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Design of Foundations for Wind Turbines

Svensson, Henrik (2010) VSM820
Structural Mechanics
Civil Engineering (M.Sc.Eng.)
Abstract
The Swedish government has specified a goal for the Swedish wind power that in 2020 it will generate 30 TWh of energy per year. This should be compared with the present energy produced from wind power of 2.5 TWh / year. To meet these goals, several thousand new wind turbines have to be built.

Today, we build the most land-based wind turbines on strong and stiff soils, but probably in the future wind turbines will have to be built also on soils with less good properties. The ordinary and fairly simple foundation method with a concrete slab with large area, may be abandoned since it can give too large differential settlement.

This thesis is examining the foundations for onshore wind turbines where both the more convential method... (More)
The Swedish government has specified a goal for the Swedish wind power that in 2020 it will generate 30 TWh of energy per year. This should be compared with the present energy produced from wind power of 2.5 TWh / year. To meet these goals, several thousand new wind turbines have to be built.

Today, we build the most land-based wind turbines on strong and stiff soils, but probably in the future wind turbines will have to be built also on soils with less good properties. The ordinary and fairly simple foundation method with a concrete slab with large area, may be abandoned since it can give too large differential settlement.

This thesis is examining the foundations for onshore wind turbines where both the more convential method with a large concrete slab are investigated, but also alternative foundation methods are studied, mainly piled foundations.

Different types of foundations is presented and discussed in which the design procedure consists of both manual calculations and numerical analyses. A case study of an 80 meter high wind turbine with realistic loads is presented. The study includes geotechnical and structural design for three different soil profiles, in which three different foundation methods are used.
The three cases are:

1. Strong and stiff moraine soil in which the most common foundation method with a spread foundation is used.

2. A 20 m thick layer of clay that overlay the strong bedrock in which toe-bearing precast concrete piles are used. In this case only the piles are assumed to bear the load.

3. Clay soil with the bedrock at considerable depth in which precast concrete piles are used as cohesion piles. Both piles and the concrete slab are assumed to bear load in a so-called piled-raft foundation.

For the three cases above, the same foundation slab is used, but for case 2 and 3 the slab is cast on piles.

The results of this study show that all three above-mentioned foundation methods are feasible, but for the third case the differential settlements are significantly big resulting in a horizontal displacement of the tower's top of 155 mm. The first case is the cheapest and easiest to perform, and is preferred if the geotechnical conditions permit that. The second case results in a relative small total pile length of 680 m, while the third case results in 3720 m in total pile length.

The big pile length that the third case results in is an expensive and laborious foundation to construct and such should not be constructed. The design of a foundation of this type has many difficulties. In this thesis the geotechnical design was performed using a two-dimensional model in
a finite element program for geotechnical applications. Modeling of piles in two dimensions is difficult to do in a realistic way and a three-dimensional model is preferred. This, together with the difficulty of finding the right stiffness ratio between the piles and the plate can be two sources of possible error in the extremely large pile length found for case 3. (Less)
Please use this url to cite or link to this publication:
author
Svensson, Henrik
supervisor
organization
course
VSM820
year
type
H3 - Professional qualifications (4 Years - )
subject
report number
TVSM-5173
ISSN
0281-6679
language
English
id
3566985
date added to LUP
2013-08-02 10:39:08
date last changed
2013-09-06 12:55:34
@misc{3566985,
  abstract     = {The Swedish government has specified a goal for the Swedish wind power that in 2020 it will generate 30 TWh of energy per year. This should be compared with the present energy produced from wind power of 2.5 TWh / year. To meet these goals, several thousand new wind turbines have to be built. 
 
Today, we build the most land-based wind turbines on strong and stiff soils, but probably in the future wind turbines will have to be built also on soils with less good properties. The ordinary and fairly simple foundation method with a concrete slab with large area, may be abandoned since it can give too large differential settlement. 
 
This thesis is examining the foundations for onshore wind turbines where both the more convential method with a large concrete slab are investigated, but also alternative foundation methods are studied, mainly piled foundations. 
 
Different types of foundations is presented and discussed in which the design procedure consists of both manual calculations and numerical analyses. A case study of an 80 meter high wind turbine with realistic loads is presented. The study includes geotechnical and structural design for three different soil profiles, in which three different foundation methods are used.
The three cases are:

1. Strong and stiff moraine soil in which the most common foundation method with a spread foundation is used.

2. A 20 m thick layer of clay that overlay the strong bedrock in which toe-bearing precast concrete piles are used. In this case only the piles are assumed to bear the load.

3. Clay soil with the bedrock at considerable depth in which precast concrete piles are used as cohesion piles. Both piles and the concrete slab are assumed to bear load in a so-called piled-raft foundation.

For the three cases above, the same foundation slab is used, but for case 2 and 3 the slab is cast on piles.

The results of this study show that all three above-mentioned foundation methods are feasible, but for the third case the differential settlements are significantly big resulting in a horizontal displacement of the tower's top of 155 mm. The first case is the cheapest and easiest to perform, and is preferred if the geotechnical conditions permit that. The second case results in a relative small total pile length of 680 m, while the third case results in 3720 m in total pile length.

The big pile length that the third case results in is an expensive and laborious foundation to construct and such should not be constructed. The design of a foundation of this type has many difficulties. In this thesis the geotechnical design was performed using a two-dimensional model in
a finite element program for geotechnical applications. Modeling of piles in two dimensions is difficult to do in a realistic way and a three-dimensional model is preferred. This, together with the difficulty of finding the right stiffness ratio between the piles and the plate can be two sources of possible error in the extremely large pile length found for case 3.},
  author       = {Svensson, Henrik},
  issn         = {0281-6679},
  language     = {eng},
  note         = {Student Paper},
  title        = {Design of Foundations for Wind Turbines},
  year         = {2010},
}