# A Review of Road Design for Wind Farms in China
## 3 Research on Circular Curve Design
### Radius of a Circular Curve  
At present, there are two main directions to study the minimum radius of a circular curve of wind farm roads. The first is to analyze the lateral slip stability and lateral overturning stability of vehicles when driving and consider the impact of tail flick on this basis. For example, [PERSON] [17] systematically studied the influence of the traverse slip stability, traverse overturning stability, and tail flick suspension steady turning by referring to the turning stability and driving track of the typical vehicles. The research shows that the turning radius of the horizontal curve of the fan transport road is mainly affected by the tail flick suspension steady turning. The general minimum radius and the limit minimum radius of the horizontal curve of the fan transport road are recommended (Table 1). [PERSON] [20] analyzed the design requirements of the circular curve radius from three aspects of tail flick suspension steady turning, lateral sliding stability, and overturning resistance based on the representative transport vehicles and blade sizes at home and abroad at present. [PERSON] [24] and others studied the maximum value and limit value of the horizontal curve turning radius of the wind power transport road from two aspects of lateral slip and lateral overturn, taking the actual project as the background, combining the investment cost composition of the wind farm, the characteristics of wind power equipment, and the performance of transport vehicles. They concluded that the value of the horizontal curve radius of the wind power transport road is mainly affected by the vehicle driving track during the steady-state turning and the special vehicle for the transport of wind turbine blades.  
The second is to determine the minimum radius of the circular curve based on the widening limit. For example, based on the design experience of the wind farm, [PERSON] [14] proposed that the minimum radius of the inner and outer bends of the horizontal curve should be determined by no more than a 10 m increase in width of the horizontal curve, and calculated the minimum radius of 750 kW, 1500 kW, and 2500 kW fans, as shown in Table 2. [PERSON] [21] and others put forward the method of selecting the radius of the circular curve with the road widening value as the limiting condition when the filling and excavation is limited. Taking a road width of less than 3 m as an example, they put forward the recommended value of turning half diameter of \"steering rear wheel\" and \"non steering rear wheel\" vehicles under different wheelbase and angle conditions, which is 20-80 m.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Speed/(km\(\cdot\)h\({}^{-1}\))** & **General Minimum Radius/m** & **Ultimate Minimum Radius/m** \\ \hline
10 & 40 & 30 \\
12 & 60 & 35 \\
15 & 90 & 45 \\
18 & 130 & 60 \\
20 & 160 & 80 \\
22 & 190 & 100 \\
25 & 250 & 120 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Recommended minimum radius of a horizontal curve [17].  
Table 1 shows the recommended values of the minimum radius under the conditions of lateral sliding and overturning, and Table 2 shows the recommended values of the minimum radius under the conditions of widening limit when the speed is 15 km/h. It can be found from the values in the two tables that the radius in Table 2 is smaller. In addition, considering the lateral slip and overturning conditions, it is applicable to roads with high speed. The terrain conditions of the wind farm are relatively complex. If it is impossible to change the geometric layout of the road or remove the obstacles that restrict visibility, the designer will reduce the speed limit [25]. The design speed of the wind farm road is generally 15 km/h, and the impact of lateral sliding and overturning is small. However, due to the large size of the wind power equipment, it has higher requirements for the horizontal traffic space of the road. Therefore, the minimum radius determination method limited by widening is more suitable for the road design of wind farms.