##### Document Text Contents

Page 1

Master’s Dissertation

Structural

Mechanics

HENRIK SVENSSON

DESIGN OF FOUNDATIONS

FOR WIND TURBINES

Page 80

70

5.5 Case 3 - Piled foundation with cohesion piles

In situation 3, where the bedrock is at great distance, the piles are functioning as cohesion piles.

The shear strength of the soil is then determining the bearing capacity of a pile and the stiffness of

the soil and the piles are governing the settlements. This is a foundation where both the piles and

the plate are bearing load a so called piled-raft foundation, more detailed described in chapter

3.3.2. One big challenge in the piled-raft foundation method is to determine the stiffness

parameters of the piles and the slab. The magnitude of these parameters is of major importance

for the load distribution. This is why this model often is associated with large uncertainty.

5.5.1 Geotechnical design

In this case the complete geotechnical design is done in Plaxis. In a clay soil the settlements tend

to be rather big [25], why the settlement calculation will determine the piling. The model consists

of a plain strain model, and as for case 1 a one meter wide shred of an equivalent quadratic area is

modeled instead of the circular, see figure 5.7 (case 1). The piles are modeled with plate elements,

where the stiffness parameters are defined per meter depth. Stiffness is added to the piles with an

axial stiffness (EA) and a flexural stiffness (EI).

EtEA , mN /

12

3Et

EI , mNm /2

Where E is Young’s modulus for the plate material

t is the thickness of the beam

If the piles are positioned with one meter distance, the stiffness in Plaxis is the actual stiffness of

the pile, that is:

mGNEbEA / 4057.227.01033 292

mMNm

b

EEI / 6146.14

12

27.0

1033

12

2

4

9

4

If the piles are positioned with half the distance between them, the stiffness parameters above are

doubled and so on.

The interaction foundation-soil, and pile-soil are modeled with interface elements with strength

parameter 0.9 for surfaces facing friction soil and 0.8 for surfaces facing the clay. The interface

elements are extended through corners to avoid stress concentrations around corners, see section

4.3.3.2.

A set up with 56 piles with a length of 60 m at the outmost perimeter and 12 vertical inner piles

with a length of 30 m giving reasonable settlements. Figure 5.21 and 5.22 are showing the Plaxis

model and figure 5.23 showing the deformed mesh after the calculation. The piles at the outermost

perimeter is inclined inwards and outwards alternately both with the inclination angle 4.47 degrees.

Inside the rectangle that encloses the foundation (figure 5.21 and 5.23) the mesh is refined

causing a more exact result. The standard fixities described in chapter 4.3.1 are applied to the

model.

Page 81

71

Figure 5.21: The geotechnical model

Figure 5.22: Enlarged picture of the foundation

Clay

Fill

Compacted crushed

material

Concrete

Horizontal fixity

Total fixity

Page 160

LVIII

Page 161

LIX

Master’s Dissertation

Structural

Mechanics

HENRIK SVENSSON

DESIGN OF FOUNDATIONS

FOR WIND TURBINES

Page 80

70

5.5 Case 3 - Piled foundation with cohesion piles

In situation 3, where the bedrock is at great distance, the piles are functioning as cohesion piles.

The shear strength of the soil is then determining the bearing capacity of a pile and the stiffness of

the soil and the piles are governing the settlements. This is a foundation where both the piles and

the plate are bearing load a so called piled-raft foundation, more detailed described in chapter

3.3.2. One big challenge in the piled-raft foundation method is to determine the stiffness

parameters of the piles and the slab. The magnitude of these parameters is of major importance

for the load distribution. This is why this model often is associated with large uncertainty.

5.5.1 Geotechnical design

In this case the complete geotechnical design is done in Plaxis. In a clay soil the settlements tend

to be rather big [25], why the settlement calculation will determine the piling. The model consists

of a plain strain model, and as for case 1 a one meter wide shred of an equivalent quadratic area is

modeled instead of the circular, see figure 5.7 (case 1). The piles are modeled with plate elements,

where the stiffness parameters are defined per meter depth. Stiffness is added to the piles with an

axial stiffness (EA) and a flexural stiffness (EI).

EtEA , mN /

12

3Et

EI , mNm /2

Where E is Young’s modulus for the plate material

t is the thickness of the beam

If the piles are positioned with one meter distance, the stiffness in Plaxis is the actual stiffness of

the pile, that is:

mGNEbEA / 4057.227.01033 292

mMNm

b

EEI / 6146.14

12

27.0

1033

12

2

4

9

4

If the piles are positioned with half the distance between them, the stiffness parameters above are

doubled and so on.

The interaction foundation-soil, and pile-soil are modeled with interface elements with strength

parameter 0.9 for surfaces facing friction soil and 0.8 for surfaces facing the clay. The interface

elements are extended through corners to avoid stress concentrations around corners, see section

4.3.3.2.

A set up with 56 piles with a length of 60 m at the outmost perimeter and 12 vertical inner piles

with a length of 30 m giving reasonable settlements. Figure 5.21 and 5.22 are showing the Plaxis

model and figure 5.23 showing the deformed mesh after the calculation. The piles at the outermost

perimeter is inclined inwards and outwards alternately both with the inclination angle 4.47 degrees.

Inside the rectangle that encloses the foundation (figure 5.21 and 5.23) the mesh is refined

causing a more exact result. The standard fixities described in chapter 4.3.1 are applied to the

model.

Page 81

71

Figure 5.21: The geotechnical model

Figure 5.22: Enlarged picture of the foundation

Clay

Fill

Compacted crushed

material

Concrete

Horizontal fixity

Total fixity

Page 160

LVIII

Page 161

LIX