## 1 Introduction  
As an active reinforcement technology, applying prestressed anchor cables is an important technological method used to reinforce a slope [1; 2; 3; 4; 5; 6]. On the one hand, prestressed anchor cables induce rock and soil masses on the surface of slopes to be under triaxial compression, thus increasing their strength. On the other hand, it restricts the sliding of potential sliding masses in slopes, thus improving slope stability. Research on the reinforcement mechanism of prestressed anchor cables on slopes contributes to grasping the practical effect of prestressed anchor cables. More importantly, it is conducive to improving the design of reinforcement schemes using prestressed anchor cables and enhancing the reinforcement effect of prestressed anchor cables, thus reducing unnecessary expense and decreasing the difficulty of their construction. Therefore, exploring the reinforcement mechanism of prestressed anchor cables on slopes exerts theoretical and practical significance. [PERSON] et al. [7] introduced cable prestress into the transfer coefficient method and they suggested that prestress generates compressive stress at the bottom, thereby increasing the sliding resistance. [PERSON] et al. [8] proposed an advanced limit equilibrium method, considering the tensile resistance of the reinforcement, to study the stability of a nailed slope. In their study, the normal and tangential force on and along the sliding surface were all considered, which means that they also believed the compressive stress on a sliding surface was nonnegligible. [PERSON] et al. [9; 10] pointed out that prestressed anchor cables provide sliding resistance for slopes and analysed the stability of slopes reinforced by using prestressed anchor cables from the perspective of stress on the whole sliding mass. Through numerical simulation, [PERSON] et al. [11] investigated the characteristics of stress distribution in rock mass in the tensioning of prestressed anchor cables and explained why the stability of high slopes in Three Gorges Shiplock improves under the reinforcement effect of prestressed anchor cables. By conducting simulations and numerical analysis on the anchoring effect of prestressed anchor cables, [PERSON] et al. [12] revealed the underlying universal working mechanism of anchoring technology. It can be seen that current research into the reinforcement mechanism of prestressed anchor cables on slopes has been carried out mainly from the perspective of stress state. On the one hand, it is thought that prestressed anchor cables directly strengthen the sliding resistance of slopes, thus improving the slope stability. On the other hand, prestressed anchor cables are believed to be able to improve the stress state of rock and soil mass within a certain range on the surface of slopes and control the slope deformation, thus enhancing slope stability. Based on understanding on relationships of stress field and displacement field of slopes with slope stability [13], some scholars proposed new design methods for reinforcing slopes by using cables. [PERSON] et al. [14] put forward an active slope reinforcement based on stress control on the sliding surface, which indirectly reflected the reinforcement effect of prestressed anchor cable in recovering the stress state of sloping rock and soil masses. [PERSON] et al. [15; 16; 17] proposed an optimisation design method of prestressed anchor cables for reinforcing slopes based on stress and displacement fields. They pointed out that prestressed anchor cables are used for improving the stress state of slope rock and soil masses so as to control slope deformation. [PERSON] et al. [18] showed that, to increase slope stability, it is necessary to reinforce key parts by applying reinforcement according to the distribution and magnitude of any unbalanced force. For high and steep slopes, the authors suggested that the reinforcement mechanism of prestressed anchor cables on slopes remains unclear.