# Numerical Evaluation of Structural Safety for Aged Onshore Wind Foundation to Extend Service Life
## 3 Structural Safety Evaluation for Aged Wind Foundation
### Soil Resistance, Overturning, and Sliding  
(1) Soil resistance  
The onshore wind foundation should be installed on a good soil layer, and it is important to ensure the bearing capacity of the soil layer against the maximum vertical load acting on the foundation ground. In other words, the vertical ground reaction force generated on the bottom of the foundation due to the ultimate turbine load should lie below the allowable bearing capacity of the foundation ground [13; 14; 15]. In this paper, the soil resistance of foundation soil layer was evaluated by comparing the maximum vertical load (q\({}_{\text{max}}\)) generated at the bottom of the foundation with the allowable bearing capacity (q\({}_{\text{a}}\)) of the foundation ground calculated by the design criteria [13; 14; 15], as indicated in Equation (4). The targeted ground condition of this study is soft rock (basal), and the allowable bearing capacity of this soil layer is given as 200 kN/m\({}^{2}\) in the design condition. \[\text{R}_{\text{q}}=\text{q}_{\text{max}}/\text{q}_{\text{a}} \tag{4}\]  
As a result of the evaluation, the maximum vertical reaction force generated on the bottom of the foundation resulted in 74.1 kN/m\({}^{2}\), and the allowable bearing capacity of the foundation soil layer was calculated to be 200 kN/m\({}^{2}\) by design condition. Therefore, the maximum vertical reaction force generated on the bottom of the foundation due to the ultimate turbine load was estimated to be about 37% of the allowable bearing capacity of the foundation soil layer and the supporting resistance of the soil layer was evaluated to be sufficient. (2) Overturning  
Another important aspect of the safety of the onshore wind foundation is overturning. It is important to secure the overturning resistance to the maximum overturning moment generated in the foundation by the turbine load [24]. In other words, the overturning moment generated on the bottom of the foundation due to the ultimate turbine load should lie below the allowable overturning capacity of the foundation [13; 14; 15]. In this paper, the overturning safety of foundation was evaluated by comparing the maximum overturning moment (M\({}_{\text{max}}\)) generated at the bottom of the foundation with the allowable overturning capacity (M\({}_{\text{a}}\)) of the foundation calculated by the design criteria [13; 14; 15], as indicated in Equation (5). The allowable overturning capacity means the resistance to overturning is only from the self-weight of the foundation and it can be easily calculated by multiplying the total self-weight of the foundation and the overturning moment arm length. \[\text{R}_{\text{M}}=\text{M}_{\text{max}}/\text{M}_{\text{a}} \tag{5}\]  
As a result of the evaluation, the maximum overturning moment generated on the bottom of the foundation resulted in 10,473 kN-m and the allowable overturning capacity of the foundation was calculated to be 21,881 kN-m by the design criteria. Therefore, the maximum overturning moment generated on the bottom of the foundation due to the turbine load was estimated to be about 48% of the allowable overturning capacity of the foundation and the overturning safety of foundation was evaluated to be sufficient. (3) Sliding  
It is important to secure the sliding resistance of the foundation against the lateral load generated by the ultimate turbine load. The sliding of the onshore wind foundation resists only by the frictional force on the bottom of the foundation. Therefore, the maximum shear force generated on the bottom of the foundation due to the ultimate turbine load should lie below the allowable shear (frictional) force of the foundation ground [13; 14; 15]. In this paper, the sliding safety of foundation was evaluated by comparing the maximum shear force (V\({}_{\text{max}}\)) generated at the bottom of the foundation with the allowable shear (frictional) force (V\({}_{\text{a}}\)) of the foundation ground calculated by the design criteria [13; 14; 15], as indicated in Equation (6).