• KSCE Journal of Civil and Environmental Engineering Research
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• v.14 no.1
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• pp.171-178
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• 1994

The skin friction on single piles was investigated by using an analytical study and a numerical analysis. The emphasis was given to the variation of skin friction on piles based on the load transfer mechanism developed for the consolidation of a surrounding soft clay. Local yield or slip at the pile-soil interface was taken into account by specifying a limiting value of shear stress. The response of a single pile was analyzed and compared to the results of field case study. Based on the results obtained, it is shown that the skin friction on a pile increases as the degree of consolidation increases and the ultimate axial forces result from the long term behavior of clay corresponding to the end of the consolidation. It is also found that the analysis using one-dimensional consolidation theory as well as two or three-dimensional non-linear analysis gives relatively reasonable results.

• Journal of the Korean GEO-environmental Society
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• v.18 no.7
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• pp.37-47
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• 2017

In this study, a series of full-scale field tests on prebored and precast steel pipe piles and the corresponding numerical analysis have been conducted in order to study the characteristics of pile load-settlement relations and shear stress transfer at the pile-soil interface. Dynamic pile load tests (EOID and restrike) have been performed on the piles and the estimated design pile loads from EOID and restrike tests were analysed. Class-A type numerical analyses conducted prior to the pile loading tests were 56~105%, 65~121% and 38~142% respectively of those obtained from static load tests. In addition, design loads estimated from the restrike tests indicate increases of 12~60% compared to those estimated in the EOID tests. The EOID tests show large end bearing capacity while the restrike tests demonstrate increased skin friction. When impact energy is insufficient during the restrike tests, the end bearing capacity may be underestimated. It has been found that total pile capacity would be reasonably estimated if skin friction from the restrike tests and end bearing capacity from the EOID are combined. The load-settlement relation measured from the static pile load tests and estimated from the numerical modelling is in general agreement until yielding occurs, after which results from the numerical analyses substantially deviated away from those obtained from the static load tests. The measured pile behaviour from the static load tests shows somewhat similar behaviour of perfectly-elastic plastic materials after yielding with a small increase in the pile load, while the numerical analyses demonstrates a gradual increase in the pile load associated with strain hardening approaching ultimate pile load. It has been discussed that the load-settlement relation mainly depends upon the stiffness of the ground, whilst the shear transfer mechanism depends on shear strength parameters.

• Earthquakes and Structures
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• v.10 no.5
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• pp.1143-1179
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• 2016

Offshore wind turbines are considered as a fundamental part to develop substantial, alternative energy sources. In this highly flexible structures, monopiles are usually used as support foundations. Since the monopiles are large diameter (3.5 to 7 m) deep foundations, they result in extremely stiff short monopiles where the slenderness (length to diameter) may range between 5 and 10. Consequently, their elastic deformation patterns under lateral loading differ from those of small diameter monopiles usually employed for supporting structures in offshore oil and gas industry. For this reason, design recommendations (API and DNV) are not appropriate for designing foundations for offshore wind turbine structures as they have been established on the basis of full-scale load tests on long, slender and flexible piles. Furthermore, as these facilities are very sensitive to rotations and dynamic changes in the soil-pile system, the accurate prediction of monopile head displacement and rotation constitutes a design criterion of paramount importance. In this paper, the Fourier Series Aided Finite Element Method (FSAFEM) is employed for the determination of static impedance functions of monopiles for OWT subjected to horizontal force and/or to an overturning moment, where a non-homogeneous soil profile has been considered. On the basis of an extensive parametric study, and in order to address the problem of head stiffness of short monopiles, approximate analytical formulae are obtained for lateral stiffness $K_L$, rotational stiffness $K_R$ and cross coupling stiffness $K_{LR}$ for both rough and smooth interfaces. Theses expressions which depend only on the values of the monopile slenderness $L/D_p$ rather than the relative soil/monopile rigidity $E_p/E_s$ usually found in the offshore platforms designing codes (DNV code for example) have been incorporated in the expressions of the OWT natural frequency of four wind farm sites. Excellent agreement has been found between the computed and the measured natural frequencies.

• Journal of the Korean Geotechnical Society
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• v.23 no.4
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• pp.169-183
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• 2007

Numerical analysis was performed to investigate the behavior of rammed aggregate piers (RAP) in soft ground with various interface conditions, area replacement ratio, aspect ratio and surcharge loads of pile and soil. And field modulus load test was carried out to predict the input parameters. Field prototype (unit cell) tests are in progress to compare the result of numerical analysis. Also a modified load transfer equation of RAP on soft foundation was proposed. According to the results, the behavior of RAP depended on such as interface conditions, settlement characteristics (free strain) and stress concentration ratio. On the other hand, maximun stress concentration ratio increased as area replacement ratio and aspect ratio increased, and it was remarkably affected by surcharge loads.