Laterales Tragverhalten von in Sand einvibrierten Monopiles

Motivation

The lateral load bearing behaviour of open steel pipe piles depends, among others, on the pile diameter, the embedment length of the pile, the interface friction angle between the pile and the soil, the relative density of the soil and the installation method. During the vibratory-driven installation the pile is vertically excited by a dynamical force acting down- and upwards. This vertical excitation of the pile leads to an excitation of the soil grains which are moved not only vertically, but also horizontally (Hartung 1994). In accordance with findings by Hartung (1994) for piles vibratory driven with a frequency of 40 - 50 Hz, the soil around the pile shaft during installation can be divided into two different zones: a liquefaction zone close to the pile shaft and a compaction zone. Due to the dynamical excitation the soil grains in the liquefaction zone are moving with high velocities leading to a higher volume of the soil in this area.

Current research by e.g. Kallehave et al. (2012) or Klinkvort et al. (2013) on the lateral load bearing behaviour of monopiles focuses on the modification of current approaches, e.g. the p-y approach (DNVGL-ST-0126). In this research pile installation effects are neglected, while the influence of different lateral load situations is investigated. In literature only one large scale field test is documented investigating the lateral load bearing behaviour of vibratory-driven monopiles (cf. Moormann et al. 2016). This lack of research was motivation to conduct own experimental tests with the focus on the investigation of the influence of installation effects due to vibratory pile driving on the lateral load bearing behaviour.

Experimental investigations

The main target of the scaled model tests was to investigate the influence of different installation parameters on the lateral load bearing behaviour of vibratory-driven monopiles. Installation parameters are the frequency f and the static moment Mstat of the vibrators. Besides the installation parameters, also the density of the sand was varied. The scaled model tests were carried out under 1g conditions.

Figure 1: Set-up of the scaled model tests.

The set-up of the 1-g model scaled tests is illustrated in Fig. 1. The scaled model tests were conducted in a cylinder shaped concrete container with a diameter of 2.0 m and a height of 2.5 m. The tests have been conducted with saturated Berlin Sand. In the scaled model tests, a glass-fibre reinforced pile has been used. A picture of the pile is shown in Fig. 2. The dimensions of the pile lead to an L/D ratio of 4.2, which is typical for monopiles.

The scaled model tests consist of different steps. In the first step, the sand was filled homogeneously in the model container. In the second step, the pile was vibratory-driven to the desired embedment depth. In the third step, a lateral pile load test was carried out to investigate the lateral load bearing behaviour.

Figure 2: Photo of the model pile.

Numerical simulations

Investigating the bearing behaviour of monopiles requires knowledge about the development of stress and strains around the pile during and after installation. To gain a better understanding of the processes the soil is undergoing during installation, the complete installation of the pile is simulated numerically. These numerical simulations involve advanced modelling techniques to overcome numerical problems with the calculation of large deformations, such as the Coupled Eulerian-Lagrangian (CEL) method and the Arbitrary Lagrangian-Eulerian (ALE) method. The soil is described by a hypoplastic constitutive model (cf. von Wolffersdorff 1996) with the small strain extension by Niemunis & Herle (1997).

In Fig. 3 results of a numerical back-analysis of one scaled model test are presented. In the figure the distribution of void ratio during one vibration cycle is shown. The simulation was conducted using the CEL method together with a hypoplastic constitutive law.

Figure 3: Distribution of void ratio e.

Acknowledgements

The investigations have been partially conducted in the frame of the VibroPile II project. The IGS would like to acknowledge the founding by innogy SE.

References

DNVGL-ST-0126, April 2016: Support structures for wind turbines. DNV GL Standard.

Hartung, M. (1994). Einflüsse der Herstellung auf die Pfahltragfähigkeit in Sand. Dissertation, Heft 45, Institut für Grundbau und Bodenmechanik, Technische Universität Braunschweig.

Kallehave, D., LeBlanc, C., and Liingaard, M.A. (2012). Modification of the API p-y formulation of initial stiffness of sand. Offshore Site Investigation and Geotechnics: Integrated Technologies - Present and Future, 12-14 September. Society of Underwater Technology, London. 465–472.

Klinkvort, R.T., Hededal, O., and Springman, S. (2013). Centrifuge modelling of drained lateral pile - soil response: Application for offshore wind turbine support structures. Dissertation, Technical University of Denmark.

Moormann, C., Kirsch, F., and Herwig, V. (2016). Vergleich des axialen und lateralen Tragverhaltens von vibrierten und gerammten Stahlrohrpfählen. Proceedings of the 34. Baugrundtagung, DGGT, Bielefeld. 73–81.

Niemunis, A., and Herle, I. (1997). Hypoplastic model for cohesionless soils with elastic strain range. Mechanics of Cohesive-Frictional Materials, 2, 279–299.

von Wolffersdorff, P. A. (1996). A hypoplastic relation for granular materials with a predefined limit state surface. Mechanics of Cohesive-Frictional Materials, 1, 251–271.

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