**3. Relationship among the plant span, strength-stress ratio, and incremental cohesion of the surrounding rock**

#### **3.1 Incremental cohesion of the surrounding rock after anchor reinforcement**

As shown in **Table 2**, the physical and mechanical parameters, maximum principal stress, and anchor parameters of the underground plants of 29 hydropower stations, such as Jiangkou, Xiluodu, and Jinping I, are shown [6–36]. In **Table 2**, the maximum principal stress is taken as the maximum value near the main powerhouse. In accordance with the design principle of the "difference between arch and side wall", generally, the system anchors on the side wall of the main plant are used for statistical data. When the system anchor arrangement for the upstream and downstream side walls varies, the average value is taken as the statistical data. The incremental cohesion of the surrounding rock is calculated by Eq. (2), where *η* = 3.5, *τ<sup>s</sup>* = 200 MPa, and the strength-stress ratio in **Table 2** is a dimensionless constant.
