Design and Analysis of a New Prefabricated Foundation for Onshore Wind Turbines

Abstract

A new type of prefabricated foundation for onshore wind power was proposed in this paper. The stress and bearing mechanism of the new foundation was explored through theoretical calculation and finite element analysis. The results show that compared with the extended foundation in the same position, the amount of concrete in the new foundation is reduced by 30.00%, and the amount of rebars are reduced by 34.69%. The new prefabricated foundation has been inspected and calculated according to the specification. The calculation results indicate that the stress and initial reinforcement of the foundation meet the specification requirements. The bearing capacity, inclination rate, deformation, and stability of the foundation meet the requirements. Through finite element analysis, it is further confirmed that the structure meets the requirements of wind turbine operation, and the overall force meets the requirements of various indicators. The stress distribution of the foundation concrete and rebars is reasonable and uniform, and the load transfer is great. Also, the maximum stress of rebars and concrete does not exceed the specification limit, and the concrete remains intact without cracking or damage. The new foundation structural design and theoretical analysis were reasonable and accurate, and can be put into application in the future.

1. Introduction

At present, a new round of global energy and technological revolutions is in the ascendant. Vigorously developing renewable energy has become a major strategic direction for global energy transformation and the response to climate change. The global primary energy structure is rapidly moving towards diversification, cleanliness, and low carbon [1,2,3,4]. With the continuous growth of global energy transformation, wind power has developed rapidly and is poised to play a vital role in accelerating the global energy transition [5,6]. According to the Global Wind Report 2023 by the Global Wind Energy Council (GWEC), by the end of 2022, the cumulative installed capacity of global wind power reached 837 GW, of which onshore wind power was 780 GW and offshore wind power was 57 GW [6]. With more and more countries around the world developing the wind power business and wind power costs continuing to decline, the global wind energy industry will maintain a rapid development trend, and the global wind power installed capacity will continue to grow. In 2022, 77.6 GW of new wind power capacity was added worldwide, of which the installed capacity of onshore wind power was 68.8 GW. The global wind energy market is expected to grow by 15% on average per year (Figure 1) [6]. Compared with offshore wind power, the development cost of onshore wind power is about a quarter of offshore wind power, and the operation and maintenance costs are also lower [7].

The continuous growth in overall energy demand and the related environmental impacts play a significant role in the large sustainable and green global energy transition [8,9,10,11]. The development mode and trend of wind power show the characteristics of gradual large-scale development and base construction, which puts forward higher requirements for the cost, construction period, and convenient installation of wind turbine foundations [12]. The traditional cast-in-place concrete construction technology is inefficient, polluting, and difficult to ensure quality, making it unsuitable for the high-quality development of the wind power industry [13,14,15]. Changing the traditional form of cast-in-place reinforced concrete foundations and developing efficient, economic, and green prefabricated foundations is the inevitable trend of economic construction and sustainable development in the new period. It has great potential and market benefits, and will improve the infrastructure [16,17]. The concept of assembled wind turbine foundations is in line with the development requirements of building industrialization. The assembled wind turbine foundation adopts the construction method of standardized design and factory mass production, and it can solve the quality and discontinuous pouring problems caused by on-site mixing in remote mountainous areas due to the non-transportation of commercial mixing. While effectively ensuring the quality of the foundation, it significantly shortens the construction period, saves consumables, reduces energy consumption, and minimizes environmental pollution, which is conducive to the early production and power generation of the wind turbine. It can be recycled after the service period [12,18,19,20].

Currently, the main structural forms of onshore wind power assembled foundations include the assembled box girder foundation, the assembled raft foundation, the assembled spread foundation, the assembled multi-footing foundation, the assembled braced foundation, and the assembled tubular foundation (Figure 2) [12]. RUTE Foundation Systems has developed a fully assembled box girder foundation. The beam is divided into three or more different box-type prefabricated components, and has strong flexibility in the design and construction of the foundation [21]. The assembled raft foundation of onshore wind power has the advantages of a clear force transmission path, easy division of prefabricated components, and a small connection area. It is the mainstream form of onshore wind power assembled foundation. RWE proposed a square prefabricated raft foundation with prefabricated beams and plates, which is divided into four types of prefabricated modules [22]. ANKER has developed a variety of fully assembled raft foundation structures [23]. Wu Xiangguo proposed to divide the complete main body of the extended foundation into two types of prefabricated concrete members through modularizing the extended foundation [24]. RUTE Foundation Systems has developed a prefabricated multi-footing foundation with post-tensioned prestressed connections. There are three different beam end support methods, the structure can significantly decrease material and time costs, and the overall stability is good. However, the shear strength of the joint surface of the pillar and rib beam does not meet the requirements under certain working conditions [25,26]. Ma Renle proposed a prefabricated tubular foundation. It has been proved that the structure is simple, the construction speed is fast, the cost is low, and it has good bending resistance [27,28]. The onshore wind power assembly support foundation originates from the Spanish company ESTEYCO. The foundation consists of a base, support, central tube, and roof. Only the support structure is prefabricated, and the on-site engineering quantity is still large. Compared to the traditional cast-in-place foundation, the construction period is shorter [29].

In this paper, we combined the mechanical characteristics of the existing assembled foundation structure with the advantages of raft foundation, taking into account the ease of transportation and construction. We proposed a new type of prefabricated foundation for onshore wind power to replace the existing raft foundation type, which effectively spreads the force and decreases the overall weight of the structure. It has certain advantages in transportation, economy, and hoisting. The main research contents of this paper are as follows: A new prefabricated foundation structure for onshore wind power was proposed, and its design and layout explained. The force and bearing mechanism of the new prefabricated foundation was explored through theoretical calculation and finite element analysis.

2. New Prefabricated Foundation for Onshore Wind Turbines

2.1. Foundation Structure

An onshore wind farm is situated in the hilly and gully area of Loess Plateau, with the ground elevation of the wind turbine ranging from 1763 to 1980 m. Most of the wind turbines in the field are located in the cultivated land formed through man-made transformation of loess beams, mound tops, or loess beams and mounds. The foundation form is mainly expansion foundation. In order to overcome the problems of a large engineering volume and complicated construction of the extended foundation, and combined with the structural form of prefabricated single-ribbed raft foundation (Figure 3), a new prefabricated assembled foundation is innovatively proposed in this paper. Compared to the single-ribbed raft foundation, the new prefabricated foundation integrates the prefabricated girder slab foundation and the table pillar section as a whole, reducing the number and types of prefabricated components and improving construction efficiency.

The new prefabricated foundation consists of 16 blocks, and each block consists of two rib beams and a pillar. The diameter of the base is 21.6 m, the diameter of the pillar is 6.8 m, the thicknesses of the baseplate and rib beams are 0.4 m and 0.25 m, and the volume of one block is 22.55 m³. The structural form and dimensions are shown in Figure 4.

The concrete consumption of one prefabricated foundation is approximately 360.84 m³ and 45.28 t of rebars. The typical circular extended foundation for this site is 3.5 m in height and 18 m in diameter, and is connected with the upper tower through prestressed anchors. The extended foundation at the same position requires approximately 515.47 m³ of concrete and 52.46 t of rebars. The comparison of the two foundations in terms of material consumption is shown in

Table 1. Comparison of two foundation materials.

Material Types	Extended Foundation	New Prefabricated Foundation	Reduce (%)
C40 concrete (m3)	515.47	360.47	30.00
rebar (t)	52.46	45.28	13.69

. The new foundation reduces concrete by 30.00% and rebars by 13.69% compared with the extended foundation. Since the prefabricated blocks of the new foundation are fabricated in the factory and tested for quality before being sent to the site, the site only needs to be installed and spliced according to the requirements, so the construction efficiency and engineering quality of the new foundation are controllable and can be guaranteed.

The anchor bolt hole should be reserved in one single block, and the positioning and temporary internal support should prevent the displacement and deformation of the concrete during the pouring process. Before pouring the concrete, it is necessary to adjust the level and verticality of the anchor bolt hole and connect the new foundation with the up and down anchor plates through the anchor bolt. During the pouring process, continuous pouring should be carried out in layers without leaving construction joints, and the dimensions of each part of the component should be strictly controlled and uniformly accepted. The convex and concave shear keys between the pillars can be embedded with each other. The top, middle, and bottom layers of the pillars are equipped with six 1860-grade circumferential prestressed steel strands with a diameter of 15.2 mm. The six tension control stresses are 1280 MPa, and the ring anchors are fixed. The prestressed steel strands are protected with anti-corrosion grease and sheaths. The prestressed system should be tensioned in the order of middle-bottom-top layers. After tensioning the prefabricated block, the open space needs to be filled and repaired with high-level elevation concrete.

2.2. Construction Process of the New Prefabricated Foundation

The schematic diagram of each part of the structure is shown in Figure 5.

The construction process is shown in Figure 6: assembly of the new prefabricated foundation blocks, anchor rods connecting the up and down anchor plates with the foundation, grouting between pillars, prestressing tensioning, grouting on the top of the pillars, and anchor tensioning are performed in the sequence of operations.

The 16 prefabricated blocks of the new foundation are positioned and placed, and embedded using shear keys for initial connection. Before grouting between pillars, each component needs to be connected and tightened using anchors. During the grouting process, it is important to check the fasteners. Before grouting, clean up the debris in the gap and pour the grouting between pillars. Then the anchors can be passed through the reserved circumferential holes. When the strength of the grouting material between the pillars reaches 80%, the tension of the circumferential prestressed system can be carried out to enhance the integrality of the foundation.

Finally, the up and down anchor plates and prefabricated foundation blocks are connected using anchors and bolts, grouting at the top of the pillars, and then tensioning the anchor bolts, which plays a positive role in the safety, stability, and durability of the later operation of the wind turbine.

In addition, the new foundation adopts large-scale intensive production, which can save consumables, and reduce energy consumption and construction waste to a certain extent. The new prefabricated foundation plate effectively solves the problems of a long construction period, slow construction speed, and difficult formwork support, and has the advantages of a simple structure and convenient installation. Its mechanized installation in the construction process can reduce air, noise, wastewater discharge, and other pollution, as well as reduce carbon emissions throughout the building’s life cycle.

3. Theoretical Calculations

3.1. Wind Turbine Loads

The operating wind turbine is subjected to 360° loads and large eccentric forces, which will produce great loads on the tower and the foundation [31,32]. From the viewpoint of the structure system, wind turbines belong to structures with a “heavy head” [31]. The complex load on the top of the wind turbine is transmitted to the foundation top through the tower, and the load is transformed into horizontal load, vertical load, bending moment, and torque composite load. The wind turbine foundation GL coordinate system is shown in Figure 7.

The standard foundation load values for 4.20 MW wind turbines at an onshore wind farm were used for the calculation and analysis, as shown in

Table 2. Standard values for wind turbine loads (k₀ = 1.0).

Loading Conditions	Wind Turbine Loads
	Horizontal Load Fxy (kN)	Vertical Load Fz (kN)	Bending Moment Mxy (kN·m)	Torsion Mz (kN·m)
Normal operation of the wind turbine	626.979	4035	63,142.9	821.28
Extreme operation of the wind turbine	798.868	4029.39	83,612.3	743.42

.

Since the installed capacity of the wind turbine is 4.2 MW, greater than 2.5 MW, it meets the safety level of first-level infrastructure required by the specification. Therefore, when calculating the structure under the extreme operating conditions basic combination, it is necessary to take the structural importance coefficient into account. The specific load combination is shown in

Table 3. Calculated load combinations for wind turbine foundations [33].

Conditions	Operational State	Load Combinations	Structural Importance Coefficient k0
1	Normal operating loads	Standard combination	Disregard
2	Extreme loads	Standard combination	Disregard
3	Extreme loads	Basic combination	1.1

.

3.2. Theoretical Calculations

The design of a wind turbine foundation adopts the limit state design method so that the overall size meets the specification construction requirements, but also to carry out the foundation bearing capacity calculation, foundation stability calculation, rebars calculation of the foundation structure, material strength calculation, and crack width calculation [34].

3.2.1. Calculation of Structural Stability

The raft foundation mainly resists the deformation of the foundation through the stiffness of the beams and resists the overturning moment through the self-weight of the filling between the foundation and the ribbed beams. The calculation method is similar to the design method of gravity-type extended foundation [33]. According to the Code for Design of Wind Turbine Foundation for Onshore Wind Power Projects (NB/T 10311-2019), the calculation formulas of eccentric load on a circular foundation are as follows:

$$P_{\mathrm{kmax}}=\frac{N_{\mathrm{k}}+G_{\mathrm{k}}}{A}+\frac{M_{\mathrm{k}}}{W},$$

(1)

$$P_{\mathrm{kmin}}=\frac{N_{\mathrm{k}}+G_{\mathrm{k}}}{A}-\frac{M_{\mathrm{k}}}{W},$$

(2)

When calculated for the standard combination of normal load conditions, P_kmin > 0, the bottom surface of the foundation is not separated from the foundation. The characteristic value of the bearing capacity of the foundation bearing layer should be greater than 220 kPa. The actual bearing capacity of the surface foundation on site is about 153 kPa, and the foundation treatment is required.

Under the standard combination of extreme load conditions, P_kmin < 0, the bottom surface of the foundation is separated from the foundation, and the ratio of the base separation area is 3.49%. For the raft foundation, it is subjected to eccentric load outside the core area. The bottom surface of the foundation is partially separated, and the separation area is not more than 1/4 of the total area. The bottom pressure of the circular foundation of the wind turbine is calculated according to Formula (3), and the resulting bottom pressure of the foundation is 244.39 kPa.

$$P_{\mathrm{kmax}}=\frac{N_{\mathrm{k}}+G_{\mathrm{k}}}{\xi R^2},$$

(3)

The anti-sliding force of the foundation bottom surface is calculated using Formula (4). Under the standard combination of normal operating load conditions, the anti-sliding force of the foundation bottom surface F_R is calculated as 12,536.95 kN. Under the standard combination of extreme load conditions, the foundation’s bottom surface anti-sliding force F_R is 12,534.71 kN. The friction coefficient between the bottom of the foundation and the ground is 0.4 (the lower limit value) [35].

$$F_{\mathrm{R}}=\mu \times N_{\mathrm{k}},$$

(4)

$$\frac{F_{\mathrm{R}}}{{\gamma }_0{F}_{\mathrm{S}}}\ge {\gamma }_{\mathrm{d}},$$

(5)

The anti-sliding force and sliding force on the most dangerous sliding surface of anti-sliding stability should satisfy Formula (5). The ratios of F_R and F_s are 18.18 and 14.26, which are greater than the structural coefficient of 1.3 (${\gamma }_{\mathrm{d}}$). Therefore, the foundation’s anti-slip stability meets the requirements.

The anti-overturning moment of the foundation should be calculated according to Formula (6). The foundation’s anti-overturning moment is calculated to be 338,497.76 kN·m under the standard combination of normal operating load conditions. Under the standard combination of extreme load conditions, the foundation’s anti-overturning moment is 338,437.17 kN·m. The anti-sliding force and sliding force on the most dangerous sliding surface of anti-sliding stability should meet Formula (7). The ratios of M_R and M_s are 4.68 and 3.54, which are greater than the structural coefficient of 1.6 (${\gamma }_{\mathrm{d}}$). Therefore, the foundation’s anti- overturning stability meets the requirements.

$$M_{\mathrm{R}}=N_{\mathrm{k}}\times R,$$

(6)

$$\frac{M_{\mathrm{R}}}{{\gamma }_0{M}_{\mathrm{S}}}\ge {\gamma }_{\mathrm{d}},$$

(7)

In calculating the foundation settlement deformation, the circular cross section of the foundation is equated to a rectangular cross section by area for ease of calculation, and its side length D is calculated as:

$$D=\sqrt{\frac{\pi d^2}{4}},$$

(8)

where d is the diameter of the new prefabricated foundation, so the equivalent rectangular section side length D is 19.14 m. According to the Code for Design of Wind Turbine Foundation for Onshore Wind Power Projects (NB/T 10311-2019), the settlement value can be calculated using the Formulas (9) and (10):

$$s={\psi }_s{s}^{\prime }={\psi }_s{\displaystyle \sum _{i=1}^n\frac{p_{0k}}{E_{si}}}(z_i\overline{{\alpha }_i}-z_{i-1}\overline{{\alpha }_{i-1}}),$$

(9)

$$p_{0k}=\frac{F_{zk}+G_k}{A_s},$$

(10)

The additional stress P_0k at the base of the foundation is calculated to be 86.77 kPa. The empirical coefficient of pile settlement ψ_s at this point is calculated to be 0.99 through linear interpolation, resulting in a final settlement deformation s of 42.04 mm for the foundation. The tower height of the designed wind turbine is 107 m. According to the Design Code for Wind Turbine Foundations for Onshore Wind Farm Engineering (NB/T 10311-2019), the allowable value of foundation settlement is 100 mm when 90 m < H ≤ 120 m. The foundation’s settlement is 42.04 mm, which is in line with the code requirements.

The poor soil quality is not enough for the stress requirements of the foundation, so it is not suitable to use natural ground. Considering the calculation of the base processing of the prototype, f_spk = 257.32 kPa and the characteristic value of the bearing capacity of the composite foundation is considered to be 250 kPa.

3.2.2. Calculation of Rebars

When the rebars of the foundation are determined, the load effect is taken according to the basic combination of the load under the ultimate state of bearing capacity. The bottom rebars of the baseplate are calculated according to the two working conditions of the bearing capacity of the one-way plate and the minimum rebar ratio of the one-way plate. The baseplate’s bending capacity is calculated according to the fixed one-way plate at both ends, and the plate strip with a width of 1 m in the middle is calculated. According to the Static Calculation Handbook for Practical Structure [36], the maximum net reaction force at the base edge P_jmax is calculated as 163.98 kPa according to Formula (11).

Formula (12) calculates the net reaction force at the bottom of the foundation P_1′, which equals 121.52 kPa. The net reaction width of the foundation bottom surface a_c^′ is 12.42 m, the circumferential bending moment of the bottom surface is 37.74 kN·m, the circumferential reinforcement area of the bottom surface of the baseplate is 285.40 mm² (structural reinforcement), and the radial reinforcement area of the bottom surface of the baseplate is 600 mm² (minimum reinforcement). Therefore, the design of the reinforcing baseplate bottom circumferential reinforcement is 16@150 (unit length of reinforcement area of 1340 mm²), and the bottom radial reinforcement is 3 roots 16 (unit arc length radial reinforcement area of 603 mm²).

Similarly, the reinforcement calculation of the top surface of the baseplate is carried out. Under the action of self-weight, the pressure on the tensile side is 99.87 kPa, and the circumferential bending moment of the top surface is 31.02 kN·m. The circumferential reinforcement area of the top surface of the baseplate is 234.24 mm², and the radial reinforcement area of the top surface of the baseplate is 600 mm². Therefore, the circumferential reinforcement of the top surface of the baseplate is 14@150 (unit length of reinforcement area of 1026 mm²), and the radial reinforcement of the top surface of the bottom surface is 3 roots 16 (unit arc length radial reinforcement area of 603 mm²).

$$P_{\mathrm{jmax}}=P_{\mathrm{kmax}}-\frac{G_{\mathrm{k}}}{A},$$

(11)

$$P_1^{\prime }=\frac{a_{\mathrm{c}}^{\prime }-\frac{D}{2}(1-\cos \frac{\pi }{n})-h}{a_{\mathrm{c}}^{\prime }}{P}_{{ }_{\mathrm{jmax}}}^{ },$$

(12)

The rib beam reinforcement is divided into a bottom and top reinforcement calculation, rib beam oblique section shear, and a rib beam bending bearing capacity calculation. The height of the beam at the root of the ribbed beam is 3500 mm, the beam width is 250 mm, and the reinforcement area at the bottom of the ribbed beam is 6141.08 mm². Therefore, the reinforcement at the bottom of the ribbed beam is designed to be 9 roots 32, and the reinforcement area is 7238 mm². The reinforcement area at the top of the ribbed beam is 3129.22 mm², so the reinforcement at the top of the ribbed beam is designed to be 4 roots 32, and the reinforcement area is 3217 mm². It is calculated that the shear force at the root of the ribbed beam is 1474.69 kN, the stirrup of the ribbed beam is 12@200, and the design value of the shear capacity is 8166.37 kN, which is greater than the shear force at the root of the ribbed beam. The bending capacity is 7531.77 kN·m, and the shear capacity of the inclined section is 2465.97 kN, both of which are greater than the design value, so the reinforcement is reasonable and meets the requirements.

4. Structural FEM Analysis

4.1. Finite Element Model

We used the finite element software ABAQUS (https://www.3ds.com/products/simulia/abaqus) to establish the prototype finite element model including foundation, rebars, and soil. The whole finite element model is shown in Figure 8. Specific details and dimensions are shown in Figure 4: the diameter of the base is 21.6 m, the diameter of the pillar is 6.8 m, and the thicknesses of the baseplate and rib beams are 0.4 m and 0.25 m.

The foundation is simulated using Concrete Damaged Plasticity (CDP), and the foundation and soil are meshed using an eight-node reduced integral solid element (C3D8R). The foundation material is C40 concrete and the cushion material is C20 concrete. The steel structure and rebars are simulated using a linear elastic model, and the solid element (C3D8R) and the truss element (T3D2) are used for meshing. The meshing is shown in Figure 9, with a total number of 151,507 elements, 40,665 linear hexahedral elements of type C3D8R, and 110,842 linear line elements of type T3D2.

The physical parameters of the soil in the actual project are taken, and the ideal elastic–plastic constitutive model based on the Mohr–Coulomb yield criterion is used for simulation. The soil parameters are shown in

Table 4. Soil parameters.

Layer of Soil	Soil Height (m)	Density	Elastic		Mohr Coulomb Plasticity
		Mass Density (kg·m−3)	Young’s Modulus (MPa)	Poisson’s Ratio	Friction Angle (°)	Plasticity Angle (°)
1	4	1420	26.70	0.35	30.30	0.1
2	9	2000	77.85	0.35	49.80	0.1
3	3	2000	90.45	0.35	48.45	0.1
4	4.5	1540	44.40	0.35	27.60	0.1
5	13.5	1594	48.72	0.35	29.52	0.1
6	6	1668	54.11	0.35	28.70	0.1

. The diameter of the soil body is 60 m and the height is 40 m. The bottom surface of the soil is fully fixed (U1 = U2 = U3 = UR1 = UR2 = UR3 = 0) and the side surfaces are horizontally restrained (U1 = U2 = UR3 = 0).

In the actual construction process, it is necessary to grout the pillar part. In order to make the finite element analysis more consistent with the actual situation, tie contact is used between the pillars in the finite element model, and no contact is set between the baseplates. Surface-to-surface contact is used between the soil and the foundation cushion to simulate the interaction. The friction coefficient is 0.35.

4.2. Structural Finite Element Analysis

Foundation structure forces under normal standard loads and extreme operating conditions were calculated. The results are shown in Figure 10. From Figure 10a, it can be seen that under normal operating conditions, the vertical displacement at the bottom of the prototype foundation and the vertical displacement at the top flange are 10.562 mm and 4.379 mm, and the tilt rates are 0.49‰ and 0.89‰. The allowable value of the tilt rate of the onshore wind turbine foundation with 90 < H ≤ 120 m is 3‰, so the tilt rate of the new prefabricated foundation meets the requirements.

Figure 10b shows that under extreme operating conditions, the maximum compressive stress of the foundation is 18.80 MPa, which is less than the design value of C40 concrete compressive strength f_ck = 19.1 MPa and meets the requirements. The maximum tensile stress of the foundation is 2.82 MPa, which is greater than the design value of C40 concrete tensile strength of C40 concrete f_tk = 2.39 MPa. Therefore, the crack width is further checked. The formula is:

$${\omega }_{\max }={\alpha }_{cr}\psi \frac{{\sigma }_s}{E_s}\left(1.9c_s+0.08\frac{d_{eq}}{{\rho }_{te}}\right)$$

(13)

The calculated maximum crack width at the top of the beam ${\omega }_{\max }$ is 0.029 mm, which is less than 0.2 mm, meeting the requirement.

From Figure 10, it is clear that the maximum stress on the compression side is located at the intersection of the grout layer and the pillar. This is because the annular grouting layer bears the wind turbine load transmitted from the up anchor plate, resulting in a relatively concentrated stress at this position. In addition, the compressive stress of the concrete on the compression side is evenly spread across the adjacent foundation blocks. This shows that the new preassembled foundation structure is still a whole structure and that the foundation blocks have not moved or separated. In the new prefabricated foundation, in addition to the large compressive stress at the junction of the grouting layer and the pillar, there is also a large compressive stress at the junction of the column and the rib beam, which is manifested as the trend of diffusion along the junction of the pillar and the rib beam to the baseplate. It is further explained that the stress distribution of the foundation structure is relatively uniform and reasonable, and there is no obvious local stress concentration or abnormal situation. Such stress distribution characteristics ensure the stability and safety of the infrastructure, providing reliable support for the normal operation of the wind turbine.

The stress range of the up and down anchor plates is 2.28~201.60 MPa, which meets the requirements. The maximum soil pressure at the bottom of the foundation is 276.30 kPa, and the edge of the bottom plate to the main wind direction is a large area of soil pressure. The theoretical calculation of the maximum soil pressure of the foundation is 257.32 kPa, with an error of 6.97%.

The stress range of the main rebars in the foundation is 9.16~109.90 MPa. The stress contour of the rebars in each part of the foundation is shown in Figure 11. It can be seen from Figure 8 that the overall stress distribution of the new prefabricated foundation structure is uniform and reasonable, and it is symmetrically distributed along the loading direction. The stress of foundation rebars diffuses uniformly along the loading direction and is similar to the stress distribution of concrete. The maximum stress positions of the annular and radial rebars in the up and down layers of the baseplate are relatively close, and the rebars stress are 39.26 MPa and 39.22 MPa, which indicates that the reinforcement design of the baseplate is reasonable, and the load transfer law is good. In the prototype structure of the new prefabricated foundation, the stress is relatively concentrated at the junction of the pillar and rib beam. Therefore, in the later indoor scale model test, we should pay attention to this position to ensure the stability and safety of this area.

From the above finite element analysis, it is evident that the design of the new prefabricated foundation structure is reasonable, and it can be put into production and application.

5. Conclusions

In this paper, the rationality and superiority of the designed new prefabricated foundation are verified through theoretical calculations and finite element analysis, and the force characteristics of the new prefabricated foundation are also studied.

(1)

A new type of prefabricated foundation for onshore wind turbines is proposed. The new prefabricated foundation consists of 16 prefabricated blocks. Compared with the extended foundation at the same position, the concrete amount is reduced by 30.00%, and the rebars amount is reduced by 34.69%, significantly reducing the amount of reinforcing steel and concrete. At the same time, the new prefabricated foundation blocks can be precast in bulk indoors by means of a precast concrete plant, which can significantly reduce costs and ensure quality. The new prefabricated foundation combines the prefabricated girder slab foundation and the table pillar section as a whole, reducing the number and type of prefabricated components while improving construction efficiency.

(2)

Based on the specification, the new prefabricated foundation was inspected and calculated. The calculation results indicate that the foundation’s stress and reinforcement meet the specification requirements. Additionally, structural calculations based on several specifications show that the new prefabricated foundation meets the design requirements of the specifications in terms of bearing capacity, stability against overturning, stability against sliding, settlement, and other factors. The new prefabricated foundation is stable and safe enough.

(3)

Based on the finite element analysis software ABAQUS, the rebars stress, concrete stress, and foundation inclination rate of the new prefabricated foundation were analyzed. Through the analysis of the stress distribution of foundation concrete and rebars, it can be found that the stress distribution is reasonable and uniform, and the load transfer is fine. The maximum stress of the rebars and concrete does not exceed the specification limit, and the concrete does not crack.

(4)

The tilt rates of the bottom flange of the foundation and the bottom flange of the fan are 0.49‰ and 0.89‰, respectively, which do not exceed the normative permissible value of 3‰, and the tilt rate of the foundation meets the specification requirements. The design of the new prefabricated foundation is reasonable. It meets the specification requirements and the actual bearing demand, and it has great mechanical properties and stability. It provides theoretical support for subsequent measurement of indoor experiments and field construction.