Preliminary Analysis and Design of a Deep Foundation Supporting a Wind Turbine

Design of the Monopile

In this open-ended assignment you are asked to perform a preliminary analysis and design of a deep foundation supporting a wind-turbine, applying all the relevant theoretical and analytical tools offered in this module, to model the soil behaviour and the Soil-StructureInteraction effects. The available data are limited to the design loads and some basic information regarding the soil material. You must complete the missing information and select reasonable design parameters, justified through your own research in the literature. Your understanding, analytical capabilities, engineering judgement, as well as your personal reflections, need to be evident in your final report.

A deep foundation system is needed to support the wind turbine, taking into account the properties of the 10m silty sand layer and the soft clay layer underneath. Two foundation options need to be explored: a monopile foundation and a piled raft foundation with reinforced concrete piles. Split your analysis into the following steps:

1) Design of the monopile: your design (i.e. selection of diameter and length) needs to satisfy the ultimate vertical bearing capacity requirements with a factor of safety equal to 2.5. Monopiles are hollow steel sections – refer to the literature for ranges of parameters needed. You will also need to calculate the soil strength parameters, for the sand and the clay material.

2) Then analyse the performance of your monopile under the working loads, i.e. analyse the settlement and the deflection profiles and the corresponding internal forces (axial and shear force and bending moment diagrammes). To do that you have to consider two cases:

1. Apply the relevant elastic solutions, assuming constant (averaged) soil properties – the approximations are left to your discretion – to get a rough theoretical estimation. In order to do that, you have to analyse the stiffness parameters of the soil materials.
2. Solve again, for variable soil properties, this time applying a numerical solution with finite differences. Use Matlab to solve the system of equations (alternatively, you can use Ms Excel, if you have difficulties in Matlab).

3) Model the problem in Plaxis 2D, using the “Embedded Beam Row” element for yourmonopile. Regarding the soil material models, you can use linear Mohr-Coulomb or nonlinear hardening model for the sand and linear Mohr-Coulomb or non-linear soft-soil model for the clay. You are strongly encouraged to do both and compare the differences. Compare also with your elastic analysis in question (2).

4) Now consider an alternative foundation, with a 3-by-3 pile group (9 piles overall) connected to a stiff pile cap. You need to design again the pile group (diameter, length and spacing between the piles). To estimate the efficiency ratio of the pile group, you can apply the elastic interaction factors method.

5) Last step, model your pile group in Plaxis 2D, using “Embedded Beam Row” elements for the piles and a beam element for the pile cap. In this analysis you can use only one of the soil model combinations (in you have created both a linear and a non-linear set). Applythe loads in a sequence: first the vertical force, then the horizontal and the bending moment, and compare the effect on the results. Comment on the pile group interaction effects.

Figure 1. Working loads for the wind turbine and soil stratigraphy

Figure 2. Experimental curve from 1-D consolidation oedometer test