Experimental Investigation of the Behavior of Monopile under Asymmetric Two-Way Cyclic Lateral Loads
Publication: International Journal of Geomechanics
Volume 21, Issue 3
Abstract
The monopile is the most common form of foundation employed in offshore wind turbines. These foundations are subjected to millions of repeated load cycles, owing to wind and wave action. In this study, a series of six cyclic lateral load tests and two monotonic tests were performed on an aluminum model pipe pile with an outer diameter of 63.5 mm and a wall thickness of 2.5 mm. A model monopile was embedded in medium dense sand (Dr = 55%) and subjected to asymmetric two-way cyclic loading. From the experimental investigations, the effects of embedded length and the asymmetric two-way cyclic loads on the lateral pile head displacements and the cyclic secant stiffness of the soil–pile system were studied. Linear regression analysis was also performed to fit the conventional degradation parameters using the minimum number of critical constraints, which included the loading conditions and the flexibility parameters of the soil–pile system. From the test results, it was observed that asymmetric two-way loading causes a reversal of accumulated displacement for a pile embedded at a greater depth (L > 1.91T) under relatively lower amplitudes (ζb < 0.37). The cyclic secant stiffness was observed to increase at a relatively constant rate (Aκ) with the logarithm of the number of cycles. The study also revealed that the magnitude of initial cyclic secant stiffness, in comparison with the monotonic stiffness, exhibited a critical drop near the specific load character, ζc = −0.38.
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Acknowledgments
The authors express their gratitude toward Science and Engineering Research Board SERB (Sanction Order No. SERB/F/9086/2016-17), DST, Government of India, for providing financial assistance.
Notation
The following symbols are used in this paper:
- Aκ
- coefficient of stiffness rate;
- B
- diameter of pile;
- Dr
- relative density;
- D50
- effective grain diameter;
- e/B
- relative loading eccentricity;
- EpIp
- bending stiffness of pile;
- Es
- soil modulus;
- Hus
- ultimate static capacity of pile defined at a prescribed displacement of 0.1B;
- L
- embedded length of pile;
- L/B
- slenderness ratio of pile;
- L/T
- nondimensional embedded length;
- pmax,n
- maximum lateral load in the nth cycle;
- pmin,n
- minimum lateral load in the nth cycle;
- T
- relative stiffness of soil–pile system;
- Xb, Xc
- nondimensional functions relating to maximum accumulated displacement;
- ymax,n
- lateral pile head displacement corresponding to maximum load in the nth cycle;
- ymin,n
- lateral pile head displacement corresponding to minimum load in the nth cycle;
- yn
- maximum lateral pile displacement in the nth cycle;
- α
- degradation parameter;
- ζb
- cyclic load amplitude normalized with the static capacity;
- ζc
- load character of the asymmetric two-way cyclic load;
- κc
- nondimensional function of load character related to cyclic secant stiffness;
- κn
- cyclic secant stiffness in the nth cycle; and
- κs
- monotonic stiffness.
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Information & Authors
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Published In
International Journal of Geomechanics
Volume 21 • Issue 3 • March 2021
Copyright
© 2021 American Society of Civil Engineers.
History
Received: Oct 1, 2019
Accepted: Sep 15, 2020
Published online: Jan 6, 2021
Published in print: Mar 1, 2021
Discussion open until: Jun 6, 2021
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