## 4 Optimisation of Reinforcement Approach of Prestressed Anchor Cables on the Slope  
By analysing the changes in cable force under different prestresses, with the growth of the strength reduction factor of slope rock and soil masses, the force in cables increased from the slope foot to the slope crest. However, the incremental force closer to the slope foot was greater. By contrast, the force in cables in the upper-third did not change much, therefore they played little part in improving the safety factor of the slope and could be removed, so as to save money and reduce construction difficulty. To propose an optimised reinforcement scheme of prestressed anchor cables, the slope was divided into three parts from slope foot to the slope crest: the lower third is represented by D, the middle-third by M, and the upper-third by U. Figures 11 and 12 show the relationships between the displacement at the slope foot and the strength reduction factor with different numbers of cables under different prestresses. The legends All-500 kN and All-300 kN represent prestresses on all constructed cables from the lower to upper parts of the slopes as being 500 and 300 kN, respectively. The legend D-500 kN + M-100 kN means that the prestress on cables in the lower-third of the slope is 500 kN, the equivalent in the middle-third is 100 kN, and there are no cables in the upper-third. The meanings of the other legends are inferred accordingly. Figures 11b and 12b are enlarged drawings of the enlarged sections in Figures 11a and 12a, which helps when differentiating between the safety factors of slopes under different working conditions. By comparing two curves under All-300 kN and D-500 kN + M-100 kN, it can be seen that after removing cables in the upper-third, the safety factor of the slope rose from 1.47 to 1.53 by increasing the cable prestress in the lower-third and decreasing the cable prestress in the middle-third. This law further proves the above conclusion that the anchor cables near the foot of the slope is useful for strengthening the slope stability. It means that during the slip process the anchor cables in the lower part are first stressed and play a major role in anti-sliding, while the cables in the middle part also play a certain role in anti-sliding, but the cables in the upper part do not play an obvious role in anti-sliding. This is because in the sliding process of the slope, the internal force of the cable in the lower part increases obviously, and the closer to the top of the slope, the less obvious the increase of the internal force of the anchor cables becomes. Therefore, the safety factor of the slope can be increased by improving the cable prestress in the lower part. By comparing three curves under D-500 kN + M-100 kN, D-400 kN + M-200 kN and D-300 kN + M-300 kN, it can be found that when having no cables within the upper-third, although the total cable prestress was unchanged, the safety factor of the slope gradually declined (from 1.53 to 1.48 and finally 1.42) after reducing the cable prestress in the lower part and increasing the cable prestress in the middle part. This further implied that cables at the lower part of the slope played an important role in improving the safety factor. It is worth noting that, by comparing curves under All-300 kN, D-400 kN + M-200 kN, and D-300 kN + M-300 kN, it can be seen that the cable force during sliding of the slope did not change to any significant extent and that at the slope crest was slightly reduced. However, it was undeniable that directly removing cables in the upper part of the slope would decrease the safety factor there (comparing curves under All-300 kN and D-300 kN + M-300 kN). The main reason for this was that for sliding mass in the slope, cables at the upper part of the slope also provided a certain sliding resistance, however, this could be compensated for by increasing cable prestress in the lower part (curves under All-300 kN and D-400 kN + M-200 kN were similar).