 , pp. 1220 - 1235-1220 - 1235.
  • Bhattacharya S, Carrington T, Aldridge T. (2009) 'Observed increases in offshore pile driving resistance'. Telford Proceedings of the Institution of Civil Engineers-Geotechnical engineering, 162 Article number 1 , pp. 71-80.
  • Dash SR, Govindaraju L, Bhattacharya S. (2009) 'A case study of damages of the Kandla Port and Customs Office tower supported on a mat–pile foundation in liquefied soils under the 2001 Bhuj earthquake'. Soil Dynamics and Earthquake Engineering, 29 Article number 2 , pp. 333 - 346-333 - 346.
  • Bhattacharya S, Madabhushi G. (2008) 'A critical review of methods for pile design in seismically liquefiable soils'. Bulletin of Earthquake Engineering, , pp. 407-446.
  • Bhattacharya S, Dash SR, Adhikari S. (2008) 'On the mechanics of failure of pile-supported structures in liquefiable deposits during earthquakes'. CURRENT SCIENCE ASSOC/INDIAN ACADEMY OF SCIENCES CURRENT SCIENCE-BANGALORE-, 94 Article number 5 , pp. 605-605.
  • Bhattacharya S, Kerciku A. (2007) 'Discussion on “Buckling behaviour of single pile and pile bent in the Scotch Road Bridge'. Geomechanics and Geo-engineering: An International Journal (Taylor and Francis), 2 (4), pp. 317-318.
  • Bhattacharya S. (2006) 'Safety assessment of existing piled foundations in liquefiable soils against buckling instability'. ISET journal for Earthquake Engineering, 43 (4)
  • Bhattacharya S, Oxford U, Carrington TM, Aldridge TR, others . (2006) 'Design of FPSO piles against storm loading'. Offshore Technology Conference,
  • Bhattacharya S, Bolton M, Madabhushi G. (2005) 'A reconsideration of the safety of piled bridge foundations in liquefiable soils'. Soils and Foundations, 45 (4), pp. 13-26.
  • Bolton MD. (2004) 'An alternative mechanism of pile failure in liquefiable deposits during earthquakes'. ICE Géotechnique, 54, pp. 203-213(10)-203-213(10).

Conference papers

  • Cui L , Bhattacharya S , Nikitas G , Vimalan J. (2017) 'Predicting Long Term Performance of OWT Foundation using Cyclic Simple Shear Apparatus and DEM Simulations'. Society for Underwater Technology Proceedings of OSIG 2017 conference, London, UK: Offshore Site Investigation & Geotechnics Committee 8th International Conference: Smarter Solutions for Future Offshore Developments
    [ Status: Accepted ]

    Abstract

    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.

  • Cui L , Bhattacharya S , Nikitas G . (2017) 'Micromechanics of soil responses in cyclic simple shear tests'. EDP Sciences EPJ Web of Conferences, Montpellier: Powders & Grains 2017 140 Article number 02008

    Abstract

    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.

  • Bhattacharya S . (2012) 'Dynamic Soil-Structure Interaction issues in designing offshore wind turbines'. Hangzhou: Proceedings of International Symposium on Coastal and Offshore Geotechnics

    Abstract

    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).

  • Bhattacharya S. (2006) 'Design of FPSO piles against storm loading'. Houston: Offshore Technology Conference (OTC)

Books

  • Bhattacharya S, Alexander N, Lombardi D, Ghosh S. (2015) Fundamentals of Engineering Mathematics. ICE Publishing

Book chapters

  • Lopez-Querol S, Cui L, Bhattacharya S. (2017) 'Numerical Methods for SSI Analysis of Offshore Wind Turbine Foundations'. in (ed.) Wind Energy Engineering: A Handbook for Onshore and Offshore Wind Turbines 1st Edition. Academic Press Article number 14 , pp. 275-297.
    [ Status: In preparation ]
  • Lombardi D, Bhattacharya S, Nikitas G. (2017) 'Physical Modeling of Offshore Wind Turbine Model for Prediction of Prototype Response'. in (ed.) Wind Energy Engineering: A Handbook for Onshore and Offshore Wind Turbines 1st Edition. Academic Press Article number 17 , pp. 353-374.
    [ Status: In preparation ]
  • Bhattacharya S. (2017) 'Civil Engineering Aspects of a Wind Farm and Wind Turbine Structures'. in (ed.) Wind Energy Engineering: A Handbook for Onshore and Offshore Wind Turbines 1st Edition. Academic Press Article number 12 , pp. 221-242.
    [ Status: In preparation ]
  • Bhattacharya S, Wang L, Liu J, Hong Y. (2017) 'Civil Engineering Challenges Associated With Design of Offshore Wind Turbines With Special Reference to China'. in (ed.) Wind Energy Engineering: A Handbook for Onshore and Offshore Wind Turbines 1st Edition. Academic Press Article number 13 , pp. 243-273.
    [ Status: In preparation ]
  • Jalbi S, Shadlou M, Bhattacharya S. (2017) 'Practical Method to Estimate Foundation Stiffness for Design of Offshore Wind Turbines'. in (ed.) Wind Energy Engineering: A Handbook for Onshore and Offshore Wind Turbines 1st Edition. Academic Press Article number 16 , pp. 329-352.
    [ Status: In preparation ]
  • Bhattacharya S . (2012) 'Model container design for Soil-Structure Interaction Studies'. in (ed.) Role of Seismic Testing Facilities in Performance-Based Earthquake Engineering Springer Netherlands , pp. 135-158.

    Abstract

    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).

Patents

  • Szyniszewski S, Bhattacharya S, Cavarretta I. Apparatus and method for controlling a resonant frequency of a wind turbine generator.

Reports

  • Bhattacharya S. (2011) Pile design in seismic areas. http://www.seced.org.uk/index.php/newsletters/39-seced-newsletter-vol-23-no-2 :

Theses and dissertations

  • Bhattacharya S. (2003) Pile instability during earthquake liquefaction.

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