Volume 9, Issue 2

June 2024

Directors’ research and development activities

Performance of winged monopile in calcareous sand ground
Vu Anh Tuan
Associate Professor, Civil Engineering Department
Le Quy Don Technical University, Hanoi, Vietnam

Abstract
This study aims to investigate the behaviors of winged monopiles subjected to cyclic loading in a calcareous sand throughnumerical analysis. Axial load tests (push-in or pull-out) of monopiles with and without wings in a calcareous ground are
simulated using a three-dimensional finite element software, PLAXIS 3D. Prior to these simulations, triaxial tests on acalcareous sand sampled from Vietnam were carried out to investigate the mechanism behaviors of the calcareous sand,aiding in the selection of an appropriate soil model and the evaluation of the corresponding soil parameters. The
hypoplastic model was adopted to simulate the sand. Interface elements were employed to simulate the slippage
between the pile shaft and the soil. The analyzed results show that the resistance of the winged pile is considerably higher
than that of the pile without wings.
1. Introduction
In the sea areas of Southeast Asia, especially around Truong Sa Island in Vietnam, calcareous sand is quite commonlydistributed. A problematic feature of the calcareous sand is high crushability of soil particles when subjected to highconfining pressure, shear stress, and cyclic loading. A monopile foundation is usually used for supporting offshore
structures subjected to cyclic loading caused by sea waves and/or winds as well as to vertical loading. To improve theshaft resistance, the use of a winged monopile, where reinforcement wings are implemented along the outer surface ofthe pile, is a feasible solution.
The physical and mechanical characteristics of calcareous sands have been summarized in [1, 2]. In a natural state,calcareous sediments behave differently from terrestrial silica sands, the most significant distinguishing feature being
their tendency to exhibit volume reduction upon shearing, even at relatively low normal stresses. It was pointed out in[2] that the tendency for volume reduction due to shearing plays a dominant role in the foundations of calcareous
sediments. A numerical study was conducted on the behavior of a single pile subjected to cyclic loading in the coral sand[3]. Al-Douri and Poulos [4] compared the predicted and observed performance of cyclic load tests on small-scale single
piles jacked into dry calcareous sand to study the accumulation of permanent displacement of the piles.
This study investigates the behavior of winged monopile subjected to cyclic loading in calcareous ground throughnumerical analysis. In the analyses, axial load tests (push-in or pull-out) of monopiles with and without wings in a
calcareous ground are simulated using a three-dimensional finite element program, PLAXIS 3D [5]. Triaxial tests on acalcareous ground sampled in Vietnam are carried out prior to the analyses of the load tests to investigate the mechanicalbehavior of the calcareous ground and to select an appropriate soil model as well as to evaluate the corresponding soil
parameters. In this study, the hypoplastic model [6] is used to model the ground. Interface elements are employed tosimulate the slippage between the pile shaft and the soil. The analyzed results show that the resistance of the winged
pile is considerably higher than that of the pile without wings.
2. Triaxial Tests of The Calcareous Sand and FEM Simulation
The calcareous sand collected from Truong Sa Island in Vietnam was used in this study. Only soil particles having a particle
diameter, d, smaller than 4.75 mm were used in the experiments. Triaxial monotonic and cyclic CD compression tests on
calcareous sand with a relative density, Dr, of 70% were conducted at a confining pressure, p0, of 150 kPa. Cylindrical
specimens with a height of 100 mm and a diameter of 50 mm were used in the tests.
Fig. 1 and 2 show the experimental and simulation results of the triaxial tests for monotonic and cyclic loading,respectively. The experimental results indicate that post-peak softening behavior is observed, and dilatancy decreases
with increasing axial strain. Specifically, in the case of cyclic loading (Fig. 2a), the accumulative strain increases with thenumber of loading cycles.
Simulations of the triaxial tests were carried out to select an appropriate soil model and to evaluate the soil parameters.
In this study, soil models such as Mohr-Coulomb (MC), Hardening Soil (HS) [7] and Hypoplastic (HP) were used for the10Volume 9, Issue 2

June 2024

simulations. The FEM simulations results of the triaxial tests are also shown in Fig. 1 and 2. It is evident that MC and HSmodels cannot simulate the post-peak softening behavior (Figs. 1a and 2a) and the reduction of dilatancy (Figs. 1b and2b) with increasing axial strain, εa. In particular, the MC and HS models are unable to simulate the accumulative strain
due to cyclic loading (see Fig. 2b). Meanwhile, the HP model [6] generally simulated the measured q vs. εa of thecalcareous sand well, both qualitatively and quantitatively, although there are differences in dilatancy curves between
the experiments and the simulations. The hypoplastic model will be employed for model ground in numerical analyses ofmonopiles, which are presented in Section 3.

Volumetric strain, εvol (%)

Deviatoric stress, q (kPa)

1200
1000
800
600
400
20000

mono load
p0= 150 kPa

2468Experiment
FEM_HP
FEM_HS
FEM_MC10 12 14 16 18 20

Axial strain, εa (%)-7-6-5-4-3-2-10
1230Experiment
FEM_HP
FEM_HS
FEM_MC

246mono load
p0= 150 kPa

810 12 14 16 18 20

Axial strain, εa (%)(1b) Volumetric strain εvol versus axial strain εa(1a) Deviatoric stress q versus axial strain εa

1200

Volumetric strain, εvol (%)

Deviatoric stress, q (kPa)

Fig. 1. The experimental and simulation results of the triaxial tests for monotonic loading.

1000
800
600
400
20000cyclic load
p0=150 kPa

2468Experiment
FEM_HP
FEM_HS
FEM_MC10 12 14 16 18 20

Axial strain, εa (%)(1a) Deviatoric stress q versus axial strain εa-7-6-5-4-3-2-101230

Experiment cyclic load
p0=150 kPa
FEM_HP
FEM_HS
FEM_MC

246810 12 14 16 18 20Axial strain, εa (%)(1b) Volumetric strain εvol versus axial strain εaFig. 2. The experimental and simulation results of the triaxial tests for cyclic loading.
3. Numerical Analysis of Monopiles in Calcareous Sand
3.1. Description of Numerical Model
Case studies considered in this paper are close-ended monopiles with and without wings, subjected to vertical loading(push-in or pull-out loading) in calcareous sand ground. The monopiles made of steel have a total length of 25 m, with 24m embedded and 1 m free-standing. They have an outer diameter of 1 m and a pile wall thickness of 0.015 m. The

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Volume 9, Issue 2

June 2024

reinforcement wings of the winged pile have a width of 0.5 m (0.5D), with an opening angle of 120 degrees and a thicknessof 0.01 m. The wings are spaced at 1 m interval, as shown in Fig. 3.
Model ground in this numerical study consists of calcareous sand, with properties presented in the Section 2.
Groundwater influences the initial effective stress conditions, although for simplicity, a fully-drained condition wasassumed during loading, resulting in uncoupled analyses. Fig. 4 shows the dimensions of the model ground. The sideboundary extends laterally to 6 m (6D, D is the pile diameter) from the pile axis with restrained (fixed) horizontal
displacements applied. The base boundary, located 10 m (10D) from the pile tip, has restrained vertical and horizontal
displacements applied.
To model the pile, a hybrid model was used, consisting of a beam element surrounded by solid elements, according toKimura and Zhang [8]. In the hybrid model of this paper, beam element carried a large proportion of the axial stiffness,
EA, and bending stiffness, EI of the pile.
Interface elements were assigned along the pile shaft to simulate the slippage between the pile and the ground. An
innterface cohesion of 1 kN/m2 and an interface angle of 32 degrees were assumed, based on the triaxial test results andthe strength interface reduction factor of 0.67.

Fig 4. Dimensions of the model ground.

Fig 3. Dimensions of reinforcement wings.
3.2. Numerical Results and Discussions

Fig. 5 shows the analyzed results for the monopiles (with and without wings) subjected to push-in loading including
monotonic and cyclic load. Clearly, the winged-pile shows much greater resistance than the pile without wings underboth monotonic and cyclic loading. Firstly, consider the load-settlement curves of the pile without wings. It is interesting
to notice the difference in the pile resistance, which is influenced by loading types. The resistance of the pile after thecyclic loading decreases compared with that of the monotonic loading. Meanwhile, in the case of winged-pile, the pile
resistance is almost the same between the monotonic and cyclic loading cases.
Fig. 6 shows the load-displacement curves of the piles in the case pull-out loading, in which the pile resistance iscontributed by only the pile shaft resistance. Clearly, the resistance of the wing-pile is much greater than that of the pile
without wings. As for the single pile without wings, the shaft resistance attained to the ultimate value of around 2300 kNafter the pile head displacement reached about 0.01 m (w/D = 0.01). Meanwhile, the resistance of the winged pileincreased with the increase of the pile head displacement and attained a stable behavior even until the displacement
reached 0.1 m (w/D= 0.1). The resistance of the winged pile at w/D = 0.1 is 17000 kN which is over 7 times larger thanthat of the pile without wings, indicating a significant effect of the reinforcement wings on the pile shaft resistance. Note12

Volume 9, Issue 2June 2024

0.00

0Pile head push-in load, Phead (kN)

0.02
0.04

5000 10000 15000 20000 25000 30000
Push-in test
Mono_No wings
Mono_wings
Cyclic_No wings
Cyclic_wings

0.06
0.08
0.10

18000

Pile head pull-out load, Phead (kN)

Pile head downward disp., whead (m)here that the resistance of a pile at w/D = 0.1 is considered as the pile capacity in design specification of many countries.
The differences in pile resistance between monotonic and cyclic loading are insignificant.

16000
14000
12000

Pull-out test
Mono_No wings
Mono_Wings
Cyclic_No wings
Cyclic_Wings

10000Fig 5. Load-displacement curves in push-in test.

8000
6000
4000
2000
00.00

0.02

0.04

0.06

0.08

Pile head upward disp., whead (m)0.10

Fig 6. Load-displacement curves in pull-out test.

4. Conclusion
In this study, triaxial tests of a calcareous sand sampled from Truong Sa Island of Vietnam were conducted to investigate
the mechanism behaviors under monotonic and cyclic loading. After that, numerical simulations of the triaxial tests werecarried out to select an appropriate soil model and evaluate the corresponding parameters for numerical analyses on
behaviors of monopiles in the calcareous sand ground.
The results from both experimental and simulation of triaxial tests indicate that the Hypoplastic soil model is moresuitable than the Mohr-Coulomb or Hardening soil models for simulating calcareous sand, particularly under cyclicloading.
The numerical results show that the winged pile has considerably greater resistance than the normal pile without wings,
both in pull-out and push-in loading tests, indicating its high performance efficiency of the winged pile. However, in thisnumerical study, the pile installation effect was not considered. Consequently, conducting experiments through smallscale models and/or full-scale models, wherein the pile installation effect is considered, are necessary to evaluate theperformance efficiency of the winged pile in practical applications.
References
1. Murff, J.D. Pile capacity in calcareous sands; State of the art. Journal of Geotechnical Engineering, 113(5), 490-570 (1987).
2. Poulos, H.G. The mechanics of calcareous sediments. Australian Geomechanics, 8-41 (1988).
3. Kusumaningsih, D.A., Vu, A.T. and Matsumoto, T. Numerical study on behaviour of single piles in a coral sand. Proc. SoftSoils 2016 Conf., Bandung, pp. D1-1 to D1-6 (2016).
4. Al-Douri, H.A. and Poulos, H.G. Predicted and observed cyclic performance of piles in calcareous sand. Journal ofGeotechnical Engineering, 121(1), 1-16 (1995).
5. Vu, AT., Matsumoto, T. Numerical Analysis on Behaviors of Winged Monopile Subjected to Cyclic Loading in a CalcareousGround. Lecture Notes in Civil Engineering, 18, 217-223 (2019).
6. Wolffersdorff, P.A. A hypoplastic relation for granular materials with a predefined limit state surface. Mechanics ofcohesive-frictional materials 1, 251-271 (1996).
7. Schanz, T., Vermeer P.A. and Bonnier, P.G. The hardening soil model: formulation and verification. In: Brinkgreve RBJ,editor. Beyond 2000 in computational geotechnics. Rotterdam: Balkema, 281-290 (1999).
8. Kimura, M. and Zhang F. Seismic evaluations of pile foundations with three different methods based on threedimensional elasto-plastic finite element analysis. Soils and Foundations. 40(5), 113-132 (2000).

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