Because offshore wind turbines are highrise structures, they transfer large horizontal loads and moments to their foundations. One of the keys to designing a foundation is determining the sensitivities and laws affecting its loadbearing capacity. In this study, this procedure was carried out for a new highrise cap pile group foundation adapted to the loading characteristics of offshore wind turbines. The sensitivities of influential factors affecting the bearing properties were determined using an orthogonal test. Through a combination of numerical simulations and model tests, the effects of the inclination angle, length, diameter, and number of side piles on the vertical bearing capacity, horizontal bearing capacity, and bending bearing capacity were determined. The results indicate that an increase in the inclination angle of the side piles will increase the vertical bearing capacity, horizontal bearing capacity, and bending bearing capacity. An increase in the length of the side piles will increase the vertical bearing capacity and bending bearing capacity. When the length of the side piles is close to the central pile, the increase is more apparent. Finally, increasing the number of piles will increase the horizontal bearing capacity; however, the growth rate is small because of the pile group effect.
Offshore wind power, which is stable and sustainable, is a new type of energy resource. This resource can generate large amounts of power and does not require the support of land resources. Furthermore, wind power has been favored by several countries in recent years [
The new highrise cap pile group foundation uses inclined piles, which have been widely used in bridge, wharf, and large transmission line foundations. The foundation’s bearing capacity has become a highly popular research topic. Currently, several domestic and foreign researchers have studied the axial and horizontal operating performance of inclined piles. In a study on axial bearing capacity, Meyerhof et al. [
For the new highrise cap pile group foundation considered in this report, the author studied the effects of different inclination angles of inclined piles on the vertical bearing capacity, horizontal bearing capacity, and flexural loading capacity and developed an optimal design for the new highrise cap pile group foundation. The vertical and horizontal bearing capacities of the new highrise cap pile group foundation were determined through laboratory model tests. The test results were analyzed using a numerical simulation method with the finite element analysis software program ABAQUS. The numerical simulation method provides the basis for the actual design of offshore wind power foundations.
The foundation of a wind turbine, unlike other common structural foundations, must bear large moments. As indicated in Figure
New type of pile group.
It is proposed that the vertical load is primarily supported by the central pile, whereas the horizontal force and bending moment are primarily supported by the side piles through the combination of a reasonable layout of the piles, a change in the inclination angle, and the length of the piles. Furthermore, the combination of the strong vertical bearing capacity of the largediameter piles and the strong horizontal bearing capacity and flexural capacity of the inclined piles allows the foundation to control the horizontal displacement and discrepancy settlement, which are a result of the effectively large horizontal force and bending moment sustained by the foundation.
There are several factors that affect the bearing capacity of the new pile group, such as the soil properties, pile length, distance between piles, number of piles, and shapes and sizes of the pile group.
To improve the bearing capacity of the foundation and determine the characteristic parameters of the bearing capacity effectively, an orthogonal experiment was performed to find the sensitivities of the various design parameters.
An orthogonal experiment using four factors and five levels was designed, which included 25 sets of tests. The four influential factors are the inclination angle of side piles
A numerical simulation method was used to complete the orthogonal experiment. The numerical analysis model and the boundary conditions are provided in Figure
Loads on finite element model used in analyses.
To eliminate the side effect [
Parameters of soils in FEM simulation.
Unit weight 
Poisson’s ratio 
Elastic modulus 
Internal friction angle 
Cohesion 

7.0  0.3  30.0  0  10.0 
The degrees of influence of the various factors on the vertical ultimate bearing capacity
Influence of factors on bearing capacity for different directions.
Bearing capacity  Sensitive degree  

Higher  High  Middle  Low  















As indicated in Table
Five different values of
Loadsettlement curves and ultimate bearing capacity of pile groups with different degrees of inclination.
Vertical loaddisplacement curves
Horizontal loaddisplacement curves
Bending resistance loaddisplacement curves
As indicated in Figure
Six different values of
Loadsettlement curves and ultimate bearing capacity of pile group with different side pile lengths.
Vertical loaddisplacement curves
Bending resistance loaddisplacement curves
As indicated in Figure
Five different values of
Loadsettlement curves of pile group with different numbers of piles.
As indicated in Figure
Thus, with an increase in
The author conducted a laboratory model test using a model of the new highrise pile group foundation. Two types of bearing capacity were studied: vertical bearing capacity and horizontal bearing capacity. Furthermore, a numerical simulation method was used to simulate the laboratory model test, and the applicability of the numerical simulation was verified.
As indicated in Figure
Map of model test apparatus.
The testing sand was sea sand, which was filled in the model tank in a stratified manner. The specific gravity of the soil grain was 2.67, and its density was 1.60 g/cm^{3}. The thickness of the soil was 1.5 m. The foundation model consisted of a seamless steel pipe. A weld was used to connect the piles and cap, as indicated in Figure
Test models.
Two types of experiments were performed: the first experiment analyzed the values of
The deformation of the soil surface observed after loading was complete is shown in Figure
Deformation of the soil surface.
Loadsettlement curve of new pile group.
As indicated in Figure
As indicated in Figure
The deformation of the soil surface after loading is complete and is presented in Figure
Deformation of the soil surface.
Loadsettlement curve of new pile group.
As indicated in Figure
As indicated in Figure
When the vertical load force at the center of the four models with different
Loadsettlement curves of inclined piles with different degrees of inclination.
As indicated in Figure
Based on the actual size of the laboratory model test, the new pile group experiment was simulated. The MohrCoulomb failure criterion was applied to the soil. The unit weight was 16.0 kN/m^{3}, Poisson’s ratio was 0.3, the elasticity modulus was 24 MPa, the internal friction angle was 32°, and the cohesion was 2 kPa. Figure
Loadsettlement curves of new type of pile group.
As indicated in Figure
Figure
Comparison between test deformation and FEM results for vertical loading of pile group.
As indicated in Figure
Figure
Comparison between test deformation and FEM results for lateral loading of pile group.
As indicated in Figure
In conclusion, the numerical simulation method established in this report is applicable to the simulation of new highrise cap pile group foundations.
An offshore wind power farm is proposed to be constructed. According to a geological survey of the seabed, the geotechnical spatial distribution is complex. From top to bottom, the seabed can be divided into 11 layers within the surveying depth. Several of these layers are soft soil or hard soil. The specific soil parameters are provided in Table
Soil parameters.
Stratum  Soil description  Thickness 
Wet weight 
Compression modulus 
Consolidated quickly shear test  

Cohesion 
Internal friction angle 

1  Sludge  8.5  17.0  2.49  11.4  11.3 
2  Clay  2.3  18.5  3.22  25.0  10.4 
3  Silty sand  5.2  20  15  0  33 
4  Silty clay  2.8  19.5  5.18  24.3  14.1 
5  Silty sand  6.7  20  18  0  37 
6  Silty clay  6.2  20.3  6.03  33.8  10.5 
7  Silt  1.6  20.5  6.76  30.7  18.8 
8  Silt  10.4  19.3  7.90  17.0  25.8 
9  Silt  4.3  19.3  7.90  17.0  25.8 
10  Silty clay  2  20.0  5.25  23.1  16.3 
11  Silty sand  10  20  21  0  38 
The load cases for 3 MW wind turbines are shown in Table
Load cases.
Load case 





Load case A  4147.0  4688.2  91524.26  3280.3 
Load case B  5153.1  5598.5  126249.76  4428.4 
In the design of the foundation structure, different load effect combinations were selected to calculate different design content. A foundation featuring one central pile and six side piles was designed. The six side piles were evenly distributed along the control circle. The diameter of the central pile was 3 m, and its total length and buried depth were 67.5 m and 50 m, respectively. The diameter of the side pile was 1.5 m, and the total side pile length and buried depth were 60 m and 42.5 m, respectively. The diameter of the control circle was 10.4 m. The side piles inclined outwardly along a line that connected the center of the cap and the center of the central pile. The rake ratio of the side piles was 1 : 6.
To verify the feasibility and safety of this foundation, the proposed numerical simulation method was performed. In the simulation, the load cases shown in Table
The tension and compression stress distributions under Load Case A are presented in Figure
Tension and compression stress distribution in the pile.
Based on FEM postprocessing, the maximum compression stress was determined to be 9604.9 kN, and the maximum tension stress was determined to be 3752.1 kN. Incorporating a structure importance coefficient of 1.1 into the calculation, the maximum compression stress was calculated to be 10565.4 kN, which is less than the value of 10757.1 MPa calculated by the Code for Pile Foundation in Harbor Engineering. The maximum tension stress was determined to be 4127.3 kN, which is less than the value of 5493.3 MPa calculated by the Code for Pile Foundation in Harbor Engineering.
The horizontal displacement of the foundation and the vertical displacement of the cap are presented in Figures
Horizontal displacement of foundation.
Vertical displacement of cap.
Figure
Thus, all of the results obtained meet the specified requirements.
Based on the characteristics of offshore wind loads, a new highrise cap pile group foundation consisting of largediameter piles and inclined piles was proposed, which takes full advantage of the strong vertical bearing capacity of the largediameter piles and the strong horizontal bearing capacity and flexural capacity of the inclined piles. The influence of certain factors on the vertical bearing capacity, horizontal bearing capacity, and bending bearing capacity was studied through laboratory experiments and a numerical simulation method. The following conclusions can be drawn.
The primary factors affecting the bearing capacity of the new highrise cap pile group foundation were determined using an orthogonal numerical simulation test. The primary factors affecting the vertical bearing capacity are the inclination angle and length of the side piles. For the horizontal bearing capacity, the primary factors are the inclination angle and number of piles. For the bending bearing capacity, the primary factors are the length and inclination angle of the side piles.
A numerical simulation method was conducted to study the primary factors affecting the bearing capacity. The results were verified by laboratory experiments. An increase in the inclination angle of the side piles increases the vertical bearing capacity, the horizontal bearing capacity, and the bending bearing capacity. An increase in the length of the side piles increases the vertical bearing capacity and the bending bearing capacity. When the length of the side piles is close to the length of the central pile, the increase in the bearing capacity is more apparent. Increasing the number of piles increases the horizontal bearing capacity; however, the rate of growth is small because of the pile group effect.
A foundation consisting of one central pile (
The authors declare that there is no conflict of interests regarding the publication of this paper.
This study was supported by the Innovative Research Groups of the National Natural Science Foundation of China (51021004), the National Key Basic Research Program of China (973) (2014CB046800), the National High Technology Research and Development Program (863) (2012AA051702), and the International Science & Technology Cooperation (2012DFA70490).