Evaluation of horizontal and moment bearing capacities of tripod bucket foundations in sand

doi:10.1016/j.oceaneng.2017.05.012

Ocean Engineering

Volume 140, 1 August 2017, Pages 209-221

https://doi.org/10.1016/j.oceaneng.2017.05.012 Get rights and content

Highlights

•
FEM analyses of bearing capacities (Ho & Mo) of tripod bucket foundations in sand.
•
N_cH(T) is proportional to S/D and reached a constant value at S/D ≥ (S/D)_critical.
•
N_cM(T) is almost linearly proportional to S/D due to the increase in moment arm.
•
Design equations were developed to evaluate horizontal and moment capacities.

Abstract

Bearing capacities of tripod bucket foundations differ from those of single bucket foundations because of the interaction among individual buckets of the tripod. This study analyzed the bearing capacities of tripod bucket foundations in medium and dense sands by performing a series of 3D finite element analyses. The sandy soil was modeled by using an elasto-plastic model following the Mohr-Coulomb failure criterion. The Young's modulus of the sands increased with depth because of the stress-dependency of the modulus. Parametric studies were performed, in which the spacing between each bucket, the embedded depth of the bucket, the bucket diameter, and the magnitude of vertical loading were varied. Results showed that the horizontal bearing capacities of tripod bucket foundations reached the maximum values at a spacing ratio of S/D=1.5 to 3.5 for the foundation with an embedment ratio of L/D=0.5 to 1 (where S is the bucket spacing, D is the bucket diameter, and L is the skirt length of the tripod bucket). However, the moment bearing capacities increased linearly as the S/D ratios increased. Finally, bearing capacity equations were proposed in consideration of bucket spacing, embedment depth, effect of foundation diameter and vertical load, and soil density.

Introduction

Offshore wind turbines are being constructed and used in Korea, and the construction of several offshore wind farms are being planned at the west sea of the country. Studies on the foundations for offshore wind turbines have been conducted extensively in European countries (e.g., UK, Germany, and Denmark). In recent years, the majority of foundations supporting offshore wind turbines have been monopiles, tripod piles, and jacket structures. The cost of the foundation for an offshore wind turbine accounts for approximately 35% of the total construction cost (Byrne and Houlsby, 2003). Therefore, reducing the cost of the foundation greatly contributes to the cost-effectiveness of constructing an offshore wind turbine.

A potential alternative foundation is the bucket foundation, which can be classified into two types: single (monopod) bucket foundation and tripod bucket foundation. A tripod bucket foundation consists of three single bucket foundations in a triangular shape. Single bucket foundations have been extensively investigated in both sand and clay soils (e.g., Hung and Kim, 2012; Hung and Kim, 2014a; Achmus et al., 2013), whereas only a few studies have examined tripod bucket foundations in clay (e.g. Hung and Kim, 2014b; Kim et al., 2014a) or the group effect on undrained bearing capacities of rigidly connected multi-footings (e.g., Martin and Hazell, 2005; Gourvenec and Steinepreis, 2007; Gourvenec and Jensen, 2009).

Several experimental studies on bearing capacities of single bucket foundations in dry dense sands were carried out by varying embedment depth and vertical load (Byrne and Houlsby, 1999, Byrne, 2000, Byrne et al., 2003, Villalobos, 2006, Villalobos et al., 2009). These works successfully investigated the failure of the bucket foundation under general loading conditions. However, investigation in saturated sand might be more practical than investigation dry sand. Therefore, several previous works investigated the failure of the bucket foundation in saturated sand, i.e., Villalobos (2006), Larsen et al. (2013), Ibsen et al. (2014), and Ibsen et al. (2015). They found that the failure surface of the bucket foundation under general loading largely depends on the embedment depth and the applied vertical load. The failure surface size increased with the increase of the embedment depth and applied vertical load.

Meanwhile, Achmus et al. (2013) conducted a parametric study on the single bucket foundation under combined loading by using numerical analysis. A significant finding of the relationship between moment and horizontal loads was presented in consideration of the foundation geometry and load eccentricity.

Tripod bucket foundations show varying bearing behaviors depending on geometric configuration. Gourvenec and Jensen (2009) stated that the horizontal bearing capacity of the skirted foundation in uniform clay would reach a constant value at a certain spacing between each individual of conjoined skirted foundation systems. Meanwhile, the moment bearing capacity was proportional to the magnitude of the spacing. The bearing capacities of tripod bucket foundations were systematically investigated for clayey soil (e.g., Hung and Kim (2014b) and Kim et al. (2014a)). They reaffirmed the foregoing findings on the bearing behaviors of tripod bucket foundations under horizontal loads and moments.

From the above discussions, the bearing behavior of single bucket foundations has been extensively investigated in both sand and clay grounds. However, no study has been investigated systematically bearing capacities of the tripod bucket foundations in sand. Offshore wind turbines with large power rates must be installed in deep waters of more than 20 m and are subjected to strong horizontal and moment loadings exerted by wind, waves, and currents (Achmus et al., 2009). The tripod bucket foundation is well-suited to support large horizontal and moment loadings of offshore wind turbines in deep waters. Therefore, this study investigates the horizontal (Ho) and moment (Mo) bearing capacities of tripod bucket foundations in medium and dense sands.

Three-dimensional (3D) finite element (FE) analysis was adopted to model the 3D geometry of the tripod bucket foundations and the appropriate soil–foundation interaction. The accuracy of the FE modeling was validated by comparing its results with published experimental ones. A parametric study was performed in which the embedment depth of the bucket and the bucket spacing were varied. The model grounds were medium and dense sands. In addition, the effects of the bucket diameter and magnitude of the vertical load on the bearing capacities were evaluated.

Section snippets

Finite element modeling and material properties

Fig. 1 shows the geometry of the tripod bucket foundation and the sign conventions adopted in this study. In the parametric study, the embedment ratio L/D (where L is the skirt length and D is the foundation diameter of a single bucket foundation) was varied as 0.5, 0.75, and 1, and the spacing ratio S/D was varied as 1, 1.5, 2, 2.5, 3, 3.5, and 4 (where S is the bucket spacing between the center of the individual buckets of the tripod bucket foundation and the center of the tower structure).

Evaluation of bearing capacities of tripod bucket foundations

To evaluate the bearing capacities of the tripod bucket foundation, the group effect of the tripod bucket was analyzed by comparing the bearing capacities of single and tripod buckets. Case 1 in Table 4 was selected to analyze the effect of L/D and S/D ratios on the bearing capacities. The diameter of the single bucket and the individual buckets of the tripod bucket foundation was fixed at D=6 m.

Development of bearing capacity equations

On the basis of the results obtained from the FE analysis, new equations were proposed to evaluate the horizontal and moment bearing capacities of tripod bucket foundations in medium and dense sands. All of the equations were developed by performing least square analysis as described in 4.1 Horizontal bearing capacity equations, 4.2 Moment bearing capacity equations.

Conclusions

Three-dimensional finite element analyses were performed to investigate the horizontal and moment bearing capacities of the tripod bucket foundations in medium and dense sands. The following conclusions were drawn:

(1)
The horizontal bearing capacity factors N_cH(T) of the tripod bucket foundations increased with an increase in S/D ratios for all foundations in both medium and dense sands, and these factors reached constant values at S/D≥(S/D)_critical. The factors in dense sand were approximately 1.1

Acknowledgments

This research was supported by the project entitled “Development of performance-based seismic design technologies for advancement in design codes for port structures,” funded by the Ministry of Oceans and Fisheries of Korea and Basic Science Research Program funded by the Ministry of Education, South Korea (NRF-2016R1A6A1A03012812).

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View full text

Evaluation of horizontal and moment bearing capacities of tripod bucket foundations in sand

Highlights

Abstract

Introduction

Section snippets

Finite element modeling and material properties

Evaluation of bearing capacities of tripod bucket foundations

Development of bearing capacity equations

Conclusions

Acknowledgments

Behavior of monopile foundations under cyclic lateral load

Comput. Geotech.

Load-bearing capacity of suction bucket foundation in sand

Appl. Ocean Res.

Standardized sign conventions and notation for generally loaded foundations

Geotechnique

Investigation of Suction Caissons in Dense Sand (Ph.D dissertation)

Foundations for offshore wind turbines

Philos. Trans. R. Soc. Lond. A

Uplift capacity of piles and pile groups in sand

Inst. Electr. Electron. Eng. (IEEE), Oceans’

Recommendations of the Committee for Waterfront Structures Harbors and Waterways

Effect of embedment and spacing of conjoined skirted foundation systems on undrained limit states under general loading

Int. J. Geomech.

Undrained limit states of shallow foundations acting in consort

Int. J. Geomech.

Suction caisson foundations for offshore wind turbines and an emometer masts

J. Wind Eng.

Field trials of suction caissons in sand for offshore wind turbine foundations

Geotechnique

Evaluation of vertical and horizontal bearing capacities of bucket foundation in clay

Ocean Eng.

Evaluation of undrained bearing capacities of bucket foundations under combined loads

Mar. Georesources Geotechnol.

Evaluation of combined horizontal-moment bearing capacities of tripod bucket foundations in undrained clay

Ocean Eng.