Advanced laboratory testing of offshore wind turbine foundations under lateral loading

Started: October 2014
Supervisor: Jardine, RJ. 
Funding: China Scholarship Council (CSC)-Imperial College London Joint Scholarship

Background

Offshore energy exploitation is crucial to many economies in Europe and Asia and extensive attention is being given to developing offshore geotechnical engineering to meet the technical challenges posed by moving into deeper water and extending the life of existing assets. Pile stiffness, capacity, cyclic response, and long-term displacement behaviour are crucial to the foundations of wind-turbines, oil and gas platforms, and other offshore structures. Piles for offshore wind turbines (Fig. 1) are usually subjected to axial, lateral and moment static and cyclic loading. However, due to the complexity of the key geomechanics issues, the prediction of pile capacity and displacement under monotonic loading remains unreliable, and it is unclear how cyclic or extended loading should be considered in design.

Fig 1
Fig 1. Industrial application: offshore wind turbines

Research aims

This research forms part of the Pile Soil Analysis (PISA), offshore Joint Industry Project (JIP), which is funded by Carbon Trust, DONG Energy and other offshore energy companies. The Academic Work Group (AWG) of the project comprises Oxford University, Imperial College London and University College Dublin. The main objective is to reduce the cost of mono-piles and other piled foundations by finding better design approaches. The strategy is to conduct advanced numerical modelling studies (David Abadías Gómez) that will allow simplified industrially applicable tools to be developed. Central to these activities are checks undertaken through comprehensive loading tests conducted at full scale in a clay till and a dense sand site. Essential to any calibration of numerical models against the field is the accurate geotechnical characterisation of the full-scale test sites. This is to be achieved by applying the advanced laboratory testing techniques and apparatus available at Imperial College London. 

Fig 2
Fig 2: Two advanced laboratory apparatuses employed in the research

Research objective

The main objective for the research is to perform high quality Triaxial Cell, Hollow Cylinder Apparatus (ICRCHCA and Mark II ICHCA) tests (Fig.2) that will both feed into the numerical analysis and provide key insights into several issues related to the full-scale foundations. This part of research is accompanied by the work of Emil Ushev, who focuses mainly on the triaxial shear strength, compressibility and stiffness characterisation of soils.

The PISA project involves two field tests, one in sand, at Dunkerque, N. France and another in glacial till at Cowden, Humberside, N.E. England. High quality samples are obtained for testing by block or rotary sampling techniques. The major objectives include:

  • Laboratory testing programme designed to model in-situ stress conditions and lateral loading piles
  • Investigation of small strain behaviour under static and cyclic loading
  • Study of the effect of rate and ageing on soil behaviour and its influence on pile response
  • Characterization of shear strength and stiffness anisotropy considering the rotation of the direction of the major principal stress axis
References
  • Hoque, E. & Tatsuoka, F. (2004) Effects of stress ratio on small-strain stiffness during triaxial shearing. Geotechnique 54(7):429-439.
  • Jardine, R. J. (2013) Advanced laboratory testing in research and practice. In Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris.
  • Bross, A. M. (2012) Study of the anisotropy of three British mudrocks using a Hollow Cylinder Apparatus. PhD thesis, Imperial College London, London.
  • Sim, W. W., Aghakouchak, A., Jardine, R. J. (2013) Cyclic triaxial tests to aid offshore pile analysis and design. Proceedings of the ICE – Geotechnical Engineering, 166(GE2): 111-121.
  • Nishimura, S. (2005) Laboratory study on anisotropy of natural London clay. PhD thesis, Imperial College London, London.
  • Tsuha, C. H. C., Foray, P. Y., Jardine, R. J., Yang, Z. X., Silva, M. & Rimoy, S. (2012) Behaviour of displacement piles in sand under cyclic axial loading. Soils and Foundations 52(3):393-410.

TINGFA LIU

Tingfa LiuPhD Candidate - Geotechnics 
Department of Civil & Environmental Engineering 
Imperial College London SW7 2AZ
t.liu14@imperial.ac.uk